KR101094080B1 - Method of fabricating high strength-high ductility light weight steel plate - Google Patents

Abstract

According to the present invention, there is provided a method for producing austenitic lightweight steel sheet, wherein the method comprises (a) wt% Mn: 20-30%, Al: 6-12%, C: 0.6-1.3%, S: 0.01 Providing a steel comprising% or less, P: 0.01% or less, balance Fe and other unavoidable impurities; (b) hot rolling the steel provided in step (a); and (c) the hot rolled steel Cooling to room temperature after homogenizing, (d) cold rolling the cooled steel to achieve a thickness reduction rate of 50% or more, and (e) roughly cold rolling steel in a temperature range of 1,000 to 1,200 ° C. Solution treatment for 30 minutes to about 1 hour, and (f) cooling the solution-treated steel material.

Description

METHOD OF FABRICATING HIGH STRENGTH-HIGH DUCTILITY LIGHT WEIGHT STEEL PLATE}

The present invention relates to a method for producing a high strength and high ductility steel sheet, and more particularly, to a method for producing a high Mn-Al-C-containing lightweight steel sheet and a low specific gravity steel sheet.

The recent record high oil prices, stricter safety regulations, and greenhouse gas emission regulations are driving the demand for high-strength and high-strength as well as lightweight steel in the steel industry. In the case of automotive steel sheets, IF (Interstitial Free) steel, DP (Dual Phase) steel, TRIP (TRansformation Induced Plasticity) steel, etc. have been developed and commercialized in response to this continuous demand. The union is only at 25,000 MPa%. According to the 2002 ULSAB-Ultra Light Steel Auto Body-Advanced Vehicle Technology (ULSAB-AVC) 2002 report, more than 40,000 MPa% of Extra Advanced High Strength Steels (X-AHSS) and 60,000 MPa% of U-AHSS (Ultra Advanced) High Strength Steels) is expected to be developed.

The high Mn-containing TWIP (Twin Induced Plasticity) steels currently in development are around 40,000-60,000 MPa%. However, in the case of TWIP steels, mechanical twinning during deformation, which is a major cause of high strength / ductility, occurs when the stacking defect energy of the austenite matrix is 50 mJ / m ² or less. Therefore, the content of light weight elements such as Al, which increases the stacking defect energy of the austenitic matrix, is limited to less than 5%, thereby limiting the weight reduction of the steel material.

Meanwhile, conventional techniques related to high Mn-containing lightweight austenitic steels include precipitation-reinforced Fe-30Mn-7.7Al-1.3C steels such as Choo (Acta Materialia, Vol. 45, No. 12 (1997) pp.4877 ~ 4885) and the TRIPLEX steel developed by Frommeyer and Brux et al. (Steel Research Institute, Vo. 77, No. 9-10 (2006), pp. 627-633). Choo et al .: Precipitation hardened Fe-30Mn-7.7Al-1.3C steels were subjected to aging treatment for 30 minutes at 980 ℃ for up to 160 hours at 550 ℃ and tensile strength 900 ~ 1,120 MPa according to aging time. , Elongation 10-60%, maximum tensile strength-ductility combination 45,000 MPa% were obtained. However, in addition to the long-term aging treatment and aging treatment conditions, other manufacturing methods have limitations in practical use by setting a single condition. In addition, TRIPLEX steel, developed by Frommeyer and Brux, is based on the alloy component Fe-18 / 28Mn-9 / 12Al-0.7 / 1.2C and consists of residual and unavoidable impurities. It is composed of 5 ~ 15% BCC ferrite and 6 ~ 9% composite carbide nanoparticles, and the maximum unit density decrease by Al addition is 17% compared to pure iron. The room temperature tensile strength and elongation of TRIPLEX steel are 1,000 MPa and 55%, respectively, for Fe-28Mn-12Al-1.2C, the largest component system of TRIPLEX steel, and the tensile strength-elongation combination is 55,000 MPa%, which is somewhat higher than TWIP steel. to be. However, in the case of TRIPLEX steel, it is difficult to control the volume fraction of BCC ferrite and composite carbides, and as a well-known fact, the elongation is limited by their presence. In addition, the major plastic deformation mechanism of TRIPLEX steel is shear band organoplasticity, and it is well known that the shear band formed during plastic deformation causes a very severe deformation locally and acts as a factor to lower the elongation.

