1000MPa-grade low-density medium manganese steel and preparation method thereof

Abstract

The invention relates to 1000 MPa-grade low-density medium manganese steel and a preparation method thereof, and belongs to the technical field of ferrous metallurgy and metal material machining, and the low-density medium manganese steel is composed of the following chemical components of, by mass, 0.80%-0.95% of C, 7.50%-9.30% of Al, 8.35%-12.87% of Mn, 0.55%-0.62% of Si, 0.001%-0.003% of B, smaller than or equal to 0.008% of P, smaller than or equal to 0.0012% of S and the balance Fe and inevitable impurities. The Fe-Mn-Al-C series low-density medium manganese steel serves as the basis, the process is optimized, the low-density medium manganese steel with excellent comprehensive performance is prepared, the production cost is low, the preparation process is short, the preparation method is simple and easy to implement, the obtained low-density medium manganese steel has good ductility, the tensile strength can reach 1000 MP, the low-density medium manganese steel is suitable for manufacturing of automobile parts, and the production cost is low. The application prospect is further widened.

Description

Technical Field

[0001] The present invention relates to a 1000 MPa-grade low-density medium manganese steel and a preparation method thereof, belonging to the technical field of steel metallurgy and metal material processing. Background Art

[0002] With the continuous improvement of global energy conservation and environmental protection requirements and the promotion of economic benefits, lightweighting of vehicles has become an urgent need of today's automotive industry and a key trend for future development. The main ways to achieve lightweighting of vehicles include adopting lightweight raw materials, using ultra-high-strength steel, and developing new high-strength, low-density medium-manganese steel. Common lightweight raw materials (such as aluminum alloys, magnesium alloys, engineering plastics, and carbon fiber composites) have limitations such as complex molding processes, high production costs, limited production scale, and insufficient overall performance. Therefore, developing a steel grade that combines high strength, high elongation, and low density has become an effective way to solve the above problems. At present, new steels represented by Fe-Mn-Al-C high-strength, low-density medium-manganese steel are gradually becoming the ideal choice for lightweighting vehicles due to their excellent mechanical properties and low density.

[0003] The preparation method of traditional low-density high-strength steel usually adopts a complex and multi-step smelting, processing and multi-step heat treatment process, and adds expensive alloy elements (such as Mo, Ni, Cr, etc.), so as to prepare a kind of steel with high strength, high elongation and low density. The invention patent CN108018494B provides an annealing process for improving the strength and plasticity of low-density steel and a production method of low-density steel, and the alloy composition percentage is: Mn 19.40-20.00%; Al 9.82-10.00%; C 0.98-1.00%; P≤0.003%; S≤0.003%, the rest is Fe and inevitable impurities, after smelting, die casting, hot rolling, solid solution treatment, cold rolling, annealing treatment (two austenitizing treatments) and aging treatment, a low-density steel with yield strength of 920 MPa, tensile strength of 1080 MPa and elongation of 48% after fracture is obtained, the process has many heat treatment steps and is complex; the invention patent CN113278896B provides a Fe-Mn-Al-C high-strength low-density steel and a preparation method thereof, and the alloy composition percentage is: C 1.0-2.0%, Mn 20-40%, Al 10-14%, 2%≤Ni+Cr≤15%, to further improve the performance of the steel, Mo 0.10-1.00%, Si 0.20-2.00%, Cu 0.50-2.50%, B 0.001-0.005%, Nb 0.05-0.50%, Ti 0.02-0.50%, V 0.02-0.35%, RE 0.001-0.005%, Ca 0.005-0.025%, the rest is Fe, after smelting, solidification casting, hot working (forging / rolling) and solid solution treatment, a low-density steel with a tensile strength of 1000 MPa is obtained, although the above invention has a simple process, but many valuable alloy elements (Ni, Cr, Mo, RE, Nb, etc.) are used in the alloy composition design, which greatly increases the product cost. The invention patent CN116065081B provides a 1000 MPa grade low-density steel plate and a preparation method thereof, and the alloy composition percentage is: C 0.31%-0.34%; Mn 12.1%-12.5%; Al 8.51%-8.55%; Ni 10.0%-15.0%; P 0.005%-0.008%; S 0.001%-0.002%; H 0.0004%-0.0005%; O 0.001%-0.008%; N 0.003%-0.005%, the rest is Fe and inevitable impurity elements, after smelting and casting, high-temperature homogenization, finishing, forging, solid solution, quenching and aging treatment, a low-density steel with a tensile strength of 1000 MPa is obtained, the process of the above invention has many heat treatment steps and is complex, and the alloy composition contains the high-cost alloy element Ni.Therefore, it is necessary to further optimize the alloy composition and preparation process to reduce production costs and improve the comprehensive performance of the material to meet the diverse needs in practical applications. Summary of the Invention

