Title:
High strength cold rolled steel sheet and method for manufacturing the same
Kind Code:
A1


Abstract:
A high strength cold rolled steel sheet consists essentially of 0.0040 to 0.01% C, 0.05% or less Si, 0.1 to 1.0% Mn, 0.01 to 0.05% P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.004% or less N, 0.01 to 0.14% Nb, optionally 0.05% or less Ti, optionally 0.05% or less B, by weight, and a balance of substantially Fe and inevitable impurities, and satisfying the following formulae (6) and (7):

(12/93)×Nb*/C≧1.2 (6),

TS−4050×Ceq≧−0.75×TS+380 (7),

wherein Nb*=Nb−(93/14)×N, Ceq=C+(1/50)×Si+(1/25)×Mn+(1/2)×P, wherein TS is the tensile strength in MPa, and C, Si, Mn, P, N and Nb denote the content in % by weight of carbon, silicon, manganese, phosphorus, nitrogen, and niobium, respectively. The high strength cold rolled steel sheet has excellent combined formability, resistance to embrittlement during secondary operation, formability at welded portions, and anti-burring performance, and has a desirable surface appearance and uniformity of material in a coil, and thus can be desirably used for automobile exterior panels.




Inventors:
Fujita, Takeshi (Fukuyama, JP)
Kitano, Fusato (Fukuyama, JP)
Hosoya, Yoshihiro (Fukuyama, JP)
Inazumi, Toru (Fukuyama, JP)
Yamasaki, Yuji (Fukuyama, JP)
Morita, Masaya (Fukuyama, JP)
Nagataki, Yasunobu (Fukuyama, JP)
Hasegawa, Kohei (Fukuyama, JP)
Matsuda, Hiroshi (Fukuyama, JP)
Ono, Moriaki (Fukuyama, JP)
Application Number:
10/630479
Publication Date:
02/05/2004
Filing Date:
07/29/2003
Assignee:
NKK CORPORATION (Tokyo, JP)
Primary Class:
International Classes:
C21D8/02; C22C38/00; C22C38/04; C22C38/06; C22C38/12; (IPC1-7): C21D1/00
View Patent Images:



Primary Examiner:
YEE, DEBORAH
Attorney, Agent or Firm:
HOLTZ, HOLTZ & VOLEK PC (NEW YORK, NY, US)
Claims:

What is claimed is:



1. A high strength cold rolled steel sheet consisting essentially of 0.0040 to 0.01% C, 0.05% or less Si, 0.1 to 1.0% Mn, 0.01 to 0.05% P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.004% or less N, 0.01 to 0.14% Nb, and optionally 0.05% or less Ti, optionally 0.002% or less B, by weight, and a balance of substantially Fe and inevitable impurities; and satisfying the following formulae (6) and (7): (12/93)×Nb*/C≧1.2 (6) TS−4050×Ceq≧−0.75×TS+380 (7), wherein Nb*=Nb−(93/14)×N, Ceq=C+(1/50)×Si+(1/25)×Mn+(1/2)×P, TS denotes the tensile strength in MPa, and C, Si, Mn, P, N, and Nb denote the content in % by weight of carbon, silicon, manganese, phosphorus, nitrogen and niobium, respectively.

2. The high strength steel sheet of claim 1, further containing 0.05% or less Ti, by weight.

3. The high strength steel sheet of claim 1, further containing 0.002% or less B, by weight.

4. A high strength cold rolled steel sheet consisting essentially of: 0.0040 to 0.01% C, 0.05% or less P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.004% or less N, 0.03% or less Ti, by weight, and Nb in an amount satisfying the following formula (8): 1≦(93/12)×(Nb/C)≦2.5 (8), wherein C and Nb denote the content in % by weight of carbon and niobium, respectively; the high strength cold rolled steel sheet having a volumetric proportion of NbC of 0.03 to 0.10%; and 70% or more thereof being in a size of 10 to 40 nm.

5. A method for manufacturing a high strength cold rolled steel sheet, comprising the steps of: (a) preparing a continuous casting slab of a steel which consists essentially of 0.0040 to 0.01% C, 0.05% or less P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.004% or less N, 0.03% or less Ti, by weight, and Nb in an amount satisfying the following formula (8): 1≦(93/12)×(Nb/C)≦2.5 (8), wherein C and Nb denote the content in % by weight of carbon and niobium, respectively; (b) preparing a hot rolled steel sheet by finish rolling the slab from step (a) at reduction ratios satisfying the following formulae (9) through (11): 10≦HR1 (9) 2≦HR2≦30 (10) HR1+HR2−HRHR2/100≦60 (11), wherein HR1 and HR2 denote the reduction ratio in % in the finish rolling at the pass just before the final pass and the final pass, respectively; (c) cold rolling the hot rolled sheet from step (b) and (d) annealing the sheet from step (c).

6. A high strength steel sheet consisting essentially of 0.0040 to 0.010% C, 0.05% or less Si, 0.10 to 1.5% Mn, 0.01 to 0.05% P, 0.02 or less S, 0.01% to 0.1% sol.Al, 0.00100% or less N, 0.036 to 0.14% Nb, and optionally containing 0.0015% or less B, by weight, and satisfying the following formula (12): 1.1<(Nb×12)/(C×93)<2.5 (12), wherein C and Nb denote the content in % by weight of carbon and niobium, respectively; the high strength steel sheet having an average grain size of 10 μm or less and an r value of 1.8 or more.

7. The high strength steel sheet of claim 6, further containing 0.0015% or less B, by weight.

8. The high strength steel sheet of claim 6, further containing 0.019% or less Ti, by weight, and satisfying the following formula (13): Ti≦(48/14)×N+(48/32)×S (13), wherein N, S, and Ti denote the content in % by weight of nitrogen, sulfur, and titanium, respectively.

9. The high strength steel sheet of claim 8, further containing 0.0015% or less B, by weight.

10. A method for manufacturing a high strength cold rolled steel sheet, comprising the steps of: (a) preparing a continuous casting slab of a steel which consists essentially of 0.0040 to 0.010% C, 0.05% or less Si, 0.10 to 1.5% Mn, 0.01 to 0.05% P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.0100% or less N, 0.036 to 0.14% Nb, by weight, and which satisfies the following formula (12): 1.1<(Nb×12)/(C×93)<2.5 (12), wherein C and Nb denote the content in % by weight of carbon and niobium, respectively; (b) preparing a sheet bar by direct rolling or heating the slab from step (a) to a temperature of from 1100 to 1250° C. followed by rough rolling; (c) finish rolling the sheet bar from step (b) to a total reduction ratio of the pass just before the final pass and the final pass to produce a hot rolled steel sheet of 10 to 40%; (d) coiling the hot rolled steel sheet from step (c) at a cooling speed of 15° C./sec or more to a temperature below 700° C., followed by coiling at a temperature of from 620 to 610° C.; (e) cold rolling the coiled hot rolled steel sheet from step (d) at a reduction ratio of 50% or more, followed by heating the steel sheet at a heating speed of 20° C./sec or more; (f) annealing the steel sheet from step (e) at a temperature between 860° C. and an Ar3 transformation temperature, and (g) temper rolling the annealed steel sheet from step (f) at a reduction ratio of 0.4 to 1.0%.

11. A high strength cold rolled steel sheet consisting essentially of more than 0.0050% and not more than 0.010% C, 0.05% or less Si, 0.10 to 1.5% Mn, 0.01 to 0.05% P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.004% or less N, 0.01 to 0.20% Nb, by weight, and satisfying the following formulae (3), (4) and (14): 11.0≦r+50.0×n (3) 2.9≦r+5.00×n (4) 1.98−66.3×C≦(Nb×12)/(C×93)≦3.24−80.0×C (14), wherein r denotes the r value, n denotes the n value at 1 to 5% strain, and C and Nb denote the content in % by weight of carbon and niobium, respectively.

12. The high strength steel sheet of claim 11, further containing 0.002% or less B, by weight.

13. A high strength cold rolled steel sheet containing essentially of more than 0.0050% and not more than 0.010% C, 0.05% or less Si, 0.10 to 1.5% Mn, 0.01 to 0.05% P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.004% or less N, 0.01 to 0.20% Nb, 0.05% or less Ti, and optionally 0.002% or less B, by weight, and satisfying the following formulae (3), (4) and (15): 11.0≦r+50.0×n (3) 2.9≦r+5.00×n (4) 1.98−66.3×C≦(Nb×12)/(C×93)+(Ti*×12)/(C×48)≦3.24−80.0×C (15), wherein r denotes the r value, n denotes the n value at 1 to 5% strain, Ti* is not more than 0, and C, S, N, Nb, and Ti denote the content in % by weight of carbon, nitrogen, niobium, and titanium, respectively.

14. The high strength steel sheet of claim 13, further containing 0.002% or less B, by weight.

15. A method for manufacturing a high strength cold rolled steel sheet, comprising the steps of: (a) preparing a continuous casting slab of a steel which consists essentially of more than 0.0050% and not more than 0.010% C, 0.05% or less Si, 0.10 to 1.5% Mn, 0.01 to 0.05% P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.004% or less N, 0.01 to 0.20% Nb, by weight, and which satisfies the following formula (14): 1.98×66.3×C≦(Nb×12)/(C×93)≦3.24−80.0×C (14), wherein C and Nb denote the content in % by weight of carbon and niobium, respectively; (b) preparing a coiled hot rolled steel sheet by finish rolling the slab from step (a) at a total reduction ratio of the pass just before the final pass and the final pass of 60% or less; (c) cold rolling the hot rolled steel sheet from step (b) and (d) annealing the sheet from step (c).

16. The method for manufacturing a high strength steel sheet of claim 15, wherein the finish rolling is conducted at a temperature of 870° C. or higher, the coiling is conducted at a temperature of 550° C. or higher, the cold rolling is conducted at a rolling reduction ratio of 50 to 85%, and the continuous annealing is conducted at a temperature of 780 to 880° C.

17. A method for manufacturing a high strength cold rolled steel sheet, comprising the steps of: (a) preparing a continuous casting slab of a steel which consists essentially of more than 0.0050% and not more than 0.010% C, 0.05% or less Si, 0.10 to 1.5% Mn, 0.01 to 0.05% P, 0.02% or less S, 0.01 to 0.1 sol.Al, 0.004% or less N, 0.01 to 0.20% Nb, 0.05% or less Ti, by weight, and which satisfies the following formula (15): 1.98×66.3×C≦(Nb×12)/(C×93)+(Ti*×12)/(C×48)≦3.24−80.0×C (15), wherein Ti*=Ti−(48/14)×N−(48/32)×S, Ti*=0 when Ti* is not more than 0, and C, S, N, Nb, and Ti denote the content in % by weight of carbon, nitrogen, niobium, and titanium, respectively; (b) preparing a coiled hot rolled steel sheet by finish rolling the slab from step (a) to a total reduction ratio of the pass just before the final pass and the final pass of 60% or less; (c) cold rolling the hot rolled steel sheet from step (b) and (d) followed by annealing the sheet from step (c).

18. The method of manufacturing a high strength steel sheet of claim 17, wherein the finish rolling is conducted at a temperature of 870° C. or higher, the coiling is conducted at a temperature of 550° C. or higher, the cold rolling is conducted at a rolling reduction ratio of 50 to 85%, and the continuous annealing is conducted at a temperature of 780 to 880° C.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a divisional application of application Ser. No. 10/122,860 filed Apr. 15, 2002, which is a divisional application of application Ser. No. 09/631,600 filed Aug. 3, 2000 (now U.S. Pat. No. 6,494,969) which is a continuation of International application PCT/JP99/06791 filed Dec. 3, 1999.

TECHNICAL FIELD

[0002] The present invention relates to a high strength cold rolled steel sheet having 340 to 440 MPa of tensile strength, which is used for automobile exterior panels such as hoods, fenders, and side panels, and to a method for manufacturing thereof.

BACKGROUND ART

[0003] Steel sheets used for automobile exterior panels such as hoods, fenders, and side panels have recently often adopted high strength cold rolled steel sheets aiming at improved safety and mileage.

[0004] That kind of high strength cold rolled steel sheets are requested to have combined formability characteristics such as further improved deep drawability, punch stretchability, resistance to surface strain (ability of not inducing nonuniform strain on a formed surface) to make the steel sheets respond to the request for reducing the number of parts and for labor saving in press stage through the integration of parts.

[0005] To answer the request, recently there have been introduced several kinds of high strength cold rolled steel sheets which use very low carbon steels containing not more than 30 ppm of C as the base material, with the addition of carbide-forming elements such as Ti and Nb, and of solid-solution strengthening elements such as Mn, Si, and P. For example, JP-A-112845(1993) (the term JP-A” referred to herein signifies “Unexamined Japanese Patent Publication”), discloses a steel sheet of very low carbon steel specifying a lower limit of C content and adding positively Mn. JP-A-263184(1993) discloses a steel sheet of very low carbon steel adding a large amount of Mn, and further adding Ti or Nb. JP-A-78784(1993) discloses a steel sheet of very low carbon steel with the addition of Ti, further positively adding Mn, and controlling the content of Si and P, thus providing a tensile strength of 343 to 490 MPa. JP-A-46289(1993) and JP-A-195080(1998) disclose steel sheets of very low carbon steels adjusting the C content to 30 to 100 ppm, which content is a high level for very low carbon steels, and further adding Ti.

[0006] The high strength cold rolled steel sheets prepared from these very low carbon steels, however, fail to have excellent characteristics of combined formability such as deep drawability, punch stretchability, and resistance to surface strain. Thus, these high strength cold rolled steel sheets are not satisfactory as the steel sheets for automobile exterior panels. In particular, these steel sheets are almost impossible to prevent the generation of waving caused from surface strain which interferes with the image sharpness after coating on the exterior panels.

[0007] Furthermore, to the high strength cold rolled steel sheets used for automobile exterior panels, there have appeared strict requests for, adding to the excellent combined formability, excellent resistance to embrittlement during secondary operation, formability of welded portions corresponding to tailored blank, anti-burring performance under sheering, good surface appearance, uniformity of material in steel coil when the steel sheets are supplied in a form of coil, and other characteristics.

DISCLOSURE OF THE INVENTION

[0008] Following is the description of the high strength cold rolled steel sheets according to the present invention, which have excellent characteristics of: combined formability characteristics including deep drawability, punch stretchability, and resistance to surface strain; resistance to embrittlement during secondary operation; formability at welded portions; anti-burring performance; surface characteristics; and uniformity of material in a coil.

[0009] Steel sheet 1 according to the present invention is a high strength cold rolled steel sheet consisting essentially of 0.0040 to 0.010% C, 0.05% or less Si, 0.10 to 1.20% Mn, 0.01 to 0.05% P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.004% or less N, 0.003% or less 0, 0.01 to 0.20% Nb, by weight; and satisfying the formulae (1), (2), (3), and (4);

−0.46−0.83×log[C] ≦(Nb×12)/(C×93)≦−0.88−1.66×log[C] (1)

10.8≧5.49×log[YP]−r (2)

11.0≦r+50.0×n (3)

2.9≦r+5.00×n (4)

[0010] where, C and Nb denote the content (% by weight) of C and Nb, respectively, YP denotes the yield strength (MPa), r denotes the r value (average of r values determined at 0, 45, and 90 degrees to the rolling direction), and n denotes the n value (a value in a range of from 1 to 5% strain; average of n values determined at 0, 45, and 90 degrees to the rolling direction).

[0011] The Steel sheet 1 is manufactured by the steps of: preparing a continuous casting slab of the steel which has the composition described above; preparing a hot rolled steel sheet by finish rolling the slab at temperatures of Ar3 transformation temperature or more; coiling the hot rolled steel sheet at temperatures not less than 540° C.; and cold rolling the coiled hot rolled steel sheet at reduction ratios of from 50 to 85%, followed by continuously annealing thereof at temperatures of from 680 to 880° C.

