A Mn-Cr-V-N series high-strength and high-ductility air-cooled hardening steel and its preparation method

Abstract

The present invention discloses an Mn-Cr-V-N series high-strength and high-ductility air-cooled hardening steel and a preparation method thereof, comprising, by mass percentage: C: 0.46-0.55%, Si: 0.5-1.0%, Mn: 1.51-2.51%, Cr: 2.51-3.51%, V: 0.21-0.36%, N: 0.02-0.10%, the contents of P and S are controlled below 0.01%, and the remaining components are Fe. The preparation process includes melting, forging, hot rolling, cold rolling, annealing, and air-cooled quenching. The present invention belongs to the technical field of ultra-high-strength steel and its processing technology, and an air-cooled hardening steel with a tensile strength higher than 2100 MPa and an elongation exceeding 10.0% is prepared. Compared with the existing technology, the material prepared by the present invention has both ultra-high strength, high toughness and oxidation resistance, and is easy to be industrially produced on a large scale, having broad development and application prospects.

Description

Technical Field

[0001] The present invention belongs to the technical field of ultra-high strength steel and its processing, and particularly relates to an Mn-Cr-V-N series high-strength and high-ductility air-cooled hardened steel and a preparation method thereof. Background Art

[0002] Automobile lightweighting is an effective way to save energy and reduce emissions and is the theme of the development of the automobile industry. Automobile lightweighting refers to achieving a reduction in the vehicle's own weight under the premise of meeting various indicators in terms of material selection and design, etc., so as to achieve the goals of safety, energy conservation, weight reduction, and emission reduction. Currently, there are mainly two research directions for lightweight materials: using low-density materials such as aluminum-magnesium alloys, composite materials, and plastics; using high-strength steel to reduce the thickness of automotive parts. Among them, high-strength steel has advanced forming processes, high R & D efficiency, and low production costs, which determines that the main research direction of current vehicle body lightweighting technology is the development and application of high-strength steel and advanced forming technology. Therefore, the research and development of a new generation of advanced high-strength steel with higher strength, higher plasticity, and better formability is the focus of the development of automobile lightweighting.

[0003] The research and development of automotive parts is the key to the development of the automotive industry. However, with the increase in the strength of steel used in automobiles, the deformation resistance of steel plates increases, the springback and cracking of parts are serious, and the shape stability becomes poor, resulting in huge challenges for traditional cold stamping forming technology. However, although hot stamping forming technology solves the problem of forming high-strength steel, it also brings high production costs, such as developing hot stamping forming equipment with rapid cooling, and the hot stamping forming process still cannot produce complex automotive parts. In recent years, air-cooled hardened steel has attracted the attention of relevant scholars. Its annealed sheet has excellent cold forming performance, and complex automotive parts can be processed through traditional cold stamping forming technology. Subsequently, after austenitization treatment and cooling in still air, a high-strength martensite structure can be obtained, meeting the requirements of lightweight design and structural parts for the new generation of automotive steel.

[0004] Chinese Invention Patent 202010795720.7 discloses "Air-cooled Hardening Steel Plate with Ultra-high Strength, High Toughness and Ultra-fine Microstructure and Its Preparation Process". The chemical composition of the air-cooled hardening steel in this patent is relatively complex (involving 13 alloying elements), and precious alloying elements such as W, Co, Ta and Nd are added. In addition, high-energy-consuming processing technologies such as warm rolling and asynchronous rolling are adopted, resulting in high production costs and being unfavorable to environmental protection. Chinese Invention Patent 201910248764.5 discloses "A Cold-rolled Low-alloy High-strength Air-cooled Strengthened Steel for Automobile and Its Preparation Method". This patent introduces expensive Mo and B elements, and its tensile strength level is relatively low, only at the 1100MPa level, with low cost performance and room for further strength improvement. Chinese Invention Patent 202210137740.4 discloses "A 1800MPa-level High-strength and Tough Hot-formed Steel and Its Heat Treatment Process", which belongs to the Mn-B series hot-formed steel, and its hardenability mainly depends on B to maintain. However, B is prone to generate BN at the grain boundaries, seriously deteriorating the performance of the experimental steel. Ti needs to be added to avoid the "boron embrittlement" phenomenon, but the addition of B and Ti will reduce the plasticity and toughness of the steel and increase its production cost. Summary of the Invention

[0005] The purpose of this part is to outline some aspects of the embodiments of the present invention and briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this part, as well as in the abstract and title of the present application, to avoid obscuring the purpose of this part, the abstract and the title. However, such simplifications or omissions shall not be used to limit the scope of the present invention.