[0004] To address the aforementioned issues in the prior art, the present invention provides a 1000 MPa-grade low-density medium-manganese steel and a method for preparing the same. Based on an Fe-Mn-Al-C-based low-density medium-manganese steel, the present invention selects the economically efficient alloying elements B and Si. By adjusting the alloy composition, processing technology, and quenching parameters, a high-strength, high-elongation, low-density medium-manganese steel is produced. This steel features low production costs, a short preparation process, and a simple and easy preparation method. The steel is particularly suitable for the manufacture of automotive parts, exhibiting excellent economic efficiency and application prospects.

[0005] A 1000 MPa grade low-density medium manganese steel comprises the following chemical components in percentage by mass: C 0.80%-0.95%, Al 7.50%-9.30%, Mn 8.35%-12.87%, Si 0.55%-0.62%, B 0.001%-0.003%, P≤0.008%, S≤0.0012%, and the balance being Fe and unavoidable impurities.

[0006] Furthermore, the microstructure of the low-density medium manganese steel is a three-dimensional interpenetrating network structure composed of ferrite and austenite, wherein the mass percentage of the austenite is 60% to 75%, and the mass percentage of the ferrite is 20% to 30%.

[0007] Furthermore, the yield strength of the low-density medium manganese steel is 810-864 MPa, the tensile strength is 1000-1038 MPa, the elongation after fracture is 25%-33%, and the density is 6.62-6.96 g / cm 3 .

[0008] The main functions of the chemical components in the 1000 MPa grade low-density medium manganese steel of the present invention are: C: C added to low-density steel is one of the most important alloying elements in steel. It mainly improves the strength of steel through solid solution strengthening. The appropriate addition of C helps to increase the hardenability of steel and improve the strengthening effect during heat treatment.

[0009] Mn: The addition of Mn can improve the strength of steel through solid solution strengthening, promote the stability of the austenite phase, help achieve excellent ductility and plasticity, and also promote the high-temperature oxidation resistance and thermal stability of steel.

[0010] Al: As a low-density element, Al is a key alloying element for achieving lightweight steel. Its addition not only helps to significantly reduce the density of the material, but also effectively increases the stacking fault energy of austenite, thereby inhibiting the martensitic phase transformation, allowing the steel to stably maintain the austenitic structure after rapid cooling to room temperature, thereby improving the plasticity of the material; in addition, Al helps to enhance high-temperature structural stability, inhibit carbide precipitation, improve corrosion resistance, and to a certain extent has a positive effect on grain refinement, while also improving the strength of the steel.

[0011] Si: Si can promote the formation of ferrite in low-density medium manganese steel, enhance the steel's oxidation resistance, and improve the steel's thermal stability. An appropriate amount of Si can accelerate the formation kinetics of κ-carbide and increase the distribution coefficient of C, promoting the enrichment of C into κ-carbide, causing local dislocation slip and dynamic strain aging during deformation, thereby enhancing the strengthening effect and improving the steel's wear resistance and high-temperature resistance.

[0012] B: Trace amounts of B can significantly improve the hardenability of steel and optimize its heat treatment properties. B can inhibit the growth of initial austenite grains through grain boundary segregation and precipitation of boron carbides, refine the microstructure and increase the stability of austenite, thereby improving the strength and hardness of steel. The addition of B can regulate the distribution of κ-carbides, thereby reducing the tendency of edge cracking during hot rolling. In addition, B promotes the distribution of carbon from martensite to austenite and ferrite during heat treatment, improving microstructural uniformity and reducing stress concentration, thereby improving fracture toughness.

[0013] P and S: P and S are harmful impurity elements in steel, which will seriously affect the strength, toughness and corrosion resistance of the steel. Their content must be strictly controlled during the smelting process and reduced to a minimum. Therefore, the present invention controls the P content to be below 0.008% and the S content to be below 0.0012%.

[0014] Another object of the present invention is to provide a preparation method for the above-mentioned 1000 MPa grade low-density medium manganese steel, comprising batching, vacuum smelting and casting, controlled rolling and controlled cooling, and quenching treatment processes, wherein the controlled rolling and controlled cooling process comprises heat treatment, rough rolling, finish rolling, and cooling steps, and the quenching treatment process comprises treating the rolled steel billet at 780-820°C for 20 minutes and then rapidly water-cooling it to room temperature.

