Title:
HIGH-STRENGTH TWIP STEEL SHEET AND METHOD OF MANUFACTURING THE SAME
Kind Code:
A1


Abstract:
The present invention features a high-strength and light TWIP steel sheet which can be used to manufacture vehicle body parts, and a method of manufacturing the same.



Inventors:
Hong, Seung Hyun (Anyang, KR)
Ko, Se Jin (Seoul, KR)
Application Number:
12/572378
Publication Date:
10/14/2010
Filing Date:
10/02/2009
Assignee:
HYUNDAI MOTOR COMPANY (Seoul, KR)
KIA MOTORS CORPORATION (Seoul, KR)
Primary Class:
Other Classes:
72/365.2, 148/329, 420/75
International Classes:
C21D8/00; B21B45/00; C22C38/02; C22C38/04; C22C38/06; C22C38/14
View Patent Images:



Primary Examiner:
YEE, DEBORAH
Attorney, Agent or Firm:
Mintz Levin/Special Group (Boston, MA, US)
Claims:
What is claimed is:

1. A method of manufacturing a high-strength TWIP steel sheet, comprising: cold-rolling a hot-rolled steel sheet having a composition including 0.15˜0.30 wt % of carbon (C), 0.01˜0.03 wt % of silicon (Si), 15˜25 wt % of manganese (Mn), 1.2˜3.0 wt % of aluminum (Al), 0.020 wt % or less of phosphorus (P), 0.001˜0.002 wt % of sulfur (S), 4.0˜5.0 wt % of titanium (Ti), and residual iron and inevitable impurities; and continuously annealing the cold-rolled steel sheet.

2. The method according to claim 1, wherein the hot-rolled steel sheet is obtained by preparing a powdered titanium-manganese (Ti—Mn) alloy, melting the titanium-manganese (Ti—Mn) alloy in a converter together with another composition and then continuously casting them to form a slab, hot-rolling the slab from 1100˜1300° C. to 850˜950° C., and then cooling the hot-rolled slab.

3. The method according to claim 2, wherein the hot-rolled slab is air-cooled from 850˜950° C. to 650˜750° C. at a cooling rate of 35˜45° C./sec.

4. A high-strength TWIP steel sheet having a composition, the composition comprising: 0.15˜0.30 wt % of carbon (C); 0.01˜0.03 wt % of silicon (Si); 15˜25 wt % of manganese (Mn); 1.2˜3.0 wt % of aluminum (Al); 0.020 wt % or less of phosphorus (P); 0.001˜0.002 wt % of sulfur (S); 4.0˜5.0 wt % of titanium (Ti); and residual iron and inevitable impurities.

5. The high-strength TWIP steel sheet according to claim 4, wherein the composition is formed into the TWIP steel sheet having a yield strength of 550 MPa or more by preparing a powdered titanium-manganese (Ti—Mn) alloy, melting the titanium-manganese (Ti—Mn) alloy in a converter together with another composition and then continuously casting them to form a slab, hot-rolling the slab to recrystallize, cold-rolling the hot-rolled slab, and then annealing the cold-rolled slab.

6. The high-strength TWIP steel sheet according to claim 5, wherein, in the hot rolling, the continuous-cast slab is hot-rolled from 1100˜1300° C. to 850˜950° C.; and then the hot-rolled slab is air-cooled from 850˜950° C. to 650˜750° C. at a cooling rate of 35˜45° C./sec.

7. A method of manufacturing a high-strength TWIP steel sheet, comprising: cold-rolling a hot-rolled steel sheet having a composition comprising carbon (C), silicon (Si), manganese (Mn), aluminum (Al), phosphorus (P), sulfur (S), titanium (Ti), and residual iron and inevitable impurities; and annealing the cold-rolled steel sheet.

8. The method of manufacturing a high-strength TWIP steel sheet of claim 7, wherein the hot-rolled steel sheet has a composition comprising 0.15˜0.30 wt % of carbon (C).

9. The method of manufacturing a high-strength TWIP steel sheet of claim 7, wherein the hot-rolled steel sheet has a composition comprising 0.01˜0.03 wt % of silicon (Si).

10. The method of manufacturing a high-strength TWIP steel sheet of claim 7, wherein the hot-rolled steel sheet has a composition comprising 15˜25 wt % of manganese (Mn).

11. The method of manufacturing a high-strength TWIP steel sheet of claim 7, wherein the hot-rolled steel sheet has a composition comprising 1.2˜3.0 wt % of aluminum (Al).

