1000MPa maritime work steel plate with excellent Bauschinger effect resistance and preparation method thereof

Abstract

The invention discloses a 1000MPa maritime work steel plate with excellent Bauschinger effect resistance and a preparation method of the 1000MPa maritime work steel plate, and belongs to the technical field of steel and iron material preparation. The 1000MPa maritime work steel plate with the excellent Bauschinger effect 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 of Fe and inevitable impurities. According to the prepared steel plate for marine engineering, the yield strength is larger than or equal to 1000 MPa, the tensile strength is 1030-1250 MPa, the ductility is larger than or equal to 16%, the Charpy impact energy at the temperature of-60 DEG C is larger than or equal to 120 J, the Bauschinger effect ratio is larger than or equal to 0.9 when the residual strain is 2%, and the service requirement of marine engineering equipment can be completely met.

Description

Technical Field

[0001] The invention belongs to the technical field of steel material preparation, and particularly relates to a 1000MPa marine engineering steel plate with excellent resistance to Bauschinger effect and a preparation method thereof. Background Art

[0002] Since the beginning of the 21st century, the field of shipbuilding and marine engineering has ushered in rapid development. In particular, with the opening of the Arctic route and the exploration and development of cold marine resources, the challenges to related technologies and equipment have become increasingly severe. The Arctic region is rich in energy such as oil and natural gas, and its development and utilization has brought new opportunities for change in the global energy structure. However, the extreme climatic conditions in this region, such as low temperature and highly corrosive environment, have put forward more stringent performance requirements for ships and marine engineering equipment, including the need to have stronger resistance to the Bauschinger effect, low temperature resistance, corrosion resistance, fatigue resistance and other characteristics. In this context, the research and development of marine steel materials has become particularly critical. New marine steels must not only be able to withstand the influence of mechanical loads in extreme environments, but also have good deformation resistance, welding performance, corrosion resistance and sufficient low-temperature toughness. At present, 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] During their service, offshore engineering platforms face complex challenges in the marine environment. In addition to conventional mechanical loads, they also need to withstand the test of natural factors such as wind and waves, tides, ice impact, and earthquakes. These factors put forward special requirements for the selection of materials, especially in high-salt and humid marine climates, where steel plates are susceptible to corrosion and biological adhesion, resulting in performance degradation and shortened life. Therefore, it is particularly urgent to develop high-quality ultra-high-strength steel for marine engineering that can work stably in extreme marine environments. This kind of steel needs to have high strength, ultra-low temperature toughness, resistance to the Bauschinger effect, excellent fatigue resistance, easy weldability, good corrosion resistance, and resistance to marine biological adhesion to ensure the long-term stable operation of offshore engineering platforms.

[0004] At present, existing marine engineering steels can meet most of the market demands in the marine engineering field, but special steels with lower ductile-brittle transition temperature and excellent deformation resistance 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 difficult.

[0005] The invention patent with publication number CN1922337B, "Steel plate or steel pipe with small manifestation of Bauschinger effect and its manufacturing method", proposes a low Bauschinger effect steel pipe containing only C, Si, Mn, P, and S elements. In terms of chemical composition and production process, it cannot solve the low Bauschinger effect problem of thick ultra-high strength steel plates. The invention patent with publication number CN102560254B, "A manufacturing method of low Bauschinger effect and low strain aging effect steel pipe", proposes a Bauschinger effect resistant steel pipe with a yield strength of 480-700Mpa, which uses Cr+Cu+Nb+V alloy composition and cannot produce thick ultra-high strength low Bauschinger effect steel plates. Therefore, the research and development of a thick 1000MPa ultra-high strength marine engineering steel plate with anti-Bauschinger effect suitable for harsh environment marine engineering has become an important topic that needs to be studied urgently. Summary of the invention

[0006] In view of this, the purpose of the present invention is to provide a 1000MPa marine engineering steel plate with excellent resistance to Bauschinger effect and a preparation method thereof. The marine engineering steel plate prepared by the present 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 -60℃ Charpy impact energy ≥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.

[0007] In order to achieve the above object, the present invention provides the following technical solutions:

[0008] The invention provides a 1000MPa marine engineering steel plate with excellent anti-Bauschinger effect. The steel plate comprises the following components by weight percentage: 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%, and the rest are Fe and unavoidable impurities, and Ceq is 1.0-1.4.

