Mn-Cr-V-N series high-strength plastic air-cooled hardened steel and preparation method thereof

Abstract

The invention discloses Mn-Cr-V-N series high-strength plastic air-cooled hardened steel and a preparation method thereof, and the Mn-Cr-V-N series high-strength plastic air-cooled hardened steel comprises the following components in percentage by mass: 0.46 to 0.55 percent of C, 0.5 to 1.0 percent of Si, 1.51 to 2.51 percent of Mn, 2.51 to 3.51 percent of Cr, 0.21 to 0.36 percent of V, 0.02 to 0.10 percent of N, less than 0.01 percent of P and S, and the balance of Fe. The preparation process comprises the steps of smelting, forging, hot rolling, cold rolling, annealing and air cooling quenching. The invention relates to the technical field of ultra-high-strength steel and processing thereof, the air-cooled hardened steel with the tensile strength higher than 2100 MPa and the ductility higher than 10.0% is prepared, and compared with the prior art, the material prepared through the method has the advantages of ultra-high strength, high toughness and oxidation resistance, industrial large-scale production is easy, and wide development and application prospects are achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of ultra-high strength steel and processing thereof, and specifically relates to a Mn-Cr-VN series high-strength plastic air-cooling hardening steel and a preparation method thereof. Background Art

[0002] Lightweighting of automobiles is an effective way to save energy and reduce emissions, and is the theme of the development of the automobile industry. Lightweighting of automobiles refers to reducing the weight of the automobile itself, achieving the goals of safety, energy saving, weight reduction and emission reduction, on the premise of meeting various indicators in terms of material selection and design. There are currently two main 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 automobile parts. Among them, high-strength steel has advanced forming technology, high R&D efficiency and low production cost, which determines that the main research direction of current 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 forming performance is the focus of the development of lightweight automobiles.

[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 automotive steel, the deformation resistance of steel plates increases, the parts rebound and crack seriously, and the shape stability deteriorates, resulting in huge challenges for traditional cold stamping technology. However, although hot stamping technology has solved the problem of high-strength steel forming, it also brings high production costs. For example, the hot stamping forming equipment with rapid cooling has been developed, and the hot stamping forming process is still unable to produce complex automotive parts. In recent years, air-cooled hardened steel has attracted the attention of relevant scholars. Its annealed plate has excellent cold forming properties. Complex automotive parts can be processed through traditional cold stamping technology. After austenitization treatment, high-strength martensitic structure can be obtained by cooling in still air, which meets the requirements of lightweight design and structural parts of the new generation of automotive steel.

[0004] Chinese invention patent 202010795720.7 discloses "Ultra-high strength, high toughness, ultra-fine microstructure, air-cooled hardened steel plate and its preparation process". The chemical composition of the air-cooled hardened steel in this patent is relatively complex (involving 13 alloy elements), and precious alloy elements such as W, Co, Ta and Nd are added. In addition, the use of high-energy-consuming processing technologies such as warm rolling and asynchronous rolling has high production costs and is not conducive to environmental protection. Chinese invention patent 201910248764.5 discloses "A kind of cold-rolled low-alloy high-strength air-cooled strengthened steel for automobiles and its preparation method". This patent introduces expensive Mo and B elements, and its tensile strength level is low, only 1100MPa level, the cost performance is not high, and the strength needs to be further improved. Chinese invention patent 202210137740.4 discloses "A kind of 1800MPa-level high-strength and toughness 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 easily generates BN at the grain boundaries, which seriously deteriorates 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 section is to summarize some aspects of embodiments of the present invention and briefly introduce some preferred embodiments. Some simplifications or omissions may be made in this section and the specification abstract and the invention title of this application to avoid blurring the purpose of this section, the specification abstract and the invention title, and such simplifications or omissions cannot be used to limit the scope of the present invention.

[0006] In view of the above problems and / or the 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-VN series high-strength and plastic air-cooled hardening steel.

[0008] In order to solve the above technical problems, the present invention provides the following technical solutions: a Mn-Cr-VN series high-strength plastic air-cooling hardening steel, characterized in that: the Mn-Cr-VN series high-strength plastic air-cooling hardening steel contains carbon, silicon, manganese, chromium, vanadium, nitrogen, phosphorus, sulfur, and the rest is iron; 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 is less than 0.01%, and the mass fraction of sulfur is less than 0.01%.

[0009] 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.

[0010] 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.

[0011] As a preferred embodiment of the air-cooling hardening steel of the present invention, the air-cooling 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%.

[0012] 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.