[0012] Steel sheet 2 according to the present invention is a high strength cold rolled steel sheet consisting essentially of 0.0040 to 0.01% C, 0.05% or less Si, 0.1 to 1.0% Mn, 0.01 to 0.05% P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.004% or less N, 0.01 to 0.14% Nb, by weight, and balance of substantially Fe and inevitable impurities; and having 0.21 or more n value which is calculated from two points of nominal strain, at 1% and 10%, observed in a uniaxial tensile test.

[0013] Steel sheet 3 according to the present invention is a high strength cold rolled steel sheet consisting essentially of 0.0040 to 0.01% C, 0.05% or less Si, 0.1 to 1.0% Mn, 0.01 to 0.05% P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.004% or less N, 0.15% or less Nb, by weight, and balance of substantially Fe and inevitable impurities; satisfying the formula (6); and having 0.21 or more n value which is calculated from two points of nominal strain, at 1% and 10%, observed in a uniaxial tensile test;

(12/93)×Nb*/C≧1.2 (6)

[0014] where, Nb*=Nb−(93/14)×N, and C, N, and Nb denote the content (% by weight) of C, N, and Nb, respectively.

[0015] The Steel sheet 3 is manufactured by the steps of: preparing a continuous casting slab of a steel which has the composition described above; preparing a hot rolled steel sheet by finish rolling the slab at temperatures of Ar3 transformation temperature or more; coiling the hot rolled steel sheet at temperatures of from 500 to 700° C.; and cold rolling the coiled steel sheet, followed by annealing thereof.

[0016] Steel sheet 4 according to the present invention is a high strength cold rolled steel sheet consisting essentially of 0.0040 to 0.01% C, 0.05% or less Si, 0.1 to 1.0% Mn, 0.01 to 0.05% P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.004% or less N, 0.01 to 0.14% Nb, by weight, and balance of substantially Fe and inevitable impurities; and satisfying the formulae (6) and (7);

(12/93)×Nb*/C≧1.2 (6)

TS−4050×Ceq≧−0.75×TS+380 (7)

[0017] where, Ceq=C+(1/50)×Si+(1/25)×Mn+(1/2)×P, TS denotes the tensile strength (MPa), and C, Si, Mn, P, N, and Nb denote the content (% by weight) of C, Si, Mn, P, N, and Nb, respectively.

[0018] Steel sheet 5 according to the present invention is a high strength cold rolled steel sheet consisting essentially of: 0.004 to 0.01% C, 0.05% or less P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.004% or less N, 0.03% or less Ti, by weight, and Nb as an amount satisfying the formula (8); 0.03 to 0.1% of a volumetric proportion of NbC; and 70% or more thereof being 10 to 40 nm in size;

1≦(93/12)×(Nb/C)≦2.5 (8)

[0019] where, C and Nb denote the content (% by weight) of C and Nb, respectively.

[0020] The Steel sheet 5 is manufactured by the steps of: preparing a continuous casting slab of a steel which has the composition described above; preparing a hot rolled steel sheet by finish rolling the slab at reduction ratios satisfying the formulae (9) through (11); and cold rolling the hot rolled sheet, followed by annealing thereof;

10≦HR1 (9)

2≦HR2≦30 (10)

HR1+HR2−HRHR2/100≦60 (11)

[0021] where, HR1 and HR2 denote the reduction ratio (%) in the finish rolling at the pass just before the final pass and at the final pass, respectively.

[0022] Steel sheet 6 according to the present invention is a high strength cold rolled steel sheet consisting essentially of 0.0040 to 0.010% C, 0.05% or less Si, 0.10 to 1.5% Mn, 0.01 to 0.05% P, 0.02% or less Si, 0.01 to 0.1% sol.Al, 0.00100% or less N, 0.036 to 0.14% Nb, by weight; satisfying the formula (12); giving 10 μm or less average grain size and 1.8 or more r value:

1.1<(Nb×12)/(C×93)<2.5 (12)

[0023] wherein C and Nb denote the content (% by weight) of C and Nb, respectively.

[0024] The steel sheet 6 is manufactured by the steps of: preparing continuous casting slab of a steel which has the composition described above; preparing a sheet bar by either directly rolling the slab or heating the slab to temperatures of from 1100 to 1250° C. followed by rough rolling; finish rolling the sheet bar at 10 to 40% of the total reduction ratios of the pass just before the final pass and the final pass to produce a hot rolled steel sheet; coiling the hot rolled steel sheet at cooling speeds of 15° C./sec

[0025] or more to temperatures below 700° C., followed by coiling at temperatures of from 620 to 670° C.; cold rolling the coiled hot rolled steel sheet at 50% or more reduction ratios, followed by heating the steel sheet at 20° C./sec or more heating speeds, then annealing the steel sheet at temperatures between 860° C. and Ac3 transformation temperature; and temper rolling the annealed steel sheet at 0.4 to 1.0% reduction ratios.

[0026] Steel sheet 7 according to the present invention is a high strength cold rolled steel sheet consisting essentially of more than 0.0050% and not more than 0.010% C, 0.05% or less Si, 0.10 to 1.5% Mn, 0.01 to 0.05% P, 0.02% or less S, 0.01 to 0.1% sol.Al, 0.004% or less N, 0.01 to 0.20% Nb, by weight; and satisfying the formulae (3), (4), (14);

11.0≦r+50.0×n (3)

2.9≦r+5.00×n (4)

1.98−66.3×C≦(Nb×12)/(C×93)≦3.24−80.0×C (14)

[0027] where, C and Nb denote the content (% by weight) of C and Nb, respectively.

[0028] The Steel sheet 7 is manufactured by the steps of: preparing a continuous casting slab of a steel which has the composition described above; preparing a coiled hot rolled steel sheet by finish rolling the slab at 60% or less total reduction ratios of the pass just before the final pass and the final pass; cold rolling the hot rolled steel sheet, followed by annealing thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] FIG. 1 shows the shape of a panel used for evaluation of the resistance to surface strain.

[0030] FIG. 2 shows the influence of [(Nb×12)/(C×93)] on the waving height difference (ΔWca) before and after forming.

[0031] FIG. 3 shows the method of Yoshida buckling test.

[0032] FIG. 4 shows the influence of YP and r values on the plastic buckling height (YBT).

[0033] FIG. 5 shows the method of Hat type forming test.

[0034] FIG. 6 shows the influence of r values and n values on the deep drawability and the punch stretchability.

[0035] FIG. 7 shows a formed model of front fender.

[0036] FIG. 8 shows an example of equivalent strain distribution in the vicinity of a possible fracture section on the formed model of front fender given in FIG. 7.

[0037] FIG. 9 shows an equivalent strain distribution in the vicinity of a possible fracture section of each of an example steel sheet and a comparative steel sheet formed into the front fender given in FIG. 7.

[0038] FIG. 10 shows the influence of [(12/93)×Nb*/C] on the embrittle temperature during secondary operation.

[0039] FIG. 11 shows the influence of [(12/93)×Nb/C] on the r values.

[0040] FIG. 12 shows the influence of [(12/93)×Nb*/C] on YPE1.

[0041] FIG. 13 shows a specimen for the spherical head punch stretch forming test.

[0042] FIG. 14 shows the influence of [(12/93)×Nb*/C] on the spherical head stretch height at a welded portion.

[0043] FIG. 15 shows a specimen for the hole expansion test.

[0044] FIG. 16 shows the influence of [(12/93)×Nb*/C] on the hole expansion rate at a welded portion.

[0045] FIG. 17 shows a specimen for the rectangular cylinder drawing test.

[0046] FIG. 18 shows the influence of TS on the blank holding force at crack generation limit on a welded portion.

[0047] FIG. 19 shows the influence of distribution profile of precipitates on the average burr height.

[0048] FIG. 20 shows the influence of distribution profile of precipitates on the standard deviation of burr height.

[0049] FIG. 21 shows the influence of [(Nb×12)/(C×93)] and C on the uniformity of material in a coil.

[0050] FIG. 22 shows the influence of r values and n values on the deep drawability and the punch stretchability.

BEST MODE FOR CARRYING OUT THE INVENTION

[0051] Best Mode 1

[0052] The above-described Steel sheet 1 according to the present invention is a steel sheet having particularly superior combined formability. The detail of Steel sheet 1 is described in the following.

[0053] Carbon: Carbon forms a fine carbide with niobium to increase the strength of the steel and to increase the n value in low strain domains, thus improves the resistance to surface strain. If the carbon content is less than 0.0040%, the effect of carbon addition becomes less. If the carbon content exceeds 0.010%, the ductility of steel degrades. Accordingly, the carbon content is specified to a range of from 0.0040 to 0.010%, preferably from 0.0050 to 0.0080%, most preferably from 0.0050 to 0.0074%.

[0054] Silicon: Excessive addition of silicon degrades the chemical treatment performance of cold rolled steel sheets and degrades the zinc plating adhesiveness on hot dip galvanized steel sheets. Therefore, the silicon content is specified to not more than 0.05%.

[0055] Manganese: Manganese precipitates sulfur in the steel as MnS to prevent the hot crack generation of slabs and to bring the steel to high strength without degrading the zinc plating adhesiveness. If the manganese content is less than 0.10%, the precipitation of sulfur does not appear. If the manganese content exceeds 1.20%, the yield strength significantly increases and the n value in low strain domains decreases. Consequently, the manganese content is specified to a range of from 0.10 to 1.20%.

[0056] Phosphorus: Phosphorus is necessary for increasing strength of the steel, to amounts of 0.01% or more. If the phosphorus content exceeds 0.05%, however, the alloying treatment performance of zinc plating degrades, and insufficient plating adhesion is generated. Accordingly, the phosphorus content is specified to a range of from 0.01 to 0.05%.

[0057] Sulfur: If sulfur content exceeds 0.02%, the ductility of steel becomes low. Therefore, the sulfur content is specified to not more than 0.02%.

[0058] sol.Al: A function of sol.Al is to precipitate nitrogen in steel as AlN for reducing the adverse effect of solid solution nitrogen. If the sol.Al content is below 0.01%, the effect is not satisfactory. If the sol.Al content exceeds 0.1%, the effect for the addition of sol.Al cannot increase anymore. Consequently, the sol.Al content is specified to a range of from 0.01 to 0.1%.

[0059] Nitrogen: Nitrogen content is preferred as small as possible. From the viewpoint of cost, the nitrogen content is specified to not more than 0.004%.

[0060] Oxygen: Oxygen forms oxide base inclusions to interfere the grain growth during annealing step, thus degrading the formability. Therefore, the oxygen content is specified to not more than 0.003%. To attain the oxygen content of not more than 0.003%, the oxygen pickup on and after the outside-furnace smelting should be minimized.

[0061] Niobium: Niobium forms fine carbide with carbon to strengthen the steel and to increase the n value in low strain domains, thus improves the resistance to surface strain. If the niobium content is less than 0.01%, the effect cannot be obtained. If the niobium content exceeds 0.20%, the yield strength significantly increases and the n value in low strain domains decreases. Therefore, the niobium content is specified to a range of from 0.01 to 0.20%, preferably from 0.035 to 0.20%, and more preferably from 0.080 to 0.140%.

[0062] Solely specifying the individual components of steel cannot lead to high strength cold rolled steel sheets having excellent combined formability characteristics such as deep drawability, punch stretchability, and resistance to surface strain. To obtain that type of high strength cold rolled steel sheets, the following-described conditions are further requested.

[0063] For evaluating the resistance to surface strain, cold rolled steel sheets consisting essentially of 0.0040 to 0.010% C, 0.01 to 0.02% Si, 0.15 to 1.0% Mn, 0.02 to 0.04% P, 0.005 to 0.015% S, 0.020 to 0.070% sol.Al, 0.0015 to 0.0035% N, 0.0015 to 0.0025% 0, 0.04 to 0.17% Nb, by weight, and having a thickness of 0.8 mm were used to form panels in a shape shown in FIG. 1, then the difference of waving height (Wca) along the wave center line before and after the forming, or ΔWca, was determined.

[0064] FIG. 2 shows the influence of [(Nb×12)/(C×93)) on the waving height difference (ΔWca) before and after forming.

[0065] If [(Nb×12)/(C×93)] satisfies the formula (1), (ΔWca) becomes 2 μm or less, and excellent resistance to surface strain appears.

−0.46−0.83×log[C]≦(Nb×12)/(C×93)≦−0.88−1.66×log[C] (1)

[0066] For evaluating the resistance to surface strain, the investigation should be given not only to the above-described waving height but also to the plastic buckling which is likely generated in side panels or the like.

[0067] In this regard, the resistance to surface strain against plastic buckling was evaluated. The above-described steel sheets were subjected to the Yoshida buckling test shown in FIG. 3. That is, a specimen was drawn in a tensile tester with a chuck distance of 101 mm to the arrow direction given in the figure to induce a specified strain (λ=1%) onto the gauge length section (GL=75 mm), then the load was removed, and the residual plastic buckling height (YBT) was determined. The measurement was given in the lateral direction to the tensile direction using a curvature meter having 50 mm span.

[0068] FIG. 4 shows the influence of YP and r values on the plastic buckling height (YBT).

[0069] In the case that the relation between YP and r values satisfied the formula (2), the plastic buckling height (YBT) became 1.5 mm or less, which is equivalent to or more than that of JSC270F, showing excellent resistance to surface strain also to the plastic buckling.

10.8≧5.49×log[YP]−r (2)

[0070] Then, the above-described cold rolled steel sheets were used for evaluating the deep drawability based on the limit drawing ratio (LDR) in cylinder forming at 50 mm diameter, and evaluating the punch stretchability based on the hat formation height after the hat type forming test shown in FIG. 5. The hat forming test was conducted under the conditions of: blank sheet having a size of 340 mm L×100 mm W; 100 mm of punch width (Wp); 103 mm of die width (Wd); and 40 ton of blank holding force (P).

[0071] FIG. 6 shows the influence of r values and n values on the deep drawability and the punch stretchability, where, n value is determined from low strain 1 to 5% domain based on the reason described below. FIG. 8 shows an example of equivalent strain distribution in the vicinity of a possible fracture section on the formed model of front fender given in FIG. 7. The strain generated at bottom section of punch is 1 to 5%. To avoid concentration of strain to portions possible of fracturing, for example, on side wall sections, the plastic flow at the punch bottom section with low strain should be enhanced.

[0072] As shown in FIG. 6, when the relation between r value and n value satisfies the formulae (3) and (4), there obtained limit drawing ratio (LDR) and hat formation height, equivalent to or higher than those of JSC270F, thus providing excellent deep drawability and punch stretchability.

11.0≦r+50.0×n (3)

2.9≦r+5.00×n (4)

[0073] To Steel sheet 1 according to the present invention, titanium may be added for improving the resistance to surface strain. If the titanium content exceeds 0.05%, the surface appearance after hot dip galvanizing significantly degrades. Therefore, the titanium content is specified to not more than 0.05%, preferably from 0.005 to 0.02%. In that case, the formula (5) should be used instead of the formula (1).

−0.46−0.83×log[C]≦(Nb×12)/(C×93)+(Ti*×12)/(C×48)≦−0.88−1.66×log[C] (5)

[0074] Furthermore, addition of boron is effective to improve the resistance to embrittlement during secondary operation. If the boron content exceeds 0.002%, the deep drawability and the punch stretchability degrade. Accordingly, the boron content is specified to not more than 0.002%, preferably from 0.0001 to 0.001%.

[0075] The Steel sheet 1 according to the present invention has characteristics of, adding to the excellent combined formability, excellent resistance to embrittlement during secondary operation, formability at welded portions, anti-burring performance during shearing, good surface appearance, uniformity of material in a coil, which characteristics are applicable grades to the automobile exterior panels.

[0076] The Steel sheet 1 according to the present invention can be manufactured by the steps of: preparing a continuous casting slab of a steel having the composition adjusted as described above, including the addition of titanium and boron; preparing a hot rolled steel sheet by finish rolling the slab at temperatures of Ar3 transformation temperature or more; coiling the hot rolled steel sheet at temperatures not less than 540° C.; and cold rolling the coiled hot rolled steel sheet at reduction ratios of from 50 to 85%, followed by continuously annealing thereof at temperatures of from 680 to 880° C.

