A medium-manganese steel with a yield strength of 1gpa level and high plasticity with high yield strength ratio and a preparation method thereof

Abstract

The application discloses a medium-manganese steel with a yield strength of 1GPa and high yield strength ratio and plasticity and a preparation method thereof, and relates to the technical field of high-strength steel processing. The method comprises the following steps: preparing a steel billet of the medium-manganese steel, uniformly treating the steel billet, and then hot-rolling the steel billet into a hot-rolled steel plate with a thickness of 5-8 mm; heating the hot-rolled steel plate to a two-phase region temperature and keeping the temperature for a certain time, and then water-quenching the hot-rolled steel plate to room temperature to obtain a partitioning steel plate; after the partitioning steel plate is pickled, the temperature of the partitioning steel plate is increased to a temperature lower than the two-phase region temperature, the temperature is kept for 10-20 min, and then the partitioning steel plate is rapidly taken out and rolled for multiple times, so that the final rolling amount is 10%-80%, and the medium-manganese steel with a yield strength of 1GPa and high yield strength ratio and plasticity is obtained. According to the application, the stability of austenite is improved, so that when the yield strength exceeds 1GPa, the yield strength ratio exceeds 0.9 and the elongation exceeds 30%.

Description

Technical Field

[0001] This invention relates to the field of high-strength steel processing technology, specifically to a medium-manganese steel with a yield strength of 1 GPa and high yield strength-to-tensile ratio and high plasticity, and its preparation method. Background Technology

[0002] With the rapid development of modern society and the significant improvement in living standards, people's demand for transportation tools such as automobiles, airplanes, and trains is increasing daily. The changes in overall production and sales growth in the automobile market are most pronounced. The continuous rapid growth in automobile sales has brought huge economic benefits and expanded production scale to major automakers, but it has also exacerbated energy consumption and environmental pollution problems. To achieve energy conservation, emission reduction, and environmental protection goals while ensuring vehicle body strength and safety, lightweighting of the vehicle body has rapidly become a key focus in automotive R&D. Research shows that vehicle weight and fuel consumption have a linear relationship; for every 10% reduction in vehicle weight, fuel consumption decreases by 6% to 8%. To achieve lightweighting, automakers have attempted to use lightweight materials such as aluminum alloys, magnesium alloys, and carbon fiber to replace traditional steel materials. However, due to drawbacks such as high cost, processing difficulty, and long process cycles, widespread adoption has been hindered. Currently, steel remains the primary material for vehicle bodies. Adding lightweight alloying elements to reduce material density and improving the comprehensive mechanical properties of steel to reduce steel sheet thickness has become the main approach in automotive material lightweighting research.

[0003] Medium-manganese steel (Mn content approximately 3-12 wt%) is a typical representative of third-generation advanced high-strength steel and is widely used in the automotive field. In 1972, Miller [MILLER RLJM, BM T. Ultrafine-grained microstructures and mechanical properties of alloy steels [J]. 1972, 3(4):905-12.] successfully developed high-strength and high-ductility medium-manganese TRIP steel for the first time. 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 microstructure with a volume fraction of 29% austenite and 71% ferrite, ultimately achieving a strength of 878 MPa and an elongation of 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 a medium-manganese steel with Fe-0.3C-8.5Mn-5.6Al. First, the material was homogenized at 1200℃ for 1 hour, then held at 650℃ for 1 hour to form a ferrite-austenite duplex structure. It was then cooled to room temperature and cold-rolled at room temperature. Finally, austenite reverse transformation occurred in the two-phase region to form a ferrite-austenite duplex structure. The final result showed a strength exceeding 900 MPa and an elongation exceeding 30%. Existing technology discloses a high-strength, high-ductility, cold-rolled medium-manganese steel and its preparation method. Utilizing microalloying combined with a rapid annealing process, medium-manganese TRIP steel with low C and Al content achieves high strength, high ductility, and a high total elongation. >50%. Existing technology also discloses a high-strength, high-ductility two-stage warm-rolled medium-manganese steel and its preparation method. This method employs a two-stage warm rolling process (two-phase zone rolling). During warm rolling, the reverse transformation of austenite and the dynamic recrystallization of ferrite occur simultaneously in the medium-manganese steel, resulting in a multiphase microstructure of lamellar + equiaxed austenite and ferrite. This avoids the long-term, multiple annealing processes required in cold rolling while ensuring a high strength-ductility product. Those skilled in the art use microalloyed cold-rolled steel sheets as raw materials and, through graded distribution treatment, achieve a more uniform and wider distribution of austenite stability gradient, ultimately obtaining a medium-manganese steel with a tensile strength ≥1280 MPa, elongation ≥33.08%, and strength-ductility product ≥44.22 GPa·s.

