Title:
Cold-rolled steel strip and hot-dip coated cold-rolled steel strip for use as building material and manufacturing method thereof
Kind Code:
A1


Abstract:
A cold-rolled steel strip or hot-dip coated cold-rolled steel strip for use as a fire-proof building member has composition consisting of 0.01-0.25 wt. % C, up to 1.5 wt. % Si, 0.05-2.5 wt. % Mn, up to 0.1 wt. % P, no more than 0.02 wt. % S, 0.005-0.1 wt. % Al, 0.05-1.0 wt. % Mo, optionally 0.005-0.2 wt. % one or more selected from Ti, Nb, V and W, optionally one or more of 0.05-0.6 wt. % Cu, 0.05-0.6 wt. % Ni, 0.05-3.0 wt. % Cr and 0.0003-0.003 wt. % B and the balance being essentially Fe except inevitable impurities. The cold-rolled steel strip is manufactured by hot-rolling, acid-pickling, cold-rolling and then annealing at a temperature above its recrystallization temperature but below 950° C. The hot-dip coated cold-rolled steel strip is manufactured in the same way but in-line heating a cold-rolled steel strip at a temperature above its recrystallization temperature but below 950° C. and then immersing it into a coating bath. The annealed or hot-dip coated steel strip may be further cold-rolled with such a slight duty to induce a plastic strain of 1-5%.



Inventors:
Higo, Yuichi (Kure-shi, JP)
Hamanaka, Seiichi (Kure-shi, JP)
Fujita, Toru (Tokyo, JP)
Application Number:
09/756245
Publication Date:
05/10/2001
Filing Date:
01/08/2001
Assignee:
HIGO YUICHI
HAMANAKA SEIICHI
FUJITA TORU
Primary Class:
Other Classes:
148/531, 148/533, 148/537, 420/9, 420/99, 420/123, 427/433, 428/653
International Classes:
C21D8/02; C22C38/04; C22C38/06; C22C38/12; C22C38/42; C22C38/44; (IPC1-7): B32B15/18; C21D9/52; C22C38/06; C22C38/12
View Patent Images:
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Primary Examiner:
RICKMAN, HOLLY C
Attorney, Agent or Firm:
Kent E. Baldauf, Sr., Esq. (Pittsburgh, PA, US)
Claims:

What is claimed is;



1. A cold-rolled steel strip for use as a fire-proof building member consisting of 0.01-0.25 wt. % C, up to 1.5 wt. % Si, 0.05-2.5 wt. % Mn, up to 0.1 wt. % P, no more than 0.02 wt. % S, 0.005-0.1 wt. % Al, 0.05-1.0 wt. % Mo and the balance being essentially Fe except inevitable impurities.

2. The cold-rolled steel strip defined in claim 1, which further contains 0.005-0.2 wt. % one or more selected from Ti, Nb, V and W.

3. The cold-rolled steel strip defined in claim 1, which further contains one or more of 0.05-0.6 wt. % Cu, 0.05-0.6 wt. % Ni, 0.05-3.0 wt. % Cr and 0.0003-0.003 wt. % B.

4. The cold-rolled steel strip defined in claim 1, which further contains 0.005-0.2 wt. % one or more selected from Ti, Nb, V and W, and one or more of 0.05-0.6 wt. % Cu, 0.05-0.6 wt. % Ni, 0.05-3.0 wt. % Cr and 0.0003-0.003 wt. % B.

5. A hot-dip coated cold-rolled steel strip for use as a fire-proof building member consisting of 0.01-0.25 wt. % C, up to 1.5 wt. % Si, 0.05-2.5 wt. % Mn, up to 0.1 wt. % P, no more than 0.02 wt. % S, 0.005-0.1 wt. % Al, 0.05-1.0 wt. % Mo and the balance being essentially Fe except inevitable impurities.

6. The hot-dip coated cold-rolled steel strip defined in claim 5, which further contains 0.005-0.2 wt. % one or more selected from Ti, Nb, V and W.

7. The hot-dip coated cold-rolled steel strip defined in claim 5, which further contains one or more of 0.05-0.6 wt. % Cu, 0.05-0.6 wt. % Ni, 0.05-3.0 wt. % Cr and 0.0003-0.003 wt. % B.

8. The hot-dip coated cold-rolled steel strip defined in claim 5, which further contains 0.005-0.2 wt. % one or more selected from Ti, Nb, V and W, and one or more of 0.05-0.6 wt. % Cu, 0.05-0.6 wt. % Ni, 0.05-3.0 wt. % Or and 0.0003-0.003 wt. % B.