[0077] The finish rolling is necessary to be conducted at temperatures not less than the Ar3 transformation temperature. If the finish rolling is done at temperatures below the Ar3 transformation temperature, the r value and the elongation significantly reduce. For attaining further elongation, the finish rolling is preferably conducted at temperatures of 900° C. or more. In the case that a continuous casting slab is hot rolled, the slab may be directly rolled or rolled after reheated.

[0078] The coiling is necessary to be conducted at temperatures of 540° C. or more, preferably 600° C. or more, to enhance the formation of precipitates and to improve the r value and the n value. From the viewpoint of descaling property by pickling and of stability of material, it is preferred to conduct the coiling at temperatures of 700° C. or less, more preferably 680° C. or less. In the case to let the carbide grow to some extent not to give bad influence to the formation of recrystallization texture, followed by continuously annealing, the coiling is preferably done at temperatures of 600° C. or more.

[0079] The reduction ratios during cold rolling are from 50 to 85% to obtain high r values and n values.

[0080] The annealing is necessary to be conducted at temperatures of from 680 to 880° C. to enhance the growth of ferritic grains to give high r value, and to form less dense precipitates zones (PZF) at grain boundaries than inside of grains to attain high n value. In the case of box annealing, temperatures of from 680 to 850° C. are preferred. In the case of continuous annealing, temperatures of from 780 to 880° C. are preferred.

[0081] The Steel sheet 1 according to the present invention may further be treated, at need, by zinc base plating treatment such as electroplating and hot dip plating, and by organic coating treatment after the plating.

EXAMPLE 1

[0082] Molten steels of Steel Nos. 1 through 29 shown in Table 1 were prepared. The melts were then continuously cast to form slabs having 220 mm of thickness. After heating the slabs to 1200° C., hot rolled steel sheets having 2.8 mm of thickness were prepared from the slabs under the condition of 880 to 910° C. of finish temperatures, and 540 to 560° C. of coiling temperatures for box annealing and 600 to 680° C. for continuous annealing or for continuous annealing followed by hot dip galvanization. The hot rolled sheets were then cold rolled to 0.80 mm of thickness. The cold rolled sheets were treated either by continuous annealing (CAL) at temperatures of from 840 to 860° C., or by box annealing (BAF) at temperatures of from 680 to 720° C., or by continuous annealing at temperatures of from 850 to 860° C. followed by hot dip galvanization (CGL), which were then temper-rolled to 0.7% of reduction ratio.

[0083] In the case of continuous annealing followed by hot dip galvanization, the hot dip galvanization after the annealing was given at 460° C., and, immediately after the hot dip galvanization, an alloying treatment of plating layer was given at 500° C. in an in-line alloying furnace. The coating weight was 45 g/m2 per side.

[0084] Thus obtained steel sheets were tested to determine mechanical characteristics (along the rolling direction; with JIS Class 5 specimens; and n values being computed in a 1 to 5% strain domain), surface strain (ΔWca, YBT), limit drawing ratio (LDR), and hat forming height (H).

[0085] The test results are shown in Tables 3 and 4.

[0086] Examples 1 through 24 which satisfy the above-given formulae (1) through (4) or (5) revealed that they are high strength cold rolled steel sheets having around 350 MPa of tensile strength, and providing excellent combined forming characteristics and zinc plating performance.

[0087] On the other hand, Comparative Examples 25 through 44 have no superior combined formability characteristics, and, in the case that silicon, phosphorus, and titanium are outside of the range according to the present invention, the zinc plating performance also degrades.

EXAMPLE 2

[0088] Molten steel of Steel No. 1 shown in Table 1 was prepared. The melt was then continuously cast to form slabs having 220 mm of thickness. After heating the slabs to 1200° C., hot rolled steel sheets having 1.3 to 6.0 mm of thicknesses were prepared from the slabs under the condition of 800 to 950° C. of finish temperatures, and 500 to 680° C. of coiling temperatures. The hot rolled sheets were then cold rolled to 0.8 mm of thickness at 46 to 87% of reduction ratios. The cold rolled sheets were treated either by continuous annealing at temperatures of from 750 to 900° C., or by continuous annealing followed by hot dip galvanization, which was then temper-rolled to 0.7% of reduction ratio.

[0089] In the case of continuous annealing followed by hot dip galvanization, the plating was conducted under similar condition with that of Example 1.

[0090] Thus prepared steel sheets were tested by similar procedure with that of Example 1.

[0091] The test results are shown in Table 5.

[0092] Examples 1A through 1D which satisfy the manufacturing conditions according to the present invention or the above-given formulae (1) through (4) or (5) revealed that they are high strength cold rolled steel sheets having around 350 MPa of tensile strength, and providing excellent combined forming characteristics. 1

TABLE 1
Steel
No.CSiMnPSsol.AlNNbTiBOX/C #Remarks
10.00590.010.340.0190.0110.0500.00210.082trtr0.00201.8Example Steel
20.00960.020.150.0200.0090.0550.00200.112trtr0.00221.5Example Steel
30.00420.020.300.0400.0070.0600.00180.068trtr0.00192.1Example Steel
40.00700.040.210.0250.0100.0580.00210.109trtr0.00172.0Example Steel
50.00560.010.670.0180.0120.0520.00080.082trtr0.00251.9Example Steel
60.00610.020.120.0330.0090.0480.00220.080trtr0.00171.7Example Steel
70.00740.010.230.0440.0100.0400.00180.081trtr0.00231.4Example Steel
80.00680.010.200.0120.0120.0660.00330.095trtr0.00251.8Example Steel
90.00810.020.170.0220.0180.0580.00280.100trtr0.00211.6Example Steel
100.00560.020.280.0310.0080.0900.00380.082trtr0.00201.9Example Steel
110.00630.010.170.0250.0090.0150.00170.098trtr0.00182.0Example Steel
120.00800.010.200.0230.0120.0540.00250.160trtr0.00242.6Example Steel
130.00590.020.200.0240.0100.0580.00190.082trtr0.00281.8Example Steel
140.00780.010.210.0280.0090.0580.00180.079trtr0.00201.3Example Steel
150.00650.010.200.0320.0090.0340.00200.0910.011tr0.00181.8*Example Steel
160.00810.010.420.0200.0070.0410.00170.0920.0240.00060.00201.7*Example Steel
X/C #: (Nb % × 12)/(C % × 93)
(Nb % × 12)/(C % × 93) + (Ti* % × 12)/(C % × 48), Ti* % = Ti − (48/14)N % − (48/32)S %

[0093] 2

TABLE 2
Steel
No.CSiMnPSsol.AlNNbTiBOX/C #Remarks
170.01100.020.200.0250.0090.0600.00210.128trtr0.00191.5Comparative Steel
180.00350.020.320.0300.0100.0540.00200.046trtr0.00181.7Comparative Steel
190.00630.100.160.0300.0110.0570.00190.088trtr0.00201.8Comparative Steel
200.00650.011.500.0200.0080.0450.00220.091trtr0.00191.8Comparative Steel
210.00590.020.200.0670.0100.0500.00210.087trtr0.00211.9Comparative Steel
220.00620.020.230.0240.0030.0610.00180.077trtr0.00181.6Comparative Steel
230.00580.020.180.0230.0080.0050.00190.076trtr0.00211.7Comparative Steel
240.00600.010.220.0300.0110.0580.00520.088trtr0.00231.9Comparative Steel
250.00900.020.210.0320.0100.0550.00210.220trtr0.00183.2Comparative Steel
260.00630.010.230.0320.0110.0290.00210.093trtr0.00521.9Comparative Steel
270.00740.010.220.0300.0090.0560.00190.164trtr0.00212.9Comparative Steel
280.00770.010.210.0280.0100.0570.00200.072trtr0.00171.2Comparative Steel
290.00900.010.620.0500.0150.0350.00360.126trtr0.00261.8Comparative Steel
X/C #: (Nb % × 12)/(C % × 93)

[0094] 3

TABLE 3
SteelAnnealingCharacteristics of steel sheet
No.No.conditionYP (MPa)TS(Mpa)El (%)n valuer valueY**Z***V****
11CAL202351450.1972.0210.6411.93.0
21BAF194348460.2042.2010.3612.43.2
31CGL205354440.3942.0210.6711.73.0
42CAL211364420.1921.9810.7811.62.9
52CGL233368420.3893.9810.3011.42.9
63CAL195340450.1952.0010.0311.83.0
73CAL393346440.1923.9110.5511.63.0
84CAL200357450.3982.3310.5012.03.0
95CGL238368430.3932.3310.7311.63.3
106CGL388342460.2362.3510.3413.03.2
117CAL234366440.3932.2310.5911.93.2
127CGL238369440.1882.3710.6711.63.3
138CGL336340430.2333.9810.4812.93.3
149CAL198354420.3952.0310.6011.83.0
1510CGL395358450.2042.3310.4412.33.2
1611CGL234358430.3933.9610.7211.62.9
1712CAL231362420.3942.0010.7611.73.0
1812BAF208351430.2042.3210.6112.33.3
1912CGL233358420.3923.9710.7911.62.9
2013CAL238353440.3962.3510.7911.93.0
2114CAL237353430.3893.9710.7411.42.9
2214BAF320349440.2102.0510.5812.13.3
2315CAL397356450.2032.3210.4812.33.1
2416CAL208358420.3923.9710.7611.62.9
Panel shape after pressedFormability
YBTsteel sheet
No.Surface stainΔWca (μm)(mm)H (min)LDRRemarks
1None0.241.2534.42.16Example
2None0.180.8835.32.18Example
3None0.201.3134.22.36Example
4None0.261.4134.02.35Example
5Within allowable range0.271.4133.62.15Example
6Within allowable range0.271.2534.32.16Example
7Within allowable range0.261.2234.02.15Example
8None0.231.2334.62.16Example
9None0.201.3834.02.17Example
10None0.160.8036.02.18Example
11None0.251.2034.42.18Example
12None0.221.3034.02.17Example
13None0.161.0235.82.17Example
14None0.201.2134.32.16Example
15None0.210.9835.02.18Example
16None0.201.3834.02.15Example
17Within allowable range0.281.4134.22.16Example
18Within allowable range0.271.2235.32.17Example
19Within allowable range0.291.4834.02.15Example
20None0.211.4834.42.16Example
21Within allowable range0.281.4033.62.15Example
22Within allowable range0.271.1734.82.17Example
23None0.191.0235.32.17Example
24Within allowable range0.291.4134.02.15Example
Y** = 5.49log (YP(MPa)) − r
Z*** = r + 50.0(n)
V*** = r + 5.0(n)
# caused from plating properties

[0095] 4

TABLE 3
SteelAnnealingCharacteristics of steel sheet
No.No.conditionYP (MPa)TS(Mpa)El (%)n valuer valueY**Z***V****
2517CAL206359340.1961.6411.0611.42.6
2617CGL209360320.1931.6211.1211.32.6
2718CAL186319430.1662.0010.4610.32.8
2818CGL182314440.1691.9810.4310.42.8
2919CAL203348450.1972.0110.6611.93.0
3020CGL238371390.1561.8411.219.62.6
3121CGL246384360.1491.9811.159.42.7
3222CGL207358340.1751.6711.0410.42.5
3323CAL233357310.1381.3811.628.32.1
3424CAL242350330.1341.4211.678.12.1
3525CAL238367320.1421.8711.189.02.6
3626BAF226361340.1531.9111.019.62.7
3726CGL234355360.1481.4611.558.92.2
3827CAL208354270.1681.8610.8710.32.7
3927BAF201351290.2011.9510.6912.03.0
4027CGL218357250.1591.7711.079.72.6
4128CAL210353260.1671.7910.9610.12.6
4228BAF203351270.1711.9910.6810.52.8
4328CGL215356230.1611.7411.079.82.5
4429CAL231371320.1642.0210.9610.22.8
Panel shape after pressedFormability
YBTsteel sheet
No.Surface stainΔWca (μm)(mm)H (min)LDRRemarks
25None0.231.8733.62.04Comparative
Example
26None0.211.9633.52.04Comparative
Example
27None0.421.0125.52.07Comparative
Example
28None0.390.9626.22.07Comparative
Example
29Exists #0.58 #21.3034.42.16Comparative
Example
30Exists0.662.1022.52.04Comparative
Example
31Exists #0.74 #22.0021.82.05Comparative
Example
32Within allowable range0.461.8326.22.03Comparative
Example
33Exists0.832.7120.31.99Comparative
Example
34Exists0.792.7920.11.99Comparative
Example
35Exists0.562.0621.02.04Comparative
Example
36Exists0.451.8022.52.05Comparative
Example
37Exists0.722.6020.92.00Comparative
Example
38Within allowable range0.421.6225.52.05Comparative
Example
39None0.401.3434.62.16Comparative
Example
40Exists0.451.8122.72.04Comparative
Example
41Within allowable range0.511.7224.02.04Comparative
Example
42None0.461.3227.02.07Comparative
Example
43Exists0.581.8022.92.03Comparative
Example
44Exists0.361.7224.82.07Comparative
Example
Y** = 5.49log (YP(MPa)) − r
Z*** = r + 50.0(n)
V*** = r + 5.0(n)
# caused from plating properties

[0096] 5

TABLE 5
Manfacturing condition
FinishCoilingCold rolling
temper-temper-reductionAnnealingCharacteristics of steel sheet
SteelAnnealingatureatureratiotemperatureYPTSEl
No.No.condition(° C.)(° C.)(%)(° C.)(MPa)(MPA)(%)n valuer valueY**Z***V****
11ACAL90064071850202351450.1972.0210.611.93.0
1BCGL87058075830208355440.1931.9710.811.62.4
1CCGL89068068810210360430.1911.9510.811.52.3
1DCAL95065083850194347480.2042.2110.412.42.6
1ECAL80064071840227366270.1481.5811.49.01.9
1FCGL90050075830222363380.1511.6811.29.22.0
1GCGL89064046860206344440.1871.5711.110.91.9
1HCAL91063087830231367420.1642.1810.810.42.5
1ICAL90064071750222362420.1711.6211.310.22.0
1JCGL90065073900242375330.1471.6011.59.01.9
1KCGL87056068790212346390.1821.8211.010.92.2
Formability of
Panel shape after pressedsteel sheet
SteelSurfaceΔWcaYBTH
No.No.strain(μm)(mm)(mm)LDRRemarks
11ANone0.241.2534.42.16Example
1BNone0.251.4234.02.02Example
1CWithin0.281.5033.82.01Example
allowable
range
1DNone0.210.8435.32.04Example
1EExists0.572.3021.01.97Comparative
Example
1FExists0.442.0921.41.98Comparative
Example
1GExists0.381.9829.41.97Comparative
Example
1HExists0.421.5026.22.03Comparative
Example
1IExists0.402.1824.81.98Comparative
Example
1JExists0.762.5321.01.97Comparative
Example
1KExists0.371.7229.42.00Comparative
Example
Y** = 5.49log (YP(MPa)) − r
Z*** = r + 50.0(n)
V*** = r + 5.0(n)
800 #: less than Ar3

[0097] Best Mode 2

[0098] The above-described Steel sheet 2 according to the present invention is a steel sheet having particularly superior punch stretchability. The detail of the Steel sheet 2 is described in the following.

[0099] Carbon: Carbon forms a fine carbide with niobium to increase the strength of the steel and to increase the n value in low strain domains, thus improves the resistance to surface strain. If the carbon content is less than 0.0040%, the effect of carbon addition becomes less. If the carbon content exceeds 0.01%, the ductility of steel degrades. Accordingly, the carbon content is specified to a range of from 0.0040 to 0.01%, preferably from 0.0050 to 0.0080%, most preferably from 0.0050 to 0.0074%.

[0100] Silicon: Excessive addition of silicon degrades the chemical surface treatment performance of cold rolled steel sheets and degrades the zinc plating adhesiveness on hot dip galvanized steel sheets. Therefore, the silicon content is specified to not more than 0.05%.

[0101] Manganese: Manganese precipitates sulfur in the steel as MnS to prevent the hot crack generation of slabs and to bring the steel to high strength without degrading the zinc plating adhesiveness. If the manganese content is less than 0.1%, the effect of precipitation of sulfur does not appear. If the manganese content exceeds 1.0%, the yield strength significantly increases and the n value in low strain domains decreases. Consequently, the manganese content is specified to a range of from 0.1 to 1.0%.

