A martensitic steel and a method for producing the same

Abstract

The present application relates to the technical field of metal materials, and particularly relates to a martensitic steel and a preparation method thereof, the martensitic steel provided by the present application comprises, by weight percentage, 0.14%<=C<=0.16%, 0.03%<=N<=0.05%, 1.1%<=Si<=1.3%, 10%<=Cr<=11.0%, 0.4%<=Mn<0.6%, 1.2%<=W<=1.3%, 0.12%<=Ta<=0.15%, 0.18%<=V<=0.2%, 0%<Zr<=0.02%, and the balance is Fe and inevitable impurities. 23 The martensitic steel provided by the present application changes the material from relying on M6 carbide strengthening to relying on nanoscale MX type (carbon) nitride strengthening with extremely high thermal stability, and is assisted by the synergistic regulation of specific micro-alloying elements, so that the martensitic steel has excellent liquid lead bismuth corrosion resistance and excellent high-temperature creep resistance.

Description

Technical Field

[0001] This invention relates to the field of metallic materials technology, and in particular to a martensitic steel and its preparation method. Background Technology

[0002] Lead-based reactors, with their inherent safety, high thermal efficiency, and excellent nuclear waste transmutation capabilities, have become the core reactor type for Generation IV advanced nuclear energy systems and accelerator-driven subcritical systems (ADS). Liquid lead-bismuth alloy (LBE), as a preferred coolant and spallation target material for such systems, possesses low vapor pressure, high thermal conductivity, high boiling point, and excellent neutronics properties. However, the performance degradation of core structural materials in a liquid lead-bismuth environment, especially the corrosion caused by liquid lead-bismuth and its synergistic effect with stress leading to liquid metal embrittlement, has become a significant technical bottleneck restricting the engineering application of lead-based reactors.

[0003] Ferritic / martensitic (F / M) steels, especially fully martensitic steels with a chromium content of 9-12 wt%, are considered important candidate structural materials for internal components of lead-based reactor cores (such as fuel cladding and core assemblies) due to their good high-temperature strength, high thermal conductivity, low coefficient of thermal expansion, and excellent resistance to radiation swelling. However, traditional commercial F / M steels (such as T91) face the dual challenges of corrosion and mechanical property degradation in the high-temperature (>550℃) and oxygen-fluctuating LBE environment: on the one hand, their chromium content is insufficient to ensure the formation of a dense oxide film with long-term protective properties, easily leading to severe corrosion; on the other hand, their long-term structural stability and creep strength at high temperatures are insufficient to meet the requirements of higher design temperatures and longer service life of reactors.

[0004] To address corrosion issues, existing technologies primarily rely on significantly increasing the chromium (Cr) and silicon (Si) content in steels such as T91 to form a stable Cr / Si-rich oxide layer on the surface. However, this alloying approach has a significant drawback: excessively high Cr and Si contents drastically increase the proportion of high-temperature δ-ferrite in the material. Therefore, existing technologies typically require maintaining a high carbon content (approximately 0.2 wt%) to suppress the formation of high-temperature δ-ferrite, making the high-temperature strengthening of these materials mainly dependent on M... 23 C6 (M is mainly Cr and Fe)-based carbides. Under long-term service conditions above 550°C, these M... 23 C6 carbides are highly susceptible to Ostwald ripening and rapid coarsening, which leads to a sharp decline in the strengthening effect of the material and a significant reduction in creep strength and fracture life.

[0005] In summary, while improving the LBE corrosion resistance, existing high-chromium and high-silicon corrosion-resistant steel grades often sacrifice high-temperature microstructure stability and long-term creep performance, making it difficult to solve the problem of synergistically improving corrosion resistance and high-temperature mechanical properties. This can no longer meet the stringent requirements for the development of lead-based reactors towards higher operating temperatures (>550°C) and long-life service, and has become the main bottleneck and core challenge in the current technical field. Summary of the Invention

[0006] The present invention provides a martensitic steel and a preparation method thereof, which is used to solve the problem that it is difficult to synergistically improve the corrosion resistance and high-temperature mechanical properties of existing high-chromium martensitic steels in a high-temperature (>550°C) liquid lead-bismuth environment.