[0009] The action mechanism of each chemical element in steel is as follows:

[0010] C is an important element in steel to improve the strength and hardness of steel plates. If the C content is too low, the C solid solution content and carbide content will be reduced, the strength and rigidity of the steel plate will be insufficient, and the ability to resist the Bauschinger effect will be poor. If the C content is too high, excessive hardening structure will be produced, and the toughness of the steel plate will be reduced, so the C content is precisely controlled at 0.06% to 0.14%.

[0011] Si can improve the strength and wear resistance of the steel plate. Appropriate addition of Si elements can improve the rigidity of the steel. At the same time, Si can reduce the O content. When the Si content is lower than 0.15%, the deoxidation effect is not obvious. When the Si content is higher than 0.4%, it will lead to poor low-temperature toughness. The Si content of the present invention is 0.15% to 0.4%.

[0012] The Mn element can be dissolved in the Fe matrix in large quantities to improve the strength of the steel plate. When the Mn content is lower than 0.9%, it contributes less to the strength of the thick steel plate. At the same time, Mn is an element that expands the austenite phase region and improves the stability of austenite. This patent adopts clean smelting to appropriately increase the Mn element content, which plays a role in improving the uniform deformation elongation of the steel plate. When the mass percentage of the Mn element is greater than 1.3%, segregation will make the low-temperature toughness of the core of the thick plate poor. The Mn content of the present invention is 0.9% to 1.3%.

[0013] P and S elements have no benefit to the mechanical properties of steel plates, especially fatigue resistance, and P should be controlled to be ≤ 0.02% and S ≤ 0.01%.

[0014] Al is the main deoxidizing element in steel. When the Al content is too low, microalloying elements such as V and Ti cannot achieve the purpose of grain refinement due to oxidation. On the contrary, when the Al content is too high, large inclusions are formed. The Als content of the present invention is 0.02% to 0.06%.

[0015] The role of Ni is to improve the toughness and hot workability of the steel plate. A large amount of Ni addition can obtain a lower tough-to-brittle transition temperature and improve the toughness and plasticity of the steel plate. At the same time, the addition of Ni can improve the hot cracking tendency of Cu in steel. Ni has a certain anti-pitting effect. The Ni content of the present invention is 7.0% to 12.0%.

[0016] Cr element in steel can effectively improve the strength and rigidity of steel plate; appropriately increasing the Cr content can improve the uniform deformation ability of steel plate, but too high Cr content will reduce the impact toughness of steel plate. The Cr content of the present invention is 0.3% to 0.45%.

[0017] The Mo element can improve the hardenability of the steel plate. At the same time, the Mo element can form fine carbides in the steel, which can effectively improve the yield strength of the steel plate. At the same time, adding a proper amount of Mo element to the heat-treated steel plate can improve the toughness of the steel plate. The Mo content of the present invention is 0.3% to 0.65%.

[0018] Cu is an important element added in the present invention. It can improve the strength of the steel plate. Cu can work together with Ni in the steel to reduce the tough-brittle transition temperature of the steel plate. Excessive Cu alone will cause hot brittleness, reduce the surface quality of the steel plate, and reduce the low-temperature toughness of the steel plate. However, a large amount of Ni is added in the present invention. Combined with the heating rolling process in the present invention, the Cu brittle problem can be effectively avoided and the role of Cu can be played. The content of Cu in the present invention is 0.3% to 1.2%.

[0019] Nb is the main added element of the present invention, which improves the strength and toughness of the steel plate while improving the uniform deformation ability of the steel plate within the elastic deformation range. When heated, the C and N compound particles of Nb that are not dissolved are distributed on the austenite grain boundary, which can hinder the growth of austenite grains when the steel is heated, refine the grains, and improve fatigue resistance and toughness, thereby improving the anti-Bauschinger effect of the steel plate; during the cooling process of the steel plate, Nb (CN) precipitates in large quantities, further promoting dislocation entanglement and grain refinement. In the present invention, the Nb content is 0.03% to 0.08%.

[0020] The V element can form V (C, N) particles in the matrix, which can play a role in refining the grains. The addition of the V element can enhance the anti-Bauschinger effect of the steel plate. The V content of the present invention is 0.08% to 0.18%.

[0021] The addition of Ti element is to make Ti and N form TiN, which can prevent the growth of grains during the heating and rolling process of the steel billet, and can improve the strength and uniform elastic deformation capacity of the steel plate. When it exceeds 0.025%, the toughness of the steel will deteriorate. Therefore, in the present invention, the Ti content is 0.01% to 0.025%.

