METHOD OF MANUFACTURING TWINNING INDUCED PLASTICITY TYPE ULTRA-HIGH STRENGTH STEEL SHEET

Abstract

The present invention features a method of manufacturing a TWIP type ultra-high strength steel sheet, which can improve the yield strength, tensile strength and elongation rate of the TWIP type ultra-high strength steel sheet by appropriately adjusting the amounts of carbon (C), silicon (Si), manganese (Mn), aluminum (Al), molybdenum (Mo), phosphorus (P) and sulfur (S).

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims under 35 U.S.C. §119(a) priority to Korean Application No. 10-2008-0087282, filed on Sep. 4, 2008, 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, generally, to a method of manufacturing a twinning induced plasticity (TWIP) type ultra-high strength steel sheet and, more particularly, to a method of manufacturing a TWIP type ultra-high strength steel sheet for vehicle body components, which can suitably increase yield strength, tensile strength and elongation rate.

2. Description of the Related Art

Generally, ultra-high strength steel sheets which are widely used as materials for automotive body components have a tensile strength of 590˜780 MPa, a yield strength of 270˜350 MPa, an elongation rate of 25˜35% and a plastic strain ratio of 0.9˜1.2.

However, when applying those ultra-high strength steel to automotive body components, cracks, corrugating, and the like, can be caused by an insufficient elongation rate at the time of press forming, which may be problematic. Thus a thick steel sheet is used in consideration of the strength of vehicle body components. Further, even though elongation is sufficiently ensured, it is generally difficult to form a steel sheet into vehicle body components because the vehicle body components are complicated and multi-functionalized. Therefore, the plastic strain ratio of a steel sheet is preferably required to be considerably increased with the development of forming technologies.

Korean Unexamined Patent Application Publication No. 2007-0018416, incorporated by reference in its entirety herein, discloses a twinning induced plasticity type ultra-high strength steel sheet, comprising: 0.15˜0.30 wt % of carbon, 0.01˜0.03 wt % of silicon, 15˜25 wt % of manganese, 1.2˜3.0 wt % of aluminum, 0.020 wt % or less of phosphorus, 0.001˜0.002 wt % of sulfur, and residual iron and other inevitable impurities.

Although the above twinning induced plasticity (TWIP) type ultra-high strength steel sheet has remarkable material properties, it is also increasingly required to have high collision strength and to be used for complicated vehicle body components. Thus, it is an object of the invention to improve the yield strength, tensile strength and elongation rate of the TWIP type ultra-high strength steel sheet together. The reason for this is because the defective fraction in the formation of a product is suitably increased when its elongation rate is low.

Accordingly, various alloy elements are required to be added to the steel sheet. However, the use of the alloy elements may be suitably limited due to the rise in the price of raw materials and due to the requirement of the use of environment-friendly materials. Therefore, there is a need in the art for the development of methods of considerably improving the material properties of a steel sheet without changing the composition of the steel sheet.

The above information disclosed in 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 method of manufacturing a TWIP type ultra-high strength steel sheet, which can suitably increase the yield strength, tensile strength and elongation rate of the TWIP type ultra-high strength steel sheet.

In preferred embodiments, the present invention provides a method of manufacturing a TWIP type ultra-high strength 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), and residual iron (Fe) and other inevitable impurities in four passes or more; suitably recovering the cold-rolled steel sheet at a temperature of 200˜220° C. after the third pass of the cold rolling; and annealing the recovered steel sheet.

In further preferred embodiments of the method, the recovering of the cold-rolled steel sheet may preferably be conducted for 5˜6 minutes.

In related embodiments, the annealing of the recovered steel sheet may preferably be conducted at a temperature of 700˜850° C. for 5˜6 minutes.

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 grain size of a TWIP type ultra-high strength steel sheet depending on annealing temperature according to preferred embodiments of the invention;

FIG. 2 is a graph showing the yield strength of a TWIP type ultra-high strength steel sheet depending on annealing time according to Examples of the present invention;

FIG. 3 is a graph showing the elongation rate of a TWIP type ultra-high strength steel sheet depending on annealing time according to Examples of the present invention; and

FIG. 4 is a graph showing the yield strength of a TWIP type ultra-high strength steel sheet depending on the elongation rate thereof according to the Examples of the present invention and the Comparative Examples.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a first aspect, the invention features a method of manufacturing a TWIP type ultra-high strength 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), and residual iron (Fe) and other inevitable impurities in four passes or more; and recovering the cold-rolled steel sheet.