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

[0005] To address the shortcomings of the aforementioned background technology, this invention primarily solves the problem of maintaining a high yield strength ratio and high plasticity in medium manganese steel at relatively high yield strengths. This invention provides a medium manganese steel with a yield strength of 1 GPa and high yield strength ratio and high plasticity, as well as its preparation method. This invention improves austenite stability, enabling it to achieve a yield strength ratio exceeding 0.9 and an elongation exceeding 30% when the yield strength exceeds 1 GPa.

[0006] The first objective of this invention is to provide a method for preparing medium-manganese steel with a yield strength of 1 GPa and high yield-to-tensile ratio and high ductility, comprising the following steps:

[0007] Steel billets for preparing medium manganese steel,

[0008] The steel billet is kept at 1050~1150℃ for 2~4 hours for homogenization treatment, and then hot-rolled to 5~8mm thick hot-rolled steel plate.

[0009] Hot-rolled steel sheets are heated to the two-phase region temperature and held for a certain time to allow C and Mn elements to fully disperse in the ferrite and austenite phases. The sheets are then water-quenched to room temperature to obtain dispersed steel sheets. The two-phase region temperature is 690~770°C. o C, the heat preservation time is 0.5~2 hours;

[0010] After pickling the steel plate, heat it to a temperature below the two-phase region and hold it for 10-20 minutes. Then quickly remove it and roll it multiple times to make the final rolling amount 10%-80%, which yields medium manganese steel with a yield strength of 1GPa, a high yield strength ratio, and high plasticity.

[0011] Preferably, during hot rolling after homogenization treatment, the initial rolling temperature is 1050~1100℃ and the rolling termination temperature is 850~900℃.

[0012] Preferably, during pickling, an acid solution prepared by hydrochloric acid and water in a volume ratio of 1:3 is used.

[0013] Preferably, when performing multiple rolling processes, the temperature is reheated to below the two-phase region temperature after every two rolling processes.

[0014] Preferably, the temperature below the two-phase region is 300~500℃.

[0015] Preferably, when preparing the billet of medium manganese steel, the following components by mass percentage are selected: C: 0.2-0.4%, Mn: 3-12%, Al: 1-3%, P: ≤0.005%, S: ≤0.005%, with the balance being Fe and unavoidable impurities.

[0016] Preferably, the billet of medium manganese steel is obtained by smelting raw materials according to the mass percentage of chemical composition.

[0017] The second objective of this invention is to provide a medium-manganese steel with a yield strength of 1 GPa and a high yield strength-to-tensile ratio and high plasticity.

[0018] The third objective of this invention is to provide an application of medium manganese steel with a yield strength of 1 GPa and high yield strength-to-tensile ratio and high plasticity in automobile manufacturing.

[0019] Compared with the prior art, the beneficial effects of the present invention are:

[0020] This invention provides a medium manganese steel with a yield strength of 1GPa and high yield strength ratio and high plasticity, and its preparation method. The medium manganese steel with a yield strength of 1GPa and high yield strength ratio and high plasticity provided by this invention achieves a yield strength of more than 1GPa without microalloying, cold rolling and annealing, etc., reducing costs and having higher economic and practical value.

[0021] The medium-manganese steel provided by this invention introduces an appropriate amount of dislocations and improves the stability of austenite through warm rolling. While achieving a yield strength of 1 GPa and a high yield strength ratio in the experimental steel, it still maintains high ductility. For automotive structural components, the improved material strength can reduce the thickness of the designed material, meet the requirements of lightweighting of automobiles, and reduce fuel consumption. Attached Figure Description

[0022] Figure 1 This is a tensile stress-strain curve of the medium manganese steel sample obtained in Example 1 of the present invention.

[0023] Figure 2 This is a tensile stress-strain curve of the medium manganese steel sample obtained in Example 2 of the present invention. Detailed Implementation

[0024] To enable those skilled in the art to better understand and implement the technical solutions of the present invention, the present invention will be further described below in conjunction with specific embodiments and accompanying drawings. However, the embodiments described are not intended to limit the present invention.

[0025] The first aspect of this invention provides a method for preparing medium-manganese steel with a yield strength of 1 GPa and high yield-to-tensile ratio and high ductility, comprising the following steps:

[0026] Steel billets for preparing medium manganese steel,

[0027] The steel billet is kept at 1050~1150℃ for 2~4 hours for homogenization treatment, and then hot-rolled to 5~8mm thick hot-rolled steel plate.

[0028] Hot-rolled steel sheets are heated to the two-phase region temperature and held for a certain time to allow C and Mn elements to fully disperse in the ferrite and austenite phases. The sheets are then water-quenched to room temperature to obtain dispersed steel sheets. The two-phase region temperature is 690~770°C. o C, the heat preservation time is 0.5~2 hours;

[0029] After pickling the steel plate, heat it to a temperature below the two-phase region and hold it for 10-20 minutes. Then quickly remove it and roll it multiple times to make the final rolling amount 10%-80%, which yields medium manganese steel with a yield strength of 1GPa, a high yield strength ratio, and high plasticity.