[0015] In the above technical solution, in the vacuum smelting and casting process, the prepared raw materials are heated and melted to obtain liquid metal, which is poured into a casting mold and cooled to obtain an ingot with a thickness of 120-180 mm.

[0016] In the above technical solution, in the heat treatment step in the controlled rolling and controlled cooling process, the ingot is heated in a soaking furnace with a heating rate of 10°C / min, a holding time in the furnace of 0.5 min / mm, and a tapping temperature of 1100~1150°C.

[0017] In the above technical solution, in the rough rolling step of the controlled rolling and controlled cooling process, the ingot after heat treatment is rough rolled using a large reduction method, the starting rolling temperature is 1050~1100℃, and after 5 rolling passes, the reduction rate of the last 3 passes is ≥20%, and the cumulative compression ratio is 60%.

[0018] Furthermore, the thickness of the ingot obtained after the rough rolling is 50-70 mm.

[0019] In the above technical solution, in the finishing rolling step in the controlled rolling and controlled cooling process, the ingot obtained after rough rolling is finish rolled, the starting rolling temperature is 880~950℃, the final rolling temperature is 780~810℃, and after 12 rolling passes, the reduction rate of each pass is 11%~20%, the cumulative reduction rate is 85%~91%, and the starting cooling temperature after the finishing rolling is 750~810℃.

[0020] In the above technical solution, in the cooling step of the controlled rolling and controlled cooling process, the ingot obtained after finish rolling is cooled to room temperature by laminar cooling to obtain a rolled steel billet with a thickness of 5 to 10 mm.

[0021] The 1000 MPa-grade low-density medium-manganese steel designed by the present invention avoids the use of high-cost precious metal elements by rationally optimizing the alloy composition design, selects the more economical alloying elements B and Si, and combines it with a simplified production process (smelting and casting, controlled rolling and cooling, and quenching treatment). While ensuring the excellent comprehensive performance of material strength, plasticity, and density, it effectively reduces production costs and process complexity, showing good economic value and application prospects.

[0022] Beneficial effects of the present invention: In terms of alloy composition design, the present invention takes Fe-Mn-Al-C low-density medium manganese steel as the basis, uses the basic design concept of low-cost alloying elements such as B and Si, and sequentially performs smelting and casting, controlled rolling and controlled cooling, and a single quenching treatment to prepare low-density medium manganese steel with excellent comprehensive performance; the present invention has low production cost, a short preparation process, and a simple and easy preparation method. The obtained low-density medium manganese steel can achieve a tensile strength of 1000 MP while ensuring good elongation, and has the advantages of both high strength and good toughness.

[0023] The present invention successfully prepares a dual-phase lightweight steel with gradient strengthening phase distribution characteristics through a specific process. After the quenching process, the present invention forms a three-dimensional interpenetrating network structure with recrystallized austenite as a continuous matrix and multi-scale ferrite as a reinforcement phase. The δ-ferrite bands directionally distributed along the rolling direction improve the load distribution efficiency through the interface stress transfer mechanism; the granular α-ferrite dispersed at the austenite grain boundary inhibits grain coarsening through the Zener pinning effect; a large number of small-sized κ-carbides are uniformly precipitated to form high-density strengthening phase particles. The synergistic effect of this multi-scale microstructure realizes the superposition of fine grain strengthening and precipitation strengthening, so that the medium manganese steel material obtained by the present invention has high strength while maintaining elongation.

[0024] The alloy elements used in the present invention have low cost and simple preparation process; the obtained product has stable performance and good industrial production potential, and is easy to be widely applied and promoted. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The figure is a flow chart for preparing the low-density medium manganese steel obtained in the present invention.

[0026] Figure 2 This is a stress-strain curve diagram of the steel in Example 4 of the present invention.

[0027] Figure 3 This is a microstructure morphology diagram of the steel in Example 4 of the present invention. DETAILED DESCRIPTION

[0028] The following non-limiting examples may enable those skilled in the art to more fully understand the present invention, but are not intended to limit the present invention in any way.

[0029] Unless otherwise specified, the experimental methods described in the following examples are conventional methods; the reagents and materials described are all commercially available unless otherwise specified.