In one embodiment, the cold-rolled steel sheet is recovered at a temperature of 200˜220° C.

In another embodiment, the cold-rolled steel sheet is recovered after the third pass of the cold rolling.

In still another embodiment, the recovering of the cold-rolled steel sheet is conducted for 5˜6 minutes.

In another particular embodiment, the method further comprises annealing the recovered steel sheet.

In one embodiment, the annealing of the recovered steel sheet is conducted at a temperature of 700˜850° C.

In a further embodiment, the annealing of the recovered steel sheet is conducted for 5˜6 minutes.

Hereinafter, a method of manufacturing a TWIP type ultra-high strength steel sheet according to preferred embodiments of the present invention will be described in detail with reference to the attached drawings.

According to certain preferred embodiments of the invention, the TWIP type ultra-high strength steel sheet has 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), and residual iron (Fe) and other inevitable impurities.

According to certain preferred embodiments of the invention, the method of manufacturing a TWIP type ultra-high strength steel sheet according to a preferred embodiment of the present invention is suitably the same as a conventional method concerning the steps of suitably melting the steel sheet composition in a converter, suitably continuous-casting the molten steel sheet composition to form a steel sheet, suitably hot-rolling the steel sheet at a temperature of 1100˜1300° C. and suitably winding the hot-rolled steel sheet.

Preferably, after the winding of the hot-rolled steel sheet, the hot rolled steel sheet is suitably cold-rolled, preferably in five passes, and then the cold-rolled steel sheet is suitably recovered at a temperature of 200˜220° C. after the third pass of the cold rolling. In further embodiments, the recovered steel sheet is suitably annealed at a temperature of 700˜850° C. In other preferred embodiments, the recovering of the cold-rolled steel sheet may be conducted for 5˜6 minutes, and the annealing of the recovered steel sheet may be conducted at a temperature of 700˜850° C. for 5˜6 minutes.

Preferably, recovering of the cold-rolled steel sheet is conducted is to suitably accelerate the generation of subgrains in the grains of an austenite matrix by inducing the combinations between dislocations and twins. In more particular embodiments, the combinations between dislocations and twins are suitably induced through the recovering of the cold-rolled steel sheet after the third pass of the cold rolling, and the combined dislocation and twins are suitably formed into subgrains in the grains of an austenite matrix through the forth and fifth passes of the cold rolling.

According to further preferred embodiments of the invention, the cold rolling having five passes may preferably be conducted at a rolling reduction ratio of 20˜30% per pass in a similar manner to conventional cold rolling having 5˜7 passes which is preferably conducted at a rolling reduction ratio of about 30% per pass, and is generally used to manufacture high-strength steel sheets as well as TWIP type ultra-high strength steel sheets.

As described herein, it was found that a recovery process must be suitably performed during the cold rolling process because twinning and slipping simultaneously occur in the deformation mechanism of a TWIP type ultra-high strength steel sheet, unlike in the case of a general steel sheet. Accordingly, when 5 passes of cold rolling are made, the recovery process is suitably performed after the third pass of the cold rolling.

Preferably, when the recovery process is suitably performed after the first or second pass of the cold rolling, the expected results cannot be suitably achieved due to the ungrown subgrains. According to further preferred embodiments of the invention, when the recovery process is performed after the fourth pass of the cold rolling, desired material properties cannot be suitably obtained at the time of annealing because a low angle boundary is formed due to the misorientation between grown subgrains. Accordingly, it is preferred that the recovery process be suitably performed after the third pass of the cold rolling.

According to further preferred embodiments, the annealing of the recovered steel sheet is preferably conducted at a temperature of 700˜850° C. for a short period of time (5˜6 minutes) in order to suitably decrease the grain size of the TWIP type ultra-high strength steel sheet to 2˜3 μm. Accordingly, the elongation rate thereof can be suitably increased by decreasing the grain size thereof.

Hereinafter, preferred embodiments of the present invention will be described in more detail with reference to the following Examples and Comparative Examples.