12. The method of manufacturing a high-strength TWIP steel sheet of claim 7, wherein the hot-rolled steel sheet has a composition comprising 0.020 wt % or less of phosphorus (P).

13. The method of manufacturing a high-strength TWIP steel sheet of claim 7, wherein the hot-rolled steel sheet has a composition comprising 0.001˜0.002 wt % of sulfur (S).

14. The method of manufacturing a high-strength TWIP steel sheet of claim 7, wherein the hot-rolled steel sheet has a composition comprising 4.0˜5.0 wt % of titanium (Ti).

15. The method of manufacturing a high-strength TWIP steel sheet of claim 7, wherein the hot-rolled steel sheet has a composition further comprising residual iron and inevitable impurities.

16. The method of manufacturing a high-strength TWIP steel sheet of claim 7, wherein annealing the cold-rolled steel sheet is performed continuously.

17. A high-strength TWIP steel sheet having a composition, the composition comprising: carbon (C); silicon (Si); manganese (Mn); aluminum (Al).; phosphorus (P); sulfur (S); titanium (Ti); and residual iron and inevitable impurities.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims under 35 U.S.C. §119(a) priority to Korean Application No 10-2009-0032093, filed on Apr. 14, 2009, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a twinning induced plasticity (TWIP) steel sheet having high strength and high workability and a method of manufacturing the same. In particular embodiments, the present invention relates to a high-strength TWIP steel sheet having excellent yield strength and elongation rate and a method of manufacturing the same.

2. Description of the Related Art

Generally, a high-strength steel sheet that has excellent formability, and which can decrease the weight of a vehicle in order to increase the vehicle fuel efficiency and prevent air pollution, is a requirement in the field of vehicle steel sheets.

In particular, when complicated vehicle body components are manufactured using the superior formability of a steel sheet material, technologies for manufacturing a twinning induced plasticity (TWIP) steel sheet having high strength have been proposed.

However, in the conventional technologies, although a high-tension steel sheet is required to increase the strength of a vehicle body, it cannot be formed into a complicated part because it has a low elongation rate, so that it is formed into several parts, and then the several parts are welded together to obtain a final product. Further, when a collision member is manufactured through the conventional technologies, since the conventional TWIP steel sheet has a yield strength of about 400 MPa, which is lower than that of a transformation induced plasticity (TRIP) steel sheet or dual phase (DP) steel sheet having a tensile strength of about 980 MPa corresponding to the yield strength of about 400 MPa, the initial collision performance is low.

Further, although the conventional TWIP steel sheet must be suitably highly pressurized at the time of cold rolling in order to increase its yield strength, concomitant problems, such as fracture, side cracking and the like, occur because it has the characteristics of ultrahigh-tension steel. As a result it is difficult to economically obtain products.

The above information disclosed in this the Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a high-strength TWIP steel sheet preferably having a yield strength of about 550 MPa and a suitably improved elongation rate, and a method of manufacturing the same.

In a preferred embodiment of the invention, in order to accomplish the above object, an aspect of the present invention preferably provides a method of manufacturing a high-strength TWIP steel sheet, comprising: cold-rolling a hot-rolled steel sheet having a composition preferably including 0.15˜0.30 wt % of carbon (C), 0.01˜0.03 wt % of silicon (Si), 15˜25 wt % of manganese (Mn), 1.2˜3.0 wt % of aluminum (Al), 0.020 wt % or less of phosphorus (P), 0.001˜0.002 wt % of sulfur (S), 4.0˜5.0 wt % of titanium (Ti), and residual iron and inevitable impurities. In further embodiments the method comprises continuously annealing the cold-rolled steel sheet.

In preferred embodiments of the method, the hot-rolled steel sheet may be suitably obtained by preparing a powdered titanium-manganese (Ti—Mn) alloy, melting the titanium-manganese (Ti—Mn) alloy in a converter together with another composition and then preferably continuously casting them to form a slab, preferably hot-rolling the slab from 1100˜1300° C. to 850˜950° C., and then suitably cooling the hot-rolled slab. In further embodiments, the hot-rolled slab may be suitably air-cooled from 850˜950° C. to 650˜750° C. at a cooling rate of 35˜45° C./sec.

In another aspect, the present invention preferably provides a high-strength TWIP steel sheet having a composition, the composition preferably comprising: 0.15˜0.30 wt % of carbon (C); 0.01˜0.03 wt % of silicon (Si); 15˜25 wt % of manganese (Mn); 1.2˜3.0 wt % of aluminum (Al); 0.020 wt % or less of phosphorus (P); 0.001˜0.002 wt % of sulfur (S); 4.0˜5.0 wt % of titanium (Ti); and residual iron and inevitable impurities.