9. A method of manufacturing a cold-rolled steel strip for use as a fire-proof building member, comprising the steps of: preparing a slab having composition consisting of 0.01-0.25 wt. % C, up to 1.5 wt. % Si, 0.05-2.5 wt. % Mn, up to 0.1 wt. % P, no more than 0.02 wt. % S, 0.005-0.1 wt. % Al, 0.05-1.0 wt. % Mo, optionally 0.005-0.2 wt. % one or more selected from Ti, Nb, V and W, optionally one or more of 0.05-0.6 wt. % Cu, 0.05-0.6 wt. % Ni, 0.05-3.0 wt. % Cr and 0.0003-0.003 wt. % B and the balance being essentially Fe except inevitable impurities; heating said slab at 1000-1250° C.; hot-rolling the heated slab at a finishing temperature of 800-950° C.; coiling the hot-rolled steel strip at 400-700° C.; acid pickling the hot-rolled steel strip; cold-rolling the pickled steel strip at a reduction ratio of 40-90%; annealing the cold-rolled steel strip at a temperature above its recrystallization temperature but below 950° C.; and optionally cold-rolling the annealed steel strip with such a slight duty to induce a plastic strain of 1-5% to said steel strip.

10. The method defined in claim 9, wherein the cold-rolled steel strip is box-annealed or continuously annealed.

11. A method of manufacturing a hot-dip coated cold-rolled steel strip for use as a fire-proof building member, comprising the steps of: preparing a slab having composition consisting of 0.01-0.25 wt. % C, up to 1.5 wt. % Si, 0.05-2.5 wt. % Mn, up to 0.1 wt. % P, no more than 0.02 wt. % S, 0.005-0.1 wt. % Al, 0.05-1.0 wt. % Mo, optionally 0.005-0.2 wt. % one or more selected from Ti, Nb, V and W, optionally one or more of 0.05-0.6 wt. % Cu, 0.05-0.6 wt. % Ni, 0.05-3.0 wt. % Cr and 0.0003-0.003 wt. % B and the balance being essentially Fe except inevitable impurities; heating said slab at 1000-1250° C.; hot-rolling the heated slab at a finishing temperature of 800-950° C.; coiling the hot-rolled steel strip at 400-700° C.; acid pickling the hot-rolled steel strip; cold-rolling the pickled steel strip at a reduction ratio of 40-90%; in-line heating the cold-rolled steel strip at a temperature above its recrystallization temperature but below 950° C. in a continuous hot-dip coating installation; immersing the in-line heated steel strip into a hot-dip coating bath; and optionally cold-rolling the hot-dip coated steel strip with such a slight duty to induce a plastic strain of 1-5% to said steel strip.

Description:

BACKGROUND OF THE INVENTION

1. The present invention relates to a cold-rolled steel strip and a hot-dip coated steel strip useful as building material, and is also concerned with a method of manufacturing these steel strips.

2. Fire-proof coating has been applied to a surface of a building which needs fire-proof construction, in order to inhibit temperature-up of steel material during a fire or the like. A so-called “fire-proof steel” which exhibits high strength at an elevated temperature is used in these days, so that a building can be kept safe even when the steel material is heated at an elevated temperature near 600° C. Use of such fire-proof steel enables reduction or omission of fire-proof coating. The high-temperature strength is generally represented by yield strength at a high-temperature.

3. Such fire-proof steel material which has been used so far as main structural members for a building is usually a relatively thick hot-rolled steel sheet, although a steel sheet with or without hot-dip coating made from a cold rolled steel strip is partially used for the purpose.

4. Construction of a building also needs steel material for secondary structural members, roofing and walls in addition to main structural members. Cold-rolled steel sheets and hot-dip coated cold-rolled steel sheets have been often used as such members. When this kind of steel material is improved in fire-proof property by enhancing its high-temperature strength, the same advantages as those of the main structural members can be expected. In this sense, there is a demand for provision of a cold-rolled steel strip or a hot-dip coated cold-rolled steel strip excellent in fire-proof property.

5. Such a cold-rolled steel sheet is manufactured by subjecting a hot-rolled steel strip to cold-rolling, annealing, hot-dip coating, etc. The steel sheet is sometimes reformed with a heavy duty in order to shape a building member necessary for an actual use. Therefore, the steel sheet shall have good formability as well as a proper high-temperature strength.

SUMMARY OF THE INVENTION

6. The present invention aims at provision of steel material useful as a fire-proof building member. The steel material may be provided as a cold-rolled steel strip or a hot-dip coated cold-rolled steel strip excellent in high-temperature strength and formability. The excellent high-temperature property is attained by controlling alloying composition of the steel strip and further improved by introduction of a plastic strain to the steel strip.