[0007] According to a first aspect of the present invention, the present invention provides a martensitic steel, which comprises the following components by weight percentage: 0.14%≤C≤0.16%, 0.03%≤N≤0.05%, 1.1%≤Si≤1.3%, 10%≤Cr≤11.0%, 0.4%≤Mn<0.6%, 1.2%≤W≤1.3%, 0.12%≤Ta≤0.15%, 0.18%≤V≤0.2%, 0%<Zr≤0.02%, with the balance being Fe and unavoidable impurities.

[0008] The martensitic steel provided by the present invention adopts a composition design strategy of low carbon and high nitrogen, high chromium and high silicon, and multi-element composite strengthening. Among them, optimizing the contents of Cr and Si ensures the formation of a continuous and stable chromium-rich / silicon-rich protective oxide film on the surface, so as to provide excellent lead-bismuth corrosion resistance; regulating the contents of C and N inhibits the formation of δ-ferrite and fundamentally reduces the easily coarsened M 23 C₆ carbides. At the same time, the combination of C, N with V and Ta elements can in-situ precipitate nano-sized (V, Ta)(C, N)-type MX phases with high thermal stability in grains, which act as the core strengthening phase for high-temperature creep resistance and improve the creep resistance of the material; adding trace Zr elements promotes the precipitation of MX phases, refines M 23 C₆ carbide particles and purifies grain boundaries, further improving the high-temperature creep performance. Combined with the solid solution strengthening of Cr, Mn and W, a strengthening system dominated by thermally stable precipitated phases and synergized by multiple mechanisms is constructed, and finally, while ensuring the corrosion resistance, the microstructure stability and creep resistance of the material under high-temperature long-term service conditions are significantly improved.

[0009] The martensitic steel according to the present invention comprises, by weight percentage, the following components: 0.145%≤C≤0.155%, 0.035%≤N≤0.045%, 1.15%≤Si≤1.28%, 10.5%≤Cr≤10.8%, 0.5%≤Mn≤0.55%, 1.25%≤W≤1.28%, 0.13%≤Ta≤0.15%, 0.19%≤V≤0.2%, 0.01%≤Zr≤0.015%, with the balance being Fe and unavoidable impurities.

[0010] According to the martensitic steel of the present invention, the chromium equivalent Creq of the martensitic steel is 100×[Cr+0.8Si+1W+4V+2Zr+1Ta-(0.4Mn+20C+20N)]<10.

[0011] This invention is based on the chromium equivalent formula of key alloying elements to balance the content of austenite and ferrite forming elements, ensuring the acquisition of a uniform and fine fully tempered martensite matrix, providing an excellent foundation for the strengthening and toughening of materials.

[0012] According to the martensitic steel of the present invention, the unavoidable impurities and their contents, by weight percentage, satisfy the following conditions: O < 0.002%, H < 0.0001%, P < 0.005%, S < 0.004%, Nb < 0.01%, Cu < 0.01%, Ni < 0.01%, Ti < 0.008%, Al < 0.01%, Co < 0.005%.

[0013] The martensitic steel provided by this invention strictly controls the content of harmful impurities and activating elements such as phosphorus, sulfur, oxygen, nickel, cobalt, aluminum, and copper to achieve high purity and low activation characteristics of the material, thereby improving the stability of mechanical properties and radiation resistance, and meeting the stringent requirements of advanced nuclear energy systems for radioactivity.

[0014] According to the martensitic steel of the present invention, at least one of the following characteristics (1)-(6) is satisfied: (1) Yield strength at room temperature ≥580MPa (preferably 580-630MPa), tensile strength ≥800MPa (preferably 800-850MPa), elongation after fracture ≥20.0% (preferably 20.0-25.0%), impact energy ≥125J (preferably 125-145J); (2) At 550℃, the yield strength is ≥350MPa (preferably 350-400MPa), the tensile strength is ≥420MPa (preferably 420-470MPa), and the elongation after fracture is ≥28.0% (preferably 28.0-33.0%). (3) At 600℃, the yield strength is ≥260MPa (preferably 260-310MPa), the tensile strength is ≥340MPa (preferably 340-390MPa), and the elongation after fracture is ≥32.0% (preferably 32.0-37.0%). (4) At 650℃, the yield strength is ≥180MPa (preferably 180-230MPa), the tensile strength is ≥260MPa (preferably 260-310MPa), and the elongation after fracture is ≥35.0% (preferably 35.0-40.0%). (5) After corrosion in liquid lead-bismuth at 600°C with saturated oxygen concentration for 1000 hours, the oxide film thickness does not exceed 35 μm (preferably 30-35 μm). (6) The time of sustained fracture under stress of 600℃ and 160MPa is not less than 600h (preferably 600-700h).