[0022] Based on the above technical solution, further, the thickness of the marine engineering steel plate is 10 to 50 mm, the effective grain size is 2 to 7 μm, the metallographic structure is tempered martensite and retained austenite, the proportion of retained austenite is 2 to 5%, the yield strength is ≥1000 MPa, the tensile strength is 1030 to 1250 MPa, the elongation is ≥16%, the uniform deformation elongation is ≥6%, the Charpy impact energy at -60°C is ≥120 J, and at 2% residual strain, the Bauschinger effect ratio is ≥0.9.

[0023] The present invention also provides a method for preparing the above-mentioned 1000MPa marine engineering steel plate with excellent anti-Bauschinger effect, comprising the following steps:

[0024] (1) Heating: The ingot is placed in a heating furnace at a furnace temperature of 600-700°C and kept warm for 60-90 minutes. The purpose is to keep the temperature of the ingot uniform in the thickness direction of the extra-thick ingot at the low temperature stage to avoid uneven heating of Cr and Cu elements and cause internal defects in the ingot; the temperature is raised to the heating section temperature of 1190-1260°C and the heating section is kept warm for 2-10 hours. The purpose of the uniform heating and keeping is to ensure that the C / N compounds of the microalloy are fully dissolved while avoiding abnormal growth of the cast structure and surface defects caused by high Cu and Mn content in the ingot;

[0025] (2) Rolling: A two-stage controlled rolling process is adopted. The first stage rolling temperature is 1150-1250℃, the average rolling reduction of a single pass in the first stage is 10-15mm, and the total rolling reduction rate in the first stage is ≥40%. The purpose of the high-temperature first stage rolling is to utilize the good plasticity of the material at high temperature. At this time, the core temperature of the billet is high and the surface temperature is low, and the hardening degree is not obvious. Therefore, large rolling reduction can be carried out. This rolling method helps to destroy the columnar structure of the core of the billet and increase the deformation of the core structure, thereby improving the microstructure of the material. The cooling process of the intermediate billet obtained by rolling is also important. Controlling the cooling rate at 5-8℃ / s for water cooling can effectively improve the rolling production efficiency of the steel plate. In addition, appropriately shortening the cooling time of the intermediate billet can prevent the grains of the structure in the core of the billet from growing again during the cooling process. The regrown grains will weaken the effect of the accumulation of grain deformation in the core of the billet in the first stage of rolling, thereby affecting the low-temperature impact toughness and fatigue resistance of the steel plate. Therefore, the optimization of the cooling process of the intermediate billet is crucial to ensure the performance of the final product.

[0026] The second stage rolling temperature is 890-1000℃, the average rolling reduction of the second stage single pass is 6-15mm, the cumulative rolling reduction rate of 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 deformation of the core of the steel plate by taking advantage of the large drop in surface temperature, improve the grain size of the core of the steel plate, and promote the flattening and refinement of austenite grains; the alloy elements such as Ni, Cu, Nb, and V work together to improve the dislocation density, improve the yield strength of the steel plate and the uniformity of elastic deformation, and the single pass reduction rate and rolling temperature are the most important for grain refinement and uniform precipitation of C / N compounds;

[0027] (3) Quenching and tempering: including sub-temperature quenching, quenching and tempering, sub-temperature quenching temperature 590 ~ 760 ℃, sub-temperature quenching holding time 1 ~ 5min / mm, quenching temperature 800 ~ 900 ℃, quenching holding time 1 ~ 2min / mm, quenching cooling rate 5 ~ 20℃ / s, tempering temperature 500 ~ 600℃, tempering holding time 150 ~ 250min; the main purpose of sub-temperature quenching is to form residual austenite in the steel, which helps to improve the toughness of the steel plate under low temperature conditions. By controlling the heating temperature, sub-temperature quenching avoids excessive growth of austenite grains, thereby maintaining the microstructural integrity of the material during the quenching process, which helps to improve the toughness of the material and reduce brittleness. High temperature quenching is to ensure that the microstructure of the steel can be fully transformed into austenite, which is the prerequisite for the formation of martensite during the quenching process. Rapid cooling at an appropriate temperature can promote the transformation of the structure into martensite, thereby improving the hardness and strength of the steel plate. The combination of sub-temperature quenching and high-temperature quenching processes can better play the role of grain refinement. By replacing the dislocation strengthening method with grain boundary strengthening, a small amount of plastic deformation caused by pre-loading is avoided, and the dislocation moves along a certain slip plane to form a dislocation tangle or cellular structure. This dislocation movement is forced to move in the opposite direction by applying a reverse force after unloading. Because there are fewer obstacles on the reverse path, the dislocation can move a larger distance under lower stress, which will cause a significant Bauschinger effect. The present 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 of the steel plate after stress. At the same time, high-temperature quenching needs to be carried out without causing cracking or deformation of the steel plate to ensure the integrity and processing performance of the material; high-temperature tempering is a key step after quenching. It can optimize the matrix structure and promote the precipitation of the second phase by adjusting the tempering temperature and holding time. This treatment method helps to reduce the strength and hardness of the steel plate, while improving its plasticity and toughness. Sufficient tempering treatment can make the tempering transformation product complete the transformation as much as possible, thereby achieving the purpose of improving the comprehensive performance of the material.