7. A newly proposed steel strip useful as a fire-proof building member essentially consists of 0.01-0.25 wt. % C, up to 1.5 wt. % Si, 0.05-2.5 wt. % Mn, up to 0.1 wt. % P, up to 0.02 wt. % S, 0.005-0.1 wt. % Al, 0.05-1.0 wt. % Mo and the balance being essentially Fe except inevitable impurities. The steel strip may contain 0.005-0.2 wt. % one or more of Ti, Nb, V and W, and/or one or more of 0.05-0.6 wt. % Cu, 0.05-0.6 wt. % Ni, 0.05-3.0 wt. % Cr and 0.0003-0.003 wt. % B.

8. A cold-rolled steel strip useful as a fire-proof building member is manufactured as follows: A slab having the specified composition is heated at 1000-1250° C., hot-rolled at 800-950° C., coiled at 400-700° C., acid-pickled, cold-rolled at a reduction ratio of 40-90% and then annealed at a temperature above a recrystallization temperature but below 950° C. Either box-type annealing or continuous annealing is applicable.

9. A hot-dip coated cold-rolled steel strip useful as a fire-proof building member is manufactured as follows: A cold-rolled steel strip processed in the same way is in-line heated at a temperature above a recrystallization temperature but below 950° C. in a continuous hot-dip coating installation and then immersed into a hot-dip coating bath.

10. Any of the cold-rolled steel strip or the hot-dip coated steel sheet may be further cold-rolled with such a slight duty of 1-5% to induce a plastic strain to the steel strip, after being annealed or hot-dip coated, respectively. Cold-rolling with a slight duty is the most effective in an industrial point of view, although a plastic strain could be induced into the steel strip by application of a stretching load or leveling. The further cold-rolling advantageously enhances the high-temperature properties of the obtained steel strip.

BRIEF DESCRIPTION OF THE DRAWINGS

11. FIG. 1 is a graph which shows a relationship between a cold-rolling reduction ratio after annealing and properties at a room temperature and 600° C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

12. The inventors have researched and examined effects of alloying elements on a high-temperature strength necessary for a fire-proof steel member without deterioration of formability.

13. A hot-rolled steel strip which has been designed for use as a fire-proof building member is improved in high-temperature property by addition of such alloying elements as Mo, W, Ti or Nb which are dissolved in a steel matrix or precipitated as intermetallic compounds. On the other hand, a cold-rolled steel strip or a hot-dip coated cold-rolled steel strip is annealed at a temperature above a recrystallization point after being cold-rolled for improvement of formability, since the steel strip as cold-rolled lacks in formability.

14. Annealing after cold-rolling effectively improves a high-temperature strength due to addition of Mo which exhibits the same effect as that in a conventional hot-rolled steel strip designed for use as a fire-proof building member. However, a high-temperature strength necessary for the purpose is not often realized.

15. The inventors suppose the poor high-temperature strength is caused by unexpected precipitation or the like in the annealing step after cold-rolling. That is, a cold-rolled steel strip or a hot-dip coated cold-rolled steel strip is in metallurgical situations different from a hot-rolled steel strip, since it is processed by cold-rolling and annealing. In this sense, it is not practical to simply apply an alloying design which has been proposed for a hot-rolled steel strip to a cold-rolled steel strip or a hot-dip coated cold-rolled steel strip with the proviso that the cold-rolled steel strip or the hot-dip coated cold-rolled steel strip is merely different in thickness from the hot-rolled steel strip.

16. Taking into consideration the metallurgical hysteresis of the cold-rolled steel strip or the hot-dip coated cold-rolled steel strip, the inventors have found addition of Mo is the most effective among alloying designs proposed so far, and reached an advantageous alloying design using dissolution and precipitation of Mo in specified composition for enhancement of high-temperature properties. The fire-proof and high-temperature properties may be further improved by combinative addition of such carbide-forming elements as Ti, Nb, V and W. Formability of a steel strip necessary for use as a building member is assured by controlled heat treatment in an annealing or hot-dip coating step.

17. The high-temperature strength is further enhanced by introduction of a plastic strain of 1-5% to the cold-rolled steel strip or the hot-dip coated cold-rolled steel strip. Introduction of such a slight plastic strain enhances a yield strength of the steel strip at a high temperature near 600° C., so as to offer steel material useful as a fire-proof building member due to its excellent fire-proof properties. Such a plastic strain is applied to the steel strip in cold state but not hot state. Practically, the plastic strain is applied to the steel strip by cold-rolling at a small reduction ratio.

