1000 mpa steel plate used for marine engineering and having excellent bauschinger effect resistance and preparation method therefor

Abstract

Disclosed are a 1000 MPa steel plate used for marine engineering and having excellent Bauschinger effect resistance and a preparation method therefor, relating to the technical field of steel material preparation. The 1000 MPa steel plate used for marine engineering and having excellent Bauschinger effect resistance according to the present invention comprises the following components: 0.06-0.14% of C, 0.15-0.4% of Si, 0.9-1.3% of Mn, less than or equal to 0.02% of P, less than or equal to 0.01% of S, 0.02-0.06% of Als, 7.0-12.0% of Ni, 0.3-0.45% of Cr, 0.3-0.65% of Mo, 0.3-1.2% of Cu, 0.03-0.08% of Nb, 0.08-0.18% of V, 0.01-0.025% of Ti, and the balance being Fe and unavoidable impurities. By means of processes such as high-cleanliness and alloying smelting, two-stage rolling, and multi-stage quenching and tempering, the prepared steel plate for marine engineering has a yield strength greater than or equal to 1000 MPa, a tensile strength of 1030-1250 MPa, an elongation rate greater than or equal to 16%, a Charpy impact energy greater than or equal to 120J at -60ºC, and a Bauschinger effect ratio greater than or equal to 0.9 at 2% residual strain, and can completely meet the service requirements of marine engineering equipment.

Description

A 1000MPa marine engineering steel plate with excellent resistance to the Bauschinger effect and its preparation method Technical Field

[0001] This invention belongs to the field of steel material preparation technology, and specifically relates to a 1000MPa marine engineering steel plate with excellent resistance to the Bauschinger effect and its preparation method. Background Technology

[0002] Since the beginning of the 21st century, the shipbuilding and marine engineering field has experienced rapid development. In particular, with the opening of the Arctic shipping routes and the exploration and development of cold-water marine resources, the challenges to related technologies and equipment have become increasingly severe. The Arctic region is rich in energy resources such as oil and natural gas, and its development and utilization have brought new opportunities for transformation to the global energy structure. However, the extreme climatic conditions in this region, such as low temperatures and highly corrosive environments, place more stringent performance requirements on ships and marine engineering equipment, including stronger resistance to the Bauschinger effect, low-temperature resistance, corrosion resistance, and fatigue resistance. Against this backdrop, the research and development of marine engineering steel materials has become particularly crucial. New marine engineering steels must not only withstand the mechanical loads of extreme environments but also possess good deformation resistance, weldability, corrosion resistance, and sufficient low-temperature toughness. Currently, research focuses on microalloying technology, controlled rolling cooling, and heat treatment processes, aiming to improve the deformation resistance, corrosion resistance, and other comprehensive properties of materials through these advanced technologies.

[0003] Offshore engineering platforms face complex marine environmental challenges during service. In addition to conventional mechanical loads, they must withstand natural elements such as wind, waves, tides, ice impacts, and earthquakes. These factors place special demands on material selection, especially in high-salt, humid marine climates where steel plates are susceptible to corrosion and biofouling, leading to performance degradation and shortened lifespan. Therefore, developing high-quality ultra-high-strength steel for marine engineering that can operate stably in extreme marine environments is particularly urgent. This steel needs to possess high strength, ultra-low temperature toughness, resistance to the Bauschinger effect, excellent fatigue resistance, weldability, good corrosion resistance, and resistance to marine biofouling to ensure the long-term stable operation of offshore engineering platforms.

[0004] Currently, existing marine engineering steels can meet most of the market demand in the marine engineering field. However, special steels with lower ductile-brittle transition temperatures and excellent resistance to deformation are still the development goals of countries around the world. The research and development of high-strength steel plates with high service safety is difficult, the production process is strict, the equipment requirements are high, and the development is very challenging.

[0005] The invention patent CN1922337B, entitled "Steel Plates or Pipes with Small Bausching Effect and Their Manufacturing Methods," proposes a low-Bausching effect steel pipe containing only C, Si, Mn, P, and S elements. However, it cannot solve the problem of low-Bausching effect in thick, ultra-high-strength steel plates in terms of chemical composition and production process. The invention patent CN102560254B, entitled "A Manufacturing Method for a Steel Pipe with Low Bausching Effect and Low Strain Aging Effect," proposes a Bausching effect resistant steel pipe with a yield strength of 480-700 MPa, using a Cr+Cu+Nb+V alloy composition. However, it cannot produce thick, ultra-high-strength steel plates with low Bausching effect. Therefore, researching and developing a thick, 1000 MPa-grade ultra-high-strength marine engineering steel plate with Bausching effect resistance suitable for harsh marine engineering environments has become an important research topic urgently needed.