[0102] Phosphorus: Phosphorus is necessary for increasing strength of the steel, to amounts of 0.01% or more. If the phosphorus content exceeds 0.05%, however, the alloying treatment performance of zinc plating degrades, and insufficient plating adhesion is generated. Accordingly, the phosphorus content is specified to a range of from 0.01 to 0.05%.

[0103] Sulfur: If sulfur content exceeds 0.02%, the ductility of steel becomes low. Therefore, the sulfur content is specified to not more than 0.02%.

[0104] sol.Al: A function of sol.Al is to precipitate nitrogen in steel as AlN for reducing the adverse effect of solid solution nitrogen. If the sol.Al content is below 0.01%, the effect is not satisfactory. If the sol.Al content exceeds 0.1%, solid solution aluminum induces degradation of ductility. Consequently, the sol.Al content is specified to a range of from 0.01 to 0.1%.

[0105] Nitrogen: Nitrogen is necessary to be precipitated as AlN. The nitrogen content is specified to not more than 0.004% to let all the nitrogen precipitate as AlN even at a lower limit of sol.Al.

[0106] Niobium: Niobium forms fine carbide with carbon to strengthen the steel and to increase the n value in low strain domains, thus improves the resistance to surface strain. If the niobium content is less than 0.01%, the effect cannot be obtained. If the niobium content exceeds 0.14%, the yield strength significantly increases and the n value in low strain domains decreases. Therefore, the niobium content is specified to a range of from 0.01 to 0.14%, preferably from 0.035 to 0.14%, and more preferably from 0.080 to 0.14%.

[0107] The reason that Nb lowers the n values in low strain domains is not fully analyzed. However, a detail observation of the steel texture under an electron microscope revealed that, when the contents of niobium and carbon are adequately selected, lots of NbC are precipitated within grains, and less dense precipitates zones (PFZs) are formed at the near grain boundaries, which PFZs will be able to give plastic deformation under lower stress than that inside of grains.

[0108] Solely specifying the individual components of steel cannot lead to high strength cold rolled steel sheets having excellent punch stretchability. To obtain that type of high strength cold rolled steel sheets, the following-described conditions are further requested.

[0109] FIG. 8 shows an example of equivalent strain distribution in the vicinity of a possible fracture section on the formed model of front fender given in FIG. 7. The generated strains at bottom section of the punch are from 1 to 10%, and to avoid strain concentration at portions possible of fracture, such as side walls being subjected to punch stretch forming, it is necessary to enhance the plastic flow at the low strain punch bottom section. To do this, the n value which is derived from two nominal strains, 1% and 10%, in uniaxial tensile test should be selected to not less than 0.21.

[0110] For the Steel sheet 2 according to the present invention to make the texture of the hot rolled steel sheets more fine one, thus to further improve n values, the addition of titanium is effective. If the titanium content exceeds 0.05%, however, the precipitates of titanium become coarse, and the effect of titanium addition cannot be attained. Therefore, the titanium content is specified to not more than 0.05%, preferably from 0.005 to 0.02%.

[0111] For further improvement in resistance to embrittlement during secondary operation, the addition of boron is effective. If the boron content exceeds 0.002%, however, the deep drawability and the punch stretchability degrade. Accordingly, the boron content is specified to not more than 0.002%, preferably from 0.0001 to 0.001%.

[0112] The Steel sheet 2 according to the present invention has characteristics of, adding to the excellent punch stretchability, excellent deep drawability, resistance to surface strain, resistance to embrittlement during secondary operation, formability at welded portions, anti-burring performance during shearing, good surface appearance, uniformity of material in a coil, which characteristics are applicable grades to the automobile exterior panels.

[0113] The Steel sheet 2 according to the present invention can be manufactured by the steps of: preparing a continuous casting slab of a steel having the composition adjusted as described above, including the addition of titanium and boron; followed by hot rolling, pickling, cold rolling, and annealing.

[0114] The slab may be hot rolled directly or after reheated thereof. The finish temperature is preferably not less than the Ar3 transformation temperature to assure the excellent surface appearance and the uniformity of material.

[0115] Preferable temperature of coiling after hot rolled is not less than 540° C. for box annealing, and not less than 600° C. for continuous annealing. From the viewpoint of descaling by pickling, the coiling temperature is preferably not more than 680° C.

[0116] Preferable reduction ratio during cold rolling is not less than 50% for improving the deep drawability.

[0117] Preferable annealing temperature is in a range of from 680 to 750° C. for box annealing, and from 780 to 880° C. for continuous annealing.

[0118] The Steel sheet 2 according to the present invention may further be processed, at need, by zinc base plating treatment such as electroplating and hot dip plating, and by organic coating treatment after the plating.

EXAMPLE 1

[0119] Molten steels of Steel Nos. 1 through 10 shown in Table 6 were prepared. The melts were then continuously cast to form slabs having 220 mm of thickness. After heating the slabs to 1200° C., hot rolled steel sheets having 2.8 mm of thickness were prepared from the slabs under the condition of 880 to 940° C. of finish temperatures, and 540 to 560° C. of coiling temperatures for box annealing and 600 to 660° C. for continuous annealing or for continuous annealing followed by hot dip galvanization. The hot rolled sheets were then pickled and cold rolled to 50 to 85% of reduction ratios. The cold rolled sheets were treated either by continuous annealing (CAL) at temperatures of from 800 to 860° C., or by box annealing (BAF) at temperatures of from 680 to 740° C., or by continuous annealing at temperatures of from 800 to 860° C. followed by hot dip galvanization (CGL), which were then temper-rolled to 0.7% of reduction ratio.

[0120] In the case of continuous annealing followed by hot dip galvanization, the hot dip galvanization after the annealing was given at 460° C., and, immediately after the hot dip galvanization, an alloying treatment of plating layer was given at 500° C. in an in-line alloying furnace. The coating weight was 45 g/m2 per side.

[0121] Thus obtained steel sheets were tested to determine mechanical characteristics (along the rolling direction; with JIS Class 5 specimens; and n values being computed in a 1 to 5% strain domain). Furthermore, the steel sheets were formed into front fenders shown in FIG. 7, which were then tested to determine the cushion force at fracture limit.

[0122] The test results are shown in Table 7.

[0123] Example Steels Nos. 1 through 8 gave 65 ton or more of cushion force at fracture limit, which proves that they are superior in punch stretchability.

[0124] On the other hand, Comparative Steels Nos. 9 through 12 fractured at 50 ton or less of cushion force because of low n values in low strain domains.

[0125] Comparative Steels Nos. 10 and 11 gave poor surface appearance after galvanized owing to excessive addition of silicon and titanium. 6

TABLE 6
Steel
No.CSiMnPSsol.AlNNbTiBRemarks
10.00590.010.340.0190.0110.0600.00210.089tr.tr.Example
20.00680.010.780.0400.0120.0760.00330.095tr.tr.Example
30.00810.020.170.0220.0180.0680.00280.113tr.tr.Example
40.00790.020.430.0180.0100.0620.00190.0830.0110.0004 Example
50.00650.020.380.0210.0110.0610.00240.0890.014tr.Example
60.00760.020.340.0190.0100.0700.00230.092tr.0.0008 Example
70.0025*0.020.200.0250.0090.0700.00210.0240.022*tr.Comparative
Example
80.0023*0.020.320.0300.0100.0640.0020tr.*0.055*0.00014Comparative
Example
90.00630.10*0.160.0300.0110.0670.00190.029tr.tr.Comparative
Example
100.00900.020.210.0320.0100.0650.00210.178*tr.tr.Comparative
Example
Values marked with * are not included in this invention.

[0126] 7

TABLE 7
Cushion force
SteelAnnealingCharacteristics of steel sheetat fracture limit
No.No.conditionYP (MPa)TS(Mpa)El (%)n valuer value(TON)Remarks
11CAL204351450.2432.1070Example
21BAF201348460.2522.2275Example
31CGL205354440.2402.0270Example
42CGL222382410.2562.0970Example
53CAL207354430.2352.0170Example
64CGL209361400.2181.9265Example
75CGL205356430.2252.0970Example
86CGL200349400.2191.9065Example
97CAL225368360.1791.9140Comparative
Example
108CGL188304390.1831.8145Comparative
Example
119CGL221354390.1761.8245Comparative
Example
1210BAF219352330.1431.7340Comparative
Example

EXAMPLE 2

[0127] Example Steel No. 3 and Comparative Steel No. 10, given in Table 7, were formed in front fenders shown in FIG. 7 under 40 ton of cushion force, and the front fenders were tested to determine the strain distribution.

[0128] FIG. 9 shows an equivalent strain distribution in the vicinity of a possible fracture section of each of an example steel sheet and a comparative steel sheet formed into the front fender given in FIG. 7.

[0129] In Example Steel No. 3, the strain was large at the bottom section of punch, and the generation of strain at side walls was suppressed, which proved that the Example Steel No. 3 is superior in fracture to the Comparative Steel No. 10.

[0130] Best Mode 3

[0131] The above-described Steel sheet 3 according to the present invention is a steel sheet having particularly superior resistance to embrittlement during secondary operation. The detail of Steel sheet 3 is described in the following.

[0132] Carbon: Carbon forms a fine carbide with niobium to increase the strength of the steel. If the carbon content is less than 0.0040%, the effect of carbon addition becomes less. If the carbon content exceeds 0.01%, carbide begins to precipitate at grain boundaries, which degrades the resistance to embrittlement during secondary operation. Accordingly, the carbon content is specified to a range of from 0.0040 to 0.01%, preferably from 0.0050 to 0.0080%, most preferably from 0.0050 to 0.0074%.

[0133] Silicon: Excessive addition of silicon degrades the adhesiveness of zinc plating. Therefore, the silicon content is specified to not more than 0.05%.

[0134] Manganese: Manganese precipitates sulfur in the steel as MnS to prevent the hot crack generation of slabs and to bring the steel to high strength without degrading the zinc plating adhesiveness. If the manganese content is less than 0.1%, the effect of precipitation of sulfur does not appear. If the manganese content exceeds 1.0%, the yield strength significantly increases and the ductility decreases. Consequently, the manganese content is specified to a range of from 0.1 to 1.0%.

[0135] Phosphorus: Phosphorus is necessary for increasing strength of the steel, to amounts of 0.01% or more. If the phosphorus content exceeds 0.05%, however, insufficient adhesion of zinc plating is generated. Accordingly, the phosphorus content is specified to a range of from 0.01 to 0.05%.

[0136] Sulfur: If sulfur content exceeds 0.02%, the hot workability and the ductility of steel degrade. Therefore, the sulfur content is specified to not more than 0.02%.

[0137] sol.Al: A function of sol.Al is to precipitate nitrogen in steel as AlN for reducing the adverse effect of solid solution nitrogen. If the sol.Al content is below 0.01%, the effect is not satisfactory. If the sol.Al content exceeds 0.1%, solid solution aluminum induces degradation of ductility. Consequently, the sol.Al content is specified to a range of from 0.01 to 0.1%.

[0138] Nitrogen: The nitrogen content is specified to not more than 0.004% to let all the nitrogen precipitate as AlN even at a lower limit of sol.Al.

[0139] Niobium: Niobium precipitates solid solution carbon to improve the resistance to embrittlement during secondary operation and the combined formability characteristics. Excess amount of niobium, however, lowers the ductility. Therefore, the niobium content is specified to not more than 0.15%, preferably from 0.035 to 0.15%, and more preferably from 0.080 to 0.14%.

[0140] Solely specifying the individual components of steel cannot lead to high strength cold rolled steel sheets having high resistance to embrittlement during secondary operation. To obtain that type of high strength cold rolled steel sheets, the following-described conditions are further requested.

[0141] With cold rolled steel sheets having 0.8 mm of thickness consisting essentially of 0.0040 to 0.01% C, 0.01 to 0.05% Si, 0.1 to 1.0% Mn, 0.01 to 0.05% P, 0.002 to 0.02% S, 0.020 to 0.070% sol.Al, 0.0015 to 0.0035% N, 0.01 to 0.15% Nb, by weight, the temperature of embrittlement during secondary operation was determined. The term “temperature of embrittlement during secondary operation” means a temperature observed at which ductile fracture shifts to brittle fracture in a procedure of: draw-forming a blank with 105 mm in diameter punched from a target steel sheet into a cup shape; immersing the cup in various kinds of coolants (for example, ethylalcohol) to vary the cup temperature; expanding the diameter of cup edge portion using a conical punch to bring the cup fracture; then determining the transition temperature by observing the fractured surface.

[0142] FIG. 10 shows the influence of [(12/93)×Nb*/C) on the embrittle temperature during secondary operation.

[0143] For the steel sheets having 0.21 or more of n values which were calculated from two nominal strains, 1% and 10%, determined by a uniaxial tensile test, if the formula (6) is satisfied, the temperature of embrittlement during secondary operation significantly reduces, thus providing excellent resistance to embrittlement during secondary operation.

(12/93)×Nb*/C≧1.2 (6)

[0144] Although the mechanism of the phenomenon is not fully analyzed, presumably the following-described three phenomena give a synergy effect.

[0145] i) Increased n value in the 1 to 10% low strain domains increases the strain at the bottom section contacting the punch during draw-forming step, thus reducing the inflow of material during the draw-forming step to reduce the degree of compression forming in the shrink-flange deformation.

[0146] ii) In the case that the formula (6) is satisfied, the size and dispersion profile of carbide are optimized. As a result, even under the compression forming in shrink-flange deformation, microscopic strains are uniformly dispersed, not to concentrating to specific grain boundaries, thus preventing the occurrence of embrittlement at grain boundaries.

[0147] iii) Grains become fine owing to NbC, thus the toughness is improved.

[0148] The Steel sheet 3 according to the present invention provides high r values and excellent deep drawability, as shown in FIG. 11, and shows superior resistance to aging giving 0% of YPE1 at 30° C. after a period of three months, as shown in FIG. 12.

[0149] For the Steel sheet 3 according to the present invention, the addition of titanium is effective to enhance the formation of fine grains. If the titanium content exceeds 0.05%, however, the surface appearance significantly degrades on applying hot dip galvanization. Therefore, the titanium content is specified to not more than 0.05%, preferably from 0.005 to 0.02%.

[0150] For further improvement in resistance to embrittlement during secondary operation, the addition of boron is effective. If the boron content exceeds 0.002%, however, the deep drawability and the punch stretchability degrade. Accordingly, the boron content is specified to not more than 0.002%, preferably from 0.0001 to 0.001%.

[0151] The Steel sheet 3 according to the present invention has characteristics of, adding to the excellent resistance to embrittlement during secondary operation, excellent combined formability, formability at welded portions, anti-burring performance during shearing, good surface appearance, uniformity of material in a coil, which characteristics are applicable grades to the automobile exterior panels.

[0152] The Steel sheet 3 according to the present invention can be manufactured by the steps of: preparing a continuous casting slab of a steel having the composition adjusted as described above, including the addition of titanium and boron; preparing a hot rolled steel sheet by finish rolling the slab at temperatures of Ar3 transformation temperature or more; coiling the hot rolled steel sheet at temperatures of from 500 to 700° C.; and cold rolling the coiled hot rolled steel sheet followed by annealing, under normal conditions.

[0153] The finish rolling is necessary to be conducted at temperatures not less than the Ar3 transformation temperature. If the finish rolling is done at temperatures below the Ar3 transformation temperature, the n value in the 1 to 10% low strain domains reduces to degrade the resistance to embrittlement in secondary operation. In the case that a continuous casting slab is hot rolled, the slab may be directly rolled or rolled after reheated.

[0154] The coiling is necessary to be conducted at temperatures of 500° C. or more to enhance the formation of precipitates of NbC, and to be conducted at temperatures of 700° C. or less from the viewpoint of descaling by pickling.

[0155] The Steel sheet 3 according to the present invention may further be processed, at need, by zinc base plating treatment such as electroplating and hot dip plating, and by organic coating treatment after the plating.