[0015] According to a second aspect of the present invention, the present invention also provides a method for preparing the above-described martensitic steel, comprising the following steps: (1) Determine the raw material ratio according to the composition of martensitic steel and smelt it to obtain steel ingots; (2) The steel ingot is subjected to heat preservation treatment; (3) The steel ingots that have undergone heat preservation treatment are forged; (4) Normalizing and tempering are performed on the forged material.

[0016] The method for preparing martensitic steel provided by this invention controls the uniformity of composition, density of microstructure, and size and distribution of precipitated phases through a synergistic process design involving smelting, heat treatment and homogenization, forging, hot working, and heat treatment. This maximizes the advantages of compositional design and translates them into comprehensive macroscopic properties. Ultimately, this achieves simultaneous optimization and significant improvement of key properties such as corrosion resistance, high-temperature creep resistance, microstructure stability, and toughness in the harsh environment of liquid lead-bismuth.

[0017] According to the method for preparing martensitic steel of the present invention, the smelting is carried out by vacuum induction melting, or by a dual process of vacuum induction melting and vacuum consumable remelting.

[0018] This invention employs a dual smelting process of vacuum induction melting or vacuum induction + vacuum consumable remelting, which can remove harmful gases such as oxygen and hydrogen, as well as non-metallic inclusions, from molten steel to the greatest extent possible, significantly reducing the content of gases and impurities, and providing a high-purity ingot base for obtaining high-performance, high-toughness martensitic steel.

[0019] According to the method for preparing martensitic steel of the present invention, when the melting is carried out by vacuum induction melting, the process conditions include: vacuum degree ≤1.0Pa (preferably 0.01-1.0Pa), refining temperature 1530℃-1630℃, refining time ≥1h (preferably 1.0-2.0h), and casting temperature 1520℃-1570℃.

[0020] According to the method for preparing martensitic steel of the present invention, when the smelting adopts a dual process of vacuum induction melting and vacuum arc remelting, the process conditions include: vacuum induction melting stage: vacuum degree ≤ 1.0 Pa (preferably 0.01-1.0 Pa), refining temperature 1530℃-1630℃, refining time ≥ 1 h (preferably 1.0-2.0 h); vacuum arc remelting stage: vacuum degree ≤ 1.0 Pa (preferably 0.01-0.5 Pa), melting rate 2 kg / min-10 kg / min, cooling water temperature difference ΔT ≤ 15℃ (preferably 5-10℃).

[0021] According to the method for preparing martensitic steel of the present invention, the heat treatment temperature is 1140℃-1160℃ and the time is ≥4h (preferably 5.0-7.0h).

[0022] This invention optimizes the temperature and time of the heat preservation treatment, which can effectively eliminate dendrite segregation and homogenize alloy elements.

[0023] According to the method for preparing martensitic steel of the present invention, the initial forging temperature is 1080℃-1110℃, the final forging temperature is not lower than 850℃ (preferably 880-920℃), and the forging ratio is 5.5-8.

[0024] This invention optimizes the forging process conditions, which can fully break up the as-cast structure and weld the internal pores, significantly improve the density of the material and refine the grains, thereby improving the strength, toughness and subsequent hot working performance of the material.

[0025] According to the method for preparing martensitic steel of the present invention, the normalizing and tempering treatments are performed by holding at 1040℃-1060℃ for 20min-40min, followed by air cooling; then holding at 740℃-760℃ for 80min-110min, followed by air cooling.