[0006] Summary of the Invention

[0007] Therefore, the purpose of this invention is to provide a 1000MPa marine engineering steel plate with excellent resistance to the Bauschinger effect and its preparation method. The marine engineering steel plate prepared by this invention has a maximum thickness of up to 50mm, an effective grain size of 2-7μm, a yield strength ≥1000MPa, a tensile strength of 1030-1250MPa, an elongation ≥16%, a uniform deformation elongation ≥6%, a Charpy impact energy at -60℃ ≥120J, and a Bauschinger effect ratio ≥0.9 at 2% residual strain. Its mechanical properties and high service safety performance can fully meet the service requirements of marine engineering equipment.

[0008] To achieve the above objectives, the present invention provides the following technical solution:

[0009] This invention provides a 1000MPa marine engineering steel plate with excellent resistance to the Bauschinger effect. By weight percentage, it comprises the following components: C: 0.06%–0.14%, Si: 0.15%–0.4%, Mn: 0.9%–1.3%, P≤0.02%, S≤0.01%, Als: 0.02%–0.06%, Ni: 7.0%–12.0%, Cr: 0.3%–0.45%, Mo: 0.3%–0.65%, Cu: 0.3%–1.2%, Nb: 0.03%–0.08%, V: 0.08%–0.18%, Ti: 0.01%–0.025%, with the remainder being Fe and unavoidable impurities. The Ceq value is 1.0–1.4.

[0010] The mechanisms of action of various chemical elements in steel are as follows:

[0011] Carbon (C) is an important element for improving the strength and hardness of steel plates. Too low a C content will lead to a decrease in C solid solution content and carbide content, resulting in insufficient strength and stiffness of the steel plate, and poor resistance to the Bauschinger effect. Too high a C content will produce excessively hardened structures, reducing the toughness of the steel plate. Therefore, it is crucial to precisely control the C content between 0.06% and 0.14%.

[0012] Si can improve the strength and wear resistance of steel plates. Appropriate addition of Si can improve the rigidity of steel. At the same time, Si can reduce the O content. When the Si content is below 0.15%, the deoxidation effect is not obvious. When the Si content is above 0.4%, it will lead to poor low-temperature toughness. The Si content of this invention is 0.15% to 0.4%.

[0013] Mn can dissolve in large quantities in the Fe matrix, improving the strength of steel plates. When the Mn content is below 0.9%, its contribution to the strength of thick steel plates is relatively small. At the same time, Mn is an element that expands the austenite phase region and improves the stability of austenite. This patent uses clean smelting, which can appropriately increase the Mn content, thereby improving the uniform deformation elongation of the steel plate. When the mass percentage of Mn is greater than 1.3%, segregation will result in poor low-temperature toughness in the core of the thick plate. The Mn content of this invention is 0.9% to 1.3%.

[0014] P and S elements have no benefit to the mechanical properties of steel plates, especially fatigue resistance, and P should be controlled to ≤0.02% and S to ≤0.01%.

[0015] Al is the main deoxidizing element in steel. When the Al content is too low, microalloying elements such as V and Ti will be oxidized and will not be able to refine the grains. On the contrary, if the Al content is too high, large inclusions will be formed. The Al content in this invention is 0.02% to 0.06%.

[0016] The role of Ni is to improve the toughness and hot workability of steel plates. Adding a large amount can achieve a lower ductile-brittle transition temperature and improve the toughness and plasticity of steel plates. At the same time, the addition of Ni can reduce the hot cracking tendency of Cu in steel. Ni has a certain anti-pitting effect. The Ni content in this invention is 7.0% to 12.0%.

[0017] Cr can effectively improve the strength and stiffness of steel plates. Appropriately increasing the Cr content can improve the uniform deformation ability of steel plates. However, excessive Cr content will reduce the impact toughness of steel plates. The Cr content of this invention is 0.3% to 0.45%.