[0006] In view of the above and / or problems existing in the prior art, the present invention is proposed.

[0007] Therefore, the purpose of the present invention is to overcome the deficiencies in the prior art and provide a Mn-Cr-V-N series high-strength and plastic air-cooled hardening steel.

[0008] To solve the above technical problems, the present invention provides the following technical solution: A Mn-Cr-V-N series high-strength and plastic air-cooled hardening steel, characterized in that: the Mn-Cr-V-N series high-strength and plastic air-cooled hardening steel contains carbon, silicon, manganese, chromium, vanadium, nitrogen, phosphorus, sulfur, and the rest is iron;

[0009] Among them, in terms of mass percentage, the mass fraction of carbon is 0.46~0.55%, the mass fraction of silicon is 0.5~1.0%, the mass fraction of manganese is 1.51~2.51%, the mass fraction of chromium is 2.51~3.51%, the mass fraction of vanadium is 0.21~0.36%, the mass fraction of nitrogen is 0.02~0.10%, the mass fraction of phosphorus <0.01%, and the mass fraction of sulfur <0.01%.

[0010] As a preferred embodiment of the air-cooling hardening steel of the present invention, the mass fraction of carbon is 0.48-0.50%, the mass fraction of silicon is 0.54-1.0%, the mass fraction of manganese is 1.56-2.00%, the mass fraction of chromium is 2.54-3.0%, the mass fraction of vanadium is 0.25-0.30%, the mass fraction of nitrogen is 0.025-0.1%, the mass fraction of phosphorus is 0.003-0.006%, the mass fraction of sulfur is 0.003-0.006%, and the rest is iron.

[0011] As a preferred embodiment of the air-cooling hardening steel of the present invention, the mass fraction of carbon is 0.48%, the mass fraction of silicon is 1.0%, the mass fraction of manganese is 1.56%, the mass fraction of chromium is 2.54%, the mass fraction of vanadium is 0.25%, the mass fraction of nitrogen is 0.025%, the mass fraction of phosphorus is 0.005%, the mass fraction of sulfur is 0.004%, and the rest is iron.

[0012] As a preferred embodiment of the air-cooling hardening steel of the present invention, the air-cooling hardening steel also has the following characteristics:

[0013] (i) The microstructure is more than 90% dislocation martensite;

[0014] (ii) tensile strength greater than 2100 MPa;

[0015] (iii) The elongation is greater than 10.0%.

[0016] Another object of the present invention is to overcome the deficiencies in the prior art and provide a method for preparing Mn-Cr-VN series high-strength and plastic air-cooled hardened steel.

[0017] As a preferred embodiment of the preparation method of the present invention, the preparation method of air-cooled hardened steel comprises:

[0018] The ingredients are prepared according to the mass percentages mentioned above, and after smelting under vacuum conditions, forging, hot rolling, cold rolling and annealing are performed to obtain annealed plates, which are heated to be completely austenitized and then air quenched to room temperature to obtain Mn-Cr-VN series high-strength and plastic air-cooled hardening steel.

[0019] As a preferred embodiment of the preparation method of the present invention, the smelting is vacuum induction smelting to produce molten steel, which is then cast into ingots.

[0020] As a preferred embodiment of the preparation method of the present invention, the forging is to heat the ingot to 1250-1350°C at 10°C / s, keep it warm for 4 hours for homogenization treatment, and then forge it, with the start forging temperature not lower than 1150°C and the final forging temperature not lower than 1000°C to obtain a forging.

[0021] As a preferred embodiment of the preparation method of the present invention, the hot rolling is as follows: the forging is heated to 1200-1250°C at a rate of 10°C / s, held for 1 h, then hot rolled. The starting rolling temperature is not lower than 1150°C, the finishing rolling temperature is not lower than 850°C, the cumulative reduction is not lower than 90%. After finishing rolling, it is cooled to the coiling temperature of 650-700°C by laminar cooling and held for 1 h, and then furnace cooled to room temperature to obtain a hot rolled plate.

[0022] As a preferred embodiment of the preparation method of the present invention, the cold rolling is as follows: the hot rolled plate is pickled and then cold rolled in multiple passes, and the cumulative deformation is 30%-70%.

