Medium manganese steel with 1GPa-grade yield strength, high yield-tensile ratio and high plasticity and preparation method of medium manganese steel

Abstract

The invention discloses medium manganese steel with 1GPa-grade yield strength, high yield-tensile ratio and high plasticity and a preparation method of the medium manganese steel, and relates to the technical field of high-strength steel processing. The method comprises the following steps: preparing a medium manganese steel billet, carrying out homogenization treatment on the billet, and then carrying out hot rolling to obtain a hot-rolled steel plate with the thickness of 5-8mm; the hot-rolled steel plate is heated to the two-phase region temperature, heat preservation is conducted for a certain time, then water quenching is conducted to the room temperature, and a partition steel plate is obtained; and after the partition steel plate is subjected to acid pickling, the temperature is increased to be lower than the two-phase region temperature, heat preservation is conducted for 10-20 min, then the partition steel plate is rapidly taken out to be rolled multiple times, the final rolling amount is 10%-80%, and the medium manganese steel with the 1GPa-level yield strength, the high yield-tensile ratio and the high plasticity is obtained. By improving the austenite stability, when the yield strength exceeds 1 GPa, the yield ratio exceeds 0.9, and the elongation exceeds 30%.

Description

Technical Field

[0001] The present invention relates to the technical field of high-strength steel processing, and in particular to a medium manganese steel having a yield strength of 1 GPa level, a high yield strength ratio and high plasticity, and a preparation method thereof. Background Art

[0002] With the rapid development of modern society and the great improvement of living standards, people's demand for transportation tools such as cars, airplanes, and trains is increasing. Among them, the changes in the overall output and sales increment of the automobile market are the most obvious. The rapid growth of automobile sales year after year has brought huge economic benefits and expanded production scale to major automobile companies, but also made energy consumption and environmental pollution problems increasingly serious. On the premise of ensuring the strength and safety of the car body, in order to achieve the goals of energy conservation, emission reduction and environmental protection, the lightweighting of the car body has quickly become the focus of attention in the field of automobile research and development. Studies have shown that there is a linear relationship between vehicle weight and fuel consumption. For every 10% reduction in vehicle weight, fuel consumption will be reduced by 6% to 8%. In order to achieve lightweighting of automobiles, car companies have tried to use lightweight materials such as aluminum alloys, magnesium alloys, and carbon fibers to replace traditional steel materials, but due to the disadvantages of high cost, difficulty in processing, and long process cycle, they have never been widely promoted. At present, the body material is still mainly steel materials. Adding lightweight alloy elements to reduce the material density and improving the comprehensive mechanical properties of steel to reduce the thickness of steel plates have become the main ideas for lightweighting research on automotive materials.

[0003] Medium manganese steel (Mn content is about 3-12 wt%) is a typical representative of the third generation of advanced high-strength steel and is widely used in the automotive field. In 1972, Miller [MILLER RLJM, BM T. Ultrafine-grainedmicrostructures and mechanical properties of alloy steels [J]. 1972, 3(4):905-12.] successfully developed high-strength and high-plasticity medium manganese TRIP steel for the first time. The medium manganese steel with a composition of Fe-0.11C-5.7Mn was cold rolled and then annealed in the two-phase region to obtain a two-phase structure with a volume fraction of 29% austenite and 71% ferrite. The final strength was 878MPa and the elongation reached 34%. 2014 Seok Su Sohn et al. [SOHN SS, CHOI K, KWAK JH, et al. Novel ferrite–austenite duplex lightweight steel with 77% ductility by transformation induced plasticity and twinning induced plasticity mechanisms[J]. Acta Materialia, 2014, 78(181-9.] studied the medium manganese steel of Fe-0.3C-8.5Mn-5.6Al. First, the material was homogenized at 1200℃ for 1h, then kept at 650℃ for 1h to form a dual-phase structure of ferrite and austenite, then cooled to room temperature, cold rolled at room temperature, and finally austenite was reversed in the two-phase region to form a dual-phase structure of ferrite and austenite. Finally, the strength exceeded 900MPa and the elongation exceeded 30%; the prior art discloses a high-strength-ductility cold-rolled medium manganese steel and its preparation method. By using microalloying and rapid annealing process, the medium manganese TRIP steel with low C and Al contents can achieve high strength, high plasticity and total elongation. >50%. The prior art also discloses a high-strength and high-plasticity two-stage warm-rolled medium manganese steel and a preparation method thereof, which adopts a two-stage warm rolling process (two-phase zone rolling). During the warm rolling process, the reverse phase transformation of austenite in the medium manganese steel and the dynamic recrystallization of ferrite occur simultaneously, thereby obtaining a lamellar + equiaxed austenite and ferrite complex phase structure, avoiding the long-term and multiple annealing process required in the cold rolling process, while ensuring high strength and plasticity product. The technicians in this field use micro-alloyed cold-rolled steel sheets as raw materials, and through graded distribution treatment, make the austenite stability gradient distribution more uniform and wider in range. The final medium manganese steel has a tensile strength of ≥1280MPa, an elongation of ≥33.08%, and a strength-plasticity product of ≥44.22GPa·%.