18. The proposed alloying design will be apparent from the following explanation.

19. C: 0.01-0.25 wt. %

20. C is an alloying element to bestow a steel with a required strength. An effect of C on the strength becomes bigger as addition of C in an amount of 0.01 wt. % or more. However, excessive addition of C above 0.25 wt. % would cause deterioration of formability or weldability.

21. Si: up to 1.5 wt. %

22. Si is an alloying element effective in improvement of strength, although excessive addition of Si above 1.5 wt. % would cause hardening of steel and poor ductility. In case of a mother sheet for hot-dip coating, addition of Si in amount above 0.3 wt. % causes remaining of uncoated surface parts. In this regard, Si content shall be controlled lower. Such defects as the remaining of uncoated surface parts can be inhibited by electroplating of Fe or a ferrous alloy to a steel strip and then passing the steel strip to a hot-dip coating installation, even if the steel strip contains Si in an amount exceeding 0.3 wt. %. In this sense, a steel strip containing Si up to 1.5 wt. % may be processed in the same way.

23. Mn: 0.05-2.5 wt. %

24. Mn is added as a deoxidizing agent to a steel in a steel making step and is also effective for inhibition of embrittlement at a high temperature caused by S included as an impurity. Mn effect becomes remarkable in case of addition of Mn in an amount of 0.05 wt. % or more. However, excessive addition of Mn above 2.5 wt. % would cause poor ductility.

25. P: up to 0.1 wt. %

26. P is an element effective for enhancing a strength and also improving corrosion resistance in combination with Cu, although excessive addition of P above 0.1 wt. % would cause embrittlement.

27. S: no more than 0.02 wt. %

28. S is a harmful element included as an inevitable impurity. Less S content is more preferable for the purpose. Allowable S content in the proposed steel is not more than 0.02 wt. %.

29. Al: 0.005-0.1 wt. %

30. Al is added as a deoxidizing agent to a steel and also effective for stabilization of N as AlN in the steel. This effect is realized by addition of Al in an amount of 0.005 wt. % or more. However, excessive addition of Al above 0.1 wt. % would cause deterioration of formability and external appearance.

31. Mo: 0.05-1.0 wt. %

32. An additive Mo is dissolved or precipitated as a carbide in a steel matrix, resulting in increase of a high-temperature strength. This Mo effect becomes remarkable by addition of Mo in an amount of 0.05 wt. % or more, but saturated at 1.0 wt. %. Excessive addition of Mo above 1.0 wt. % would rather cause hardening of a steel and poor ductility.

33. Ti, Nb, V, W: each 0.005-0.2 wt. %

34. These elements are optional additives which are precipitated as carbides effective in tensile properties at room temperature as well as a high-temperature strength. The effect on such improvement is realized by addition of Ti, Nb, V or W in an amount of 0.005 wt. % or more. However, the effect is saturated at 0.2 wt. %, and excessive addition of Ti, Nb, V or W above 0.2 wt. % would cause hardening of a steel and poor ductility.

35. Cu: 0.05-0.6 wt. %

36. Cu is an optional alloying additive which effectively improves corrosion resistance of a steel in combination with P. Such the effect is remarkable by addition of Cu in an amount of 0.05 wt. %. However, excessive addition of Cu exceeding 0.6 wt. % would rather cause promotion of high-temperature cracking during hot-rolling.

37. Ni: 0.05-0.6 wt. %

38. Ni is an optional additive effective in corrosion resistance and inhibition of high-temperature embrittlement. Such the effect is remarkable by addition of Ni in an amount of 0.05 wt. % or more. However, Ni is such an expensive element to increase a steel making cost, and a favorable effect on such properties is hardly expected regardless increase of Ni content even when Ni is added in an amount above 0.6 wt. %.

39. Cr: 0.05-3.0 wt. %

40. Cr is an optional additive effective in corrosion resistance and also increases a high-temperature strength due to precipitation of carbides. Such effects are remarkable by addition of Cr in an amount of 0.05 wt. % or more. However, excessive addition of Cr above 3.0 wt. % would rather cause hardening of a steel and poor ductility, but not further improve the corrosion resistance or the high-temperature strength.

41. B: 0.0003-0.003 wt. %

42. B is an optional additive which effectively strengthens grain boundaries. The B effect is remarkable by addition of B in an amount of 0.0003 wt. % or more, but saturated at 0.003 wt. %.

43. Steel material having the specified composition is cast to a slab by a conventional continuous casting process and then hot-rolled to a predetermined thickness.

44. In the hot-rolling step, a slab is soaked, hot-rolled and then coiled.

45. The soaking promotes dissolution of alloying elements in the steel matrix and renders the slab to a state capable of hot-rolling, when the slab is heated at a temperature of 1000° C. or higher. But, an excessively higher soaking temperature above 1250° C. would cause hot embrittlement of the slab.