[0028] Based on the above technical solution, further, the heating rate of the heating process in step (1) is controlled at 1.5-5°C / min to avoid the steel billet being heated too quickly, resulting in excessive internal and external stresses in the steel billet and causing crack defects.

[0029] Based on the above technical solution, further, in step (1), the temperature of the heating section is 1200-1250° C., and the insulation time of the heating section is 3-7 hours.

[0030] Based on the above technical solution, further, in step (2), the first-stage rolling temperature is 1170-1230° C., and the total reduction rate of the first stage is controlled at 40-55%.

[0031] Based on the above technical solution, further, in step (2), the second stage starting rolling temperature is 900-980°C, the second stage total reduction rate is controlled at 45-60%, and the final rolling temperature is 850-930°C.

[0032] Based on the above technical solution, further, in step (3), the sub-temperature quenching temperature is 600-750°C, the sub-temperature quenching holding time is 1-2 min / mm, the quenching temperature is 820-880°C, the quenching holding time is 1-1.5 min / mm, the quenching cooling rate is 6-15°C / s, the tempering temperature is 500-580°C, and the tempering holding time is 180-250 min.

[0033] Based on the above technical solution, further, the preparation of the ingot described in step (1) 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 ingots, and protecting the casting throughout the continuous casting process.

[0034] The beneficial effects of the present invention compared with the prior art are as follows:

[0035] In view of the requirements of the composition and performance of ultra-high-strength marine steel in the extremely cold deep-sea marine environment, the present invention uses the composition design of C, Si, Mn, Ni, Cr, Mo, Cu and micro-alloying elements V, Nb, Ti, and conducts a large number of systematic studies on alloy element screening and proportioning, steel cleanliness control, and efficient rolling process optimization. By combining the sub-temperature quenching and high-temperature quenching processes in the quenching and tempering process, the grain refinement can be better achieved. The grain boundary strengthening formed by the fine micro-grain structure replaces the conventional dislocation strengthening. The chemistry mechanism can significantly reduce the Bauschinger effect of the steel plate after being subjected to stress. The prepared 1000MPa marine engineering steel plate with excellent resistance to Bauschinger effect and a maximum thickness of 50mm has excellent strength, toughness and anti-Bauschinger effect properties. The yield strength of the steel plate is ≥1000MPa, the tensile strength is 1030-1250MPa, the elongation is ≥16%, the uniform deformation elongation is ≥6%, the Charpy impact energy at -60℃ is ≥120J, and the Bauschinger effect ratio is ≥0.9 at 2% residual strain, which has a broad application prospect. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] In order to more clearly illustrate the embodiments of the present invention, the drawings related to the embodiments are briefly introduced below.

[0037] Figure 1 This is the metallographic structure diagram of the marine engineering steel plate prepared in Example 1. DETAILED DESCRIPTION

[0038] The present invention is described in detail below with reference to the embodiments, but the embodiments of the present invention are not limited thereto. Obviously, the embodiments described below are only partial embodiments of the present invention. For those skilled in the art, other similar embodiments obtained without creative labor all fall within the protection scope of the present invention.

[0039] Examples 1-30

[0040] 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.

[0041] Table 1 Chemical composition and weight percentage (%) of marine engineering steel plates of Examples 1-30

[0042]

[0043]

[0044] The steps include:

[0045] (1) High cleanliness alloy smelting

[0046] The molten steel is refined through 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 ingots. The continuous casting process is fully protected.