[0013] As a preferred embodiment of the preparation method of the present invention, the preparation method of air-cooled hardened steel comprises: 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.

[0014] 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.

[0015] 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.

[0016] As a preferred embodiment of the preparation method of the present invention, the hot rolling is to heat the forging to 1200-1250°C at 10°C / s, keep it warm for 1 hour and then carry out hot rolling, the start rolling temperature is not lower than 1150°C, the final rolling temperature is not lower than 850°C, the cumulative compression amount is not lower than 90%, and after the final rolling, it is cooled to the coiling temperature of 650-700°C in a laminar cooling manner and kept warm for 1 hour, and then furnace cooled to room temperature to obtain a hot-rolled plate.

[0017] As a preferred embodiment of the preparation method of the present invention, the cold rolling is to carry out multiple cold rolling after pickling the hot rolled plate, and the cumulative deformation is 30% to 70%.

[0018] As a preferred embodiment of the preparation method of the present invention, 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 heating to 850-950°C and keeping warm for 5-15 minutes; the air-quenching conditions are: the quenching rate is 25°C / s at a high temperature stage of 743-920°C, and the quenching rate is 7°C / s at a low temperature stage of 120-428°C.

[0019] Beneficial effects of the present invention: (1) The present invention provides a Mn-Cr-VN series high-strength and plastic air-cooled hardened steel and a preparation method thereof. Through reasonable and simple composition design and optimization of preparation process parameters, the prepared annealed plate has excellent processing performance and can realize cold stamping forming of complex automotive parts. In addition, Mn, high Cr and V alloy elements are introduced to improve the hardenability of the material, ensuring that the parts do not require special quenching cooling methods after high-temperature uniform austenitization. They can be directly cooled to room temperature in the air to obtain ultra-high-strength air-cooled hardened steel with a strength of more than 2100MPa and an elongation of more 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. Since the cooling rate of the air medium is low, the residual stress inside the parts is small, and the rebound and deformation are small, so the size and shape are more stable.

[0020] (2) The present invention adopts an alloy design that introduces N elements, and adds V and N in a scientific ratio. Among them, C can ensure high strength; high Cr can improve hardenability; increasing Si content can inhibit the generation of carbides during air cooling, thereby stabilizing metastable austenite and improving plasticity; VN microalloying technology can achieve high strength at low V content, which can significantly save V usage and reduce the cost of steel, and N can promote the precipitation of V carbonitride. The vanadium carbonitride precipitation particles not only play a role in refining grains, but also can capture free hydrogen atoms in steel, playing a role in improving the hydrogen embrittlement resistance of high-strength steel.

[0021] (3) The mechanical properties of the air-cooled hardened steel annealed plate after complete austenitization and air cooling to room temperature are as follows: tensile strength>2100MPa, yield strength>1200MPa, and elongation>10.0%. Air-cooled hardened steel is used to manufacture automotive parts. 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 "austenitization + air-cooling quenching" method. In line with the requirements of the new generation of advanced high-strength steel, this material can be used not only in the automotive manufacturing field, but also in a wide range of applications in bridges, pipelines, railway transportation, aircraft carriers, and engineering machinery. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative labor. Among them: Figure 1 This is a SEM microstructure photograph of the air-cooled hardened steel quenching plate of Example 1 of the present invention; Figure 2 This is a TEM microstructure photograph of the air-cooled hardened steel quenching plate of Example 1 of the present invention; Figure 3 The engineering stress-strain curves of the air-cooled hardened steel annealing plate and the quenching plate according to Example 1 of the present invention. DETAILED DESCRIPTION

[0023] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in conjunction with the embodiments of the specification.

[0024] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0025] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The term "in one embodiment" that appears in different places in this specification does not necessarily refer to the same embodiment, nor does it refer to a separate or selective embodiment that is mutually exclusive with other embodiments.

[0026] Unless otherwise specified, all raw materials used in the examples of the present invention are commercially available, as shown in Table 1 for details.

[0027] Table 1 .

[0028] Example 1 The present invention provides a method for preparing a Mn-Cr-VN series high-strength plastic air-cooled hardened steel: In terms of 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 rest is Fe.