46. The hot-rolling is preferably finished at 800-950° C. so as to assure oversaturated dissolution of alloying elements in the steel matrix without remaining of work-induced ferritic grains. If the finish temperature is lower than 800° C., the alloying elements are partially precipitated in the steel matrix resulting in poor high-temperature strength. But, a higher finish temperature above 950° C. would wastefully consume a thermal energy and also put a heavy duty on a heating furnace.

47. The hot-rolled steel strip is coiled at 400-700° C. The controlled coiling temperature is effective for maintaining the dissolution of alloying elements without growth of intermetallic compounds or the like during coiling. Due to such dissolution, the steel strip after annealing or in-line heating in succession to cold-rolling is improved in high-temperature properties.

48. Thereafter, the hot-rolled steel strip is acid-pickled before cold-rolling.

49. The steel strip is then cold-rolled under conventional conditions. The cold-rolling is preferably performed at a reduction ratio of 40-90% in order to promote recrystallization in the following annealing or continuous hot-dip coating step. Such a controlled reduction ratio is also effective for suppressing growth into coarse crystal grains which would put harmful influences on formability.

50. In case of manufacturing a cold-rolled steel strip without hot-dip coating, the steel strip after cold-rolled is directly delivered to an annealing step. In the annealing step, the steel strip is heated at a temperature above its recrystallization temperature so as to release strains caused by the cold-rolling and to promote sufficient recrystallization; otherwise the heat-treated steel would be hard and insufficient of formability. On the other hand, overheating at a temperature above 950° C. causes growth into coarse crystal grains, although the steel strip can be softened. The growth into coarse crystal grains would reduce formability and poor external appearance of the heat-treated steel strip.

51. In case of manufacturing a hot-dip coated steel strip, the steel strip after cold-rolled is in-line heated at a temperature above its recrystallization temperature but below 950° C. in a continuous hot-dip coating installation. The in-line heating is performed in a reducing atmosphere to activate a surface layer of the steel strip and to anneal the steel strip.

52. A temperature for the in-line heating is set above a recrystallization temperature: otherwise release of strains caused by cold-rolling and recrystallization would be insufficient for softening the steel strip. Such low-temperature heating also causes insufficient activation of the steel strip, resulting in remaining of uncoated surface parts after hot-dip coating. On the contrary, overheating at a temperature above 950° C. causes growth into coarse crystal grains or occurrence of surface defects or the like.

53. The in-line heated steel strip is then immersed into a hot-dip coating bath in the continuous hot-dip coating installation. The hot-dip coating bath may be Zn, Al or Zn—Al. The steel strip coated with a Zn, Al or Zn—Al plating layer is obtained in this way.

54. The cold-rolled steel strip or the hot-dip coated cold-rolled steel strip manufactured in the way as above-mentioned may be further subjected to cold-rolling under such conditions to apply a plastic strain of 1-5% to the steel strip. The introduction of the plastic strain effectively improves a high-temperature strength of the steel strip.

55. The effect of the plastic strain on the high-temperature strength is newly discovered by the inventors. The high-temperature strength is increased in response to a degree of the plastic strain which can be represented by a reduction ratio in such a cold-rolling step with a slight duty as defined above.

56. FIG. 1 shows the effect of a cold rolling reduction ratio on mechanical properties at a room temperature and 600° C., when a steel consisting of 0.09 wt. % C, 0.05 wt. % Si, 0.55 wt. % Mn, 0.012 wt. % P, 0.006 wt. % S, 0.035 wt. % Al, 0.31 wt. % Mo, 0.07 wt. % V and the balance being Fe except inevitable impurities was hot-rolled, cold-rolled, annealed 1 min. at 800° C. and then further cold-rolled.

57. It is noted from FIG. 1 that a yield strength at 600° C. is increased as increase of a cold-rolling reduction ratio. Such improvement on the mechanical properties can be clearly recognized at a reduction ratio above 1%. Although a high yield strength at 600° C. is obtained in case of a reduction ratio above 5%, an elongation at a room temperature is unfavorably reduced. Decrease of an elongation means poor formability of the cold-rolled steel strip or the hot-dip coated cold-rolled steel strip. From these results, a reduction ratio during cold-rolling after annealing is preferably controlled within a range of 1-5%.

EXAMPLE

Example 1

58. Each steel having composition shown in TABLE 1 was melted and cast to a slab. The slab was forged and hot-rolled to a steel strip of 4.0 mm in thickness. The hot-rolled steel strip was then cold-rolled to a thickness of 1.0 mm and annealed under various conditions shown in TABLE 2.