[0004] It is worth noting that although a lot of research has been done on the process design of medium manganese steel, few people are paying attention to how to maintain high yield strength ratio and high plasticity of medium manganese steel at higher yield strength. Summary of the invention

[0005] In view of the deficiencies in the above-mentioned background technology, the present invention mainly solves the problem of how to keep medium manganese steel with high yield ratio and high plasticity at higher yield strength. The present invention provides a medium manganese steel with 1GPa level yield strength, high yield ratio and high plasticity and a preparation method thereof. The present invention improves the stability of austenite so that when the yield strength exceeds 1GPa, the yield ratio exceeds 0.9 and the elongation exceeds 30%.

[0006] The first object of the present invention is to provide a method for preparing medium manganese steel with a yield strength of 1 GPa and high yield strength ratio and high plasticity, comprising the following steps: Preparation of medium manganese steel billets, The steel billet is kept at 1050-1150℃ for 2-4 hours for homogenization treatment, and then hot-rolled to a hot-rolled steel plate with a thickness of 5-8 mm; The hot rolled steel plate is heated to the two-phase temperature and kept at this temperature for a certain period of time, so that the C and Mn elements are fully distributed in the ferrite and austenite phases, and then quenched to room temperature to obtain a partitioned steel plate; wherein the two-phase temperature is 690~770 o C, insulation time is 0.5~2h; After pickling the partitioned steel plate, the temperature is raised to a temperature below the two-phase zone temperature, kept warm for 10 to 20 minutes, and then quickly taken out for multiple rolling, so that the final rolling amount is 10% to 80%, thus obtaining a medium manganese steel with a yield strength of 1GPa level and high yield strength ratio and high plasticity.

[0007] Preferably, during hot rolling after homogenization treatment, the initial rolling temperature is 1050-1100°C and the rolling end temperature is 850-900°C.

[0008] Preferably, during pickling, an acid solution prepared by hydrochloric acid and water in a volume ratio of 1:3 is used.

[0009] Preferably, when rolling is performed multiple times, the material is reheated to a temperature below the two-phase region temperature after each rolling.

[0010] Preferably, the temperature below the two-phase region is 300-500°C.

[0011] Preferably, when preparing the medium manganese steel billet, the following components are selected in mass percentage: C: 0.2-0.4%, Mn: 3-12%, Al: 1-3%, P: ≤0.005%, S: ≤0.005%, and the balance is Fe and unavoidable impurities.

[0012] Preferably, the medium manganese steel billet is produced by weighing raw materials according to the mass percentage of chemical components and smelting them.

[0013] The second object of the present invention is to provide a medium manganese steel with a yield strength of 1 GPa level and high yield strength ratio and high plasticity.

[0014] The third object of the present invention is to provide a medium manganese steel with a yield strength of 1 GPa level and high yield strength ratio and high plasticity for use in automobile manufacturing.