The present invention is to solve the problems shown in the above-described prior art, one object of the present invention is to provide a method for producing an austenitic steel sheet that can achieve a weight reduction while containing a large amount of Mn, Al, C.

Another object of the present invention is to provide a method for producing a high Mn, Al, C-containing austenitic steel sheet in which the combination of tensile strength and elongation is significantly improved compared to conventional steel sheets.

It is still another object of the present invention to provide high elongation and tension at room temperature due to microband induced plasticity rather than shear band organic plasticity in austenitic single-phase steel sheet without BCC ferrite and precipitation The combination of strength and elongation is to provide a method for producing a lightweight steel sheet which is dramatically improved compared to the conventional steel sheet.

It is a further object of the present invention to provide a steel sheet having a specific component content range in which the excellent elongation and combination of tensile strength and elongation are significantly improved compared to conventional steel sheets.

In order to achieve the above object, according to the present invention, there is provided a method for producing austenitic lightweight steel sheet, wherein (a) Mn: 20 to 30% by weight, Al: 6 to 12%, C: 0.6 Providing a steel material composed of ˜1.0%, S: 0.01% or less, P: 0.01% or less, balance Fe and other unavoidable impurities; (b) hot rolling the steel provided in step (a); c) homogenizing the hot rolled steel and then cooling it to room temperature; (d) cold rolling the cooled steel to achieve a thickness reduction rate of 50% or more, and (e) 1,000 to 1,000 cold rolled steels. Solution treatment for approximately 30 minutes to about 1 hour in a temperature range of 1,200 ° C., and (f) cooling the solution-treated steel.

In one embodiment, the solution treatment in step (e) is preferably carried out for about 1 hour.

In another embodiment of the present invention, a method for producing austenitic single-phase lightweight steel sheet is provided, which method comprises (a) 20% by weight of Mn: 20-30%, Al: 6-12%, and C: 1.0-1.3. Providing a steel comprising%, S: 0.01% or less, P: 0.01% or less, balance Fe and other unavoidable impurities; (b) hot rolling the steel provided in step (a); and (c) Cooling to room temperature after homogenizing the hot rolled steel, and (d) cold rolling the cooled steel to achieve a thickness reduction rate of 50% or more, and (e) the cold rolled steel at a temperature of 1,200 ° C. Solving treatment for 30 minutes to 1 hour in the range, and (f) characterized in that it comprises the step of cooling the solution-treated steel.

In one embodiment, the cooling in the steps (c) and (f) may be performed by air cooling and water cooling.

In the present invention, the steel sheet according to the method is deformed by the microband organic plastic at the time of tensile deformation, the steel sheet exhibits a characteristic of elongation of at least 80%, the product of tensile strength and elongation of 75,000 MPa% or more.

According to another aspect of the present invention, there is provided a steel sheet produced according to the method described above, in terms of weight% Mn: 20-30%, Al: 6-12%, C: 0.6-1.3%, S: 0.01% or less , P: 0.01% or less, remainder Fe and other unavoidable impurities, consisting of austenite single phase, elongation of 80% or more, product of tensile strength and elongation of 75,000 MPa% or more, specific gravity 7g / cm ³ or less It is characterized by.

Steel sheet provided according to another embodiment of the present invention by weight% Mn: 20-30%, Al: 6-12%, C: 0.6-1.3%, S: 0.01% or less, P: 0.01% or less, balance Fe And other unavoidable impurities, and composed of austenite single phase, exhibiting an elongation of at least 80%, a product of tensile strength and elongation of at least 75,000 MPa%, and specific gravity of 7 g / cm ³ or less.