[0018] Mo can improve the hardenability of steel plates. At the same time, Mo can form fine carbides in steel, which can effectively improve the yield strength of steel plates. In addition, adding an appropriate amount of Mo to heat-treated steel plates can improve the toughness of steel plates. The Mo content of this invention is 0.3% to 0.65%.

[0019] Cu is an important element added in this invention. In steel, it can increase the strength of the steel plate. Cu can work synergistically with Ni to lower the ductile-brittle transition temperature of the steel plate. Adding excessive Cu alone can cause hot brittleness, reduce the surface quality of the steel plate, and decrease its low-temperature toughness. However, this invention adds a large amount of Ni, combined with the heated rolling process, which effectively avoids Cu brittleness and allows Cu to play its role. The Cu content in this invention is 0.3%–1.2%.

[0020] Nitrogen (Nb) is the main additive element in this invention, improving both the strength and toughness of the steel plate while enhancing its uniform deformation capacity within the elastic deformation range. During heating, undissolved Nb C and N compound particles are distributed along the austenite grain boundaries, hindering austenite grain growth during heating, refining the grain size, and simultaneously improving fatigue resistance and toughness, thereby enhancing the steel plate's resistance to the Bauschinger effect. During cooling, a large amount of Nb(CN) precipitates, further promoting dislocation entanglement and grain refinement. In this invention, the Nb content is 0.03%–0.08%.

[0021] V can form V(C,N) particles in the matrix, which can refine the grain size. The addition of V can improve the resistance of the steel plate to the Boussinger effect. The V content in this invention is 0.08% to 0.18%.

[0022] The addition of Ti is to form TiN with Ti and N, which prevents grain growth in the billet during heating and rolling, thereby improving the strength and uniform elastic deformation capacity of the steel plate. However, if the Ti content exceeds 0.025%, the toughness of the steel will deteriorate. Therefore, the Ti content in this invention is 0.01% to 0.025%.

[0023] Based on the above technical solution, the thickness of the marine engineering steel plate is 10-50 mm, the effective grain size is 2-7 μm, the metallographic structure is tempered martensite and retained austenite, the proportion of retained austenite is 2-5%, the yield strength is ≥1000 MPa, the tensile strength is 1030-1250 MPa, the elongation is ≥16%, the uniform deformation elongation is ≥6%, the Charpy impact energy at -60℃ is ≥120 J, and the Bauschinger effect ratio is ≥0.9 at 2% residual strain.

[0024] The present invention also provides a method for preparing the above-mentioned 1000MPa marine engineering steel plate with excellent resistance to the Bauschinger effect, comprising the following steps:

[0025] (1) Heating: The billet is loaded into the heating furnace at a furnace temperature of 600-700℃ and held for 60-90 minutes. The purpose is to keep the temperature of the billet in the thickness direction of the extra-thick billet in the low temperature stage, so as to avoid the generation of internal defects in the billet caused by uneven heating of Cr and Cu elements. The temperature is raised to the heating section temperature of 1190-1260℃ and the heating section is held for 2-10 hours. The purpose of uniform heating and holding is to ensure that the C / N compounds of the microalloy are fully dissolved, while avoiding abnormal growth of the as-cast structure and avoiding surface defects caused by high Cu and high Mn content in the billet.

[0026] (2) Rolling: A two-stage controlled rolling process is adopted. The initial rolling temperature of the first stage is 1150-1250℃, the average reduction per pass in the first stage is 10-15mm, and the total reduction rate of the first stage is ≥40%. The purpose of the high-temperature first-stage rolling is to take advantage of the good plasticity of the material at high temperatures. At this time, the core temperature of the billet is high while the surface temperature is low, and the hardening degree is not obvious. Therefore, a large reduction rolling can be carried out. This rolling method helps to destroy the columnar structure of the billet core, increase the deformation of the core structure, and thus improve the microstructure of the material. The cooling process of the intermediate billet is equally important. Water cooling at a cooling rate of 5-8℃ / s can effectively improve the rolling production efficiency of steel plates. In addition, appropriately shortening the cooling time of the intermediate billet can prevent the grains of the billet core from regrow during the cooling process. The regrowth of grains will weaken the effect of grain deformation accumulation in the core of the billet during the first stage rolling, thus affecting the low-temperature impact toughness and fatigue resistance of the steel plate. Therefore, the optimization of the intermediate billet cooling process is crucial to ensuring the performance of the final product.