[0015] Compared with the prior art, the present invention has the following beneficial effects: The present invention provides a medium manganese steel with a yield strength of 1 GPa and a high yield ratio and high plasticity and a preparation method thereof. The medium manganese steel with a yield strength of 1 GPa and a high yield ratio and high plasticity provided by the present invention achieves a yield strength of more than 1 GPa without microalloying, cold rolling annealing, etc., reduces costs, and has higher economic and practical value; The medium manganese steel provided by the present invention introduces an appropriate amount of dislocation and improves the stability of austenite by means of warm rolling. While the experimental steel reaches 1GPa yield and a high yield strength ratio, it still maintains high ductility. For automotive structural parts, the improvement of material strength can reduce the thickness of the design material, achieve the lightweight requirements of the vehicle, and reduce the use of fuel. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is a tensile stress-strain curve of the medium manganese steel sample obtained in Example 1 of the present invention.

[0017] Figure 2 This is a tensile stress-strain curve of the medium manganese steel sample obtained in Example 2 of the present invention. DETAILED DESCRIPTION

[0018] In order to enable those skilled in the art to better understand and implement the technical solution of the present invention, the present invention is further described below in conjunction with specific embodiments and drawings, but the embodiments are not intended to limit the present invention.

[0019] The first aspect of the present invention provides a method for preparing a medium manganese steel with a yield strength of 1 GPa and high yield strength ratio and high plasticity, comprising the following steps: Preparation of medium manganese steel billets, The steel billet is kept at 1050-1150℃ for 2-4 hours for homogenization treatment, and then hot-rolled to a hot-rolled steel plate with a thickness of 5-8 mm; The hot rolled steel plate is heated to the two-phase temperature and kept at this temperature for a certain period of time, so that the C and Mn elements are fully distributed in the ferrite and austenite phases, and then quenched to room temperature to obtain a partitioned steel plate; wherein the two-phase temperature is 690~770o C, insulation time is 0.5~2h; After pickling the partitioned steel plate, the temperature is raised to a temperature below the two-phase zone temperature, kept warm for 10 to 20 minutes, and then quickly taken out for multiple rolling, so that the final rolling amount is 10% to 80%, thus obtaining a medium manganese steel with a yield strength of 1GPa level and high yield strength ratio and high plasticity.

[0020] Among them, during hot rolling after homogenization treatment, the initial rolling temperature is 1050~1100℃, and the rolling termination temperature is 850~900℃.

[0021] During pickling, an acid solution prepared by hydrochloric acid and water in a volume ratio of 1:3 is used.

[0022] When multiple rolling is performed, reheating is performed to a temperature below the two-phase region after each rolling.

[0023] The temperature below the two-phase region is 300-500°C.

[0024] When preparing the steel billet of medium manganese steel, the following components are selected in mass percentage: C: 0.2-0.4%, Mn: 3-12%, Al: 1-3%, P: ≤0.005%, S: ≤0.005%, and the balance is Fe and unavoidable impurities.

[0025] The billet of medium manganese steel is produced by weighing raw materials according to the mass percentage of chemical composition and smelting them.

[0026] In one embodiment, a method for preparing a medium manganese steel with a yield strength of 1 GPa and high yield strength ratio and high plasticity, wherein the composition by mass percentage is: C: 0.2-0.4%, Mn: 3-12%, Al: 1-3%, P: ≤0.005%, S: ≤0.005%, and the balance is Fe and unavoidable impurities. The specific steps are as follows: Step 1: Smelting is performed according to the mass percentage of the chemical composition to obtain a casting billet. After smelting, the cast steel is free forged into two 350 100 60mm billet.

[0027] Step 2: Heat the steel billet to 1100℃, keep it at this temperature for 2h for homogenization, then hot-roll the steel billet from 60mm to 5mm after 7 passes, with the initial rolling temperature at 1050℃ and the final rolling temperature at 900℃. Finally, air cool it to room temperature; Step 3: Heat the hot rolled steel plate to the two-phase temperature and keep it warm for a suitable time to allow the C and Mn elements to be fully distributed in the ferrite and austenite phases, and then water quench it to room temperature. The two-phase holding temperature is 690-770 o C, insulation time is 0.5-2h.