[0030] During the hot rolling process after homogenization, the initial rolling temperature is 1050~1100℃, and the rolling termination temperature is 850~900℃.

[0031] During pickling, an acid solution prepared by mixing hydrochloric acid and water in a volume ratio of 1:3 is used.

[0032] When multiple rolling processes are performed, the temperature is reheated to below the two-phase region temperature after each two rolling processes.

[0033] The temperature below the two-phase region is 300~500℃.

[0034] When preparing billets for medium manganese steel, the following composition by mass percentage is selected: C: 0.2-0.4%, Mn: 3-12%, Al: 1-3%, P: ≤0.005%, S: ≤0.005%, with the balance being Fe and unavoidable impurities.

[0035] The billet of medium manganese steel is made by smelting raw materials according to the mass percentage of chemical composition.

[0036] In one embodiment, a method for preparing a medium-manganese steel with a yield strength of 1 GPa and high yield-to-tensile ratio and high ductility is described, wherein the composition by mass percentage is: C: 0.2-0.4%, Mn: 3-12%, Al: 1-3%, P: ≤0.005%, S: ≤0.005%, with the balance being Fe and unavoidable impurities. The specific steps are as follows:

[0037] Step 1: The steel is smelted according to the stated chemical composition by mass percentage to obtain a cast billet. After smelting, the cast steel is freely forged into two 350mm diameter sections. 100 60mm steel billet.

[0038] Step 2: Heat the steel billet to 1100℃ and hold for 2 hours for homogenization. Then, hot roll the billet from 60mm to 5mm in 7 passes. The initial rolling temperature is 1050℃, and the final rolling temperature is 900℃. Finally, air cool to room temperature.

[0039] Step 3: Heat the hot-rolled steel plate to the two-phase region temperature and hold it at that temperature for an appropriate time to allow C and Mn elements to fully distribute in the ferrite and austenite phases. Then, water quench it to room temperature, with the two-phase region holding temperature being 690-770°C. o C, the heat preservation time is 0.5-2h.

[0040] Step 4: Before rolling, the heat-treated steel plate is pickled and rust-removed using a 1:3 mixture of hydrochloric acid and water.

[0041] Step 5: Heat the pickled hot-rolled steel to a temperature below the two-phase region and hold it for 10 minutes. Then quickly remove it for rolling. To ensure that the steel plate maintains a high temperature during rolling, reheat it after two rolling cycles so that the final rolling amount is 10%~80%.

[0042] As described above, this invention regulates the rate of the TRIP effect during the deformation of medium manganese steel by controlling the stability of austenite. Specifically,

[0043] In step 3, the annealing temperature and time in the two-phase region are adjusted to regulate the distribution of carbon and manganese elements in ferrite and austenite, thereby controlling the stability of austenite. This ensures that during the deformation process of medium-manganese steel, a large portion of austenite remains untransformed at the end of deformation due to excessive austenite stability, nor does the austenite completely transform at a small deformation due to poor austenite stability. This allows the TRIP effect to proceed continuously and slowly.

[0044] In step 5, rolling at a temperature below the two-phase region increases the dislocation density in austenite, further stabilizing the austenite. On the other hand, the increase in dislocation density causes some ferrite to recrystallize, generating fine equiaxed crystals, which improves the yield strength.

[0045] The second aspect of the present invention provides a medium manganese steel with a yield strength of 1 GPa and a high yield strength-to-tensile ratio and high plasticity.

[0046] The third aspect of this invention provides the application of medium manganese steel with a yield strength of 1 GPa and high yield strength-to-tensile ratio and high plasticity in automobile manufacturing.

[0047] It should be noted that, unless otherwise specified, the experimental methods used in this invention are all conventional methods; and the reagents and materials used, unless otherwise specified, are all commercially available.

[0048] Example 1

[0049] A medium-manganese steel with a yield strength of 1 GPa and high yield-to-tensile ratio and high plasticity, with an alloy composition (mass percentage): 0.28% C, 7.05% Mn, 2.97% Al, and the remainder Fe.

[0050] Preparation process:

[0051] (1) The cast steel is obtained by smelting according to the mass percentage of the chemical composition. After smelting, the cast steel is freely forged into two 350mm diameter steel bars. 100 60mm steel billet.

[0052] (2) The steel billet is heated to 1100℃ and held for 2 hours for homogenization treatment. Then the steel billet is hot rolled from 60mm to 5mm after 7 passes. The initial rolling temperature is 1050℃ and the final rolling temperature is 900℃. Finally, it is air-cooled to room temperature to obtain hot-rolled medium manganese steel.