59. A test peace was cut off each steel strip manufactured in this way and offered to tensile tests at a room temperature and 600° C. Results are shown in TABLE 2 in combination with an annealing temperature.

60. It is noted from TABLE 2 that steel strips having specified composition and being annealed at 650-950° C. had enough ductility at a room temperature and a higher yield strength at 600° C. in comparison with comparative test pieces. Consequently, it is recognized that the steel strips in the scope of the present invention are useful as fire-proof building members excellent in high-temperature properties. 1

TABLE 1
ALLOYING COMPOSITIONS OF STEELS USED IN EXAMPLE 1
STEELCOMPONENTS AND CONTENTS (unit: wt. %)
MARKCSiMnPSAlCuNiCrMoTiNbVWBNOTE
I0.130.020.550.0110.0160.0220.010.010.01COMPARATIVE
II0.110.010.680.0060.0110.0420.020.010.020.155EXAMPLES
III0.120.350.750.0090.0020.0340.010.020.020.022
IV0.150.020.450.0080.0050.0400.020.010.010.32PRESENT
V0.120.020.480.0080.0070.0300.010.010.030.310.050INVENTION
VI0.100.010.540.0100.0040.0470.500.430.020.350.030
VII0.120.021.230.0120.0070.0290.020.010.020.520.080
VIII0.130.440.450.0090.0060.0360.010.020.010.190.090
IX0.090.031.450.0100.0060.0370.020.010.020.420.080
X0.080.060.770.0120.0090.0310.010.010.020.320.0950.015
XI0.200.052.150.0110.0040.0550.010.010.550.300.002

61. 2

TABLE 2
EFFECTS OF ANNEALING TEMPERATURE ON TENSILE PROPERTIES
OF COLD-ROLLED STEEL STRIPS
TENSILE PROPERTIES AT A ROOM
TEMPERATUREYIELD
ANNEALINGTENSILEYIELDSTRENGTH
EXAMPLESSTEELTEMP.STRENGTHSTRENGTHELONGATION(MPa)
No.MARK(° C.)(MPa)(MPa)(%)AT 600° C.NOTE
1I8204933603598COMPARATIVE EXAMPLE
2II82048037930107COMPARATIVE EXAMPLE
3III82046236432115COMPARATIVE EXAMPLE
4IV60057655511220COMPARATIVE EXAMPLE
5IV85044934135164PRESENT INVENTION
6V82050341430176PRESENT INVENTION
7VI85048138531170PRESENT INVENTION
8VII6005865539259COMPARATIVE EXAMPLE
9VII85046237135167PRESENT INVENTION
10VIII80049439132178PRESENT INVENTION
11IX78048137534172PRESENT INVENTION
12X85050141132184PRESENT INVENTION
13XI85061048826181PRESENT INVENTION

Example 2

62. Each slab having composition shown in TABLE 3 was prepared by a continuous casting process. The slab was soaked at 1150-1200° C., hot-rolled to 2.3-3.0 mm in thickness at a finishing temperature of 850-870° C. and coiled at 550-580° C. The hot-rolled steel strip was then cold-rolled to 0.8-1.2 mm in thickness.

63. One group of cold-rolled steel strips were delivered to a continuous annealing line, while the remaining of the steel strips were delivered to a continuous hot-dip coating line. In the continuous annealing line, each steel strip was heated 40 seconds at 820° C. and then cooled down to a room temperature with or without overageing treatment. In the continuous hot-dip coating line, each steel strip was in-line annealed 35 seconds at 800° C., cooled down to 500° C. near a temperature of a coating bath, and then immersed into a molten Zn or Zn-5% Al bath.

64. A test piece cut off each of the cold-rolled steel strips and the hot-dip coated steel strips was offered to tensile tests at a room temperature and 600° C. Results are shown in TABLE 4.

65. It is recognized from TABLE 4 that any of the cold-rolled steel strips or the hot-dip coated steel strips having the specified compositions and being annealed in the specified temperature range is useful as a fire-proof building member due to its excellent ductility at a room temperature as well as a high yield strength at 600° C. compared with comparative steel strips. 3

TABLE 3
ALLOYING COMPOSITIONS OF STEELS USED IN EXAMPLE 2
STEELCOMPONENTS AND CONTENTS (unit: wt. %)
MARKCSiMnPSAlCuNiCrMoTiNbVWNOTE
XII0.130.010.450.0110.0050.0220.010.010.03COMPARATIVE
EXAMPLE
XIII0.100.010.550.0100.0080.0280.010.010.400.38PRESENT
INVENTION
XIV0.080.500.870.0120.0050.0350.490.450.020.410.0750.015PRESENT
INVENTION