In certain exemplary embodiments of the invention, the composition may be suitably formed into the TWIP steel sheet having a yield strength of 550 MPa or more by suitably preparing a powdered titanium-manganese (Ti—Mn) alloy, suitably melting the titanium-manganese (Ti—Mn) alloy in a converter together with another composition and then continuously casting them to preferably form a slab, hot-rolling the slab to recrystallize, cold-rolling the hot-rolled slab, and then suitably annealing the cold-rolled slab.

Preferably, in the hot rolling, the continuous-cast slab may be hot-rolled from 1100˜1300° C. to 850˜950° C., and then the hot-rolled slab is suitably air-cooled from 850˜950° C. to 650˜750° C. at a cooling rate of 35˜45° C./sec.

It is understood that the term “vehicle” or “vehicular” or other similar term as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, Watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles and other alternative fuel vehicles (e.g. fuels derived from resources other than petroleum).

As referred to herein, a hybrid vehicle is a vehicle that has two or more sources of power, for example both gasoline-powered and electric-powered.

The above features and advantages of the present invention will be apparent from or are set forth in more detail in the accompanying drawings, which are incorporated in and form a part of this specification, and the following Detailed Description, which together serve to explain by way of example the principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a graph showing the state in which crystal grains have recrystallized through heat treatment; and

FIG. 2 is a graph showing the change in yield strength of a TWIP steel sheet of the present invention compared with that of a conventional steel sheet.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As described herein, the present invention includes a method of manufacturing a high-strength TWIP steel sheet, comprising cold-rolling a hot-rolled steel sheet having a composition comprising carbon (C), silicon (Si), manganese (Mn), aluminum (Al), phosphorus (P), sulfur (S), titanium (Ti), and residual iron and inevitable impurities, and annealing the cold-rolled steel sheet.

In one embodiment, the hot-rolled steel sheet has a composition comprising 0.15˜0.30 wt % of carbon (C).

In another embodiment, the hot-rolled steel sheet has a composition comprising 0.01˜0.03 wt % of silicon (Si).

In a further embodiment, the hot-rolled steel sheet has a composition comprising 15˜25 wt % of manganese (Mn).

In one embodiment, the hot-rolled steel sheet has a composition comprising 1.2˜3.0 wt % of aluminum (Al).

In another embodiment, the hot-rolled steel sheet has a composition comprising 0.020 wt % or less of phosphorus (P).

In a further embodiment, the hot-rolled steel sheet has a composition comprising 0.001˜0.002 wt % of sulfur (S).

In one embodiment, the hot-rolled steel sheet has a composition comprising 4.0˜5.0 wt % of titanium (Ti).

In another embodiment, the hot-rolled steel sheet has a composition further comprising and residual iron and inevitable impurities.

In a further embodiment, annealing the cold-rolled steel sheet is performed continuously.

In another aspect, the invention features a high-strength TWIP steel sheet having a composition, the composition comprising carbon (C); silicon (Si); manganese (Mn); aluminum (Al); phosphorus (P); sulfur (S); titanium (Ti); and residual iron and inevitable impurities.

In another aspect, the invention features a high-strength TWIP steel sheet having a composition, the composition comprising 0.15˜0.30 wt % of carbon (C); 0.01˜0.03 wt % of silicon (Si); 15˜25 wt % of manganese (Mn); 1.2˜3.0 wt % of aluminum (Al); 0.020 wt % or less of phosphorus (P); 0.001˜0.002 wt % of sulfur (S); 4.0˜5.0 wt % of titanium (Ti); and residual iron and inevitable impurities.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the attached drawings.

The present invention preferably provides a method of manufacturing a high-strength TWIP steel sheet having a suitable yield strength of about 550 MPa and an improved elongation rate.

In particular preferred embodiments, in the present invention, in order to suitably improve the yield strength of the TWIP steel sheet by preventing the formation of twins, titanium, which is preferably present in a crystal grain in the form of a precipitate and serves to suitably prevent twins from being formed in the early stage, may preferably be considered. Titanium is an element having high oxidizability and is a metal having a high boiling point, and when it is added at the time of refining steel, there is a danger that it is oxidized into titanium oxides (TiOx) and thus in certain examples the titanium oxides (TiOx) are present in the steel as impurities. In certain embodiments, in the case of TWIP steel, since many kinds of elements are added to the TWIP steel, the melting point of the TWIP steel is suitably lowered, so that there are many difficulties in adding titanium (Ti) to the TWIP steel. Furthermore, when temperature control is not properly conducted at the time of cooling steel after hot-rolling the steel, a precipitate, called “Ti3C8”, is suitably formed. This precipitate is preferably present in a crystal grain boundary and suitably causes the deterioration of formability, such as, but not limited to, elongation rate, plastic deformation ratio or the like.