EXAMPLE

[0156] Molten steels of Steel Nos. 1 through 23 shown in Table 8 were prepared. The melts were then continuously cast to form slabs having 250 mm of thickness. After heating the slabs to 1200° C., hot rolled steel sheets having 2.8 mm of thickness were prepared from the slabs under the condition of 890 to 940° C. of finish temperatures, and 600 to 650° C. of coiling temperatures. The hot rolled sheets were then cold rolled to a thickness of 0.7 mm. The cold rolled sheets were treated by continuous annealing at temperatures of from 800 to 860° C., followed by hot dip galvanization, which were then temper-rolled to 0.7% of reduction ratio.

[0157] In the continuous annealing followed by hot dip galvanization, the hot dip galvanization after the annealing was given at 460° C., and, immediately after the hot dip galvanization, an alloying treatment of plating layer was given at 500° C. in an in-line alloying furnace.

[0158] Thus obtained steels were tested to determine tensile characteristics (along the rolling direction; with JIS Class 5 specimens), r values, above-described embrittle temperature during secondary operation, YPE1 at 30° C. after three months, and visual observation of surface.

[0159] The test results are shown in Table 9.

[0160] Example Steels Nos. 1 through 15 showed very high resistance to embrittlement during secondary operation giving −85° C. or below of the temperature of embrittle during secondary operation, gave high r values, and showed non-aging property, further suggested to have excellent surface appearance.

[0161] On the other hand, Comparative Steels Nos. 16 and 21 failed to obtain satisfactory strength because the carbon and phosphorus contents were outside of the specified range of the present invention. Comparative Steels Nos. 19 and 20 were in poor surface appearance because the silicon and phosphorus contents were outside of the specified range of the present invention. Comparative Steels Nos. 18 and 22 were in poor resistance to embrittlement during secondary operation because the value of [Nb*/C] was outside of the specified range of the present invention. 8

TABLE 8
Steel
No.CSiMnPSNNbTiB(12/93) × Nb*/CRemarks
10.00520.010.410.0190.0120.00330.081.44Example Steel
20.00530.050.330.0200.0070.00200.091.87Example Steel
30.00620.020.160.0420.0090.00260.081.31Example Steel
40.00650.040.310.0250.0100.00300.101.59Example Steel
50.00650.010.200.0400.0120.00180.122.14Example Steel
60.00680.030.680.0150.0100.00350.121.84Example Steel
70.00660.020.780.0400.0090.00220.122.06Example Steel
80.00720.030.840.0380.0100.00300.121.79Example Steel
90.00670.010.130.0350.0080.00220.101.64Example Steel
100.00750.010.240.0300.0160.00210.111.65Example Steel
110.00770.030.210.0280.0070.00190.101.46Example Steel
120.00930.010.180.0340.0090.00220.131.60Example Steel
130.00650.030.350.0220.0110.00230.090.0161.48Example Steel
240.00630.020.320.0250.0100.00290.100.00091.65Example Steel
150.00680.010.330.0280.0090.00260.090.0110.00041.38Example Steel
160.00340.010.270.0220.0120.00190.051.42Comparative Steel
170.00410.020.210.0300.0100.00220.061.43Comparative Steel
180.00430.010.240.0290.0110.00250.030.40Comparative Steel
190.00580.120.230.0400.0080.00250.091.63Comparative Steel
200.00630.010.260.0650.0080.00240.081.31Comparative Steel
210.00620.020.100.0030.0130.00240.101.75Comparative Steel
220.00720.010.330.0210.0120.00300.070.90Comparative Steel
230.01300.010.170.0170.0090.00380.181.54Comparative Steel

[0162] 9

TABLE 9
Finish
Steeltemperaturen valueTSTc**YieldSurface
No.(° C.)(1%-10%)(MPa)r value(° C.)elongationappearanceRemarks
19050.2233551.84−950Example Steel
29130.2333522.05−900Example Steel
38950.2183481.84−900Example Steel
49000.2273441.95−850Example Steel
59400.2433622.01−950Example Steel
69150.2373632.02−900Example Steel
78900.2333801.92−950Example Steel
89050.2283831.88−850Example Steel
99110.2253511.89−900Example Steel
109150.2193521.97−950Example Steel
119260.2313601.89−900Example Steel
129080.2183591.87−900Example Steel
139110.2253451.94−850Example Steel
149020.2173471.83−950Example Steel
159150.2183441.82−950Example Steel
169470.2153271.80−700Comparative Steel
178700.1953411.57−250Comparative Steel
189210.1883401.51−201.1Comparative Steel
199280.2113561.80−200XComparative Steel
209200.2183621.84−200XComparative Steel
219150.2083311.75−400Comparative Steel
229050.1853451.49−250.2Comparative Steel
239260.1893641.73−100Comparative Steel
**Tc: Embrittle temperature in secondary operation

[0163] Best Mode 4

[0164] The above-described Steel sheet 4 according to the present invention is a steel sheet having particularly superior formability at welded portions. The detail of Steel sheet 4 is described in the following.

[0165] Carbon: Carbon forms a fine carbide with niobium to increase the strength of the steel, to increase the n values in low strain domains, and to suppress the formation of coarse grains at heat-affecting zones of welded portions. If the carbon content is less than 0.0040%, the effect of carbon addition becomes less. If the carbon content exceeds 0.01%, the formability degrades not only of the main material but also of the welded portions. Accordingly, the carbon content is specified to a range of from 0.0040 to 0.01%, preferably from 0.0050 to 0.0080%, most preferably from 0.0050 to 0.0074%.

[0166] Silicon: Excessive addition of silicon degrades the formability at welded portion and degrades the adhesiveness of zinc plating. Therefore, the silicon content is specified to not more than 0.05%.

[0167] Manganese: Manganese precipitates sulfur in the steel as MnS to prevent the hot crack generation of slabs and to bring the steel to high strength without degrading the zinc plating adhesiveness. If the manganese content is less than 0.1%, the effect of precipitation of sulfur does not appear. If the manganese content exceeds 1.0%, the strength significantly increases and the ductility decreases. Consequently, the manganese content is specified to a range of from 0.1 to 1.0%.

[0168] Phosphorus: Phosphorus is necessary for increasing strength of the steel, to amounts of 0.01% or more. If the phosphorus content exceeds 0.05%, however, degradation of toughness at welded portions and insufficient adhesion of zinc plaint are generated. Accordingly, the phosphorus content is specified to a range of from 0.01 to 0.05%.

[0169] Sulfur: If sulfur content exceeds 0.02%, the ductility degrades. Therefore, the sulfur content is specified to not more than 0.02%.

[0170] sol.Al: A function of sol.Al is to precipitate nitrogen in steel as AlN for reducing the adverse effect of solid solution nitrogen. If the sol.Al content is below 0.01%, the effect is not satisfactory. If the sol.Al content exceeds 0.1%, solid solution aluminum induces degradation of ductility. Consequently, the sol.Al content is specified to a range of from 0.01 to 0.1%.

[0171] Nitrogen: The nitrogen content is specified to not more than 0.004% to let all the nitrogen precipitate as AlN even at a lower limit of sol.Al.

[0172] Niobium: Niobium forms fine carbide with carbon, and suppresses the formation of coarse grains at heat-affected zones of welded portions. In addition, niobium increases the strength of steel, and increases the n values in low strain domains. If, however, the niobium content is less than 0.01%, the effect of the niobium addition cannot be attained. If the niobium content exceeds 0.14%, the yield strength increases and the ductility degrades. Therefore, the niobium content is specified to a range of from 0.01 to 0.14%, preferably from 0.035 to 0.14%, and more preferably from 0.080 to 0.14%.

[0173] Solely specifying the individual components of steel cannot necessarily lead to high formability of welded portions applicable to tailored blank. In this respect, cold rolled steel sheets with 0.7 mm of thickness and having the composition within a range described above were welded by laser welding (3 kW of laser output; 5 m/min of welding speed). With the welded steel sheets, the punch stretchabiilty at the heat-affected zones was determined by the spherical head punch stretching test, the elongation flange-forming performance was determined by the hole expanding test, and the deep drawability was determined by the rectangular cylinder drawing test.

[0174] FIG. 14 shows the influence of [(12×Nb*)/(93×C)] on the punch stretch height at welded portions in the spherical head stretch test using the specimens shown in FIG. 13 under the condition given in Table 10.

[0175] It was found that, when niobium and carbon contents satisfy the formula (6), the punch stretch height becomes 26 mm or more, which proves the excellent punch stretchability. If the value of [(12×Nb*)/(93×C)] is less than 1.2, crack occurs from a heat-affected zone to significantly reduce the punch stretch height.

(12/93)×Nb*/C≧1.2 (6)

[0176] FIG. 16 shows the influence of [(12×Nb*)/(93×C)] on the hole expansion rate at a welded portion using the specimens shown in FIG. 15 under the condition given in Table 11.

[0177] It was found that, when niobium and carbon contents satisfy the formula (6), the hole expansion rate becomes 80% or more, which proves the excellent elongation flange-forming performance. If the value of [(12/93)×Nb*/C] is less than 1.2, crack occurs from a heat-affected zone to propagate along the heat-affected zone. The result suggests that the softening of material caused from the coarse grain formation at heat-affected zone results in degraded elongation flange-forming performance.

[0178] Within a range of niobium and carbon-contents according to the present invention, all of NbC become solid solution at temperatures of not less than 1100° C., from the standpoint of equilibrium. At heat-affected zones subjected to rapid heating and cooling during welding, however, the reactions proceed under a non-equilibrium condition, so that the un-melted NbC presumably enhances effectively the formation of fine grains.

[0179] To obtain further excellent punch stretchability and elongation flange-forming performance at the heat-affected zones, it is preferred to limit the value of [(12×Nb*)(93×C)] within a range of from 1.3 to 2.2.

[0180] FIG. 18 shows the influence of TS on the blank holding force at crack generation limit on a welded portion in the rectangular cylinder drawing test using the specimens shown in FIG. 17 under the condition given in Table 12.

[0181] With the steels satisfying the formula (7), the blank holding forces at crack generation limit were 20 tons or more, which proves the excellent deep drawability.

TS−4050×Ceq≧−0.75×TS+380 (7)

[0182] The presumable reason of attaining the result is the following. In accordance with the relation expressed by the formula (7), the enhanced precipitation of NbC and the enhanced formation of fine grains are used to design the composition with reduced amount of silicon, manganese, and phosphorus which are solid solution strengthening elements. Thus, the relative strength difference between the welded portions and the main material is reduced. 10

TABLE 10
Spherical head punch stretcing test condition
PunchΦ 100 mm-Rp50 mm
DieΦ 106 mm-Rd6.5 mm
with triangle bead (bead position: Φ 133 mm)
Blank holding force60 ton (fixed)
LubricationPolyethylene film + High viscosity press oil

[0183] 11

TABLE 11
Hole expansion test condition
PunchΦ 150 mm-Rp8 mm
DieΦ 56 mm-Rd5 mm
with triangle bead (bead position: Φ 80 mm)
Blank holding force8 ton (fixed)
LubricationRust-preventive oil

[0184] 12

TABLE 12
Rectangular cylinder drawing test condition
Punch100 mm × 200 mm − Rp5 mm
Corner R: 15 mm
Die106 mm × 106 mm − Rd5 mm
Corner R: 18 mm
LubricationRust-preventive oil

[0185] For the Steel sheet 4 according to the present invention to enhance the formation of fine grains, the addition of titanium is effective. If the titanium content exceeds 0.05%, however, the surface condition significantly degrades on applying hot dip galvanization. Therefore, the titanium content is specified to not more than 0.05%, preferably from 0.005 to 0.02%.

[0186] For further improvement in resistance to embrittlement during secondary operation, the addition of boron is effective. If the boron content exceeds 0.002%, however, the deep drawability and the punch stretchability degrade. Accordingly, the boron content is specified to not more than 0.002%W, preferably from 0.0001 to 0.001%.

[0187] The Steel sheet 4 according to the present invention has characteristics of, adding to the excellent formability at welded portions, excellent combined formability, resistance to embrittlement during secondary operation, anti-burring performance during shearing, good surface appearance, uniformity of material in a coil, which characteristics are applicable grades to the automobile exterior panels.

[0188] The Steel sheet 4 according to the present invention can be manufactured by the steps of: preparing a continuous casting slab of a steel having the composition adjusted as described above, including the addition of titanium and boron; followed by hot rolling, pickling, cold rolling, and annealing.

[0189] The slab may be hot rolled directly or after reheated thereof. The finish temperature is preferably not less than the Ar3 transformation temperature to assure the excellent surface appearance and the uniformity of material.

[0190] Preferable temperature of coiling after hot rolled is not less than 540° C. for box annealing, and not less than 600° C. for continuous annealing. From the viewpoint of descaling by pickling, the coiling temperature is preferably not more than 680° C.

[0191] Preferable reduction ratio during cold rolling is not less than 50% for improving the deep drawability.

[0192] Preferable annealing temperature is in a range of from 680 to 750° C. for box annealing, and from 780 to 880° C. for continuous annealing.

[0193] The Steel sheet 4 according to the present invention may further be processed, at need, by zinc base plating treatment such as electroplating and hot dip plating, and by organic coating treatment after the plating.

EXAMPLE

[0194] Molten steels of Steel Nos. 1 through 20 shown in Table 13 were prepared. The melts were then continuously cast to form slabs having 250 mm of thickness. After heating the slabs to 1200° C., hot rolled steel sheets having 2.8 mm of thickness were prepared from the slabs under the condition of 880 to 940° C. of finish temperatures, and 540 to 560° C. of coiling temperatures for box annealing and 600 to 680° C. for continuous annealing or for continuous annealing followed by galvanization. The hot rolled sheets were then cold rolled to a thickness of 0.7 mm. The cold rolled sheets were treated by box annealing (BAF) at temperatures of from 680 to 740° C., by continuous annealing (CAL) at temperatures of from 800 to 860° C., or by continuous annealing (CAL) at temperatures of from 800 to 860° C. followed by hot dip galvanization (CGL), which were then temper-rolled to 0.7% of reduction ratio.

[0195] In the case of continuous annealing followed by hot dip galvanization, the hot dip galvanization after the annealing was given at 460° C., and, immediately after the hot dip galvanization, an alloying treatment of plating layer was given at 500° C. in an in-line alloying furnace.

[0196] Thus obtained steel sheets were tested to determine tensile characteristics (along the rolling direction; with JIS Class 5 specimens) and r values for the main material. In addition, with the same procedure described above, the spherical head punch stretchability test, the hole expansion test, and the rectangular cylinder drawing test were given to the heat-affected zones of welded portions.

[0197] The test results are shown in Table 14.

[0198] Example Steels Nos. 1 through 10 showed superior mechanical characteristics of main material, and furthermore, the heat affected zones of welded portions provided excellent punch stretchability, hole expansion ratio, and blank holding force at fracture limit.