[0030] One of the specific implementation methods: A method for preparing 1000 MPa grade low-density medium manganese steel specifically comprises the following steps: (1) Ingredients: Weigh the raw materials according to the following mass percentages: C 0.80%~0.95%, Al 7.50%~9.30%, Mn 8.35%~12.87%, Si 0.55%~0.62%, B 0.001%~0.003%, P≤0.008%, S≤0.0012%, and the balance is Fe and unavoidable impurities; (2) Vacuum smelting and casting: the raw materials prepared in step (1) are placed in a vacuum smelting furnace for heating and melting, the smelted metal liquid is poured into a casting mold, and cooled to obtain a low-density medium-manganese steel ingot with a thickness of 120-180 mm; (3) Controlled rolling and controlled cooling: the ingot obtained in step (2) is heated in a soaking furnace at a heating rate of 10°C / min, the holding time in the furnace is 0.5 min / mm, and the tapping temperature is 1100~1150°C; then a two-stage rolling process of rough rolling and finishing rolling is adopted, the starting rolling temperature of rough rolling is 1050~1100°C, and after 5 rolling passes, the reduction rate of the last three passes is ≥20%, and large reduction rolling is adopted, and the cumulative reduction of rough rolling is 58~63%; the starting rolling temperature of the finishing rolling stage is 880~950°C, the final rolling temperature is 780~810°C, and after 12 rolling passes, the reduction rate of each pass is 11%~20%, and the cumulative reduction rate is 85~91%; the starting cooling temperature after the finishing rolling is 750~810°C, and after rolling, laminar cooling is adopted to cool to room temperature to obtain a rolled steel billet with a thickness of 5~10 mm; (4) Quenching treatment: The rolled steel billet is heated in a high-temperature furnace at a temperature of 780-820°C for 20 min, and then rapidly cooled to room temperature with water to obtain a 1000 MPa grade low-density medium manganese steel product with a thickness of 5-10 mm.

[0031] In the method of the present invention, in step (3), the thickness of the ingot obtained after rough rolling is 50-70 mm.

[0032] In the method of the present invention, the yield strength of the 1000 MPa grade low-density medium manganese steel product is 810-864 MPa, the tensile strength is 1000-1038 MPa, the elongation after fracture is 25%-33%, and the density is 6.62-6.96 g / cm 3 .

[0033] Example 1 A method for preparing 1000 MPa-grade low-density medium manganese steel with a thickness of 5 mm, comprising the following steps: (1) Ingredients: Prepare the ingredients according to the chemical composition of Example 1 given in Table 1; (2) Smelting and casting: The prepared materials are placed in a vacuum smelting furnace for heating and melting, and the smelted metal liquid is poured into a casting mold. After cooling, a low-density medium-manganese steel ingot with a thickness of 120 mm is obtained; (3) Controlled rolling and controlled cooling: the 120 mm thick ingot obtained above is heated in a soaking furnace, the furnace holding time is 60 min, and the tapping temperature is 1100°C; then rough rolling and finish rolling are carried out, the rough rolling stage opening rolling temperature is 1050°C, and after 5 passes of rolling, the cumulative compression ratio is 58%, and the obtained intermediate blank thickness is 50.4 mm; the finish rolling stage opening rolling temperature is 880°C, and after 12 passes of rolling, the finish rolling temperature is 780°C, and the finish rolling cumulative reduction is 90.1%; the starting cooling temperature after finish rolling is 750°C-780°C, and after rolling, laminar flow cooling is used to cool to room temperature, and the rolling state steel blank with a thickness of 5 mm is obtained; (4) Quenching treatment: the rolling state steel blank with a thickness of 5 mm obtained above is heated in a high temperature furnace, the temperature of the heat treatment is 780°C, the time of the heat treatment is 20 min, and then rapid water cooling to room temperature is carried out, and a 1000 MPa grade low density medium manganese steel with a thickness of 5 mm is obtained.

[0034] Examples 2-5 Examples 2-5 are different from Example 1 in that the mass percentage content of the chemical composition in step (1) and the process parameters in steps (2)-(4) are different, and the specific mass percentage content is shown in Table 1 and the process parameters are shown in Table 2.