In the steel sheet according to the present invention, the product of elongation and tensile strength and elongation of 75,000 MPa% or more is due to microband organic plasticity.

In the steel sheet according to the present invention, the content range of C may be 0.6 to 1.0%, in which case the steel sheet is about 30 minutes to about 1 hour at a temperature of 1,000 to 1,200 ° C., preferably about 1 hour. During the solution treatment process.

In a steel sheet according to another embodiment of the present invention, the content range of C may be 1.0 to 1.3%, in which case the steel sheet is about 30 minutes to about 1 hour at about 1,200 ° C., preferably about 1 It can be prepared through a process of solution treatment for a time.

The steel according to the present invention exhibits a strength-elongation combination of not less than 75,000 MPa%, which is expressed as a product of elongation at break of 80% or more and tensile strength and elongation at break, and shows specific gravity of 7 g / cm3 or less, which is compared with conventional steels. Its properties are significantly improved. That is, according to the present invention, it is possible to provide an austenitic single-phase lightweight steel sheet exhibiting an excellent strength-elongation combination.

Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings. In the following description, the detailed description is omitted for the processes already known in the art such as hot rolling, cold rolling and the like.

The present inventors have conducted studies to overcome problems such as the elongation limit and the weight reduction limit of the conventional steel sheet with respect to high Mn-containing austenite steel. As a result of this study, Mn: 20 to 30%, Al: 6 to 12%, C: 0.6 to 1.3%, S: 0.01% or less, P : Steel composed of 0.01% or less, balance Fe and other unavoidable impurities is composed of austenite single phase through appropriate mechanical heat treatment, thereby producing a lightweight steel sheet having a very good combination of room temperature strength and corresponding tensile strength-elongation. I found it.

Starting from this point of view, the present invention has been proposed by weight% Mn: 20-30%, Al: 6-12%, C: 0.6-1.3%, S: 0.01% or less, P: 0.01% or less, balance Fe and Austenitic single phase with excellent strength and elongation by performing a series of mechanical heat treatments including hot-dip and cold rolling of other inevitable impurities in the ingot or slab state, and solution treatment within 1 hour at 1,000 ~ 1,200 ℃. Provides a method for manufacturing lightweight steel sheet.

The reason for numerical limitation of the chemical component of the said steel plate is demonstrated concretely below.

In the steel sheet according to the present invention Mn is contained in the range of 20% to 30% by weight. Mn is an austenite stabilizing element, and may be contained in an amount of 10% or more in order to stabilize the austenite phase stable at high temperature even at room temperature. However, when adding 6% or more of Al, which is a ferrite stabilizing element, in the present invention, since it is difficult to obtain single-phase austenite at room temperature, it is preferable to add 20% or more. On the other hand, when Mn is added in excess of 30%, the sum of atomic mole fractions of Mn, Al, and C is 0.5 or more when Al is added to the upper limit of 12% in the alloy component system of the present invention. Mn is preferably contained at 30% or less in the steel sheet of the present invention since the steel inherent properties are lost.

In the steel sheet according to the present invention, Al is contained in the range of 6 to 12% by weight. Al is a ferrite stabilizing element and is an element that increases the stacking defect energy of the austenite single phase. When Al is less than 6% in the steel sheet of the present invention, an increase in stacking defect energy is not sufficient, and as described below, a microband for exhibiting high elongation, which is one of the objects of the present invention, in a microstructure. Should not be added, so at least 6% should be added. On the other hand, Al is a ferrite stabilizing element as described above, and when Al is added in excess of 12%, the austenite stabilization region is reduced, so that BCC ferrite is formed, Al is less than 12% in the steel sheet of the present invention. It is preferable to contain.