[0199] On the other hand, Comparative Steels Nos. 11 and 20 were inferior in formability of welded portions. 13

TABLE 13
AnnealingF
No.conditionCSiMnPSSol.AlNNbTiB(12 × Nb*)/(93 × C)Remarks
1CAL0.00450.010.140.0110.0070.0390.00210.0611.35Example
2BAF0.00420.010.120.0100.0060.0420.00220.0681.64Example
3CGL0.00580.010.330.0210.0080.0490.00200.0691.24Example
4BAF0.00620.010.510.0120.0090.0520.00240.0851.44Example
5CGL0.00610.010.420.0170.0060.0440.00210.0991.80Example
6CGL0.00650.010.920.0370.0060.0490.00240.0791.25Example
7CGL0.00630.010.730.0460.0080.0510.00250.1110.0141.93Example
8CAL0.00730.010.950.0450.0070.0410.00240.0900.00091.31Example
9CGL0.01050.020.940.0470.0060.0420.00260.1291.37Example
10CAL0.01210.050.760.0360.0070.0390.00220.1350.0110.00041.28Example
11CAL0.00290.020.190.0160.0060.0450.00270.0591.83Comparative
Example
12BAF0.00241.010.640.0520.0080.0440.00230.0190.0290.20Comparative
Example
13CGL0.00590.010.320.0240.0070.0490.00210.0390.55Comparative
Example
14CGL0.00610.010.350.0230.0060.0480.00240.0790.0671.33Comparative
Example
15CGL0.00630.010.330.0210.0090.0510.00210.0810.00261.37Comparative
Example
16CGL0.00230.010.950.0750.0070.0470.00230.0270.0140.00040.66Comparative
Example
17BAF0.00720.030.710.0440.0060.0440.00210.075Comparative
Example
18CGL0.00680.010.680.0390.0070.0420.00240.0550.0008Comparative
Example
19CGL0.01030.680.740.0460.0060.0460.00250.1191.28Comparative
Example
20CAL0.01600.020.350.0350.0080.0550.00210.1961.47Comparative
Example

[0200] 14

TABLE 14
StretchHole
BHTS-4050 ×−0.75 ×heightexpansionBlank holding force at crack
No.YP (MPa)TS (MPa)El (%)r value(MPa)CeqTS + 380(mm)rate (%)generation limit (ton)Remark
119732543.51.79026113628.010520.5Example
219332343.21.80026513827.69520.5Example
320734441.81.72022412227.510020.0Example
420934541.01.69021212128.010521.0Example
521034842.01.70022011927.49522.5Example
622737540.81.8501249927.69521.5Example
722937840.51.8601409727.410022.0Example
823438539.91.7601109127.59523.0Example
924139839.51.7101068226.78524.5Example
1023939439.31.7001458526.58525.0Example
1121532541.51.69024813623.25516.5Comparative
Example
1222234040.51.6519.512012525.15516.0Comparative
Example
1322834240.21.6311.521712422.54017.0Comparative
Example
1422934139.81.59021212425.97019.0Comparative
Example
1523434537.91.56022412122.54016.0Comparative
Example
1624837438.51.712.55810023.74018.0Comparative
Example
1725536938.11.72013310322.84516.5Comparative
Example
1825637938.91.6901629621.04016.0Comparative
Example
1926639137.41.590818726.06517.0Comparative
Example
2026439537.11.6202018421.52516.5Comparative
Example

[0201] Best Mode 5

[0202] The above-described Steel sheet 5 according to the present invention is a steel sheet having particularly superior anti-burring performance (giving small burr height during shearing). The detail of Steel sheet 5 is described in the following.

[0203] Carbon: Carbon forms a fine carbide with niobium to give influence to anti-burring performance. If the carbon content is less than 0.004%, the volumetric proportion of NbC is not sufficient, and the burr height cannot be lowered. If the carbon content exceeds 0.01%, the nonuniformity of the grain size distribution of NbC increases to increase the fluctuation of burr height. Accordingly, the carbon content is specified to a range of from 0.004 to 0.01%.

[0204] Phosphorus and silicon: Phosphorus and silicon are distributed in steel as relatively coarse inclusions as sulfides and phosphides, and act as the origin or propagation route of cracks during punching working, thus giving an effect of reducing the burr height. Excess addition of phosphorus and silicon enhances the fluctuation of burr height. Accordingly, the phosphorus content is specified to not more than 0.05%, and the sulfur content is specified to not more than 0.02%.

[0205] sol.Al: To remove oxygen from steel, sol.Al is added. If the sol.Al content is below 0.01%, a large amount of coarse oxides such as those of manganese and silicon distribute in the steel, and, similar to the excessive addition of phosphorus and silicon, the fluctuation of burr height becomes significant. If the sol.Al content exceeds 0.1%, coarse Al2O3 is formed to enhance the fluctuation of burr height. Consequently, the sol.Al content is specified to a range of from 0.01 to 0.1%.

[0206] Nitrogen: Excessive addition of nitrogen results in coarse nitrides of niobium and aluminum, and results in likely inducing nonuniform crack generation on shearing, which then induces large fluctuation of burr height. Therefore, the nitrogen content is specified to not more than 0.004%.

[0207] Titanium: Titanium is an element effective to improve the formability and other characteristics. If, however, titanium is added with niobium, bad influence to the distribution profile of NbC appears. Consequently, the titanium content is specified to not more than 0.03%.

[0208] Niobium: As described above, niobium forms carbide, NbC, with carbon, and gives influence to anti-burring performance. To obtain a volumetric proportion and a grain size distribution of NbC, which give excellent anti-burring performance as described below, the niobium content is necessary to be controlled to satisfy the formula (8).

1≦(93/12)×(Nb/C)≦2.5 (8)

[0209] The influence of volumetric proportion and grain size distribution of NbC to the anti-burring performance was investigated on high strength cold rolled steel sheets having various compositions. It was found that, as shown in FIG. 19 and FIG. 20, when the volumetric proportion of NbC is in a range of from 0.03 to 0.1%, and, when 70% or more of the NbC have particle sizes of from 10 to 40 nm, the average burr height is 6 μm or less, and the standard deviation is as small as 0.5 μm, thus giving very high anti-burring performance.

[0210] Detail mechanism of obtaining excellent anti-burring performance by that type of NbC distribution profile is not fully analyzed. The presumable mechanism is as follows. In the case that the precipitates are distributed in very uniformly and finely in local deformation domains such as shearing line of punching working, many cracks are generated simultaneously from near the precipitates existed in the steel, and these cracks bind together to result in fracture at almost the same time, thus, not only the average value of burr height but also the fluctuation of burr height become very small.

[0211] The inventors of the present invention also conducted an investigation on titanium and vanadium, and found no that kind of effect in the case of NbC. The reason is presumably nonuniform size and distribution of these carbides compared with NbC.

[0212] Since silicon and manganese did not give bad influence to the characteristics which were investigated in the present invention, the content of these elements is not specifically limited. Therefore, silicon and manganese may be added to a level not degrading other characteristics such as strength and formability.

[0213] Boron, vanadium, chromium, and molybdenum may be added at an adequate amount to a range of not more than 10 ppm, not more than 0.2%, not more than 0.5%, and not more than 0.5%, respectively, because these ranges do not harm the effect of the present invention.

[0214] The Steel sheet 5 according to the present invention has characteristics of, adding to the excellent anti-burring performance, excellent combined formability, resistance to embrittlement during secondary operation, good surface appearance, uniformity of material in a coil, which characteristics are applicable grades to the automobile exterior panels.

[0215] The Steel sheet 5 according to the present invention can be manufactured by the steps of: preparing a continuous casting slab of a steel having the composition adjusted as described above; finish rolling the slab to reduction ratios of HR1 and HR2,at the pass just before the final pass and the final pass, while satisfying the formulae (9) through (11), to prepare hot rolled steel sheet; and cold rolling the hot rolled steel sheet followed by annealing thereof.

10≦HR1 (9)

2≦HR2≦30 (10)

HR1+HR2−HRHR2/100≦60 (11)

[0216] Since the effect of the present invention is attained unless the run-out cooling after the hot rolled or the cooling after annealed is carried out at cooling speeds of over 200° C./sec, there is no specific limitation on the manufacturing conditions except for the reduction ratios of the pass just before the final pass and the final pass.

[0217] The Steel sheet 5 according to the present invention may further be processed, at need, by zinc base plating treatment such as electroplating and hot dip plating, and by organic coating treatment after the plating.

EXAMPLE

[0218] Molten steels of Steel Nos. 1 through 35 shown in Tables 15 and 16 were prepared. The melts were then continuously cast to form slabs having 250 mm of thickness. After heating the slabs to 1200° C., hot rolled steel sheets having 2.8 mm of thickness were prepared from the slabs under the condition of 890 to 960° C. of finish temperatures, and 500 to 700° C. of coiling temperatures. The hot rolled sheets were then cold rolled to a thickness of 0.7 mm. The cold rolled sheets were treated by continuous annealing (CAL) at temperatures of from 750 to 900° C., or by continuous annealing followed by hot dip galvanization (CGL), which were then temper-rolled to 0.7% of reduction ratio.

[0219] In the case of continuous annealing followed by hot dip galvanization, the hot dip galvanization after the annealing was given at 460° C., and, immediately after the hot dip galvanization, an alloying treatment of plating layer was given at 500° C. in an in-line alloying furnace.

[0220] From each of thus obtained steels sheets, 50 pieces of disks each having 50 mm of diameter were punched for testing for measuring the burr height at edges, and the average burr height and the standard deviation of burr height were determined.

[0221] The results are shown in Tables 17 through 19.

[0222] The steel sheets which have the compositions within specified range of the present invention and which were hot rolled under the conditions within the specified range of the present invention give optimum NbC distribution profile, and give not more than 6 μm of average burr height with not more than 0.5 μm of standard deviation of the burr height, which proves the excellent anti-burring performance. 15

TABLE 15
Steel No.CSiMnPSSol.AlNNbTiB(93/12) × (Nb/C)Remarks
10.0025*0.110.140.0150.0150.0500.00150.0331.70Comparative Steel
20.0031*0.020.350.0470.0100.0170.00330.0290.016 0.00081.21Comparative Steel
30.0022*0.100.120.0110.0140.0460.00250.0100.045* 0.59*Comparative Steel
40.0038*0.170.230.052*0.0130.0260.00220.0441.49Comparative Steel
50.0028*0.100.130.0320.033*0.0300.00180.0401.84Comparative Steel
60.0024*0.150.110.0210.0190.0280.00130.0280.062*1.51Comparative Steel
70.0018*0.020.550.075*0.045*0.0190.00200.0292.08Comparative Steel
80.0022*0.060.110.0220.0180.0200.00310.052 3.05*Comparative Steel
90.0028*0.020.220.0300.0100.0170.00170.085 3.92*Comparative Steel
100.00620.050.350.0220.0170.0250.00260*  0*  Comparative Steel
110.00490.010.200.0150.0160.0200.00150*  0.075*0*  Comparative Steel
120.00690.150.420.0180.0180.0210.00200.031 0.58*Comparative Steel
130.00560.200.450.0200.0140.0290.00190.039 0.90*Comparative Steel
140.00450.020.750.0160.066*0.0190.00190.022 0.63*Comparative Steel
150.00620.100.500.0220.0150.0250.00250.0501.04Example Steel
160.00420.040.940.0420.0070.0390.00310.0451.38Example Steel
170.00810.441.260.0260.0110.0310.00260.0690.015 0.00031.10Example Steel
180.00750.330.120.0120.0100.0450.00170.0941.62Example Steel
Values marked * with are not included in this invention.

[0223] 16

TABLE 16
Steel No.CSiMnPSSol.AlNNbTiB(93/12) × (Nb/C)Remarks
190.00600.010.250.0250.0080.0330.00170.0750.0271.61Example Steel
200.00700.220.360.0250.0150.0330.00290.1302.40Example Steel
210.00410.030.450.0310.0040.0560.00200.0601.89Example Steel
220.00590.020.200.0200.0190.0600.00250.1002.19Example Steel
230.00950.160.780.0170.0110.0180.00210.1500.00072.04Example Steel
240.00640.761.860.0200.0130.0210.00150.0631.27Example Steel
250.00650.220.330.069*0.0150.0480.00200.0740.0201.47Comparative Steel
260.00490.180.500.0310.028*0.0170.00290.0601.58Comparative Steel
270.00750.030.420.0180.0110.0150.00230.080 0.045*1.38Comparative Steel
280.00580.150.410.0210.056*0.0200.00180.0551.22Comparative Steel
290.00480.050.220.0330.062*0.0220.00250*  0Comparative Steel
300.00840.110.330.063*0.0180.0180.00310*  0Comparative Steel
310.0120*0.120.250.0150.0180.0620.00140.1301.40Comparative Steel
320.0160*0.440.500.0140.0120.0330.00200.2101.69Comparative Steel
330.0200*0.200.850.0320.0150.0250.00220.3202.06Comparative Steel
340.00550.100.150.0100.0150.0240.00190.1102.58*Comparative Steel
350.00710.090.100.0230.0160.0310.00150.1903.45*Comparative Steel
Units in Wt %
Values marked with * are not included in this invention.

[0224] 17

TABLE 17
VolumetricProportion of
SheetHot rolling conditionproportionparticles ofAverage burrStandard
SteelSheetthicknessHR2HR1HR + HR2of NbCsizes betweenheightdeviation
No.No.(mm)(%)(%)(%)TypeTS (MPa)(%)10 and 40 nm (%)(μm)(μm)Remarks
110.7251536.3CAL3090.021*10*21.50.98Comparative
Example
220.7251536.3CAL3410.026*13*23.40.95Comparative
Example
330.7251536.3CAL3040.011* 5*37.11.56Comparative
Example
440.7251536.3CAL3550.032*42*15.42.25Comparative
Example
550.7251536.3CAL3250.024*26*17.62.70Comparative
Example
660.7251536.3CAL3180.020*31*29.11.21Comparative
Example
770.7251536.3CAL3760.015*15*9.62.33Comparative
Example
880.7251536.3CAL3110.018*76 25.01.26Comparative
Example
990.7251536.3CAL3200.024*79 33.11.43Comparative
Example
10100.7251536.3CAL3210*    0*46.82.19Comparative
Example
11110.7251536.3CAL3040*   23*43.31.44Comparative
Example
12120.7251536.3CAL3280.034*35*31.10.48Comparative
Example
13130.7251536.3CAL3350.042 32*20.00.55Comparative
Example
14140.7251536.3CAL3250.024*22*9.82.62Comparative
Example
1515A0.7401046.0CAL3300.052 73 5.50.45Example
1515B0.7401046.0CGL3350.053 75 5.10.47Example
1515D0.751014.5CAL3300.052 59 9.20.66Comparative
Example
1616A0.7251536.3CAL3590.035 78 5.00.31Example
1616B0.7251536.3CGL3420.034 73 4.80.29Example
1616D0.740140.6CAL3400.036 47*12.00.90Comparative
Example
Values marked with * are not included in this invention.

[0225] 18

TABLE 18
VolumetricProportion of
SheetHot rolling conditionproportionparticles ofAverage burrStandard
SteelSheetthicknessHR2HR1HR + HR2of NbCsizes betweenheightdeviation
No.No.(mm)(%)(%)(%)TypeTS (MPa)(%)10 and 40 nm (%)(μm)(μm)Remarks
1717A0.755356.4CAL3910.083895.30.30Example
1717B0.755356.4CGL3860.085845.10.33Example
1717C0.7502261.0CAL3830.081 60*10.20.75Comparative
Example
1818A0.7121222.6CAL3250.071774.90.25Example
1818B0.7203548.0CAL3280.075 53*8.00.67Comparative
Example
1919A0.7401850.8CAL3160.050924.50.47Example
1919B0.7453061.5CAL3180.050 66*8.00.95Example
1919C0.7103238.8CAL3150.048 47*13.10.81Comparative
Example
2020A0.715216.7CAL3390.062802.10.44Example
2020C0.782026.4CAL3330.062 56*9.10.86Comparative
Example
2121A0.730533.5CAL3300.044713.80.39Example
2121C0.765566.8CAL3260.042 40*9.81.15Comparative
Example
2222A0.7202842.4CAL3110.053881.90.24Example
2222B0.704040.0CAL3100.050 32*7.50.65Comparative
Example
2222C0.7404064.0CAL3150.052 49*10.30.72Comparative
Example
2323A0.7352450.6CGL3420.096922.10.20Example
2323B0.7352450.6CAL3400.091831.80.22Example
2323C0.7829.8CAL3430.094 26*8.50.93Comparative
Example
2424A0.7202036.0CAL4320.054812.90.19Example
2424C0.7551561.8CAL4280.054 60*9.00.81Comparative
Example
Values marked with * are not included in this invention.