[0023] As a preferred embodiment of the preparation method of the present invention, the annealing obtains an annealed plate. After heating to complete austenitization, it is air quenched to room temperature. The annealing conditions are as follows: in an inert gas, the annealing temperature is 650-750°C, held for 2-6 h and then air cooled to room temperature; complete austenitization is heating to 850-950°C and held for 5-15 min; the air quenching conditions are that the quenching rate is 25°C / s in the high temperature stage of 743-920°C and 7°C / s in the low temperature stage of 120-428°C.

[0024] Advantages of the present invention:

[0025] (1) The present invention provides a Mn-Cr-V-N series high-strength and high-ductility air-cooled hardening steel and its preparation method. Through reasonable and simple composition design and optimization of preparation process parameters, the obtained annealed plate has excellent processing performance and can realize cold stamping forming of complex automotive parts. And Mn, high Cr and V alloying elements are introduced to improve the hardenability of the material, ensuring that the parts do not require special quenching and cooling methods after uniform austenitization at high temperature, and can be directly air cooled to room temperature to obtain an ultra-high-strength air-cooled hardening steel with a strength exceeding 2100 MPa and an elongation higher than 10.0%, which has the characteristics of low cost, high efficiency and excellent performance. In addition, the final quenching of the parts is carried out in the air. Due to the small cooling rate of the air medium, the internal residual stress of the parts is small, the springback and deformation are small, so its size and shape are more stable.

[0026] (2) The present invention adopts an alloy design with the introduction of N element, and the alloy contents of V and N are compounded and added according to a scientific ratio. Among them, C can ensure high strength; high Cr can improve hardenability; increasing the Si content can inhibit the generation of carbides during air cooling, thereby stabilizing metastable austenite and improving plasticity; the V-N microalloying technology can obtain high strength at a low V content, significantly saving the V dosage and reducing the cost of steel. Moreover, N can promote the precipitation of V carbonitrides, and the precipitated particles of vanadium carbonitride not only play a role in refining grains, but also can capture free hydrogen atoms in the steel, playing a role in improving the hydrogen embrittlement resistance of high-strength steel.

[0027] (3) The mechanical properties of the air-cooled hardened steel annealed plate after being completely austenitized and air-cooled to room temperature are as follows: tensile strength > 2100 MPa, yield strength > 1200 MPa, elongation > 10.0%. The air-cooled hardened steel is used to manufacture automotive parts, and its annealed plate has excellent forming and processing properties, and can be used to prepare complex parts. Subsequently, the service performance of the parts can be greatly improved by a simple method of "austenitization + air-cooling quenching". Meeting the requirements of the new generation of advanced high-strength steel, this material can not only be used in the field of automotive manufacturing, but also be widely applied to industries such as bridge pipelines, railway transportation, aircraft carriers and construction machinery. Brief Description of the Drawings

[0028] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings. Among them:

[0029] Figure 1 It is the SEM microstructure photograph of the air-cooled hardened steel quenched plate of Embodiment 1 of the present invention;

[0030] Figure 2 It is the TEM microstructure photograph of the air-cooled hardened steel quenched plate of Embodiment 1 of the present invention;

[0031] Figure 3 It is the engineering stress-strain curve of the air-cooled hardened steel annealed plate and quenched plate of Embodiment 1 of the present invention. Detailed Description of the Invention

[0032] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and understandable, the following will make a detailed description of the specific embodiments of the present invention in conjunction with the embodiments of the specification.

[0033] In the following description, numerous specific details are set forth to provide a thorough understanding of the present invention. However, the present invention may be practiced in other ways different from those described herein. Persons skilled in the art can make similar extensions without departing from the spirit of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed below.

[0034] Secondly, as used herein, "an embodiment" or "embodiments" refer to specific features, structures, or characteristics that may be included in at least one implementation of the present invention. The appearances of "in an embodiment" in different places in this specification do not all refer to the same embodiment, nor are they separate or alternative embodiments that exclude each other.

[0035] Unless otherwise specified, the raw materials used in the embodiments of the present invention are all commercially available. Details are shown in Table 1.

[0036] Table 1

[0037] 。

[0038] Example 1

[0039] The present invention provides a method for preparing an Mn-Cr-V-N series high-strength and high-ductility air-cooled hardening steel:

[0040] By mass percentage, it includes the following components: 0.48% C, 1.0% Si, 1.56% Mn, 2.54% Cr, 0.25% V, 0.025% N, 0.005% P, 0.004% S, and the balance is Fe.