Accordingly, the present invention features, in preferred embodiments, two methods of suitably applying titanium to a TWIP steel sheet. In a first embodiment, interstitial free (IF) steel to which titanium (Ti) is preferably added through a low-cost process is suitably melted in a vacuum furnace, and then carbon and other components are added to the melted IF steel. Preferably, in this method, the oxidization of the titanium (Ti) added to the IF steel can be suitably prevented, and titanium (Ti) is not required to be further added thereto, so that this method may be a suitably economical method.

In a second embodiment, a powdered titanium-manganese (Ti—Mn) alloy is preferably prepared and then suitably added to steel at the time of refining the steel. Preferably, when the steel is made in the form of a metal matrix composite (MMC), the steel can have a desired component ratio without being influenced by its composition. In further preferred embodiments, when this MMC steel is suitably charged into the vacuum furnace, it is easily melted because the titanium-manganese (Ti—Mn) alloy powder has high surface energy (large surface area). Accordingly, in further related embodiments, it is suitably easily supercooled. This method is advantageous in terms of component control.

In certain preferred embodiments of the present invention, the two methods were suitably employed, and final components and their material properties did not suitably differ from each other.

In other preferred embodiments of the invention, in order to suitably manufacture a high-strength TWIP steel sheet, the added amounts of manganese (Mn), carbon (C) and aluminum (Al) must be suitably adjusted and the microstructure thereof must be suitably controlled because the characteristics of TWIP steel are preferably exhibited in an austenite single phase. In further embodiments, in order to obtain a complete austenite single phase, the amounts of manganese (Mn) and carbon (C), which are elements stabilizing austenite, must be suitably optimized. Preferably, when twins are formed through the optimization of the amounts of manganese (Mn) and carbon (C), the twin formation rate must be suitably controlled by aluminum (Al). Moreover, in further embodiments, in order to increase the yield strength of the TWIP steel sheet, the amount of titanium must be suitably optimized.

In further preferred embodiments of the invention, in order to suitably manufacture a TWIP steel sheet having improved yield strength and elongation rate while utilizing the characteristics of the constituents thereof, these constituents are preferably required to be combined with each other in a suitably appropriate combination ratio. Accordingly, the present inventors have discovered the appropriate combination ratio through many trials and errors, and, as a result, that the present invention demonstrated that the following composition ratio is appropriate according to the preferred embodiments of the present invention. The results thereof are given in Table 1.

TABLE 1
Components
CSiMnAlPSTiFe
Contents0.15~0.300.01~0.0315.0~25.01.20~3.000.0200.001~0.0024.00~5.00balance
(wt %)or less

According to certain preferred embodiments, the reasons for the numerical ranges of the composition of the TWIP steel sheet are described as follows)

(i) Carbon (C) 0.15˜0.30 wt %

According to preferred embodiments of the invention, since carbon (C) contributes to the stabilization of an austenite phase, it is advantageous that the content thereof suitably increases. Preferably, when the content thereof is less than 0.15 wt %, a′-martensite is suitably formed at the time of deforming a TWIP steel sheet, and machining cracks are also suitably formed, thereby deteriorating the ductility of the TWIP steel sheet. In other embodiments, when the content thereof is more than 0.30 wt %, the stability of the austenite phase is suitably increased, and thus the transition of deformation mechanism of the TWIP steel sheet can suitably occur due to slip deformation. In related embodiments, the reason why the transition of deformation mechanism of the TWIP steel sheet occurs is that the formability of the TWIP steel sheet is suitably decreased due to the increase in laminated defect energy.)

(ii) Silicon (Si) 0.01˜0.03 wt %

According to preferred embodiments of the invention, silicon (Si) is a substitutional solid solution element and serves to suitably improve the strength of a material by maintaining the state of a solid solution structure to that at room temperature when the material is suitably heated to a melting point or higher and then cooled. Preferably, when the content of silicon (Si) is preferably less than 0.01 wt %, solute effect is suitably decreased, and thus it is difficult to suitably improve the strength of the material. In other certain embodiments of the invention, when the content thereof is preferably more than 0.03 w %, many defects can be caused at the time of welding.