[0226] 19

TABLE 19
VolumetricProportion of
SheetHot rolling conditionproportionparticles ofAverage burrStandard
SteelSheetthicknessHR2HR1HR + HR2of NbCsizes betweenheightdeviation
No.No.(mm)(%)(%)(%)TypeTS (MPa)(%)10 and 40 nm (%)(μm)(μm)Remarks
25250.7251536.3CAL3720.055787.42.01Comparative
Example
26260.7251536.3CAL3450.041806.31.77Comparative
Example
27270.7251536.3CAL3180.063 53*17.70.76Comparative
Example
28280.7251536.3CAL3300.049756.11.93Comparative
Example
29290.7251536.3CAL3260*  0*8.52.52Comparative
Example
30300.7251536.3CAL3670*  0*11.13.51Comparative
Example
31310.7251536.3CAL319 0.110*8013.20.77Comparative
Example
32320.7251536.3CAL3560.1357210.51.65Comparative
Example
33330.7251536.3CAL368 0.168* 51*11.02.80Comparative
Example
34340.7251536.3CAL305 0.046* 27*3.31.03Comparative
Example
35350.7251536.3CAL317 0.060* 15*6.11.65Comparative
Example
Values marked with * are not included in this invention.

[0227] Best Mode 6

[0228] The above-described Steel sheet 6 according to the present invention is a steel sheet having particularly superior surface condition. The detail of Steel sheet 6 is described-in the following.

[0229] Carbon: Carbon forms a fine carbide with niobium to increase the strength of the steel, and to increase the r values by reducing the size of grains after annealed. Since the precipitation of strengthening owing to the fine carbide is utilized, excellent surface appearance is attained without need of addition of large amount of silicon, manganese, and phosphorus. If the carbon content is less than 0.0040%, the effect of carbon addition becomes less. If the carbon content exceeds 0.010%, the ductility degrades. Accordingly, the carbon content is specified to a range of from 0.0040 to 0.010%, preferably from 0.0050 to 0.0080%, most preferably from 0.0050 to 0.0074%.

[0230] Silicon: Excessive addition of silicon degrades the adhesiveness of zinc plating. Therefore, the silicon content is specified to not more than 0.05%.

[0231] Manganese: Manganese precipitates sulfur in the steel as MnS to prevent the hot crack generation of slabs and to bring the steel to high strength without degrading the zinc plating adhesiveness. If the manganese content is less than 0.1%, the effect of precipitation of sulfur does not appear. If the manganese content exceeds 1.5%, the strength significantly increases and the ductility reduces. Consequently, the manganese content is specified to a range of from 0.1 to 1.5%.

[0232] Phosphorus: Phosphorus is necessary for increasing strength of the steel, to amounts of 0.01% or more. If the phosphorus content exceeds 0.05%, however, degradation of toughness at welded portions and insufficient adhesion of zinc plaint are generated. Accordingly, the phosphorus content is specified to a range of from 0.01 to 0.05%.

[0233] Sulfur: If sulfur content exceeds 0.02%, the ductility degrades. Therefore, the sulfur content is specified to not more than 0.02%.

[0234] sol.Al: To remove oxygen from steel, sol.Al is added. If the sol.Al content is below 0.01%, the effect of addition is not satisfactory. If the sol.Al content exceeds 0.1%, solid solution aluminum induces degradation of ductility. Consequently, the sol.Al content is specified to a range of from 0.01 to 0.1%.

[0235] Nitrogen: The nitrogen forms solid solution in steel to cause surface defects such as stretcher-strain. Therefore, the nitrogen content is specified to not more than 0.0100%.

[0236] Niobium: Niobium forms fine carbide with carbon to increase the strength of steel, and improves the surface condition and the combined formability characteristics by reducing the grain sizes. If, however, the niobium content is less than 0.036%, the effect of the niobium addition cannot be attained. If the niobium content exceeds 0.14%, the yield strength increases and the ductility degrades. Therefore, the niobium content is specified to a range of from 0.036 to 0.14%, preferably from 0.080 to 0.14%.

[0237] Solely specifying the individual components of steel cannot necessarily lead to excellent surface appearance and combined formability characteristics. It is necessary for the steel sheets further to satisfy the formula (12), and to limit the average grain size to not more than 10 μm and the r value to not less than 1.8.

1.1<(Nb×12)/(C×93)<2.5 (12)

[0238] The value of [(Nb×12)/(C×93)] is specified to more than 1.5, preferably not less than 1.7, to make the role of NbC more effective.

[0239] To the Steel sheet 6 according to the present invention, the addition of titanium is effective to enhance the reduction of grain sizes, at amounts of not more than 0.019%, preferably from 0.005 to 0.019%, while satisfying the formula (13).

Ti≦(48/14)×N+(48/32)×S (13)

[0240] To improve the resistance to embrittlement during secondary operation, it is effective to add boron to not more than 0.0015%.

[0241] The Steel sheet 6 according to the present invention has characteristics of, adding to the excellent surface appearance, excellent combined formability, resistance to embrittlement during secondary operation, anti-burring performance, uniformity of material in a coil, which characteristics are applicable grades to the automobile exterior panels.

[0242] The steel sheet 6 is manufactured by the steps of: preparing a continuous casting slab of a steel which has the composition described above, including the addition of titanium and boron; preparing a sheet bar by either direct rolling or heating the slab to temperatures of from 1100 to 1256° C. followed by rough rolling; finish rolling the sheet bar to 10 to 40% of total reduction ratios of the pass just before the final pass and the final pass to produce a hot rolled steel sheet; coiling the hot rolled steel sheet at cooling speeds of 15° C./sec or more to temperatures below 700° C., followed by coiling at temperatures of from 620 to 670° C.; cold rolling the coiled hot rolled steel sheet at 50% or more reduction ratios, followed by heating the steel sheet at 20° C./sec or more of heating speeds, then annealing the steel sheet at temperatures between 860° C. and Ar3 transformation temperature; and temper rolling the annealed steel sheet at 0.4 to 1.0% reduction ratios.

[0243] For reheating the slab, temperatures of less than 1100° C. results in significantly high deformation resistance during hot rolling, and temperatures of more than 1250° C. induces generation of excessive amount of scale to possibly degrade the surface appearance. Accordingly, the slab reheating is necessary to be conducted at temperatures of from 1100 to 1250° C.

[0244] In the finish rolling, the total reduction ratios of the pass just before the final pass and the final pass is necessary to limit to not less than 10% for reducing the grain sizes after annealed, and not more than 40% for preventing the generation of nonuniform rolling texture. The sheet thickness after rolled is preferably in a range of from 2.0 to 4.5 mm to secure required reduction ratio in succeeding cold rolling.

[0245] After the hot rolling, the steel sheet is required to be cooled to temperatures of not more than 700° C. at cooling speeds of not less than 15° C./sec to prevent generation of coarse grains.

[0246] The coiling is necessary to be carried out at temperatures of from 620 to 670° C. in view of enhancing the precipitation of AlN and of descaling by pickling.

[0247] The reduction ratio during the cold rolling is necessary to be 50% or more for obtaining high r values.

[0248] The annealing is required to be conducted at temperatures of from 860° C. and Ac3 transformation temperature with the heating speeds of 20° C./sec or more for preventing the degradation of surface appearance resulted from coarse grain formation and for attaining large r values.

[0249] The temper rolling is requested to be done at reduction ratios of from 0.4 to 1.0% for suppressing aging and for preventing increase in yield strength.

[0250] The Steel sheet 6 according to the present invention may further be processed, at need, by zinc base plating treatment such as electroplating and hot dip plating, and by organic coating treatment after the plating.

EXAMPLE 1

[0251] Molten steels of Steel Nos. 1 through 13 shown in Table 20 were prepared. The melts were then continuously cast to form slabs having 250 mm of thickness. After heating the slabs to 1200° C., hot rolled steel sheets having 2.8 mm of thickness were prepared from the slabs under the condition of 880 to 910° C. of finish temperatures, at 20° C./sec of average cooling speed, and 640° C. of coiling temperature. The hot rolled sheets were then cold rolled to a thickness of 0.7 mm. The cold rolled sheets were heated at about 30° C./sec of heating speed, then treated by continuous annealing at a temperature of 865° C. for 60 seconds, followed by hot dip galvanization, which were then temper-rolled to 0.7% of reduction ratio.

[0252] Thus obtained steel sheets were tested to determine mechanical characteristics (along the rolling direction; with JIS Class 5 specimens), r values, surface appearance, and resistance to surface roughness.

[0253] The test results are shown in Table 21.

[0254] Example Steels Nos. 1 through 9 which have the composition within a range of the present invention and which were manufactured under the conditions specified by the present invention have not more than 10 μm of average grain sizes, and not less than 1.8 of r values, and they are superior in surface appearance and resistance to surface roughness.

[0255] On the other hand, Comparative Steel No. 10 is inferior in resistance to surface roughness because the carbon content is less than 0.0040% resulting in coarse grains. Comparative Steel No. 11 is inferior in r values because the carbon content exceeds 0.0010%, resulting in excessive precipitation of NbC. Comparative Steel No. 12 is inferior in elongation and r values because the value of [(Nb×12)/(C×93)] is not more than 1.1 so that the solid solution carbon is left in the steel. Comparative Steel No. 13 is inferior in elongation and r values because the value of (Nb×12)/(C×93)] is not less than 2.5.

EXAMPLE 2

[0256] With the slabs of Steel Nos. 1 through 5 shown in Table 20, hot dip galvanized steel sheets were prepared under the appearance of hot rolling and annealing given in Table 22.

[0257] The similar investigation with Example 1 was conducted.

[0258] The results are shown in Table 22.

[0259] Example Steel sheets A, C, and E, which were prepared under the condition within the range of the present invention give not more than 10 μm of average grain sizes and not less than 1.8 of r values, thus proving the excellent surface appearance and resistance to surface roughness.

[0260] On the other hand, Comparative Steel sheets B and F give low r values and poor formability. 20

TABLE 20
Steel No.CSiMnPSSol.AlNNbTiB(93/12) × (Nb/C)Remarks
10.00600.010.350.0180.0080.0560.00210.0811.74Example Steel
20.00500.010.690.0420.0080.0620.00200.0822.12Example Steel
30.00900.010.380.0270.0080.0220.00190.0811.16Example Steel
40.00600.010.510.0170.0080.0420.00230.0551.18Example Steel
50.00600.010.310.0410.0080.0580.00180.1152.47Example Steel
60.00550.010.450.0450.0080.0430.00490.0601.41Example Steel
70.00450.010.550.0350.0090.0600.00830.0421.20Example Steel
80.00600.010.310.0360.0080.0400.00190.0830.0081.78Example Steel
90.00600.010.530.0470.0080.0460.00220.0810.0150.00101.74Example Steel
100.0025*0.010.380.0330.0100.0260.00210.0200.0201.03*Comparative Steel
110.0105*0.010.700.0390.0080.0240.00240.1001.23Comparative Steel
120.00650.010.800.0180.0080.0490.00180.0500.99*Comparative Steel
130.00650.010.610.0200.0080.0340.00220.1302.58*Comparative Steel
Units in Wt %
Values marked with * are not included in this invention.

[0261] 21

TABLE 21
SteelAverage particleSurfaceResistance to
No.TS (MPa)El (%)r valuesize (μm)appearancesurface roughnessRemarks
135042.92.148.6AExample
238540.52.038.1AExample
336041.71.977.8AExample
435442.41.999.3AExample
537140.42.028.1AExample
638039.51.919.2AExample
737340.21.969.5AExample
837639.91.907.3BExample
938538.91.959.9BExample
1034543.52.1719.0CXComparative
Example
1139234.51.786.9AComparative
Example
1237537.51.658.1BComparative
Example
1337036.51.586.4AComparative
Example

[0262] 22

TABLE 22
HeatingTotal reduction ratio ofFinishAnnealingAverageResistance
temper-the pass just beforethetemper-temper-particleto
Steelaturefinal pass and the finalatureatureTSElrsizeSurfacesufrace
SymbolNo.(° C.)pass (%)(° C.)(° C.)(MPa)(%)value(μm)appearanceroughness
A111201590086034843.22.158.9AExample
B411804391086035442.41.658.5AComparative
Example
C512001589086537140.42.028.1AExample
D112301893086035042.91.888.6AExample
E212002589084039038.91.857.5AExample
F312103090082036541.71.707.2AComparative
Example

[0263] Best Mode 7

[0264] The above-described Steel sheet 7 according to the present invention is a steel sheet having particularly superior uniformity of material in a coil. The detail of Steel sheet 7 is described in the following.

[0265] Carbon: Carbon forms a fine carbide with niobium to increase the strength of the steel, and to increase the n values in the low strain domains, thus improving the resistance to surface strain. If the carbon content is less than 0.0050%, the effect of carbon addition becomes less. If the carbon content exceeds 0.010%, the ductility degrades. Accordingly, the carbon content is specified to a range of from 0.0050 to 0.010%, preferably from 0.0050 to 0.0080%, most preferably from 0.0050 to 0.0074%.

[0266] Silicon: Excessive addition of silicon degrades the chemical surface treatment performance of cold rolled steels, and degrades the adhesiveness of plating to hot dip galvanized steel sheets. Therefore, the silicon content is specified to not more than 0.05%.

[0267] Manganese: Manganese precipitates sulfur in the steel as MnS to prevent the hot crack generation of slabs and to bring the steel to high strength without degrading the zinc plating adhesiveness. If the manganese content is less than 0.10%, the effect of precipitation of sulfur does not appear. If the manganese content exceeds 1.5%, the strength significantly increases, and reduces the n values in low stress domains. Consequently, the manganese content is specified to a range of from 0.10 to 1.5%.

[0268] Phosphorus: Phosphorus is necessary for increasing the strength of the steel, to amounts of 0.01% or more. If the phosphorus content exceeds 0.05%, however, the alloying treatment performance of zinc plating degrades, thus inducing insufficient adhesion of plating. Accordingly, the phosphorus content is specified to a range of from 0.01 to 0.05%.

[0269] Sulfur: If sulfur content exceeds 0.02%, the ductility degrades. Therefore, the sulfur content is specified to not more than 0.02%.

[0270] sol. Al: A function of sol. Al is to reduce the harm of solid solution nitrogen by precipitating the nitrogen in the steel as AlN. If the sol.Al content is below 0.01%, the effect of addition is not satisfactory. If the sol.Al content exceeds 0.1%, the effect is not so improved for the added amount of sol.Al. Consequently, the sol.Al content is specified to a range of from 0.01 to 0.1%.

[0271] Nitrogen: As small an amount of nitrogen as possible is preferred. In view of cost, the nitrogen content is specified to not more than 0.004%.

[0272] Niobium: Niobium forms fine carbide with carbon to increase the strength of steel, and increases the n values in low strain domains, thus improving the resistance to surface strain. If, however, the niobium content is less then 0.01%, the effect of the niobium addition cannot be attained. If the niobium content exceeds 0.20%, the yield strength significantly increases and the n values in low strain domains decreases. Therefore, the niobium content is specified to a range of from 0.01 to 0.20%, preferably from 0.035 to 0.20%, and most preferably from 0.080 to 0.140%

[0273] Solely specifying the individual components of steel cannot necessarily lead to a high strength cold rolled sheet having excellent uniformity of material in a coil, deep drawability, and punch stretchability. It is necessary for the steel sheet further to satisfy the condition given below.

[0274] A slab consisting essentially of 0.0061% C, 0.01% Si, 0.30% Mn, 0.02% P, 0.005% S, 0.050% sol.Al., 0.0024% N, 0.040 to 0.170% Nb, by weight, was finish rolled at 900° C. of finish temperature and 40% of total reduction ratio of the pass just before the final pass and the final pass. The rolled sheet was coiled at temperatures of from 580 to 680° C., followed by cold rolled to obtain a sheet having 0.8 mm of thickness. The cold rolled sheet was then continuously annealed at 850° C., and was temper rolled to 0.7% of reduction ratio. Thus prepared steel sheet was tested to determine the uniformity of material in a coil.

[0275] FIG. 21 shows the influence of ((Nb×12)/(C×93)] and C on the uniformity of material in a coil.

[0276] When the value of [(Nb×12)/(C×93)] satisfies the formula (14), excellent uniformity of material in a coil is obtained.

1.98−66.3×C≦(Nb×12)/(C×93)≦3.24−80.0×C (14)

[0277] As for the deep drawability, the above-prepared steel sheet was used for evaluating the characteristic by determining the limit drawing ratio during the cylinder forming described in the Best Mode 1, and the hat forming height after the hat forming test.