[0047] (2) Heating

[0048] The ingot from step (1) is placed in a heating furnace at a temperature of 600-695°C, kept warm for 60-90 min, and then heated to a heating section temperature at a heating rate of 1.5-5°C / min, the heating section temperature is 1200-1250°C, and the heating section is kept warm for 3-7 h;

[0049] (3) Rolling

[0050] Two-stage rolling is adopted, the starting rolling temperature of the first stage is 1170-1230°C, the average rolling reduction of a single pass in the first stage is 10-15mm, and the total rolling reduction rate of the first stage is ≥40%; the intermediate billet is water-cooled at a cooling rate of 5-8°C / s; the starting rolling temperature of the second stage is 900-975°C, the average rolling reduction of a single pass in the second stage is 6-15mm, the cumulative rolling reduction rate of the second stage is ≥45%, and the final rolling temperature is ≥850°C.

[0051] (4) Tempering

[0052] It includes sub-temperature quenching, quenching and tempering. 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.

[0053] Table 2 Heating process parameters of the offshore steel plate preparation process of Examples 1-30

[0054]

[0055]

[0056] Table 3 Rolling process parameters of the offshore steel plate preparation process of Examples 1-30

[0057]

[0058]

[0059] Table 4 Quenching and tempering process parameters of the offshore steel plate preparation process of Examples 1-30

[0060]

[0061] The mechanical properties of the 1000MPa marine engineering steel plates prepared in Examples 1-30 are shown in Table 5.

[0062] Table 5 Mechanical properties of marine engineering steel plates of Examples 1-30

[0063]

[0064] The metallographic structure of the marine engineering steel plate prepared in Example 1 is as follows: Figure 1 The structure of the steel plate at 1 / 2 of the thickness is tempered martensite and 2-5% residual austenite, the effective grain size is 2-7 μm, and the mechanical properties are good. The present invention uses grain boundary strengthening formed by extremely fine microscopic grain structure to replace the conventional dislocation strengthening mechanism, which can significantly reduce the Bauschinger effect of the steel plate after being stressed. At the same time, the presence of 2-5% residual austenite in the steel can also effectively alleviate the dislocation slip and entanglement caused by the pre-deformation process, reducing the adverse effect of the Bauschinger effect on the strength of the steel plate.

[0065] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements 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 1000MPa marine steel plate with excellent resistance to Bauschinger effect, characterized in that: Calculated 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%, and the rest are Fe and unavoidable impurities, and Ceq is 1.0-1.

4.

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

3. The method for preparing the 1000MPa marine engineering steel plate with excellent anti-Bauschinger effect according to claim 1 or 2, characterized in that: The steps include: (1) Heating: Place the ingot into a heating furnace at a temperature of 600-700°C, keep it warm for 60-90 minutes, and heat it to the heating section temperature of 1190-1260°C. Keep it warm for 2-10 hours. (2) Rolling: A two-stage controlled rolling process is adopted, the first-stage rolling temperature is 1150-1250°C, the average rolling reduction of a single pass in the first stage is 10-15 mm, the total rolling reduction rate in the first stage is ≥40%, and the intermediate billet is water-cooled at a cooling rate of 5-8°C / s; The second stage rolling temperature is 890-1000℃, the average rolling reduction of the second stage single pass is 6-15mm, the second stage cumulative rolling reduction rate is ≥45%, and the final rolling temperature is ≥850℃; (3) Quenching and tempering: including sub-temperature quenching, quenching and tempering, sub-temperature quenching temperature 590 ~ 760 ℃, sub-temperature quenching holding time 1 ~ 5min / mm, quenching temperature 800 ~ 900 ℃, quenching holding time 1 ~ 2min / mm, quenching cooling rate 5 ~ 20℃ / s, tempering temperature 500 ~ 600℃, tempering holding time 150 ~ 250min.

4. The preparation method according to claim 3, characterized in that: The heating rate of the heating process in step (1) is controlled at 1.5-5°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° C., and the insulation time of the heating section is 3-7 hours.

6. The preparation method according to claim 3, characterized in that: In step (2), the first-stage rolling temperature is 1170-1230° C., 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 second stage rolling temperature is 900-980°C, the total reduction rate of the second stage is controlled at 45-60%, and the final rolling temperature is 850-930°C.

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

9. The preparation method according to claim 3, characterized in that: The preparation of the ingot described in step (1) comprises the following steps: refining molten steel through a converter, LF furnace, RH or VD furnace, and continuously casting the ingot, wherein the continuous casting process is protected throughout the casting process.

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