[0027] The second-stage rolling temperature is 890–1000℃, the average reduction per pass in the second stage is 6–15 mm, the cumulative reduction rate in the second stage is ≥45%, and the final rolling temperature is ≥850℃. The purpose of the second-stage low-temperature rolling is to increase the core deformation of the steel plate by taking advantage of the greater decrease in surface temperature, improve the core grain size of the steel plate, and promote the flattening and refinement of austenite grains. The alloying elements such as Ni, Cu, Nb, and V work together to refine the grains, improve dislocation density, and increase the yield strength and elastic deformation uniformity of the steel plate. The single-pass reduction rate and rolling temperature are the most important for grain refinement and uniform precipitation of C / N compounds.

[0028] (3) Tempering: This includes sub-critical quenching, quenching, and tempering. Sub-critical quenching temperature is 590–760℃, with a holding time of 1–5 min / mm. Quenching temperature is 800–900℃, with a holding time of 1–2 min / mm and a cooling rate of 5–20℃ / s. Tempering temperature is 500–600℃, with a holding time of 150–250 min. The main purpose of sub-critical quenching is to form retained austenite in the steel, which helps improve the toughness of the steel plate under low-temperature conditions. By controlling the heating temperature, sub-critical quenching avoids excessive growth of austenite grains, thus maintaining the integrity of the material's microstructure during quenching, which helps improve the material's toughness and reduce brittleness. High-temperature quenching is to ensure that the microstructure of the steel can be fully transformed into austenite, which is a prerequisite for the formation of martensite during quenching. Rapid cooling at an appropriate temperature can promote the transformation of the microstructure into martensite, thereby improving the hardness and strength of the steel plate. Combining sub-critical and high-temperature quenching processes can better refine grain size. By using grain boundary strengthening instead of dislocation strengthening, the formation of dislocation tangles or cellular structures due to dislocation movement along a slip plane caused by minor plastic deformation under pre-loading is avoided. This dislocation movement, when subjected to a reverse force after unloading, forces the dislocations to move in the opposite direction. Because there are fewer obstacles on the reverse path, dislocations can move a greater distance under lower stress, resulting in a significant Bauschinger effect. This invention uses grain boundary strengthening formed by particularly fine micro-grain structure to replace the conventional dislocation strengthening mechanism, which can significantly reduce the Bauschinger effect in steel plates under stress. Simultaneously, high-temperature quenching must be carried out without causing cracking or deformation of the steel plate to ensure material integrity and processing performance. High-temperature tempering is a crucial step after quenching; by adjusting the tempering temperature and holding time, it can optimize the matrix structure and promote the precipitation of the second phase. This treatment helps reduce the strength and hardness of the steel plate while improving its plasticity and toughness. Sufficient tempering treatment allows the tempering transformation products to complete their transformation as much as possible, thereby improving the overall material properties.

[0029] Based on the above technical solution, further, the heating rate of the heating process in step (1) is controlled at 1.5 to 5℃ / min to avoid the billet being heated too quickly, which would cause excessive internal and external stress and crack defects.

[0030] Based on the above technical solution, further, in step (1), the temperature of the heating section is 1200~1250℃, and the heating section is kept warm for 3~7h.

[0031] Based on the above technical solution, further, in step (2), the first-stage rolling temperature is 1170~1230℃, and the first-stage total reduction rate is controlled at 40~55%.

[0032] Based on the above technical solution, further, in step (2), the second-stage rolling temperature is 900-980℃, the total reduction rate in the second stage is controlled at 45-60%, and the final rolling temperature is 850-930℃.

[0033] Based on the above technical solution, further, in step (3), the sub-temperature quenching temperature is 600~750℃, the sub-temperature quenching holding time is 1~2min / mm, the quenching temperature is 820~880℃, the quenching holding time is 1~1.5min / mm, the quenching cooling rate is 6~15℃ / s, the tempering temperature is 500~580℃, and the tempering holding time is 180~250min.

[0034] Based on the above technical solution, the preparation of the billet in step (1) further includes the following steps: refining the molten steel through a converter, LF furnace, RH or VD furnace to further reduce the content of P, S and non-metallic inclusions, and continuously casting it into a billet, with the continuous casting process being protected during casting.