[0035] Examples 6-7 A method for preparing a 1000 MPa grade low density medium manganese steel with a thickness of 8 mm and 10 mm, specifically comprising the following steps: (1) batching: batching according to the chemical composition of Examples 6 and 7 given in Table 1; (2) smelting and casting: placing the prepared materials into a vacuum smelting furnace respectively for heating and melting, pouring the obtained molten metal into a casting mold respectively, and obtaining low density medium manganese steel ingots with thicknesses of 160 mm and 180 mm after cooling treatment; (3) controlled rolling and controlled cooling: heating the 160 mm and 180 mm thick ingots in a soaking furnace respectively, the furnace holding time is 60 min and 90 min respectively, and the tapping temperature is 1138°C and 1150°C respectively; then rough rolling and finish rolling are carried out, the rough rolling stage opening rolling temperature is 1085°C and 1100°C respectively, and after 5 passes of rolling, the cumulative reduction is 61.5% and 62.1% respectively, and the obtained intermediate blank thickness is 61.6 mm and 68.6 mm respectively, the finish rolling stage opening rolling temperature is 900°C and 950°C, and after 12 passes of rolling, the cumulative reduction is 87% and 85.4% respectively, and the finish rolling temperature is 790°C and 810°C respectively; the starting cooling temperature after finish rolling is 780°C and 810°C respectively, and after rolling, laminar flow cooling is used to cool to room temperature, and rolling state steel blanks with thicknesses of 8 mm and 10 mm are obtained respectively; (4) Quenching treatment: The rolled steel billets with thicknesses of 8 mm and 10 mm were heated in a high-temperature furnace at temperatures of 800°C and 820°C for 20 min, respectively. The billets were then rapidly cooled to room temperature with water, thereby obtaining 1000 MPa grade low-density medium manganese steel with thicknesses of 8 mm and 10 mm, respectively.

[0036] Comparative Example 1 The main difference between this comparative example and Example 1 is that the quenching temperature in this comparative example is 750°C.

[0037] Comparative Example 2 The main difference between this comparative example and Example 1 is that the quenching temperature in this comparative example is 850°C.

[0038] Comparative Example 3 The main difference between this comparative example and Example 1 is that the content of alloying element Si in this comparative example is 0.30%.

[0039] Comparative Example 4 The main difference between this comparative example and Example 1 is that the content of alloying element C in this comparative example is 0.50%.

[0040] The chemical composition of the steels obtained in Examples 1 to 7 and Comparative Examples 1 to 4 of the present invention is shown in Table 1, and the main process parameters are shown in Table 2. The performance tests were carried out with reference to GB / T 228.1-2021. The yield strength, tensile strength, and elongation after fracture of the steels obtained above were tested, and the mechanical property results are shown in Table 3.

[0041] Table 1 Chemical composition of Examples and Comparative Examples (mass percentage)

[0042] Table 2 Process parameters of Examples and Comparative Examples

[0043] Table 3 Performance parameters of the medium manganese steel obtained in the examples and comparative examples

[0044] Figure 2 This is the stress-strain curve of the steel in Example 4 of the present invention. It can be seen that the steel obtained in Example 4 has a good match between strength and toughness, with a yield strength of 855 MPa, a tensile strength of 1027 MPa, and an elongation after fracture of 25.2%. This shows that the material has a certain plasticity while maintaining a high strength, showing excellent comprehensive mechanical properties.

[0045] Figure 3The microstructure morphology of the steel in Example 4 of the present invention is shown in the figure. It can be seen that the microstructure of the steel obtained in Example 4 is mainly composed of a continuous matrix of recrystallized austenite, with multi-scale ferrite distributed within the phase to form an interpenetrating network structure. Among them, the banded δ-ferrite is oriented along the rolling direction, which enhances the isotropic strengthening characteristics of the structure; the granular α-ferrite is dispersed at the austenite grain boundaries, effectively pinning the austenite grains, inhibiting the growth of recrystallized grains, and improving the stability of the structure. In addition, a large number of fine κ-carbides are uniformly precipitated in the austenite matrix, forming a high-density strengthening particle, which significantly improves the overall strength level of the material.

[0046] This complex phase microstructure system composed of recrystallized austenite, δ / α-ferrite and κ-carbide achieves the coordinated regulation of grain refinement, uniform distribution of strengthening phase and stress-coordinated deformation, ensuring high strength while taking into account good ductility.

[0047] By comparing Example 1 with Comparative Examples 1 and 2, it can be seen that the reasonable control of quenching temperature has a decisive influence on the mechanical properties of the experimental steel. When the quenching temperature is too high, the austenite grains grow significantly, the number of grain boundaries decreases, and the proportion of high-angle grain boundaries increases. Although the elongation of the material can be improved to a certain extent, the yield strength and tensile strength caused by grain coarsening are significantly reduced, thereby deteriorating the overall mechanical properties. Conversely, when the quenching temperature is too low, austenitization is insufficient, resulting in a high content of residual ferrite in the matrix and the presence of a large amount of undissolved carbides. The synergistic effect of the two will cause a double reduction in material strength and plasticity, which is not conducive to performance optimization.