[0041] (1) The prepared raw materials are melted in a vacuum induction melting furnace to obtain molten steel, and cast into billets under the protection of inert gas. The introduction of nitrogen element is through melting chromium nitride and iron nitride particles; subsequently, the billets are heated to 1300 °C and held for 4 h, taken out for forging, the starting forging temperature is 1200 °C, and the final forging temperature is 1150 °C;

[0042] (2) The forged billets are reheated to 1200 °C and held for 1 h, taken out for hot rolling, the starting rolling temperature is 1180 °C, the final rolling temperature is 900 °C, the reduction rate of the last pass is 20%, and after final rolling, it is cooled to the coiling temperature of 680 °C by laminar cooling and held for 1 h, and then furnace-cooled to room temperature to obtain a 3-mm-thick hot-rolled plate;

[0043] (3) The hot-rolled steel strip is pickled and then cold-rolled in multiple passes. The reduction rate in the last cold-rolling pass is controlled at 10% to obtain a cold-rolled sheet with a thickness of 1.5 mm. Subsequently, the cold-rolled sheet is heated to 680 °C and held for 4 h, and then air-cooled to room temperature. Finally, the annealed sheet is heated to 900 °C and held for 15 min to be fully austenitized, and then quenched in air to room temperature. Among them, the quenching rate of air quenching in the high-temperature stage (743 - 920 °C) is 25 °C / s, and the quenching rate in the low-temperature stage (120 - 428 °C) is 7 °C / s.

[0044] Example 2

[0045] The difference from Example 1 is that its Cr content is 3.0 wt.%.

[0046] Example 3

[0047] The difference from Example 1 is that its N content is 0.1 wt.%.

[0048] Comparative Example 1

[0049] The difference from Example 1 is that after the annealed sheet is fully austenitized, it is quenched to room temperature with cold water, that is, its quenching method is water quenching, the water temperature is 25 °C at room temperature, and the water temperature of the steel sheet can reach 80 °C after quenching. The quenching rate of water quenching is about 550 °C / s.

[0050] Comparative Example 2

[0051] The difference from Example 1 is that after the annealed sheet is fully austenitized, it is quenched to room temperature with quenching oil, that is, its quenching method is oil quenching, the oil temperature is 25 °C at room temperature, and the quenching oil used is a general-purpose quenching oil. The quenching rate of oil quenching is about 20 °C / s.

[0052] Comparative Example 3

[0053] The difference from Example 2 is that after the annealed sheet is fully austenitized, it is quenched to room temperature with cold water, that is, its quenching method is water quenching, the water temperature is 25 °C at room temperature, and the water temperature of the steel sheet can reach 80 °C after quenching. The quenching rate of water quenching is about 550 °C / s.

[0054] Comparative Example 4

[0055] The difference from Example 2 is that after the annealed sheet is fully austenitized, it is quenched to room temperature with quenching oil, that is, its quenching method is oil quenching, the oil temperature is 25 °C at room temperature, and the quenching oil used is a general-purpose quenching oil. The quenching rate of oil quenching is about 20 °C / s.

[0056] The mechanical properties (tensile strength, yield strength, elongation) of Examples 1 to 3 and Comparative Examples 1 to 4 were tested. The testing equipment was a universal testing machine, with models CMT5105 and CMT5605 respectively. The tensile tests of the annealed steel plates with a tensile strength lower than 1000 MPa were carried out on the CMT5105 tensile machine, and the quenched steel plates were carried out on the CMT5605 tensile machine. The tensile specimens were cut along the rolling direction by wire cutting with a gauge length of 25 mm, and the width of the parallel section was 6.0 mm. The shape and size parameters of the specimens referred to the national standard "Metallic materials - Tensile testing - Part 1: Method of test at room temperature" (GB / T 228.1-2010). The uniaxial tensile speed was 1.0 mm / min, and the test condition was at room temperature. The main mechanical property indexes of the specimens were tensile strength, total elongation and yield strength (for continuous yielding, the stress value corresponding to a plastic strain of 0.2% was taken, and for discontinuous yielding, the lower yield strength was taken).