(iii) Manganese (Mn) 15˜25 wt %

According to preferred embodiments of the invention, like carbon (C), manganese (Mn) is an element for suitably stabilizing austenite. Preferably, manganese (Mn) forms a′-martensite worsening formability when the content of manganese (Mn) is less than 15 wt %. In further embodiments, it is verified that the strength of the TWIP steel sheet is suitably increased, but it cannot be expected that the TWIP steel sheet has high ductility, which is a characteristic of TWIP steel.

In related embodiments, the content of manganese (Mn) is preferably limited to a lower limit of 15 wt %. Preferably, the formation of twins is prevented even when the content thereof is more than 25 wt %. This phenomenon is directly related to the deterioration of ductility. In further embodiments, as the content thereof is suitably increased, the TWIP steel sheet is easily cracked at the time of hot rolling, and the production cost of the TWIP steel sheet is suitably increased due to the addition of expensive elements, so that it is preferred that the content thereof be preferably limited to 25 wt %.

Accordingly, in further embodiments, the production cost of the TWIP steel sheet can be suitably decreased by appropriately adjusting the content of carbon (C) such that the content of manganese (Mn), which is an expensive element, is suitably minimized, and the deformation of twins can be suitably induced by adjusting the content of carbon (C) and aluminum (Al).

(iv) Aluminum (Al) 1.2˜3.0 wt %

According to further preferred embodiments of the invention, and generally, aluminum (Al) is preferably added in order to suitably deoxidize the TWIP steel sheet. IN certain preferred embodiments of the present invention, it is related to suitably increasing the ductility of the TWIP steel sheet preferably, unlike carbon (C) or manganese (Mn), aluminum (Al) is an element for preferably stabilizing ferrite, but serves to suitably increase the ductility of the TWIP steel sheet by increasing laminated defect energy and preventing the formation of e-martensite. In further embodiments, aluminum (Al) contributes to the minimization of the content of manganese (Mn) because it preferably prevents the suitable formation of e-martensite even when the content of manganese (Mn) is approximate to a lower limit of 15 wt %. Preferably, when the content of aluminum (Al) is suitably less than 1.2 wt %, the ductility of the TWIP steel sheet is decreased regardless of the increase in strength thereof because e-martensite is formed. In other preferred embodiments, when the content of aluminum (Al) is preferably more than 3.0 wt %, the ductility of the TWIP steel sheet is suitably decreased by preventing the formation of twins, and the surface quality of the TWIP steel sheet is suitably deteriorated by decreasing continuous castability and causing the oxidization of the hot-rolled TWIP steel sheet.

(v) Phosphorus (P) 0.020 wt % or Less, Sulfur (S) 0.001˜0.002 wt %

According to preferred embodiments of the invention, although phosphorus (P) is an element which is preferably added at the time of manufacturing steel, it suitably causes segregation, thus suitably decreasing workability. Therefore, according to certain embodiments of the invention, it is advantageous that the content of phosphorus (P) be suitably low, and it is preferred that the content of phosphorus (P) be preferably limited to 0.020 wt % or less.

According to further embodiments, since sulfur (S) causes cracks by forming coarse manganese sulfide (MnS) and decreases hole expandability, in certain preferred embodiments it must be suitably suppressed. Preferably, like phosphorous (P), it is advantageous for the content of sulfur (S) to be suitably low, and it is preferred that the content of sulfur (S) be preferably limited to 0.001˜0.002 wt %.

(vi) Titanium (Ti) 4.0˜5.0 wt %

According to further preferred embodiments of the invention, titanium (Ti), which is present in crystal grains in the form of a precipitate, serves to suitably prevent the formation of twins in the initial stage and functions to improve the yield strength of the TWIP steel sheet by increasing the initial deformation resistance of the TWIP steel sheet, that is, by preventing the formation of the twins. Accordingly, due to the addition of titanium (Ti), crystal grains can be suitably miniaturized and precipitates can be formed in the crystal grains or at a crystal grain boundary. The optimal titanium content ratios according to certain preferred embodiments of the invention are given in Table 2 below.