[0026] This invention optimizes the process conditions for normalizing and tempering, which can eliminate forging residual stress and obtain a uniform fully tempered martensite structure. At the same time, it can precisely control and optimize the size and dispersion distribution of nanoscale MX precipitates, ultimately achieving a simultaneous leap in the material's corrosion resistance, high-temperature creep performance, and toughness.

[0027] The martensitic steel and its preparation method provided by this invention, through a compositional design of low carbon and high nitrogen, high chromium and high silicon, and multi-component composite strengthening, promotes a dense Cr / Si-rich protective oxide film on the surface to provide excellent resistance to liquid lead bismuth corrosion. At the same time, it inhibits easily coarsened carbides and precipitates in situ a nanoscale MX phase with high thermal stability. Combined with impurity control with high purity and low activation characteristics, and an integrated synergistic process of melting-homogenization-forging-heat treatment, a uniform, fine, fully tempered martensitic matrix without δ-ferrite is finally obtained. Thus, without sacrificing corrosion resistance, a comprehensive and synergistic leap is achieved in the material's high-temperature creep resistance, toughness, long-term structural stability and radiation resistance, perfectly meeting the stringent engineering application requirements of advanced lead-based reactors under higher design temperatures and long service life conditions. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0029] Figure 1 This is a metallographic diagram of a martensitic steel provided in Embodiment 2 of the present invention.

[0030] Figure 2 This is a metallographic diagram of a martensitic steel provided in Comparative Example 2 of the present invention.

[0031] Figure 3 This is a diagram showing the thickness and cross-sectional morphology of the corrosion layer of martensitic steel after 1000 hours of corrosion in saturated oxygen liquid lead-bismuth at 600°C, as provided in Example 1 of this invention.

[0032] Figure 4 The image shows the thickness and cross-sectional morphology of the corrosion layer of the martensitic steel provided in Comparative Example 1 of this invention after 1000 hours of corrosion by liquid lead-bismuth at 600°C with saturated oxygen concentration. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0034] Example 1 This embodiment provides a martensitic steel with the following chemical composition by weight percentage: C: 0.146%, N: 0.04%, Si: 1.16%, Cr: 10.6%, Mn: 0.52%, W: 1.25%, Ta: 0.13%, V: 0.2%, Zr: 0.015%, O: 0.0018%, H: 0.00005%, P: 0.004%, S: 0.001%, Nb: 0.008%, Cu: 0.005%, Ni: 0.009%, Ti: 0.007%, Al: 0.008%, Co: 0.004%, with the balance being Fe and unavoidable impurities, wherein Creq = 9.81 < 10.

[0035] This embodiment also provides a method for preparing the martensitic steel, including the following steps: (1) The raw materials are smelted and cast in a vacuum induction furnace according to the composition ratio to obtain steel ingots: vacuum degree 0.1Pa purification, 1560℃ refining time 1.5h, 1560℃ casting.

[0036] (2) The obtained steel ingots are homogenized at high temperature: held at 1150℃ for 6 hours.

[0037] (3) The high-temperature homogenized steel ingot is forged: the initial forging temperature is 1107℃, the final forging temperature is 890℃, the forging ratio is 6.2, and the ingot is air-cooled to room temperature after forging to obtain the desired result. 30mm forged bar.

[0038] (4) Take relevant performance samples from the forged material and perform heat treatment: first, keep it at 1050℃ for 30 minutes and then air cool it, and then keep it at 750℃ for 90 minutes and air cool it.

[0039] Example 2 This embodiment provides a martensitic steel with the following chemical composition by weight percentage: C: 0.152%, N: 0.04%, Si: 1.28%, Cr: 10.8%, Mn: 0.55%, W: 1.28%, Ta: 0.15%, V: 0.19%, Zr: 0.012%, O: 0.0016%, H: 0.00005%, P: 0.004%, S: 0.0009%, Nb: 0.007%, Cu: 0.006%, Ni: 0.008%, Ti: 0.005%, Al: 0.009%, Co: 0.004%, with the balance being Fe and unavoidable impurities, wherein Creq = 9.978 < 10.

[0040] This embodiment also provides the same method for preparing the martensitic steel as in Embodiment 1.