66. 4

TABLE 4
EFFECTS OF ANNEALING TEMPERATURE ON TENSILE PROPERTIES OF COLD-ROLLED STEEL
STRIPS AND HOT-DIP COATED COLD-ROLLED STEEL STRIPS
TENSILE PROPERTIES AT A ROOM
THICKNESSTEMPERATUREYIELD
HEATINGOF A TESTTENSILEYIELDSTRENGTH
EXAMPLESTEELTEMP.*PIECESTRENGTHSTRENGTHELONGATION(MPa)
No.MARKPROCESS*(° C.)(mm)(MPa)(MPa)(%)AT 600° C.NOTE
14XII18201.044534433101COMPARATIVE
15XII28001.04483623195EXAMPLES
16XIII18200.843937432164PRESENT
17XIII28000.844938529163INVENTION
18XIII38000.845238128171
19XIV18201.049339230176
20XIV28001.048838928181
NOTE:
Process 1 means annealing a cold-rolled steel strip without hot-dip coating.
Process 2 means hot-dip coating a cold-rolled steel strip with a Zn layer.
Process 3 means hot-dip coating a cold-rolled steel strip with a Zn-5% Al layer.
The heating temperature is an annealing temperature or a in-line heating temperature before hot-dip coating.

Example 3

67. Each steel having composition shown in TABLE 5 was melted, cast, forged and then hot-rolled to a steel strip of 4.0 mm in thickness. The hot-rolled steel strip was then cold-rolled to a thickness of 1.0 mm. The cold-rolled steel strip was annealed by heating 1 min. at 800° C. and cooling in an opened atmosphere. One group of the annealed steel strips were further cold-rolled with a slight duty to induce plastic strains.

68. A test piece was cut off each steel strip and offered to tensile tests at a room temperature and 600° C. Results are shown in TABLE 6. It is noted from TABLE 6 that the steel strips having the specified compositions and being bestowed with plastic strains of 1-5% are useful as fire-proof members due to their excellent ductility at a room temperature as well as high yield strength at 600° C. compared with comparative steel strips. 5

TABLE 5
ALLOYING COMPOSITIONS OF STEELS USED IN EXAMPLE 3
STEELCOMPONENTS AND CONTENTS (unit: wt. %)
MARKCSiMnPSAlCuNiCrMoTiNbVWBNOTE
XV0.130.040.550.0110.0060.0280.010.010.02COMPARATIVE
EXAMPLE
XVI0.150.020.450.0130.0050.0300.020.010.010.32PRESENT
INVENTION
XVII0.050.340.680.0080.0100.0420.020.010.030.045COMPARATIVE
EXAMPLE
XVIII0.110.021.450.0100.0080.0310.010.010.020.420.08PRESENT
INVENTION
XIX0.080.010.540.0110.0040.0330.020.010.030.350.030PRESENT
INVENTION
XX0.200.020.480.0090.0060.0300.010.010.020.310.050PRESENT
INVENTION
XXI0.050.040.540.0120.0070.0350.050.440.010.30PRESENT
INVENTION
XXII0.080.030.770.0140.0060.0380.010.010.020.320.0950.01PRESENT
INVENTION
XXIII0.130.031.440.0100.0040.0430.010.010.030.35PRESENT
INVENTION
XXIV0.120.020.750.0090.0030.0330.020.020.020.1550.022COMPARATIVE
EXAMPLE
XXV0.050.042.150.0130.0070.0530.010.010.550.300.002PRESENT
INVENTION
XXVI0.080.031.230.0140.0120.0290.020.010.030.520.080PRESENT
INVENTION
XXVII0.100.550.450.0110.0060.0360.010.020.020.120.09PRESENT
INVENTION

69. 6

TABLE 6
EFFECTS OF COLD-ROLLING REDUCTION RATIO ON TENSILE PROPERTIES
AT A ROOM TEMPERATURE AND 600° C.
COLD-ROLLINGTENSILE PROPERTIES AT A ROOM
REDUCTIONTEMPERATUREYIELD
RATIO AFTERTENSILEYIELDSTRENGTH
EXAMPLESTEELANNEALINGSTRENGTHSTRENGTHELONGATION(MPa)
No.MARK(%)(MPa)(MPa)(%)AT 600° C.NOTE
21XV2.54133703898COMPARATIVE EXAMPLE
22XVI254247429257PRESENT INVENTION
23XVI757654514270COMPARATIVE EXAMPLE
24XVII044938132114COMPARATIVE EXAMPLE
25XVII646344113230COMPARATIVE EXAMPLE
26XVIII2.552146526239PRESENT INVENTION
27XIX245640128189PRESENT INVENTION
28XX1.562253123267PRESENT INVENTION
29XXI348142526202PRESENT INVENTION
30XXI749947613219COMPARATIVE EXAMPLE
31XXII2.545340129201PRESENT INVENTION
32XXIII1.563055223248PRESENT INVENTION
33XXV259650824241PRESENT INVENTION
34XXVI2.556350728236PRESENT INVENTION
35XXVII251146429198PRESENT INVENTION