On the other hand, in the steel sheet according to the invention C is contained in the range of 0.6% to 1.3% by weight. C is an invasive solid solution element in steel, and P is the most effective element for increasing strength, and is an austenite stabilizing element. In the present invention, even when 20% or more of Mn, which is an austenite stabilizing element, is added, when 6% or more of Al, which is a ferrite stabilizing element, is reduced, the stable austenite region is reduced. It is preferred to add at least 0.6% for further expansion. However, when C is added in excess of 1.3%, a layered structure composed of austenite-composite carbides, rather than austenite single phase, appears during the solution treatment, so that C is preferably contained in the steel sheet of the present invention at 1.3% or less. Do.

On the other hand, S in the steel sheet according to the present invention is contained in 0.01% or less. S is an element susceptible to segregation at grain boundaries, and when segregated at grain boundaries, the grain boundaries become brittle and cause brittleness at high temperature rolling. In addition, it forms with MnS inclusions in combination with Mn to reduce the amount of Mn dissolved in the austenite matrix, and the presence of such MnS inclusions lowers the elongation, so it is preferable to manage it at 0.01% or less. In addition, P in the steel sheet according to the present invention also contains 0.01% or less. In other words, P is the most severe elemental segregation during steelmaking, and the segregation zone remains during hot rolling of the sheet, which is a factor of lowering toughness and ductility.

Referring to the method of manufacturing a steel sheet having excellent strength and elongation at the same time using the steel consisting of the above components as follows.

Hot rolling and cold rolling of the ingot or slab material which consist of the above-mentioned component are performed according to the shape of a final board material. However, cold rolling requires a thickness reduction rate of 50% or more because recrystallization of uniform grain size is required during the solution treatment.

On the other hand, in order to obtain a solid solution of austenite single phase, it is necessary to quench below the room temperature after solution treatment in the temperature range of 1,000-1,200 degreeC. At 1,000 ° C. or lower, a three-phase structure composed of ferrite, austenite, and unused composite carbide is formed, so that the microstructure of the present invention cannot be obtained. Further, at temperatures higher than 1,200 ° C, grain coarsening occurs and the strength decreases. On the other hand, the solution treatment time seems to be preferably 30 minutes or more for uniform grain distribution, and within 1 hour for suppressing grain coarsening.

Hereinafter, the present invention will be described in more detail with reference to various examples.

Example 1

A 50 kg ingot steel with the chemical composition of Table 1 was produced in a vacuum induction melting furnace and then processed into a sheet 11 mm thick by hot rolling. The hot rolled sheet material was homogenized at 1,000 ° C., then air cooled to room temperature, and then cold rolled to a thickness of 3 mm. The cold rolled material was subjected to a solution treatment for 1 hour at 700 to 1,200 ° C., followed by water cooling. After cooling, the steel phase was confirmed by X-ray analysis and transmission electron microscope (TEM).

Steel grade C Mn Al P S Fe A 0.79 27.8 9.06 0.003 0.0037 Remainder B 0.98 28.3 9.95 0.003 0.0037 Remainder C 1.17 28.4 9.93 0.003 0.0039 Remainder

FIG. 1 shows the results of X-ray diffraction analysis according to the solution treatment temperature of steel grade A, indicating that ferrite is present at 800 ° C. or lower. 2 shows the results of X-ray diffraction analysis according to the solution treatment temperature of steel grade B. It was confirmed that steel grade B was all austenite single phase at 1,000 ° C. or higher.

3 is a transmission electron microscope tissue photograph of the steel grade C solution-treated at 1,000 ℃ it was confirmed that the fine carbide precipitated. In this way, the structural phases according to the solution treatment temperature of steel grades A, B, and C were confirmed, and the results are summarized in Table 2. As shown in Table 2, the austenitic single phase was obtained by solution treatment at 900 to 1,200 ° C. for 1 hour in steel grades A and B, and the austenitic single phase was obtained only at 1,200 ° C. in steel grade C.

On the other hand, the specific gravity of A, B, and C steels was measured using the specific gravity measuring device using the Alchemedes principle. As a result, the average specific gravity of all steels was 6.85 ± 0.13 g / cm ³ and the specific gravity was less than 7 g / cm ³ . That is, it was confirmed that the steel grade can be reduced in weight according to the present invention.