[0053] (3) Place the hot-rolled medium manganese steel in a muffle furnace and heat it at 1... o Heat to 710 C / s o C, and hold at that temperature for 1 hour, then quench in water to room temperature to obtain a steel plate;

[0054] (4) Before cold rolling the steel plate, pickle the heat-treated steel plate with a 1:3 ratio of hydrochloric acid and water;

[0055] (5) Heat the pickled steel plate to 300°C below the two-phase zone temperature and keep it at that temperature 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 rolling twice so that the final rolling amount is 40%.

[0056] Mechanical performance test results, such as Figure 1 As shown, by stabilizing austenite, the yield strength ratio of medium manganese steel is 1, the yield strength is 1046 MPa, and the plasticity is 33.6%, so that medium manganese steel can maintain high plasticity even with high yield strength and high yield strength ratio.

[0057] Example 2

[0058] A medium-manganese steel with a yield strength of 1 GPa and high yield-to-tensile ratio and high plasticity, with an alloy composition (mass percentage): 0.28% C, 7.05% Mn, 2.97% Al, and the remainder Fe.

[0059] Preparation process:

[0060] (1) The cast steel is obtained by smelting according to the mass percentage of the chemical composition. After smelting, the cast steel is freely forged into two 350mm diameter steel bars. 100 60mm steel billet.

[0061] (2) The steel billet is heated to 1100℃ and held for 2 hours for homogenization treatment. Then the steel billet is hot rolled from 60mm to 5mm after 7 passes. The initial rolling temperature is 1050℃ and the final rolling temperature is 900℃. Finally, it is air-cooled to room temperature to obtain hot-rolled medium manganese steel.

[0062] (3) Place the hot-rolled medium manganese steel in a muffle furnace and heat it at 1... o Heat to 710 C / s o C, and hold at that temperature for 1 hour, then quench in water to room temperature to obtain the steel plate;

[0063] (4) Before cold rolling the steel plate, pickle the heat-treated steel plate with a 1:3 ratio of hydrochloric acid and water;

[0064] (5) Heat the pickled steel plate to 500°C below the two-phase zone temperature and keep it at that temperature 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 rolling twice so that the final rolling amount is 50%.

[0065] Mechanical performance test results, such as Figure 2 As shown, by stabilizing austenite, the yield strength ratio of medium manganese steel is 1, the yield strength is 1235 MPa, and the plasticity is 33.2%, so that medium manganese steel can maintain high plasticity even with high yield strength and high yield strength ratio.

[0066] This invention describes preferred embodiments and their effects. However, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to include both the preferred embodiments and all changes and modifications falling within the scope of this invention.

[0067] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which 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-to-tensile ratio and high plasticity, characterized in that, Includes the following steps: Steel billets for preparing medium manganese steel, The steel billet is kept at 1050~1150℃ for 2~4 hours for homogenization treatment, and then hot-rolled to 5~8mm thick hot-rolled steel plate. Hot-rolled steel sheets are heated to the two-phase region temperature and held for a certain time to allow C and Mn elements to fully disperse in the ferrite and austenite phases. The sheets are then water-quenched to room temperature to obtain dispersed steel sheets. The two-phase region temperature is 690~770°C. o C, the heat preservation time is 0.5~2 hours; After pickling the steel plate, heat it to a temperature below the two-phase region and hold it for 10-20 minutes. Then quickly take it out and roll it multiple times to make the final rolling amount 10%-80%, which yields medium manganese steel with a yield strength of 1GPa and high yield strength ratio and high plasticity. When performing multiple rolling operations, the temperature is reheated to below the two-phase region temperature after every two rolling operations; The temperature below the two-phase region is 300~500℃; When preparing billets for medium manganese steel, the following composition by mass percentage is selected: C: 0.2-0.4%, Mn: 3-12%, Al: 1-3%, P: ≤0.005%, S: ≤0.005%, with the balance being Fe and unavoidable impurities.

2. The method for preparing medium-manganese steel with a yield strength of 1 GPa and high yield-to-tensile ratio and high plasticity according to claim 1, characterized in that, After homogenization treatment, the initial rolling temperature during hot rolling is 1050~1100℃, and the rolling termination temperature is 850~900℃.

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

4. The method for preparing medium-manganese steel with a yield strength of 1 GPa and high yield-to-tensile ratio and high plasticity according to claim 1, characterized in that, The billet of medium manganese steel is made by smelting raw materials according to the mass percentage of chemical composition.

5. A medium-manganese steel with a yield strength of 1 GPa and high yield strength-to-tensile ratio and high plasticity, prepared by the method of any one of claims 1 to 4.

6. The application of a medium manganese steel with a yield strength of 1 GPa and high yield strength-to-tensile ratio and high plasticity as described in claim 5 in automobile manufacturing.

Images