In the Examples and Comparative Examples described herein, a TWIP type ultra-high strength steel sheet is manufactured according to certain preferred embodiments of the present invention using the composition given in Table 1, and then the mechanical properties thereof were suitably measured through tension testing, and the grain size thereof was suitably analyzed through electron back scattered diffraction (EBSD).

	TABLE 1
	C	Si	Mn	Al	P	S	Fe
Chemical	0.15~0.30	0.01~0.03	15.0~25.0	1.20~3.00	0.020	0.001~0.002	residual
components					or less
(wt %)

The results obtained from the measurement and analysis are given in Table 2 and Table 3. In particular preferred embodiments of the invention, for example as shown in the Examples, a slab, which had been prepared by suitably melting the composition in a converter and then continuous-casting the molten composition, was hot-rolled from 1300° C. to 1100° C., was cooled from 900° C. to 600° C. preferably at a cooling rate of 40° C./sec and then winded, was cold-rolled, preferably through five passes at a rolling reduction ratio of 30% or less per pass during which the slab was preferably heat-treated at 200˜220° C. for 5 minutes after the third pass of cold rolling and then the residual two passes thereof were preferably performed, and was then suitably annealed at 700˜850° C. for 5 minutes using a continuous annealing furnace, thereby suitably decreasing the grain size thereof.

	TABLE 2
	Recovery	Recovery		Annealing	Annealing	Yield	Tensile		Average
	temperature	time	Recovery	temperature	time	strength	strength	Elongation	grain
	(° C.)	(min)	pass	(° C.)	(min)	(MPa)	(MPa)	rate (%)	size (μm)
Ex. 1	200	5	3	700	5	580	1020	53	2.1
Ex. 2	200	5	3	750	5	580	1020	53.2	2.3
Ex. 3	200	5	3	800	5	560	992	52.1	2.5
Ex. 4	200	5	3	850	5	520	989	52.1	2.9
Ex. 5	220	5	3	700	5	592	1008	52.3	2.0
Ex. 6	220	5	3	750	5	590	1010	52.2	2.12
Ex. 7	220	5	3	800	5	577	998	52.8	2.6
Ex. 8	220	5	3	850	5	580	992	53.1	2.88

According to further embodiments of the invention as described herein, the Comparative Examples are similar to or the same as Examples, except that the cold rolling was preferably performed at a rolling reduction ratio of 30% or less per pass through five passes and then the annealing was preferably performed at 850° C. for 8˜10 hours using a box furnace.

	TABLE 3
	Recovery	Recovery		Annealing	Annealing	Yield	Tensile	Elongation	Average
	temperature	time	Recovery	temperature	time	strength	strength	rate	grain
	(° C.)	(min)	pass	(° C.)	(min)	(MPa)	(MPa)	(%)	size (μm)
Comp. Ex. 1	—	—	—	850	480	510	978	48.2	6.83
Comp. Ex. 2	—	—	—	850	540	502	978	48.5	9.35
Comp. Ex. 3	—	—	—	850	600	490	950	48.8	12.1
Comp. Ex. 4	200	5	3	850	480	505	980	48	7.0
Comp. Ex. 5	200	5	3	850	540	493	960	48.2	11.1
Comp. Ex. 6	200	5	3	800	600	462	963	48.5	12.4
Comp. Ex. 7	220	5	3	850	480	499	942	46.5	8.3
Comp. Ex. 8	220	5	3	850	540	493	931	47.3	9.2
Comp. Ex. 9	220	5	3	800	600	460	922	48.1	12.4
Comp. Ex.	200	4	3	700	5	530	980	42.1	3.3
10
Comp. Ex.	200	4	3	850	5	510	977	43.2	4.2
11
Comp. Ex.	220	4	3	700	5	523	977	42.8	3.5
12
Comp. Ex.	220	4	3	850	5	499	963	44.6	3.9
13
Comp. Ex.	200	7	3	700	5	510	977	41.2	3.8
14
Comp. Ex.	200	7	3	850	5	503	973	40.2	4.1
15
Comp. Ex.	220	7	3	700	5	511	974	45.1	3.9
16
Comp. Ex.	220	7	3	850	5	482	958	42.6	4.7
17

According to still further embodiments of the invention, in order to suitably determine the annealing temperatures of the Examples, the TWIP type ultra-high strength steel sheet was preferably heat-treated from 600° C. to 920° C. for 5 minutes, and then the grain size thereof was measured. The results thereof according to certain preferred embodiments of the invention as described herein are shown in FIG. 1. Referring to FIG. 1, it can be seen that the grain size thereof at a temperature range of 700˜850° C. is about 2˜3 μm.