As shown in Table 2, Examples 1 to 12 show the mechanical properties of the TWIP steel sheet including 4˜5 wt % of titanium, Comparative Examples 1 to 16 show the mechanical properties of the TWIP steel sheet including less than 4 wt % of titanium, and Comparative Examples 17 to 18 show the mechanical properties of the TWIP steel sheet including more than 5 wt % of titanium. As shown here and according to certain preferred embodiments of the invention, the mechanical properties may preferably include, but are not limited to, yield strength, which is a major object of the present invention, tensile strength, and elongation rate.

In these Examples and Comparative Examples, a slab preferably prepared through continuous casting using a converter was suitably hot-rolled from 1300° C. to 900° C. and then suitably air-cooled from 900° C. to 700° C. at a cooling rate of 40° C./sec to form a hot-rolled coil, and then the hot-rolled coil was cold-rolled seven times.

In further related embodiments, the cold-rolled coil was suitably heat-treated at a temperature of 700˜850° C. for 5 minutes using a continuous annealing furnace to recrystallize crystal grains and thus to recover elongation rate. According to further related embodiments, the fact that the crystal grains were recrystallized and the elongation rate was recovered can be seen from FIG. 1. Referring to FIG. 1, it can be seen that the recrystallization of the crystal grains was suitably completed after 180 seconds.

TABLE 2
Average
YieldTensileElongationcrystal
TiMnAlstrengthstrengthrategrain size
(wt %)(wt %)(wt %)(MPa)(MPa)(%)(m)
Exp. 14.0015.001.256198763.62.1
Exp. 24.5015.001.257098361.02.3
Exp. 35.0015.001.257799461.02.5
Exp. 44.0025.001.2559101969.32.9
Exp. 54.5025.001.2588100968.52.0
Exp. 65.0025.001.2587106263.22.12
Exp. 74.0015.003.055198367.12.6
Exp. 84.5015.003.055099866.82.88
Exp. 95.0015.003.055299868.42.66
Exp. 104.0025.003.0601101860.32.51
Exp. 114.5025.003.056199763.62.6
Exp. 125.0025.003.0559101466.72.3
Comp. Exp. 10.0015.001.241097842.36.83
Comp. Exp. 20.0025.001.240297847.09.35
Comp. Exp. 30.0015.003.049095048.212.1
Comp. Exp. 40.0025.003.050598047.47.0
Comp. Exp. 51.5015.001.249396042.311.1
Comp. Exp. 61.5025.001.246296348.212.4
Comp. Exp. 71.5015.003.049994247.18.3
Comp. Exp. 81.5025.003.049393151.39.2
Comp. Exp. 93.0015.001.246092254.112.4
Comp. Exp. 103.0025.001.243098056.33.3
Comp. Exp. 113.0015.003.041097766.24.2
Comp. Exp. 123.0025.003.042397763.03.5
Comp. Exp. 133.8815.001.246996366.73.9
Comp. Exp. 143.8825.001.241097768.93.8
Comp. Exp. 153.8815.003.040397368.34.1
Comp. Exp. 163.8825.003.041197469.23.9
Comp. Exp. 175.1515.001.251295858.33.7
Comp. Exp. 185.1525.001.2hot-rolled crack

Referring to Table 2, in Examples 1 to 6, a TWIP steel sheet includes 1.2 wt % of aluminum, 15˜25 wt % of manganese, and 4˜5 wt % of titanium. Preferably, the yield strength of the TWIP steel sheet of Examples 1 to 6 is in a range of 561˜588 MPa, which is suitably increased by 185 MPa compared to that of the TWIP steel sheet of the following Comparative Examples. In further embodiments, the elongate rate of the TWIP steel sheet of Examples 1 to 6 is preferably in a range of 61˜69.3 wt %, which is also suitably increased compared to that of the steel sheet of the following Comparative Examples.

Preferably, in Examples 7 to 12, a TWIP steel sheet includes 3.0 wt % of aluminum, 15˜25 wt % of manganese, and 4˜5 wt % of titanium. According to further preferred embodiments, the yield strength of the TWIP steel sheet of Examples 7 to 12 is in a range of 550˜601 MPa, which is suitably increased by 201 MPa compared to that of the TWIP steel sheet of the following Comparative Examples. Further, the elongate rate of the TWIP steel sheet of Examples 7 to 12 is in a range of 60.3˜68.4 wt %, which is also increased compared to that of the steel sheet of the following Comparative Examples.

Generally, as yield strength is suitably increased, elongation rate is decreased. However, in preferred embodiments of the present invention, it was found that crystal grains suitably formed by the addition of titanium is miniaturized, thus suitably increasing the elongation rate. Accordingly, in preferred embodiments of the present invention, it was determined that the yield strength is improved by delaying the formation of initial twins, the delay being caused by titanium precipitates.