Temperature＼Steel A B C 700 ℃ A + F A + F A + F + C 800 ℃ A + F A + F A + F + C 900 ℃ A A A + C 1,000 ℃ A A A + C 1,100 ℃ A A A + C 1,200 ℃ A A A

A: austenite, F: ferrite, C: fine carbide

Example 2

After the solution treatment at 700 ~ 1,200 ° C for 1 hour, room temperature tensile test was performed at 10 ^-3 s ^-1 for water-cooled A, B, and C steels, and the results are shown in Table 3 below.

Steel grade Solvent Treatment
Configuration Room temperature tensile properties
Remarks
Temperature (℃) Minutes YS (MPa) UTS (MPa) El. (%) UTS × El.
(MPa%)

A

700 60 A + F 749 1,034 52 53,768 Comparative material 800 60 A + F 605 953 61 58,133 Comparative material 1,000 5 A 633 955 71 67,875 Comparative material 1,000 10 A 539 903 82 74,046 Comparative material 1,000 60 A 440 843 100 84,300 Invention
B
1,000 60 A 572 874 99 86,526 Invention 1,100 60 A 483 813 106 86,178 Invention 1,200 60 A 446 776 111 86,136 Invention
C
1,000 60 A + C 707 919 79 72,601 Comparative material 1,100 60 A + C 672 868 82 71,175 Comparative material 1,200 60 A 524 784 103 80,752 Invention

In the case of steel A, when the solution treatment was performed at a temperature of 1,000 ° C. or lower or at 1,000 ° C. for less than 1 hour, in particular, less than 30 minutes, the tensile strength (UTS) x elongation (El.) All showed 75,000 MPa% or less. , Elongation was 100%, tensile strength (UTS) × elongation (El.) Was about 84,000 MPa% for 1 hour solution treatment at 1,000 ℃.

In the case of steel B, the solution treatment at 1,000 to 1,200 ° C. for 1 hour resulted in an excellent strength-elongation combination of 100% or more elongation (UTS) × elongation (El.) Of about 85,000 MPa% or more.

In the case of C steel, elongation was about 80% when 1 hour solution treatment was performed at 1,000 ℃ and 1,100 ℃ where fine carbides were precipitated, but tensile strength (UTS) × elongation (El.) Was 75,000 MPa% or less. Elongation was more than 100%, tensile strength (UTS) x elongation (El.) 75,000 MPa% or more only at the solution treatment temperature of 1,200 ℃.

In conclusion, in the case of the steel having a component in the above-described range, in order to obtain an austenite single phase exhibiting elongation of at least 80% and tensile strength (UTS) x elongation (El.) Of 75,000 MPa% or more, 1% or less of carbon is contained. Approximately 30 minutes or more, preferably about 1 hour, of solution solution is required at 1,000 to 1,200 ° C, while containing 1% or more of carbon, approximately 30 minutes or more, preferably about 1 hour of solution solution, at 1,200 ° C It was confirmed that treatment was necessary.

Example 3

In order to determine what mechanism is the combination of good strength and ductility of the steel according to the invention shown in Table 3, the microstructure during tensile deformation was investigated using transmission electron microscopy. Figure 4 shows the microstructure according to the degree of tensile strain of the steel grade A subjected to solution treatment for 1,000 hours, 1 hour in accordance with the present invention. It can be seen that at low strains of 5% and 10%, plastic deformation occurs due to planar sliding of dislocations, and microbands are formed at 40%. In addition, the microstructure at high deformation of 100% shows that almost all microstructures are composed of microbands, and the grains are refined by their crossing. Therefore, it was confirmed that the combination of the ductility of the excellent strength of the steel according to the present invention is due to the microband organic plasticity unlike the plastic deformation mechanism of the conventional steel.