[0041] Comparative Example 1 This comparative example provides a martensitic steel with the following chemical composition by weight percentage: C: 0.23%, Si: 1.20%, Cr: 10.7%, Mn: 0.60%, W: 1.17%, Ta: 0.11%, V: 0.19%, P: 0.004%, S: 0.0020%, Nb: 0.01%, Cu < 0.01%, Ti: 0.002%, Al < 0.005%, Co: 0.002%, with the balance being Fe and unavoidable impurities, wherein Creq = 8.86 < 10.

[0042] Comparative Example 2 This comparative example provides a martensitic steel with the following chemical composition by weight percentage: C: 0.12%, N: 0.02%, Si: 1.2%, Cr: 10.9%, Mn: 0.50%, W: 1.10%, Ta: 0.10%, V: 0.18%, Zr: 0.014%, O: 0.0015%, H: 0.0005%, P: 0.004%, S: 0.002%, Nb: 0.007%, Cu: 0.004%, Ni: 0.009%, Ti: 0.006%, Al: 0.009%, Co: 0.002%, with the balance being Fe and unavoidable impurities, wherein Creq = 10.808 > 10.

[0043] Comparative Example 3 This comparative example provides a martensitic steel with the following chemical composition by weight percentage: C: 0.142%, N: 0.042%, Si: 1.13%, Cr: 10.5%, Mn: 0.48%, W: 1.22%, Ta: 0.14%, V: 0.19%, O: 0.001%, H: 0.0005%, P: 0.004%, S: 0.001%, Nb: 0.005%, Cu: 0.002%, Ni: 0.005%, Ti: 0.003%, Al: 0.006%, Co: 0.003%, with the balance being Fe and unavoidable impurities, wherein Creq = 9.652 < 10.

[0044] The metallographic morphologies of Example 2 and Comparative Example 2 are as follows: Figure 1 and Figure 2 As shown, the microstructure of the martensitic steel in Example 2 is 100% fully martensitic, while obvious blocky ferrite can be found in the microstructure of the martensitic steel in Comparative Example 2. Calculations show that the Cr equivalent value of the steel in Example 2 is 9.978, while the Cr equivalent value of the martensitic steel in Comparative Example 2 is 10.808. Therefore, the Cr equivalent value must be strictly controlled to not exceed 10 to obtain a fully martensitic microstructure, thereby achieving excellent mechanical properties.

[0045] Strength performance tests were conducted on the martensitic steels of the examples and comparative examples, according to the national standards: GB / T228.1-2021 "Metallic materials, tensile testing—Part 1: Test method at room temperature", GB / T 228.2-2015 "Metallic materials, tensile testing—Part 2: Test method at high temperature", and GB / T 229-2020 "Metallic materials, Charpy pendulum impact test method". The test results are shown in Table 1. A comparison reveals that the room temperature and high temperature (550℃, 600℃, 650℃) strengths of the martensitic steels of the examples and comparative examples are basically equivalent, but the room temperature impact energy values ​​are significantly different. The martensitic steels of this invention (Examples 1 and 2) adopt an alloying approach of reducing carbon and increasing nitrogen, thereby reducing the amount of nitrogen (M) in the steel. 23 The amount and size of C6 precipitates significantly improve the room temperature impact energy.

[0046] Table 1

[0047] The creep rupture times of the above embodiments and comparative examples under 600℃ and 160MPa stress are shown in Table 2. Comparing Example 1 and Comparative Example 1, it can be found that although the short-term strength of Example 1 and Comparative Example 1 is basically the same, the long-term creep rupture performance of Example 1 is significantly improved. Therefore, the martensitic steel of the present invention can significantly improve the high-temperature creep resistance of the material. Comparing Example 1 and Comparative Example 3, it can be found that the only significant difference in composition is the Zr content. However, the creep rupture time of the martensitic steel of Comparative Example 3 is slightly worse than that of the martensitic steel of Example 1. Therefore, the addition of an appropriate amount of Zr element to the martensitic steel of the present invention is essential.