[0035] The advantages of this invention over the prior art are as follows:

[0036] To address the compositional and performance requirements of ultra-high-strength marine steel in the extremely cold deep-sea environment, this invention utilizes a compositional design that combines C, Si, Mn, Ni, Cr, Mo, Cu with microalloying elements V, Nb, and Ti. Extensive and systematic research was conducted on various aspects, including alloy element screening and proportioning, steel cleanliness control, and optimization of efficient rolling processes. By combining sub-critical and high-temperature quenching processes in the tempering process, the grain refinement effect is significantly improved. The grain boundary strengthening formed by the fine micrograin structure replaces conventional dislocation strengthening. The chemical mechanism can significantly reduce the Bauschinger effect of steel plates under stress. The resulting 1000MPa marine engineering steel plate with a maximum thickness of 50mm exhibits excellent resistance to the Bauschinger effect and possesses superior strength, toughness, and resistance to the Bauschinger effect. The steel plate has a yield strength ≥1000MPa, tensile strength 1030~1250MPa, elongation ≥16%, uniform deformation elongation ≥6%, Charpy impact energy at -60℃ ≥120J, and a Bauschinger effect ratio ≥0.9 at 2% residual strain, showing broad application prospects. Attached Figure Description

[0037] To more clearly illustrate the embodiments of the present invention, the accompanying drawings related to the embodiments will be briefly described below.

[0038] Figure 1 shows the metallographic structure of the marine engineering steel plate prepared in Example 1. Detailed Implementation

[0039] The present invention will be described in detail below with reference to the embodiments. However, the implementation of the present invention is not limited thereto. Obviously, the embodiments described below are only some embodiments of the present invention. For those skilled in the art, other similar embodiments can be obtained without creative effort and all fall within the protection scope of the present invention.

[0040] Examples 1-30

[0041] This embodiment provides a method for preparing a 1000MPa marine engineering steel plate with excellent resistance to the Bauschinger effect. The chemical composition and weight percentage of the 1000MPa marine engineering steel plate are shown in Table 1.

[0042] Table 1. Chemical composition and weight percentage (%) of marine engineering steel plates from Examples 1-30

[0043] Includes the following steps:

[0044] (1) High-purity alloying smelting

[0045] Molten steel is refined in a converter, LF furnace, RH or VD furnace to further reduce the content of P, S and non-metallic inclusions, and then continuously cast into billets. The continuous casting process is protected throughout the casting process.

[0046] (2) Heating

[0047] The billet from step (1) is loaded into the heating furnace at a furnace temperature of 600-695℃ and held for 60-90 minutes. Subsequently, the temperature is increased to the heating section temperature at a heating rate of 1.5-5℃ / min. The heating section temperature is 1200-1250℃ and the heating section holding time is 3-7 hours.

[0048] (3) Rolling

[0049] Two-stage rolling is adopted. The first stage rolling temperature is 1170-1230℃, the average reduction per pass in the first stage is 10-15mm, and the total reduction rate in the first stage is ≥40%. The intermediate billet is water-cooled at a cooling rate of 5-8℃ / s. The second stage rolling temperature is 900-975℃, the average reduction per pass in the second stage is 6-15mm, the cumulative reduction rate in the second stage is ≥45%, and the final rolling temperature is ≥850℃.

[0050] (4) Conditioning

[0051] This includes sub-critical quenching, quenching, and tempering. The sub-critical quenching temperature is 600–750℃, and the sub-critical quenching holding time is 1–2 min / mm. The quenching temperature is 820–880℃, and the quenching holding time is 1–1.5 min / mm. The quenching cooling rate is 6–15℃ / s. The tempering temperature is 500–580℃, and the tempering holding time is 180–250 min.

[0052] Table 2 Heating process parameters for the preparation of marine engineering steel plates in Examples 1-30

[0053] Table 3 Rolling process parameters for the preparation of marine engineering steel plates in Examples 1-30

[0054] Table 4. Tempering process parameters for the preparation of marine engineering steel plates in Examples 1-30

[0055] The mechanical properties of the 1000MPa marine engineering steel plates prepared in Examples 1-30 are shown in Table 5.