[0028] Step 4: Before rolling, the heat-treated steel plate is pickled and derusted with 1:3 hydrochloric acid and water.

[0029] Step 5: Heat the pickled hot-rolled steel to a temperature below the two-phase zone temperature and keep it warm for 10 minutes, then quickly take it out for rolling. To ensure that the steel plate maintains a high temperature during rolling, reheat it after two rollings so that the final rolling amount is 10%~80%. As described above, the present invention regulates the speed of the TRIP effect during deformation of medium manganese steel by regulating the stability of austenite. Specifically, In step 3, the two-phase annealing temperature and annealing time are regulated, the distribution of carbon and manganese elements in ferrite and austenite is regulated, and then the stability of austenite is regulated, so that during the deformation process of the medium manganese steel, most of the austenite will not be transformed due to excessive stability of the austenite at the end of the deformation, nor will the austenite be completely transformed at a small deformation amount due to poor stability, so that the TRIP effect can be carried out continuously and slowly. In step 5, the dislocation density in austenite is increased by rolling at a temperature below the two-phase region, thereby further stabilizing the austenite. On the other hand, the increase in dislocation density causes part of the ferrite to recrystallize, generating fine equiaxed grains, thereby increasing the yield strength.

[0030] A second aspect of the present invention provides a medium manganese steel having a yield strength of 1 GPa level, a high yield strength ratio and high plasticity.

[0031] A third aspect of the present invention provides a medium manganese steel having a yield strength of 1 GPa and high yield strength ratio and high plasticity for use in automobile manufacturing.

[0032] It should be noted that the experimental methods used in the present invention are all conventional methods unless otherwise specified; the reagents and materials used are all commercially available unless otherwise specified.

[0033] Example 1 A medium manganese steel with a yield strength of 1 GPa and high yield strength ratio and high plasticity, the alloy composition (mass percentage) is: 0.28% C, 7.05% Mn, 2.97% Al, and the rest is Fe.

[0034] Preparation process: (1) Smelting is performed according to the mass percentage of the chemical composition to obtain a cast ingot. After smelting, the cast steel is free forged into two 350 100 60mm billet.

[0035] (2) The steel billet is heated to 1100°C and kept at this temperature for 2 hours for homogenization treatment. The steel billet is then hot rolled from 60 mm to 5 mm after 7 passes. The initial rolling temperature is 1050°C and the final rolling temperature is 900°C. Finally, it is air-cooled to room temperature to obtain hot-rolled medium manganese steel; (3) Place the hot-rolled medium manganese steel in a muffle furnace at 1 o C / s heating to 710 o C, and keep it warm for 1h, then quench with water to room temperature to obtain a steel plate; (4) Before cold rolling the steel plate, the heat-treated steel plate is pickled with 1:3 hydrochloric acid and water; (5) The pickled steel plate is heated to 300°C below the two-phase zone temperature and kept at this temperature for 10 minutes, then quickly taken out for rolling. To ensure that the steel plate maintains a high temperature during rolling, it is reheated after two rollings to make the final rolling amount 40%. Mechanical properties test results, such as Figure 1 As shown, by stabilizing austenite, the yield ratio of medium manganese steel is 1, the yield strength is 1046MPa, and the plasticity is 33.6%, so that the medium manganese steel still maintains high plasticity under the conditions of high yield strength and high yield ratio. Example 2 A medium manganese steel with a yield strength of 1 GPa and high yield strength ratio and high plasticity, the alloy composition (mass percentage) is: 0.28% C, 7.05% Mn, 2.97% Al, and the rest is Fe.

[0036] Preparation process: (1) Smelting is performed according to the mass percentage of the chemical composition to obtain a cast ingot. After smelting, the cast steel is free forged into two 350 100 60mm billet.