[0057] Comparative Example 5

[0058] The Mn-B series hot-formed steel reported in the doctoral thesis "Research on the Strengthening and Toughening Mechanism and Application Technology of 2000MPa Grade Hot-formed Steel" of the University of Science and Technology Beijing was selected. The chemical composition of this steel was as follows in mass percentage: 0.37%C, 1.48%Si, 1.58%Mn, 0.035%Al, 0.91%Cr, 0.0037%B, 0.07%Ti, 0.048%Nb, and the rest were Fe and unavoidable impurities. The quenching method was quenching with nitrogen, and the cooling rate was 45°C / s. This steel improved the hardenability through the B element and strictly avoided the introduction of the N element, while the B element was removed in the example, and good hardenability was ensured through the Cr element, and the N element was introduced to promote the precipitation of vanadium carbonitride; in addition, in the comparative example, a nano-scale NbC precipitation phase was formed by the Nb element and the C element to play the role of refining the microstructure, while in the example, the V-N microalloying technology was used, and the vanadium carbonitride precipitation particles not only played the role of refining the grains and pinning the dislocation movement, but also could capture the free hydrogen atoms in the steel to play the role of improving the hydrogen embrittlement resistance of high-strength steel.

[0059] Comparative Example 6

[0060] Select the Mn-Cr series hot-formed steel reported in the Chinese invention patent 202111187719.7, "High-strength hot-formed steel substrate with tensile strength greater than 2200 MPa and preparation method". The chemical composition of this steel is as follows in mass percentage: 0.42% C, 0.40% Si, 1.46% Mn, 1.99% Cr, 0.005% P, 0.004% S, 0.05% Nb, 0.15% V, 0.02% Y, and the rest is Fe and inevitable impurities. The quenching method is also through nitrogen, and the cooling rate is 10 °C / s. The main difference between this steel and the example lies in the composite addition of Nb and V micro-alloying elements to form nano-scale (V, Nb)C precipitation phases with C elements, which play the role of refining the microstructure and binding hydrogen atoms. While in the example, the V-N micro-alloying technology is used, and the vanadium carbonitride precipitation phase not only plays the role of refining grains and pinning dislocations, but also can capture free hydrogen atoms in the steel, playing the role of improving the hydrogen embrittlement resistance of high-strength steel; in addition, this steel adopts a low Si element, which cannot inhibit the generation of carbides during quenching and tempering, and a fully martensitic structure is obtained. In the example, in order to obtain excellent plasticity, a higher Si element is introduced to inhibit the generation of carbides during air cooling and stabilize a certain volume fraction of metastable austenite, thereby ensuring an elongation rate higher than 10.0%.

[0061] Comparative Example 7

[0062] Select the air-cooled strengthened steel reported in the Chinese invention patent 201910248764.5, "A cold-rolled low-alloy high-strength air-cooled strengthened steel for automobiles and preparation method". The chemical composition of this steel is as follows in mass percentage: 0.06% C, 1.05% Mn, 0.17% Si, 0.6% Cr, 0.13% Mo, 0.03% Al, 0.02% Nb, 0.02% V, 0.02% Ti, 0.002% B, P < 0.03%, S < 0.03%, and the rest is Fe and inevitable impurities. After austenitizing at 900 °C and then air-cooling to room temperature, high-strength and good-toughness automotive parts can be obtained. The main difference between this comparative example and the example is that its tensile strength is only at the 1100 MPa level, and the strength needs to be further improved. In addition, a variety of micro-alloying elements are introduced, and the cost performance is not high.

[0063] Table 2 Mechanical properties of examples and comparative examples

[0064] ,

[0065] Figure 1 This is the SEM micrograph of the quenched plate of the air-cooled hardened steel in Example 1 of the present invention. Figure 3Engineering stress-strain curves of the air-cooled hardened steel annealed plate and quenched plate in Example 1 of the present invention. It can be seen from the mechanical properties of the examples and comparative examples in Table 2 that after heating the cold-rolled plate of air-cooled hardened steel to 650-750 °C and holding for 2-6 h and then air-cooling to room temperature, the annealed plates of the examples and comparative examples all have good formability, which is conducive to their cold stamping forming. Subsequently, heat treatment processes with different quenching methods (air quenching, water quenching, oil quenching) were carried out after complete austenitization of the annealed plates. The results show that due to the excellent hardenability of the test steel, more than 90% of dislocation-type martensite can be obtained during air quenching in the examples (see Figure 2 ), achieving a good performance match with a tensile strength > 2100 MPa and an elongation > 10.0%. Martensite ensures that the test steel has excellent strength, and due to the cooling rate of air quenching being much lower than that of water quenching and oil quenching, a small amount of metastable austenite is retained to room temperature, thereby improving the plastic toughness of the test steel. In the comparative examples, after water quenching and oil quenching, the strength is relatively high, but the elongation is poor, mainly because the quenching speed is fast and the residual stress is large, thus deteriorating the plasticity of the test steel. In summary, the alloy composition of this invention patent is optimized and the preparation method is simple, and air-cooled hardened steel with a strength above 2100 MPa can be prepared, which is of great significance in realizing the lightweight of high-strength automotive steel and reducing production costs, etc.