According to further embodiments, it was found that the TWIP type ultra-high strength steel sheet was not suitably recrystallized at a temperature of less than 700° C., and that, according to other further embodiments, the elongation rate of a final product did not reach 20%. Accordingly, the annealing was not performed at a temperature of less than 700° C..

In other further embodiments of the invention and referring to the results given in Table 2 and Table 3, it was found that the yield strengths of the Examples of the present invention were suitably increased by 30 MPa˜100 MPa compared to those of Comparative Examples 1 to 3, and that the elongation rates of the Examples of the present invention were also suitably increased by 3˜4% compared to those of Comparative Examples 1 to 3. Generally, according to certain preferred embodiments, elongation rate is suitably decreased with an increase in strength. However, in the case of certain preferred Examples of the present invention as described herein, both strength and elongation rate were suitably increased due to the twins existing in subgrains generated through the recovery process.

From Comparative Examples 4 to 9, it can be seen that according to further embodiments of the invention, there is no effect when the annealing conditions are suitably the same as conventional annealing conditions even though the recovery process during the cold rolling process is preferably conducted the same as in the Examples. In further preferred embodiments, and from Comparative Examples 10 to 17, it can be seen that it is most effective when the recovery time during the cold rolling process is 5 minutes.

According to still other embodiments of the present invention, the reason why the annealing time of the Examples is preferably set to 5 minutes is that, when the annealing time is less than 5 minutes, the TWIP type ultra-high strength steel sheet is suitably slightly recrystallized, and thus the increase in the elongation rate thereof cannot be expected.

In other particular embodiments, when the annealing time is above 5 minutes or excessively above 5 minutes, the increase in the elongation rate of the TWIP type ultra-high steel sheet can be expected, but the strength thereof is rapidly decreased due to the overgrowth of grains. According to exemplary embodiments of the invention, and as shown in FIG. 2, a reason for this can be verified from FIG. 2 showing the suitable decrease in the yield strength of the TWIP type ultra-high strength steel sheet preferably depending on annealing time at 700° C.. Accordingly, in certain embodiments of the invention, it is preferred that the annealing be conducted for 5˜6 minutes, more preferably 5 minutes. Accordingly, in other exemplary embodiments of the invention and as shown in FIG. 3, a reason for this can also be verified from FIG. 3 showing the change in the elongation of the TWIP type ultra-high strength steel sheet preferably depending on annealing time. Accordingly, referring to FIG. 3, it can be seen that a preferred elongation rate of 50% or more preferably can be obtained when the annealing time is 5˜6 minutes.

	TABLE 4
	Inter.	Inter.	Inter.						Average
	heat	heat	heat	Annealing	Annealing	Yield	Tensile	Elongation	grain
	treatment	treatment	treatment	temperature	time	strength	strength	rate	size
	temp. (° C.)	time (min)	pass	(° C.)	(min)	(MPa)	(MPa)	(%)	(μm)
Ex. 1	200	5	3	700	5	580	1020	53	2.1
Ex. 4	200	5	3	850	5	520	989	52.1	2.9
Ex. 5	220	5	3	700	5	592	1008	52.3	2.0
Ex. 7	220	5	3	800	5	577	998	52.8	2.6
Comp.	200	5	4	700	5	492	977	46.3	4.1
Ex. 18
Comp.	200	5	4	850	5	488	976	46.3	3.9
Ex. 19
Comp.	220	5	4	700	5	479	943	45.5	4.0
Ex. 20
Comp.	220	5	4	800	5	482	930	46.1	4.6
Ex. 21
Comp.	200	5	2	700	5	490	975	47.1	3.7
Ex. 22
Comp.	200	5	2	850	5	490	975	47.3	3.6
Ex. 23
Comp.	220	5	2	700	5	483	950	43.5	4.2
Ex. 24
Comp.	220	5	2	800	5	480	945	46.2	3.9
Ex. 25