Example 4

70. Each slab having composition shown in TABLE 7 was prepared by a continuous casting process. The slab was soaked at 1180-1210° C., hot-rolled to a steel strip of 2.3-3.0 mm in thickness at a finishing temperature of 840-870° C. and then coiled at 530-580° C. The hot-rolled steel strip was cold-rolled to a thickness of 0.6-1.0 mm.

71. One group of the cold-rolled steel strips manufactured in this way were delivered to a continuous annealing line, while the remaining group of the steel strips were delivered to a hot-dip coating line. In the annealing line, each steel strip was heated 40 sec. at 800° C. and then cooled down to a room temperature with or without overageing treatment. In the hot-dip coating line, each steel strip was in-line heated 35 sec. at 800° C., cooled down to 500° C. near a temperature of a coating bath and then immersed into the coating bath. A molten Zn or Zn-5% Al pool was used as the coating bath.

72. Each steel strip was cold-rolled with a slight duty after annealing or hot-dip coating, so as to induce a plastic strain.

73. A test piece was cut off each of the cold-rolled steel strips and the hot-dip coated steel strips and offered to tensile tests at a room temperature and 600° C. Results are shown in TABLE 8. It is noted from TABLE 8 that the steel strips having the specified compositions and being bestowed with plastic strains of 1-5% are useful as fire-proof members due to their excellent ductility at a room temperature as well as high yield strength at 600° C. compared with comparative steel strips. 7

TABLE 7
ALLOYING COMPOSITIONS OF STEELS USED IN EXAMPLE 4
STEELCOMPONENTS AND CONTENTS (unit: wt. %)
MARKCSiMnPSAlCuNiCrMoTiNbVWNOTE
XXVIII0.090.010.510.0140.0040.0290.010.010.030.0500.040COMPARATIVE EXAMPLE
XXIX0.070.410.520.0120.0080.0310.550.460.400.35PRESENT INVENTION
XXX0.080.030.770.0110.0060.0380.020.020.020.350.0650.010
XXXI0.110.251.080.0080.0100.0390.020.010.010.450.025

74. 8

TABLE 8
EFFECTS OF COLD-ROLLING REDUCTION RATIO ON TENSILE PROPERTIES OF COLD-ROLLED
STEEL STRIPS AND HOT-DIP COATED STEEL STRIPS AT A ROOM TEMPERATURE AND 600° C.
TENSILE PROPERTIES AT A ROOM
THICKNESSTEMPERATUREYIELD
REDUCTIONOF TESTTENSILEYIELDELONGA-STRENGTH
EXAMPLESTEELRATIO*PIECESTRENGTHSTRENGTHTION(MPa)
No.MARKPROCESS*(%)(mm)(MPa)(MPa)(%)AT 600° C.NOTE
36XXVIII201.059551028121COMPARATIVE
EXAMPLE
37XXVIII231.060156225140COMPARATIVE
EXAMPLE
38XXIX130.846242029200PRESENT
INVENTION
39XXIX160.847845414227COMPARATIVE
EXAMPLE
40XXX12.50.846842927214PRESENT
INVENTION
41XXX180.848746513222COMPARATIVE
EXAMPLE
42XXX21.50.846743827208PRESENT
INVENTION
43XXX23.50.847745026212PRESENT
INVENTION
44XXX320.846143125207PRESENT
INVENTION
45XXXI120.655251324238PRESENT
INVENTION
NOTE:
Process 1 means annealing a cold-rolled steel strip without hot-dip coating.
Process 2 means hot-dip coating a cold-rolled steel strip with a Zn layer.
Process 3 means hot-dip coating a cold-rolled steel strip with a Zn-5% Al layer.
Reduction ratio is a value during cold-rolling after annealing.

75. A cold-rolled steel strip or a hot-dip coated cold-rolled steel strip according to the present invention is excellent in formability and high-temperature properties necessary for use as a fire-proof building member. Since there is not required any special changes in manufacturing process from a steel making step to an annealing or hot-dip coating step, the steel strip is advantageously manufactured in an industrial point of view. A fire-proof property of the steel strip is further improved by cold-rolling the steel strip with such a slight duty to induce a plastic strain of 1-5% after annealing or hot-dip coating.