[0066] It should be noted that the above examples are only used to illustrate the technical solutions of the present invention and are not restrictive. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered within the scope of the present invention.

Claims

1. A Mn-Cr-V-N series high-strength and high-ductility air-cooled hardening steel, characterized in that: It contains carbon, silicon, manganese, chromium, vanadium, nitrogen, phosphorus, sulfur, and the rest is iron; Wherein, in terms of mass percentage, the mass fraction of carbon is 0.48-0.50%, the mass fraction of silicon is 0.54-1.0%, the mass fraction of manganese is 1.56-2.00%, the mass fraction of chromium is 2.54-3.0%, the mass fraction of vanadium is 0.25-0.30%, the mass fraction of nitrogen is 0.025-0.1%, the mass fraction of phosphorus is 0.003-0.006%, the mass fraction of sulfur is 0.003-0.006%, and the rest is iron; The Mn-Cr-VN series high-strength plastic air-cooled hardening steel also has the following characteristics: (i) The microstructure is more than 90% dislocation martensite; (ii) tensile strength greater than 2100 MPa; (iii) The elongation is greater than 10.0%.

2. The Mn-Cr-V-N series high-strength and high-ductility air-cooled hardening steel according to claim 1, wherein: The mass fraction of carbon is 0.48%, the mass fraction of silicon is 1.0%, the mass fraction of manganese is 1.56%, the mass fraction of chromium is 2.54%, the mass fraction of vanadium is 0.25%, the mass fraction of nitrogen is 0.025%, the mass fraction of phosphorus is 0.005%, the mass fraction of sulfur is 0.004%, and the rest is iron.

3. A preparation method of an Mn-Cr-V-N series high-strength and high-ductility air-cooled hardening steel, characterized in that: include, The ingredients are prepared according to the mass percentages as described in any one of claims 1 to 2, and after smelting under vacuum conditions, forging, hot rolling, cold rolling and annealing are performed to obtain annealed plates, which are heated to be completely austenitized and then air quenched to room temperature to obtain Mn-Cr-VN series high-strength and plastic air-cooled hardening steel.

4. The preparation method according to claim 3, characterized in that: The smelting is vacuum induction smelting to produce molten steel, which is then cast into ingots.

5. The preparation method according to claim 4, characterized in that: The forging is to heat the ingot to 1250-1350°C at 10°C / s, keep it warm for 4 hours for homogenization, and then forge it, with the start forging temperature not lower than 1150°C and the final forging temperature not lower than 1000°C to obtain a forging.

6. The preparation method according to claim 5, characterized in that: The hot rolling is to heat the forging to 1200-1250°C at 10°C / s, keep it warm for 1h, then hot roll it, the start rolling temperature is not less than 1150°C, the final rolling temperature is not less than 850°C, the cumulative compression is not less than 90%, after the final rolling, cool it to the coiling temperature of 650-700°C in a laminar cooling manner and keep it warm for 1h, then furnace cool it to room temperature to obtain a hot rolled plate.

7. The preparation method according to claim 3, characterized in that: The cold rolling is to pickle the hot-rolled plate and then perform multiple cold rolling, with the cumulative deformation amount being 30% to 70%.

8. The preparation method according to claim 3, characterized in that: The annealing obtains an annealed plate, which is heated to be completely austenitized and then air-quenched to room temperature, wherein the annealing conditions are: under inert gas, the annealing temperature is 650-750°C, and the plate is kept warm for 2-6 hours before air-cooling to room temperature; the complete austenitization is heated to 850-950°C and kept warm for 5-15 minutes; the air-quenching conditions are: the quenching speed is 25°C / s at a high temperature stage of 743-920°C, and the quenching speed is 7°C / s at a low temperature stage of 120-428°C.

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