In other embodiments of the invention, in order to suitably verify the change in the material properties of the TWIP type ultra-high strength steel sheet at the time of recovering the TWIP type ultra-high strength steel sheet manufactured using the composition given in Table 1 during the cold rolling process, the material properties of the TWIP type ultra-high strength steel sheet of Examples 1, 4, 5 and 7 and Comparative Examples 18 to 25 are given in Table 4. In further embodiments, and referring to Table 4, from Comparative Examples 18 to 25, it can be seen that the material properties of the TWIP type ultra-high strength steel sheet are not influenced by the recovery process suitably performed after the fourth pass or second pass of the cold rolling process. Therefore, according to further preferred embodiments of the invention, it is preferred that the recovery process preferably be performed after the third pass of the cold rolling process.

According to the above described methods of manufacturing the TWIP type ultra-high strength steel sheet according to preferred embodiments of the present invention, the yield strength of the TWIP type ultra-high strength steel sheet can be suitably increased by a maximum of 100 MPa compared to that of conventional steel sheets, the elongation rate thereof can be suitably increased by 3˜4% compared to that of conventional steel sheets to obtain an elongation rate of 50% or more, and the tensile strength thereof can also be suitably increased to 980 MPa. Preferably, a TWIP type ultra-high strength steel sheet, which has suitably high collision strength and can be suitably formed into complicated vehicle body components, can be suitably manufactured. Accordingly, preferred embodiments of the present invention are show in FIG. 4, where FIG. 4 shows the yields strengths and elongation rates of the Examples and the Comparative Examples. Referring to FIG. 4, from data A, it can be seen that in certain exemplary embodiments, the yield strengths of the Examples are 520˜592 MPa and the elongation rates thereof are 50% or more. In other embodiments, from data B, it can be seen that the yield strengths of the Comparative Examples are 520 MPa or less and the elongation rates thereof are 50% or less.

As described herein, preferred methods of manufacturing a TWIP type ultra-high strength steel sheet according to the preferred embodiments of present invention are advantageous in that the yield strength, tensile strength and elongation rate of the TWIP type ultra-high strength steel sheet can be simultaneously improved, and thus the defective fraction in the formation of a vehicle body component can be decreased.

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.

Claims

1. A method of manufacturing a TWIP type ultra-high strength 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), and residual iron (Fe) and other inevitable impurities in four passes or more;

recovering the cold-rolled steel sheet at a temperature of 200˜220° C. after the third pass of the cold rolling; and

annealing the recovered steel sheet.

2. The method of manufacturing a TWIP type ultra-high strength steel sheet according to claim 1, wherein the recovering of the cold-rolled steel sheet is conducted for 5˜6 minutes.

3. The method of manufacturing a TWIP type ultra-high strength steel sheet according to claim 1, wherein the annealing of the recovered steel sheet is conducted at a temperature of 700˜850° C. for 5˜6 minutes.

4. A method of manufacturing a TWIP type ultra-high strength 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), and residual iron (Fe) and other inevitable impurities in four passes or more; and

recovering the cold-rolled steel sheet.

5. The method of manufacturing a TWIP type ultra-high strength steel sheet of claim 4, wherein the cold-rolled steel sheet is recovered at a temperature of 200˜220° C..

6. The method of manufacturing a TWIP type ultra-high strength steel sheet of claim 5, wherein the cold-rolled steel sheet is recovered after the third pass of the cold rolling.

7. The method of manufacturing a TWIP type ultra-high strength steel sheet according to claim 6, wherein the recovering of the cold-rolled steel sheet is conducted for 5˜6 minutes.

8. The method of manufacturing a TWIP type ultra-high strength steel sheet of claim 4, further comprising annealing the recovered steel sheet.

9. The method of manufacturing a TWIP type ultra-high strength steel sheet according to claim 8, wherein the annealing of the recovered steel sheet is conducted at a temperature of 700˜850° C..

10. The method of manufacturing a TWIP type ultra-high strength steel sheet according to claim 8, wherein the annealing of the recovered steel sheet is conducted for 5˜6 minutes.