[0029] (1) The prepared raw materials are melted in a vacuum induction melting furnace to obtain molten steel, and then cast into ingots under the protection of inert gas, wherein the nitrogen element is introduced by melting chromium nitride and iron nitride particles; then, the ingots are heated to 1300°C and kept at this temperature for 4 hours, and then taken out for forging, with the starting forging temperature being 1200°C and the final forging temperature being 1150°C; (2) The forged billet was reheated to 1200°C and kept at this temperature for 1 hour. After being taken out, it was hot rolled. The starting rolling temperature was 1180°C, the final rolling temperature was 900°C, and the last pass reduction rate was 20%. After the final rolling, it was cooled to the coiling temperature of 680°C by laminar cooling and kept at this temperature for 1 hour. Then, it was furnace cooled to room temperature to obtain a 3 mm thick hot rolled plate. (3) After pickling, the hot-rolled steel strip was cold-rolled for multiple passes, with the last pass cold-rolling reduction rate controlled at 10% to obtain a 1.5 mm thick cold-rolled sheet; then, the cold-rolled sheet was heated to 680°C and kept at this temperature for 4 h, and then air-cooled to room temperature; finally, the annealed sheet was heated to 900°C and kept at this temperature for 15 min to completely austenitize it, and then quenched in air to room temperature. The quenching rate of air quenching in the high temperature stage (743~920°C) was 25°C / s, and the quenching rate in the low temperature stage (120~428°C) was 7°C / s.

[0030] Example 2 The difference from Example 1 is that the Cr content is 3.0 wt.%.

[0031] Example 3 The difference from Example 1 is that the N content is 0.1 wt.%.

[0032] Comparative Example 1 The difference from Example 1 is that after the annealed plate is completely austenitized, it is quenched to room temperature using cold water, that is, the quenching method is water quenching, the water temperature is room temperature 25°C, the water temperature after the steel plate quenching is completed can reach 80°C, and the quenching speed of water quenching is about 550°C / s.

[0033] Comparative Example 2 The difference from Example 1 is that after the annealed plate is completely austenitized, it is quenched to room temperature using quenching oil, that is, the quenching method is oil quenching, the oil temperature is room temperature 25°C, the quenching oil used is a universal quenching oil, and the quenching speed of the oil quenching is about 20°C / s.

[0034] Comparative Example 3 The difference from Example 2 is that after the annealed plate is completely austenitized, it is quenched to room temperature using cold water, that is, the quenching method is water quenching, the water temperature is room temperature 25°C, the water temperature after the steel plate quenching is completed can reach 80°C, and the quenching speed of water quenching is about 550°C / s.

[0035] Comparative Example 4 The difference from Example 2 is that after the annealed plate is completely austenitized, it is quenched to room temperature using quenching oil, that is, the quenching method is oil quenching, the oil temperature is room temperature 25°C, the quenching oil used is a universal quenching oil, and the quenching speed of the oil quenching is about 20°C / s.

[0036] 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, model CMT5105 and CMT5605, respectively. The annealed steel plates with a tensile strength lower than 1000 MPa were tested on a CMT5105 tensile machine, and the quenched steel plates were tested on a CMT5605 tensile machine. The tensile specimens were cut along the rolling direction by wire cutting with a gauge length of 25 mm, and the parallel section width was 6.0 mm. The shape and size parameters of the specimens were based on the national standard for tensile tests of metal materials, Part 1: Room temperature test methods (GB / T 228.1-2010). The uniaxial tensile speed was 1.0 mm / min, and the test conditions were room temperature. The main mechanical properties of the specimens were tensile strength, total elongation, and yield strength (the continuous yield was taken as the stress value corresponding to the plastic strain reaching 0.2%, and the discontinuous yield was taken as the lower yield strength).

[0037] Comparative Example 5 The Mn-B hot-forming steel reported in the doctoral dissertation "Research on the Strengthening and Toughening Mechanism and Application Technology of 2000MPa Hot-Forming Steel" of Beijing University of Science and Technology was selected. The chemical composition of the steel is as follows: 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 is Fe and unavoidable impurities. The quenching method is quenching with nitrogen, and the cooling rate is 45℃ / s. The steel improves hardenability by using the B element and strictly avoids the introduction of the N element, while the embodiment removes the B element, ensures good hardenability by using the Cr element, and introduces the N element to promote the precipitation of vanadium carbonitride; in addition, the comparative example forms a nano-scale NbC precipitation phase by using the Nb element and the C element to refine the structure, while the embodiment utilizes the VN microalloying technology, and the vanadium carbonitride precipitation particles not only refine the grains and pin the dislocation movement, but also capture the free hydrogen atoms in the steel, thereby improving the hydrogen embrittlement resistance of the high-strength steel.