[0048] A comparison of Example 1 and Comparative Example 3 shows that reducing the Si content also has an adverse effect on microstructural evolution and performance. As a typical ferrite-forming element, reducing Si content inhibits ferrite formation, thereby weakening the thermal stability of the experimental steel. Furthermore, reducing Si also affects the precipitation behavior of κ-carbides, slowing their precipitation kinetics and causing carbides to coarsen during high-temperature dwelling. This is detrimental to the refinement of precipitation strengthening effects, ultimately negatively impacting mechanical properties.

[0049] A comparison of Example 1 and Comparative Example 4 shows that reducing C, a strong austenite-stabilizing element, weakens the thermal stability of austenite, resulting in a decrease in austenite content and an increase in the proportion of δ-ferrite, thereby reducing material plasticity. Furthermore, reducing C content inhibits the precipitation of κ-carbides, weakening the precipitation strengthening mechanism and resulting in a decrease in material strength. Furthermore, a low C content is also detrimental to the deformation-induced martensitic transformation (TRIP effect), reducing strain hardening capacity and ductility.

Claims

1. A 1000 MPa grade low-density medium manganese steel, characterized by: The low-density medium manganese steel is composed of the following chemical components in percentage by mass: C 0.80%-0.95%, Al 7.50%-9.30%, Mn 8.35%-12.87%, Si 0.55%-0.62%, B 0.001%-0.003%, P≤0.008%, S≤0.0012%, and the balance is Fe and unavoidable impurities.

2. The 1000 MPa grade low-density medium manganese steel according to claim 1, characterized in that: The microstructure of the low-density medium manganese steel is a three-dimensional interpenetrating network structure composed of ferrite and austenite, wherein the mass percentage of the austenite is 60% to 75%, and the mass percentage of the ferrite is 20% to 30%.

3. The 1000 MPa grade low-density medium manganese steel according to claim 1, characterized in that: The low-density medium manganese steel has a yield strength of 810-864 MPa, a tensile strength of 1000-1038 MPa, an elongation after fracture of 25%-33%, and a density of 6.62-6.96 g / cm 3 .

4. The method for preparing the 1000 MPa grade low-density medium manganese steel according to any one of claims 1 to 3, characterized in that: The preparation method includes batching, vacuum smelting and casting, controlled rolling and controlled cooling, and quenching treatment processes. The controlled rolling and controlled cooling process includes heat treatment, rough rolling, finish rolling, and cooling steps. The quenching treatment process is to treat the rolled steel billet at 780-820°C for 20 minutes and then quickly water-cool it to room temperature.

5. The preparation method according to claim 4, characterized in that: In the vacuum smelting and casting process, the prepared raw materials are heated and melted to obtain liquid metal, which is poured into a casting mold and cooled to obtain an ingot with a thickness of 120-180 mm.

6. The preparation method according to claim 4, characterized in that: In the heat treatment step of the controlled rolling and controlled cooling process, the ingot is heated in a soaking furnace at a heating rate of 10°C / min, a holding time in the furnace of 0.5 min / mm, and a tapping temperature of 1100-1150°C.

7. The preparation method according to claim 4, characterized in that: In the rough rolling step of the controlled rolling and controlled cooling process, the heat-treated ingot is rough rolled using a large reduction method, the starting rolling temperature is 1050-1100°C, and the rolling is repeated for 5 passes, with the reduction rate of the last 3 passes being ≥20%, and the cumulative reduction ratio is 60%.

8. The preparation method according to claim 7, characterized in that: The thickness of the ingot obtained after the rough rolling is 50-70 mm.

9. The preparation method according to claim 4, characterized in that: In the finishing rolling step of the controlled rolling and controlled cooling process, the ingot obtained after rough rolling is finished rolling, the starting rolling temperature is 880~950℃, the final rolling temperature is 780~810℃, and after 12 rolling passes, the reduction rate of each pass is 11%~20%, the cumulative reduction rate is 85%~91%, and the starting cooling temperature after the finishing rolling is 750~810℃.

10. The preparation method according to claim 4, characterized in that: In the cooling step of the controlled rolling and controlled cooling process, the ingot obtained after finish rolling is cooled to room temperature by laminar cooling to obtain a rolled steel billet with a thickness of 5 to 10 mm.

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