Although the present invention has been described above with reference to preferred embodiments, it should be noted that the present invention is not limited to the above-described embodiments. That is, the present invention can be variously modified and modified within the scope of the following claims, all of which fall within the scope of the present invention. Accordingly, the invention is limited only by the constructions and equivalents described in the claims.

1 is a view showing the results of X-ray diffraction analysis of the steel grade A solution solution treated for about 1 hour at 700 ~ 1,000 ℃.

FIG. 2 is a diagram showing the result of X-ray diffraction analysis of the steel grade B solvated for about 1 hour at 1,000 ~ 1,200 ℃.

FIG. 3 is a transmission electron microscope (TEM) photograph of steel grade C, solution treated at about 1,000 ° C. for about 1 hour.

4 is a micrograph showing the microstructure according to the degree of tensile deformation of the steel grade A solution-treated at about 1,000 ℃ for about 1 hour.

Claims (17)

(a) Provide steel materials consisting of Mn: 20-30%, Al: 6-12%, C: 0.6-1.0%, S: 0.01% or less, P: 0.01% or less, balance Fe and other unavoidable impurities in weight percent To do that, (b) hot rolling the steel provided in step (a); (c) homogenizing the hot rolled steel and cooling it to room temperature; (d) cold rolling the cooled steel to achieve a thickness reduction rate of 50% or more; (e) solution treatment of the cold-rolled steel for 30 minutes to 1 hour at a temperature range of 1,000 to 1,200 ° C., (f) cooling the solution-treated steel Light weight steel sheet manufacturing method comprising a. The method of claim 1, wherein the solution treatment in step (e) is performed for 1 hour. delete (a) Provide steel materials consisting of Mn: 20-30%, Al: 6-12%, C: 1.0-1.3%, S: 0.01% or less, P: 0.01% or less, balance Fe and other unavoidable impurities in weight percent. To do that, (b) hot rolling the steel provided in step (a); (c) homogenizing the hot rolled steel and cooling it to room temperature; (d) cold rolling the cooled steel to achieve a thickness reduction rate of 50% or more; (e) solution treatment of the cold-rolled steel for 30 minutes to 1 hour in a temperature range of 1,200 ℃, (f) cooling the solution-treated steel Light weight steel sheet manufacturing method comprising a. delete The method of claim 1 or 4, wherein the cooling in the steps (c) and (f) is performed by air cooling and water cooling, respectively. The method according to any one of claims 1, 2 and 4, wherein the steel sheet according to the method is deformed by microband organic plasticity upon tensile deformation. The method of claim 7, wherein the steel sheet has a property of 80% or more elongation, and the product of tensile strength and elongation is 75,000 MPa% or more. delete By weight% Mn: 20-30%, Al: 6-12%, C: 0.6-1.3%, S: 0.01% or less, P: 0.01% or less, balance Fe and other unavoidable impurities, in austenite single phase A steel sheet, characterized in that the elongation is at least 80%, the product of tensile strength and elongation is 75,000 MPa% or more, specific gravity 7g / cm ³ or less. The steel sheet according to claim 10, wherein the product of elongation and tensile strength and elongation of 75,000 MPa% or more is due to microband organic plasticity. The steel plate according to claim 10, wherein the content range of C is 0.6 to 1.0%. The steel sheet of claim 12, wherein the steel sheet is manufactured through a process of solution treatment for 30 minutes to 1 hour at a temperature of 1,000 to 1,200 ° C. The steel sheet of claim 13, wherein the steel sheet is manufactured through a process of solution treatment for 1 hour at a temperature of 1,000 to 1,200 ° C. The steel plate according to claim 10, wherein the content range of C is 1.0 to 1.3%. The steel sheet of claim 15, wherein the steel sheet is manufactured through a process of solution treatment at 1,200 ° C for 30 minutes to 1 hour. The steel sheet of claim 16, wherein the steel sheet is manufactured through a process of solution treatment at 1,200 ° C for 1 hour.

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