[0038] Comparative Example 6 The Mn-Cr hot-formed steel reported in the Chinese invention patent 202111187719.7 "High-strength hot-formed steel substrate with tensile strength greater than 2200MPa and preparation method" was selected. The chemical composition of the steel is as follows by 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 unavoidable impurities. The quenching method is also nitrogen, and the cooling rate is 10℃ / s. The main difference between this steel and the embodiment is that the composite addition of Nb and V microalloying elements and C elements forms a nanoscale (V, Nb)C precipitation phase, which plays a role in refining the structure and binding hydrogen atoms, while the embodiment uses VN microalloying technology, and the vanadium carbonitride precipitation phase not only plays a role in refining grains and pinning dislocations, but also can capture free hydrogen atoms in the steel, playing a role in improving the hydrogen embrittlement resistance of high-strength steel; in addition, the steel adopts low Si elements, which cannot suppress the generation of carbides during quenching and tempering, and obtains a full martensitic structure. In order to obtain excellent plasticity, the embodiment introduces a higher Si element, suppresses the generation of carbides during air cooling, stabilizes a certain volume fraction of metastable austenite, and thus ensures an elongation higher than 10.0%.

[0039] Comparative Example 7 The air-cooled strengthened steel reported in Chinese invention patent 201910248764.5 "A cold-rolled low-alloy high-strength air-cooled strengthened steel for automobiles and its preparation method" is selected. The chemical composition of the steel is calculated by 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 unavoidable impurities. The component is austenitized at high temperature of 900 ° C and then air-cooled to room temperature to obtain high-strength and good toughness automotive parts. The main difference between this comparative example and the embodiment is that its tensile strength is only at the level of 1100 MPa, the strength needs to be further improved, and a variety of micro-alloy elements are introduced, and the cost performance is not high.

[0040] Table 2 Mechanical properties of examples and comparative examples ,

[0041] Figure 1 This is a SEM microstructure photograph of the air-cooled hardened steel quenching plate of Example 1 of the present invention. Figure 3 The engineering stress-strain curves are for the annealed plate and quenched plate of the air-cooled hardened steel of Example 1 of the present invention. From the mechanical properties of the examples and comparative examples in Table 2, it can be seen that after the air-cooled hardened steel cold-rolled plate is heated to 650-750°C and kept warm for 2-6 hours and then air-cooled to room temperature, the annealed plates of the examples and comparative examples have good forming properties, which is conducive to cold stamping. Subsequently, the annealed plates were subjected to a heat treatment process with different quenching methods (air quenching, water quenching, oil quenching) after complete austenitization. The results show that due to the excellent hardenability of the test steel, 90% or more dislocation martensite can be obtained in the example during the air quenching process (see Figure 2 ), achieving a good performance match of tensile strength > 2100MPa and elongation > 10.0%. Martensite ensures that the experimental steel has excellent strength, and the air quenching has a much lower cooling rate than water quenching and oil quenching, so that a small amount of metastable austenite is retained to room temperature, thereby improving the plasticity and toughness of the experimental steel. The comparative example has higher strength but poorer elongation after water quenching and oil quenching, mainly due to the fast quenching speed and large residual stress, which deteriorates the plasticity of the experimental steel. In summary, the alloy composition of the patent of the present invention is optimized and the preparation method is simple. Air-cooled hardened steel with a strength of more than 2100MPa can be prepared, which plays a very important role in achieving lightweight high-strength automotive steel and reducing production costs.

[0042] 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 preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention can be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the present invention.

Claims

1. A Mn-Cr-VN series high strength and plastic air-cooling hardening steel, characterized in that: It contains carbon, silicon, manganese, chromium, vanadium, nitrogen, phosphorus, sulfur, and the rest is iron; 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 is less than 0.01%, and the mass fraction of sulfur is less than 0.01%.

2. The Mn-Cr-VN high strength and plastic air-cooling hardening steel according to claim 1, characterized in that: 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.

3. The Mn-Cr-VN high strength and plastic air-cooling hardening steel according to claim 2, characterized in that: 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.

4. The Mn-Cr-VN high strength and plastic air-cooling hardening steel according to claim 1, characterized in that: It also has the following features: (i) The microstructure is more than 90% dislocation martensite; (ii) tensile strength greater than 2100 MPa; (iii) The elongation is greater than 10.0%.

5. A method for preparing Mn-Cr-VN series high-strength plastic 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 4, 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.

6. The preparation method according to claim 5, characterized in that: The smelting is vacuum induction smelting to produce molten steel, which is then cast into ingots.

7. The preparation method according to claim 6, 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.

8. The preparation method according to claim 7, 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.

9. The preparation method according to claim 8, 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%.

10. The preparation method according to claim 5, 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.

Images