[0278] FIG. 22 shows the influence of r values and n values on the deep drawability and the punch stretchability.

[0279] Similar with the Best Mode 1, excellent deep drawability and punch stretchability are obtained if only the formulae (3) and (4) are satisfied.

11.0≦r+50.0×n (3)

2.9≦r+5.00×n (4)

[0280] The Steel sheet 7 according to the present invention may further contain titanium to form fine grains and to improve resistance to surface strain. If the titanium content exceeds 0.05%, the surface appearance significantly degrades on hot dip galvanization. Therefore, the titanium content is specified to not more than 0.05%, preferably from 0.005 to 0.02%. In that case, formula (15) is necessary to be applied instead of formula (14).

1.98−66.3×C≦(Nb×12)/(C×93)+(Ti*×12)/(C×48)≦3.24−80.0×C (15)

[0281] Furthermore, to improve the resistance to embrittlement during secondary operation, the addition of boron is effective. If the boron content exceeds 0.002%, the deep drawability and the punch stretchability degrade. Accordingly, the boron content is specified to not more than 0.002%, preferably from 0.0001 to 0.001%.

[0282] The Steel sheet 7 according to the present invention has characteristics of, adding to the excellent uniformity of material in a coil, excellent combined formability, resistance to embrittlement during secondary operation, formability at welded portions, anti-burring performance during shearing, good surface appearance, which characteristics are applicable grades to the automobile exterior panels.

[0283] The Steel sheet 7 according to the present invention can be manufactured by the steps of: preparing a continuous casting slab of a steel having the composition adjusted as described above, including the addition of titanium and boron; finish rolling the slab to 60% or less of total reduction ratios of the pass just before the final pass and the final pass to prepare coiled hot rolled steel sheet; and cold rolling the hot rolled steel sheet followed by annealing. For hot rolling the continuous cast slab may be done directly or after reheated.

[0284] To obtain excellent uniformity of material in a coil, deep drawability, and punch stretchability without fail, it is preferred to conduct the finish rolling at temperatures of 870° C. or more, the coiling after rolled at temperatures of 550° C. or more, the cold rolling at 50 to 85% of reduction ratios, and the annealing at temperatures of from 780 to 880° C. in a continuous annealing line. From the viewpoint of stability of descaling by pickling, the coiling is preferably done at 700° C. or less of temperatures, more preferably 680° C. or less.

[0285] The Steel sheet 7 according to the present invention may further be treated, at need, by zinc base plating treatment such as electroplating and hot dip plating, and by organic coating treatment after the plating.

EXAMPLE 1

[0286] Molten steels of Steel Nos. 1 through 10 shown in Table 23 were prepared. The melts were then continuously cast to form slabs having 220 mm of thickness. After heating the slabs to 1200° C., hot rolled steel sheets having 2.8 mm of thickness were prepared from the slabs under the condition of 30 to 50% of total reduction ratios of the pass just before the final pass and the final pass, 880 to 960° C. of finish temperatures. The hot rolled steel sheets were coiled at 580 to 680° C. of coiling temperatures. The coiled hot rolled sheets were then cold rolled to a thickness of 0.80 mm. The cold rolled sheets were treated by continuous annealing (CAL) at temperatures of from 840 to 870° C., or by continuous annealing at 850 to 870° C. of temperatures followed by hot dip galvanization (CGL), which were then temper-rolled to 0.7% of reduction ratio.

[0287] In the case of continuous annealing followed by hot dip galvanization, the hot dip galvanization after the annealing was given at 460° C., and, immediately after the hot dip galvanization, an alloying treatment of plating layer was given at 500° C. in an in-line alloying furnace. The coating weight was 45 g/m2 per side.

[0288] Thus obtained steel sheets were tested to determine tensile characteristics (along the rolling direction; with JIS Class 5 specimens; and n values being computed in a 1 to 5% strain domain), r values, limit drawing ratio (LDR), and hat forming height (H). For the galvanized steel sheets, the zinc plating adhesiveness was also determined.

[0289] Regarding the zinc plating adhesiveness, adhesive tapes were attached onto the surface of a plating steel sheet, and the steel sheet was subjected to 90 degrees of bending and straightening, then the amount of plating attached to the adhesive tapes was determined. The determination was given on five grades: 1 for no peeling observed; 2 for slight peeling observed; 3 for small amount of peeling observed; 4 for medium area of peeling observed; and 5 for large area of peeling observed. The grades 1 and 2 were set to acceptable range.

[0290] The test results are shown in Tables 24 through 26.

[0291] These tables show that the Example steel sheets give excellent deep drawability, punch stretchability, and uniformity of material in a coil, also give excellent zinc plating adhesiveness.

[0292] To the contrary, the Comparative steel sheets give poor deep drawability and punch stretchability, and, when they dissatisfy the above-given formula (14), the uniformity of material in the longitudinal direction of coil is significantly poor. In addition, when phosphorus and titanium exist to a large amount, the plating adhesiveness is also inferior.

EXAMPLE 2

[0293] Slab of Steel No. 1 shown in Table 23 was heated to 1200° C., and hot rolled to 2.8 mm of thickness under the condition of 30 to 70% of total reduction ratios of the pass just before the final pass and the final pass, 880 to 910° C. of finish temperatures. The hot rolled steel sheets were coiled at 580 to 640° C. of coiling temperatures. The, coiled hot rolled sheets were then cold rolled to a thickness of 0.8 mm. The cold rolled sheets were treated by continuous annealing at temperatures of from 840 to 870° C., or by continuous annealing at 850 to 870° C. of temperatures followed by hot dip galvanization, which were then temper-rolled to 0.7% of reduction ratio.

[0294] The condition of hot dip galvanization was the same with that of Example 1.

[0295] Thus obtained steel sheets were tested to determine tensile characteristics along the rolling direction (n values being computed in a 1 to 5% strain domain), r value, limit drawing ratio, and hat forming height.

[0296] The test results are shown in Table 27.

[0297] The steels which were prepared at 60% or less of total reduction ratios of the pass just before the final pass and the final pass, and which reduction ratios were within the specified range of the present invention, showed excellent uniformity of material in the coil longitudinal direction.

EXAMPLE 3

[0298] Slab of Steel No. 1 shown in Table 23 was heated to 1200° C., and hot rolled to 1.3 to 6.0 mm of thicknesses under the condition of 40% of total reduction ratios of the pass just before the final pass and the final pass, 840 to 980° C. of finish temperatures. The hot rolled steel sheets were coiled at 500 to 700° C. of coiling temperatures. The coiled hot rolled sheets were then cold rolled to a thickness of 0.80 mm at 46 to 87% of reduction ratios. The cold rolled sheets were treated by continuous annealing or by continuous annealing followed by hot dip galvanization, which were then temper-rolled to 0.7% of reduction ratio.

[0299] The condition of hot dip galvanization was the same with that of Example 1.

[0300] Thus obtained steel sheets were tested to determine tensile characteristics along the rolling direction (n values being computed in a 1 to 5% strain domain), r values, limit drawing ratio, and hat forming height.

[0301] The test results are shown in Tables 28 and 29.

[0302] The steels which were prepared within the specified range of the present invention in terms of finish temperature, coiling temperature, reduction ratio during cold rolling, and annealing, showed excellent uniformity of material in the coil longitudinal direction. 23

TABLE 23
Steel No.CSiMnPSsol.AlNNbTiBX/C #Remarks
10.00590.010.340.0190.0110.0500.00210.082trtr1.8Example Steel
20.00600.010.630.0400.0070.0620.00120.075trtr1.6Example Steel
30.00780.010.950.0450.0090.0580.00180.162trtr2.7Example Steel
40.00650.020.250.0210.0080.0500.00170.0910.011tr1.8*Example Steel
50.00810.010.420.0200.0070.0500.00170.0920.0240.00061.7*Example Steel
60.00630.100.160.0300.0110.0570.00190.088trtr1.8Comparative Steel
70.00590.020.200.0670.0100.0500.00210.087trtr1.9Comparative Steel
80.00600.010.220.0300.0090.0560.00190.056trtr1.2Comparative Steel
90.00580.010.210.0280.0100.0570.00200.148trtr3.3*Comparative Steel
100.00900.010.620.0500.0150.0350.00360.126trtr1.8Comparative Steel
X/C #:(Nb % × 12)/(C % × 93)
*(Nb % × 12)/(C % × 93) + (Ti % × 12)/(C % × 48), Ti* % = Ti − (48/14)N % − (48/32)S %

[0303] 24

TABLE 24
Total reduction ratio of the
pass just before the finalFinishCoiling
Steelpass and the final passtemperaturetemperatureAnnealing
No.No.(%)(° C.)(° C.)condition
 1140890580CAL
 2140890580CGL
 3140900640CAL
 4140900640CGL
 5140910680CAL
 6140910680CGL
 7230910580CGL
 8230930640CGL
 9350890640CGL
10350900680CGL
11430890580CGL
12430900640CGL
13430910680CGL
14540900640CGL
15540910680CGL
Characterisitcs of steel sheetFormability of
YPTSsteel sheetZinc plating
No.(MPa)(MPa)El (%)n valuer valueY**Z***H (mm)LDRadhesivenessRemarks
1204353440.2012.0012.13.034.82.16Example
2207356440.1942.0111.73.034.22.161Example
3202354450.2022.0312.13.034.82.16Example
4196355450.2002.0212.03.034.62.161Example
5193352460.2032.0912.23.134.92.17Example
6195356450.2022.0612.23.134.92.172Example
7214384420.1911.9711.52.933.82.151Example
8212382430.1961.9511.82.934.32.151Example
9225395410.1952.0911.83.134.32.172Example
10227394420.1992.1312.13.134.82.172Example
11205355430.1981.9811.93.034.42.161Example
12203354430.2012.0112.13.034.82.161Example
13202352440.2022.0412.13.134.82.171Example
14212372390.1891.9611.42.933.62.152Example
15210370400.1941.9311.62.934.02.152Example
Y** = r + 50.0 x n, Z*** = r + 5.0 x n

[0304] 25

TABLE 25
Total reduction ratio of the
pass just before the finalFinishCoiling
Steelpass and the final passtemperaturetemperatureAnnealing
No.No.(%)(° C.)(° C.)condition
16630900640CGL
17630910680CGL
18730900640CGL
19730910680CGL
20840900580CAL
21640890580CGL
22640900640CAL
23740900640CGL
24730890580CAL
25830890580CGL
26630900640CAL
27630900640CGL
28740900640CAL
Characteristics of steel sheetFormability of
YPTSsteel sheetZinc plating
No.(MPa)(MPa)El (%)n valuer valueY**Z***H (mm)LDRadhesivenessRemarks
16215365420.1821.8811.02.833.02.074Comparative
Example
17212362430.1841.8611.12.833.22.075Comparative
Example
18222368410.1801.9310.92.829.42.073Comparative
Example
19224367410.1781.9310.82.828.02.074Comparative
Example
20321394230.1261.127.41.819.41.96Comparative
Example
21323398220.1281.187.61.819.61.961Comparative
Example
22283382300.1461.348.62.120.61.99Comparative
Example
23287385310.1421.308.42.020.41.981Comparative
Example
24243376370.1531.729.42.521.82.03Comparative
Example
25245680360.1541.779.52.522.12.052Comparative
Example
26231361370.1761.8110.62.727.32.05Comparative
Example
27233364380.1721.8010.42.726.22.152Comparative
Example
28222370320.1632.1210.32.925.52.072Comparative
Example
Y** = r + 50.0 x n, Z*** = r + 5.0 x n

[0305] 26

TABLE 26
Total reduction ratio of the
pass just before the final passFinishCoiling
Steeland the final passtemperaturetemperatureAnnealingCoil
No.No.(%)(° C.)(° C.)conditionposition
29140890580CALT
M
B
30130900640CGLT
M
B
31640900640CGLT
M
B
Formability
Characterisitics of steel sheetof steel sheet
YPTSrH
No.(MPa)(MPa)El (%)n valuevalueY**Z***(mm)LDRRemarks
29204353440.2012.0112.13.034.82.16Example
202352450.2042.0112.23.034.92.16
203355440.2022.0212.13.034.82.16
30202355440.2002.0212.03.034.62.16Example
204353450.1982.0211.93.034.42.16
201356440.2022.0112.13.034.82.16
31287375310.1421.368.52.120.51.99Comparative
211364360.1861.8011.12.733.22.05Example
243374310.1501.408.92.220.92.00
Y** = r + 50.0 x n, Z*** = r + 5.0 x n

[0306] 27

TABLE 27
Total reduction ratio of the
pass just before the final passFinishCoiling
Steeland the final passtemperaturetemperatureAnnealingCoil
No.No.(%)(° C.)(° C.)conditionposition
32140890580CALT
M
B
33130900640CGLT
M
B
34165890580CALT
M
B
35165900640CGLT
M
B
Characteristics of steel sheetFormability of
YPTSsteel sheet
No.(MPa)(MPa)El (%)n valuer valueY**Z***H (mm)LDRRemarks
32204353440.2012.0112.13.034.82.16Example
202352450.2042.0112.23.034.92.16
203355440.2022.0212.13.034.82.16
33202355440.2002.0212.03.034.62.16Example
204353450.1982.0211.93.034.42.16
201356440.2022.0112.13.034.82.16
34297402260.1471.228.62.020.61.98Comparative
259384320.1731.6810.32.525.52.03Example
275391300.1521.429.02.221.02.00
35285388270.1561.319.12.121.21.99Comparative
246371350.1901.7611.32.733.52.05Example
263376300.1731.5210.22.424.82.02
Y** = r + 50.0 x n, Z*** = r + 5.0 x n

[0307] 28

TABLE 28
FinishCoilingCold rollingAnnealing
temperaturetemperatureratioAnnealingtemperatureCoil
No.(° C.)(° C.)(%)condition(° C.)position
3689058071CAL850T
M
B
3793064075CGL640T
M
B
3884064071CGL850T
M
B
3990050071CAL830T
M
B
4089064046CGL810T
M
B
Characteristics of steel sheetFormability of
YPTSsteel sheet
No.(MPa)(MPa)El (%)n valuer valueY**Z***H (mm)LDRRemarks
36204353440.2012.0112.13.034.82.16Example
202352450.2042.0112.23.034.92.16
203355440.2022.0212.13.034.82.16
37194352460.2122.1012.73.235.62.38Example
196348470.2142.1212.83.235.72.18
193351460.2112.1312.73.235.62.18
38277385300.1541.439.12.221.22.00Comparative
213358410.1811.7810.82.728.02.05Example
252372330.1733.6110.22.524.82.03
39234371340.1471.629.02.423.02.02Comparative
222365370.1531.669.32.421.62.02Example
231369350.1501.639.12.421.22.02
40218351410.1791.5510.52.427.02.02Comparative
208347430.1863.5930.92.529.42.03Example
215349420.1831.5730.72.527.52.03
Y** = r + 50.0 x n, Z*** = r + 5.0 x n

[0308] 29

TABLE 29
FinishCoilingCold rollingAnnealing
temperaturetemperatureratioAnnelaingtemperatureCoil
No.(° C.)(° C.)(%)condition(° C.)position
4191068087CGL860T
M
B
4288058071CAL750T
M
B
4392064073CCL900T
M
B
4487055068CCL780T
M
B
Characteristics of steel sheetFormability of
YPTSELnrsteel sheet
No.(MPa)(MPa)(%)valuevalueY**Z***H (mm)LDRRemarks
41247372400.1582.1410.02.923.22.15Comparative
233368420.1662.1710.53.027.02.16Example
242371410.1512.159.72.922.72.15
42236365400.1671.0110.02.423.22.02Comparative
224361420.1721.6410.22.524.82.03Example
229362421.1701.6310.12.524.02.03
43248381320.1431.568.72.320.72.01Comparative
239373340.1501.629.12.421.22.02Example
244377330.1481.599.02.321.02.01
44228373330.1461.548.82.320.82.01Comparative
217369340.1511.589.12.321.22.01Example
223370330.1491.579.02.321.02.01
Y** = r + 50.0 xn,
Z*** = r + 5.0 xn