In Comparative Examples 1 to 16, a TWIP steel sheet includes 1.2˜3.0 wt % of aluminum, 15˜25 wt % of manganese, and less than 4 wt % of titanium. It can be seen that the yield strength of the TWIP steel sheet of Comparative Examples 1 to 16 is increased to 499 MPa, which is low compared to the yield strength (500 Mpa) of a conventional TWIP steel sheet or dual phase steel sheet. From this result, it can be seen that the content of titanium is suitably determined depending on the appropriate conditions.

In Comparative Examples 17 to 18, a TWIP steel sheet includes 1.2 wt % of aluminum, 15˜25 wt % of manganese, and more than 4 wt % of titanium. From Comparative Example 17, it can be seen that the yield strength of the TWIP steel sheet of Comparative Example 17 is increased to the maximum yield strength and is then decreased to about 500 MPa, and that the elongation rate and tensile strength thereof is decreased. The reason for this has been found to be that titanium precipitates are coarse.

In Comparative Example 18, since side cracks occur during hot rolling when the content of manganese becomes 25 wt %, which is the maximum value of the conventional TWIP steel sheet, a final product cannot be produced. According to preferred embodiments of the invention, the reason for this was determined to be that the strength of a hot-rolled steel sheet is suitably increased according to the increase in content of titanium and manganese.

For example, in certain embodiments of the invention, when titanium is added in an amount of 4 wt % or more, the mechanical properties of the TWIP steel sheet are rapidly improved. The titanium carbide (TiC) precipitates formed in grains or grain boundaries cause increase in yield strength by preventing the movement of initial dislocation, that is, by delaying the formation of twins, but allows elongation rate to be maintained in a plastic region because they do not prevent the formation of twins in the plastic region. As a result, when alloying processes are used to manufacture, the TWIP steel sheet according to the present invention, a high-strength steel strip or sheet which can be cold-formed such that it is suitable to manufacture a vehicle body panel and which is light can be obtained.

Therefore, according to preferred embodiments of the invention, it is preferred that the optimum content of titanium be in a range of 4˜5 wt %.

Accordingly, the high-strength TWIP steel sheet has a preferred composition including 0.15˜0.30 wt % of carbon (C), 0.01˜0.03 wt % of silicon (Si), 15˜25 wt % of manganese (Mn), 1.2˜3.0 wt % of aluminum (Al), 0.020 wt % or less of phosphorus (P), 0.001˜0.002 wt % of sulfur (S), 4.0˜5.0 wt % of titanium (Ti), and residual iron and inevitable impurities.

According to further preferred embodiments of the invention, the composition is hot-rolled, coiled, cold-rolled, and then continuously annealed to manufacture a high-strength TWIP steel sheet having a suitably improved yield strength and excellent elongation rate. A preferred method of manufacturing high-strength TWIP steel sheet according to preferred embodiments of the present invention is described as follows.

In preferred embodiments in the method of manufacturing the high-strength TWIP steel sheet, the preferred composition including 0.15˜0.30 wt % of carbon (C), 0.01˜0.03 wt % of silicon (Si), 15 25 wt % of manganese (Mn), 1.2˜3.0 wt % of aluminum (Al), 0.020 wt % or less of phosphorus (P), 0.001˜0.002 wt % of sulfur (S), 4.0˜5.0 wt % of titanium (Ti) and residual iron and inevitable impurities is suitably melted in a converter and then continuously cast, and then the cast is suitably hot-rolled, preferably from 1100˜1300° C. to 850˜950° C. and then air-cooled from 850˜950° C. to 650˜750° C. at a cooling rate of 35˜45° C./sec to suitably obtain the high-strength TWIP steel sheet.

Preferably, when the hot rolling of the cast is suitably completed in a preferred range of 850˜950° C., the formation of Ti3C8 can be suitably prevented, but when it is completed at about 800° C., Ti3C8 is suitably formed instead of TiC, and thus the formability of the TWIP steel sheet can be suitably deteriorated. In further embodiments, when the hot-rolled cast is preferably air-cooled to a coiling temperature, a reason why the cooling rate is preferably maintained at 35˜45° C./sec is that this cooling rate is a suitably optimal cooling rate at which precipitates are suitably uniformly distributed in crystal grain boundaries. Preferably, in further preferred embodiments, when the cooling rate is preferably more than 45° C./sec, added elements are present in the form of a solid solution phase in which they are oversaturated in crystals, so that they are suitably reprecipitated during subsequent cold-rolling and annealing processes, with the result that unexpected material properties changes, for example, increase of elongation rate and the like, can occur. In other certain embodiments, when the cooling rate is less than 35° C./sec, crystal grains of a hot-rolled plate are excessively grown, so that the size of crystal grain of a final plate can be two fold or more of that of a general metal. Accordingly, surface defects, such as orange peel and the like, can suitably occur at the time of forming vehicle parts.