[0037] (2) The steel billet is heated to 1100°C and kept at this temperature for 2 hours for homogenization treatment. The steel billet is then hot rolled from 60 mm to 5 mm after 7 passes. The initial rolling temperature is 1050°C and the final rolling temperature is 900°C. Finally, it is air-cooled to room temperature to obtain hot-rolled medium manganese steel; (3) Place the hot-rolled medium manganese steel in a muffle furnace at 1 o C / s heating to 710 o C, and keep warm for 1h, then quench with water to room temperature to obtain a steel plate; (4) Before cold rolling the steel plate, the heat-treated steel plate is pickled with 1:3 hydrochloric acid and water; (5) The pickled steel plate is heated to 500°C below the two-phase zone temperature and kept warm for 10 minutes, then quickly taken out for rolling. To ensure that the steel plate maintains a high temperature during rolling, it is reheated after two rollings to make the final rolling amount 50%.

[0038] Mechanical properties test results, such as Figure 2 As shown, by stabilizing austenite, the yield ratio of medium manganese steel is 1, the yield strength is 1235MPa, and the plasticity is 33.2%, so that the medium manganese steel still maintains high plasticity under the conditions of high yield strength and high yield ratio.

[0039] The present invention describes preferred embodiments and their effects. However, those skilled in the art may make additional changes and modifications to these embodiments once they are aware of the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention.

[0040] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing medium manganese steel with a yield strength of 1 GPa and high yield strength ratio and high plasticity, It is characterized in that The following steps are involved: Preparation of medium manganese steel billets, The steel billet is kept at 1050-1150℃ for 2-4 hours for homogenization treatment, and then hot-rolled to a hot-rolled steel plate with a thickness of 5-8 mm; The hot rolled steel plate is heated to the two-phase temperature and kept at this temperature for a certain period of time, so that the C and Mn elements are fully distributed in the ferrite and austenite phases, and then quenched to room temperature to obtain a partitioned steel plate; wherein the two-phase temperature is 690~770 o C, insulation time is 0.5~2h; After pickling the partitioned steel plate, the temperature is raised to a temperature below the two-phase zone temperature, kept warm for 10 to 20 minutes, and then quickly taken out for multiple rolling, so that the final rolling amount is 10% to 80%, thus obtaining a medium manganese steel with a yield strength of 1GPa level and high yield strength ratio and high plasticity.

2. The method for preparing the medium manganese steel with 1 GPa level yield strength, high yield strength ratio and high plasticity according to claim 1, It is characterized in that During hot rolling after homogenization treatment, the initial rolling temperature is 1050~1100℃ and the rolling end temperature is 850~900℃.

3. The method for preparing the medium manganese steel with 1 GPa level yield strength, high yield strength ratio and high plasticity according to claim 1, It is characterized in that During pickling, an acid solution prepared by hydrochloric acid and water in a volume ratio of 1:3 is used.

4. The method for preparing the medium manganese steel with 1 GPa level yield strength, high yield strength ratio and high plasticity according to claim 1, It is characterized in that When multiple rolling is performed, reheating is performed to a temperature below the two-phase region after each rolling.

5. The method for preparing the medium manganese steel with 1 GPa level yield strength, high yield strength ratio and high plasticity according to claim 4, It is characterized in that The temperature below the two-phase region is 300-500°C.

6. The method for preparing the medium manganese steel with 1 GPa level yield strength, high yield strength ratio and high plasticity according to claim 1, It is characterized in that When preparing the steel billet of medium manganese steel, the following components are selected in mass percentage: C: 0.2-0.4%, Mn: 3-12%, Al: 1-3%, P: ≤0.005%, S: ≤0.005%, and the balance is Fe and unavoidable impurities.

7. The method for preparing the medium manganese steel with 1 GPa level yield strength, high yield strength ratio and high plasticity according to claim 1, It is characterized in that The billet of medium manganese steel is produced by weighing raw materials according to the mass percentage of chemical composition and smelting them.

8. A medium manganese steel having a yield strength of 1 GPa and high yield strength ratio and high plasticity, obtained by the method according to any one of claims 1 to 7.

9. Use of the medium manganese steel with a yield strength of 1 GPa level and high yield strength ratio and high plasticity as claimed in claim 8 in automobile manufacturing.

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