[0048] Table 2

[0049] Figure 3 and Figure 4 The images show the corrosion layer thickness and cross-sectional morphology of the martensitic steels of Example 1 and Comparative Example 1 after corrosion for 1000 hours in a liquid lead-bismuth corrosion environment with saturated oxygen concentration at 600°C. There is no significant difference in the corrosion cross-sectional morphology between the two. The corrosion layer thickness of the martensitic steel of the present invention (Example 1) is approximately 32 μm, while the corrosion layer thickness of the martensitic steel of Comparative Example 1 is approximately 36 μm. This demonstrates that the martensitic steel of the present invention exhibits superior resistance to liquid lead-bismuth corrosion.

[0050] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A martensitic steel, characterized in that, In terms of weight percentage, it comprises the following components: 0.14%≤C≤0.16%, 0.03%≤N≤0.05%, 1.1%≤Si≤1.3%, 10%≤Cr≤11.0%, 0.4%≤Mn<0.6%, 1.2%≤W≤1.3%, 0.12%≤Ta≤0.15%, 0.18%≤V≤0.2%, 0%<Zr≤0.02%, with the balance being Fe and unavoidable impurities.

2. The martensitic steel according to claim 1, characterized in that, In terms of weight percentage, it comprises the following components: 0.145%≤C≤0.155%, 0.035%≤N≤0.045%, 1.15%≤Si≤1.28%, 10.5%≤Cr≤10.8%, 0.5%≤Mn≤0.55%, 1.25%≤W≤1.28%, 0.13%≤Ta≤0.15%, 0.19%≤V≤0.2%, 0.01%≤Zr≤0.015%, with the balance being Fe and unavoidable impurities.

3. The martensitic steel according to claim 1, characterized in that, The chromium equivalent Creq of the martensitic steel satisfies: Creq=100×[Cr+0.8Si+1W+4V+2Zr+1Ta-(0.4Mn+20C+20N)]<10.

4. The martensitic steel according to claim 1, characterized in that, In terms of weight percentage, said unavoidable impurities and their contents satisfy the following conditions: O<0.002%, H<0.0001%, P<0.005%, S<0.004%, Nb<0.01%, Cu<0.01%, Ni<0.01%, Ti<0.008%, Al<0.01%, Co<0.005%.

5. The martensitic steel according to any one of claims 1-4, characterized in that, It satisfies at least one of the following features (1) to (6): (1) At room temperature, the yield strength is ≥580MPa, the tensile strength is ≥800MPa, the elongation after fracture is ≥20.0%, and the impact energy is ≥125J; (2) At 550°C, the yield strength is ≥350MPa, the tensile strength is ≥420MPa, and the elongation after fracture is ≥28.0%; (3) At 600°C, the yield strength is ≥260MPa, the tensile strength is ≥340MPa, and the elongation after fracture is ≥32.0%; (4) At 650°C, the yield strength is ≥180MPa, the tensile strength is ≥260MPa, and the elongation after fracture is ≥35.0%; (5) After corrosion in liquid lead-bismuth at 600°C with saturated oxygen concentration for 1000h, the thickness of the oxide film does not exceed 35μm; (6) Under the conditions of 600°C and 160MPa stress, the creep rupture time is not less than 600h.

6. The method for preparing martensitic steel according to any one of claims 1-5, characterized in that, It comprises the following steps: (1) Determining the raw material ratio according to the composition components of the martensitic steel and performing smelting to obtain a steel ingot; (2) Subjecting said steel ingot to a heat preservation treatment; (3) Performing forging processing on the steel ingot after the heat preservation treatment; (4) Performing normalizing and tempering treatments on the material after forging processing.

7. The method for preparing martensitic steel according to claim 6, characterized in that, Said smelting adopts vacuum induction smelting, or a duplex process of vacuum induction smelting and vacuum consumable remelting.

8. The method for preparing martensitic steel according to claim 6, characterized in that, The temperature of said heat preservation treatment is 1140°C-1160°C, and the time is ≥4h.

9. The method for preparing martensitic steel according to claim 6, characterized in that, The initial forging temperature of said forging processing is 1080°C-1110°C, the final forging temperature is not lower than 850°C, and the forging ratio is 5.5-8.

10. The method for preparing martensitic steel according to claim 6, characterized in that, The normalizing and tempering processes are performed by holding at 1040℃-1060℃ for 20-40 minutes, followed by air cooling; then holding at 740℃-760℃ for 80-110 minutes, followed by air cooling.

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