[0056] Table 5 Mechanical properties of marine engineering steel plates in Examples 1-30

[0057] The metallographic structure of the marine engineering steel plate prepared in Example 1 is shown in Figure 1. The structure at half the plate thickness consists of tempered martensite and 2-5% retained austenite, with an effective grain size of 2-7 μm, exhibiting good mechanical properties. This invention utilizes grain boundary strengthening formed by exceptionally fine microstructures instead of the conventional dislocation strengthening mechanism, significantly reducing the Bauschinger effect on the steel plate under stress. Simultaneously, the presence of 2-5% retained austenite in the steel effectively mitigates dislocation slip and entanglement caused by the pre-deformation process, reducing the adverse effects of the Bauschinger effect on the steel plate's strength.

[0058] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A 1000 MPa steel plate for marine structures, which is excellent in the Bauschinger effect, characterized in that, By weight percentage, it includes the following components: C: 0.06%–0.14%, Si: 0.15%–0.4%, Mn: 0.9%–1.3%, P≤0.02%, S≤0.01%, Als: 0.02%–0.06%, Ni: 7.0%–12.0%, Cr: 0.3%–0.45%, Mo: 0.3%–0.65%, Cu: 0.3%–1.2%, Nb: 0.03%–0.08%, V: 0.08%–0.18%, Ti: 0.01%–0.025%, with the remainder being Fe and unavoidable impurities, and Ceq being 1.0–1.

4.

2. The 1000 MPa marine steel plate excellent in anti-Bauschinger effect according to claim 1, characterized by, The marine engineering steel plate has a thickness of 10-50 mm, an effective grain size of 2-7 μm, a metallographic structure of tempered martensite and retained austenite, a retained austenite ratio of 2-5%, a yield strength ≥1000 MPa, a tensile strength of 1030-1250 MPa, an elongation ≥16%, a uniform deformation elongation ≥6%, a Charpy impact energy at -60℃ ≥120 J, and a Bauschinger effect ratio ≥0.9 at 2% residual strain.

3. The method of producing a 1000 MPa marine steel plate excellent in the Bauschinger effect according to claim 1 or 2, characterized by, Includes the following steps: (1) Heating: The billet is loaded into the heating furnace at a furnace temperature of 600-700℃, held for 60-90 minutes, and then heated to the heating section temperature of 1190-1260℃. The heating section holding time is 2-10 hours. (2) Rolling: A two-stage controlled rolling process is adopted. The initial rolling temperature of the first stage is 1150~1250℃, the average reduction per single pass of the first stage is 10~15mm, the total reduction rate of the first stage is ≥40%, and the intermediate billet is controlled to be water-cooled at a cooling rate of 5-8℃ / s. The initial rolling temperature for the second stage is 890–1000℃, the average reduction per single pass in the second stage is 6–15 mm, the cumulative reduction rate in the second stage is ≥45%, and the final rolling temperature is ≥850℃. (3) Tempering: including sub-temperature quenching, quenching and tempering. The sub-temperature quenching temperature is 590~760℃, the sub-temperature quenching holding time is 1~5min / mm, the quenching temperature is 800~900℃, the quenching holding time is 1~2min / mm, the quenching cooling rate is 5~20℃ / s, the tempering temperature is 500~600℃, and the tempering holding time is 150~250min.

4. The production method according to claim 3, characterized by, The heating rate in step (1) is controlled at 1.5 to 5 °C / min.

5. The preparation method according to claim 3, characterized in that, In step (1), the temperature of the heating section is 1200-1250℃, and the heating section is kept warm for 3-7 hours.

6. The preparation method according to claim 3, characterized in that, In step (2), the initial rolling temperature is 1170-1230℃, and the total reduction rate in the first stage is controlled at 40-55%.

7. The preparation method according to claim 3, characterized in that, In step (2), the initial rolling temperature of the second stage is 900-980℃, the total reduction rate of the second stage is controlled at 45-60%, and the final rolling temperature is 850-930℃.

8. The preparation method according to claim 3, characterized in that, In step (3), the sub-temperature quenching temperature is 600-750℃, the sub-temperature quenching holding time is 1-2 min / mm, the quenching temperature is 820-880℃, the quenching holding time is 1-1.5 min / mm, the quenching cooling rate is 6-15℃ / s, the tempering temperature is 500-580℃, and the tempering holding time is 180-250 min.

9. The preparation method according to claim 3, characterized in that, The preparation of the cast slab in step (1) includes the following steps: refining the molten steel through a converter, an LF furnace, an RH or VD furnace, continuous casting into a cast slab, and full-continuous- casting protection casting.

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