Hereinafter, the method of manufacturing the high-strength TWIP steel sheet will be described in detail.

In a preferred embodiment, the above-mentioned composition is suitably melted in a converter and then suitably continuously cast. Preferably, interstitial free (IF) steel to which titanium (Ti) is added through a low-cost process is suitably melted in a vacuum furnace, and then carbon and other components are preferably added to the melted IF steel, thereby preventing the oxidation of the titanium (Ti) added to the IF steel. In further embodiments, a powdered titanium-manganese (Ti—Mn) alloy is preferably, prepared and then added to the composition at the time of refining, so that the composition can have a suitably desired component ratio without being influenced by its constituents.

Accordingly, the reason for this according to preferred embodiments of the invention, is that titanium can be suitably oxidized to titanium oxides, which are, impurities, when it is preferably added at the time of refining because it is an element having strong oxidizability and high melting point, and that, in the case of TWIP steel, it is suitably difficult to add titanium thereto because the melting point of the TWIP steel is suitably lowered due to the addition of many elements.

According to further preferred embodiments, the cast begins to be hot-rolled at a temperature that is preferably between 1100˜1300° C., and the hot rolling of the cast is suitably completed at a temperature that is preferably 900° C. In further embodiments, the hot rolled cast is suitably air-cooled from 900° C. to 700° C. at a cooling rate of 40° C./sec. Preferably, through these procedures, dislocations accumulated through the hot rolling are suitably released, preferably, entirely released, and thus recrystallization can be completely conducted. According to preferred embodiments of the invention, the recrystallization is necessary to suitably manufacture a high-strength steel sheet. Accordingly, in other embodiments, when the recrystallization is not completely conducted, desired rolling reduction cannot be suitably obtained in a subsequent cold rolling process, and thus a final product having desired thickness cannot be suitably obtained.

According to further preferred embodiments, after the hot rolled cast is air-cooled to 700° C., a cold rolling process is suitably performed through a coiling process. Preferably, in the cold rolling process, rolling reduction is suitably maintained at 30˜90%, which may, be preferably changed depending on the use. In particular preferred embodiments, when the TWIP steel sheet is suitably used to manufacture vehicle parts, it is preferred that the rolling reduction be maintained at 50˜75%. In other preferred embodiments of the present invention, it may not be preferred that the rolling reduction be suitably limited to predetermined values.

Preferably, after the cold rolling process, an annealing process is suitably performed. According to further embodiments, in the annealing process, the cold-rolled cast is completely recrystallized by heat-treating it, preferably at a temperature of 700˜900° C., and preferably for 3˜5 minutes, using a continuous annealing furnace. In related embodiments, the recrystallization thereof can be observed through its texture photograph and hardness analysis, and is suitably the same as in a conventional process of producing a TWIP steel sheet.

Further, and according to other preferred embodiments, FIG. 2 shows the change in yield strength of a TWIP steel sheet of the present invention compared with that of a conventional steel sheet.

Preferably, as shown in FIG. 2, when a high-strength TWIP steel sheet is suitably manufactured according to the present invention, the yield strength of the portion “A” in FIG. 2 is suitably increased by 180 MPa compared to that of conventional TWIP steel sheets, and the elongate rate thereof is suitably increased by 60% or more compared to conventional TWIP steel sheets, so that its collision performance can be effectively ensured, and complicated vehicle body parts can also be easily formed, thereby manufacturing a suitably ultrahigh-strength steel sheet for vehicle parts.

As described in the embodiments above, the high-strength TWIP steel sheet according to the present invention is preferably advantageous in that its yield strength can be suitably increased by 180 MPa and its elongation rate can be suitably increased by 60% or more, compared to conventional TWIP steel sheets, and thus its collision performance can be effectively ensured.

According to further embodiments, the high-strength TWIP steel sheet according to the present invention is advantageous in that, when it is preferably used as a material for vehicle body parts, complicated vehicle body parts can also be suitably easily formed, high strength-high formability characteristics can be suitably ensured, and the manufacture of light vehicles can be preferably realized.

Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.