A method for designing high-strength martensitic stainless steel based on phase transition toughening

Abstract

The application provides a method for designing high-strength martensitic stainless steel based on phase change toughening, and relates to the technical field of performance control of martensitic stainless steel. The method for designing high-strength martensitic stainless steel based on phase change toughening comprises the following steps: raw material weighing and smelting, homogenization heat treatment-oil quenching treatment, solid solution treatment-oil quenching treatment, deep cooling treatment with temperature control, aging treatment for adjusting reverse transformation austenite content. The component content of the high-strength martensitic stainless steel is as follows in percentage by mass: Cr 10-12%, Ni 7-9%, Co 4-6%, Mo 2-4%, and the high-strength martensitic stainless steel also contains Mn, V, Si, C and other elements, and the rest is Fe and inevitable impurities. The method is simple and easy to operate, green and environmentally friendly, low in cost, short in process, high in efficiency, and beneficial to industrial large-scale production and popularization.

Description

Technical Field

[0001] This invention relates to the technical field of performance regulation of martensitic stainless steel, and in particular to a method for designing high-strength martensitic stainless steel based on phase transformation toughening. Background Technology

[0002] Martensitic stainless steel is an advanced high-strength steel whose properties are controlled through heat treatment. Characterized by acid and corrosion resistance, it typically contains ≥10.5% chromium and ≤1.2% carbon. A key characteristic is that its microstructure is primarily martensitic during use. The high dislocation density of the martensitic matrix endows it with high strength and hardness, while also exhibiting good wear resistance, heat resistance, and considerable ductility and toughness. Currently, it demonstrates significant application value in the petrochemical, military equipment, and automotive manufacturing industries. With the increasingly complex service environments of modern materials, higher demands are placed on the performance of steel materials. As a representative of the first generation of advanced high-strength steels, martensitic stainless steel's ultra-high strength makes it widely applicable in extreme environments and high-load applications, attracting extensive research from scholars worldwide.

[0003] Currently, the standardized heat treatment process for martensitic aging steel ingots is solution treatment followed by aging. While the martensitic matrix provides martensitic stainless steel with ultra-high strength, it also introduces the drawback of insufficient ductility and toughness, thus limiting its application range.

[0004] Chinese patent CN114921717A discloses a 2000MPa grade high-ductility, high-corrosion-resistant martensitic aging stainless steel and its preparation method. This method requires multiple melting processes and the coordinated preparation of hot working and heat treatment, resulting in a long process, high energy consumption, and high operational difficulty, leading to high preparation costs and low efficiency. Although a homogenization process is used, its purpose is for subsequent hot working, not for immediate heat treatment. Furthermore, the high-temperature quenching process uses a 0℃ ice-water mixture, which has a different mechanism and effect than oil quenching. In addition, the cryogenic treatment temperature is difficult to predict.

[0005] Chinese patent CN118932150A discloses a heat treatment method for additive manufacturing martensitic aging steel and a high-strength and high-toughness martensitic aging steel. It requires aging and annealing of the additively manufactured martensitic aging steel to obtain the product. Although the strength is not low, the elongation is not high, and the strength and plasticity are not synergistically improved. Moreover, it is designed for additive manufacturing martensitic steel and requires software calculation to obtain the aging treatment temperature, which is not suitable for industrial large-scale preparation.

[0006] Chinese patent CN114752741A discloses a method for improving the mechanical properties of 12Cr13 martensitic stainless steel, which requires combining quenching treatment with cold rolling deformation and tempering. Although it can improve strength and toughness, the improvement is not significant, and its strength and plasticity are not high. Summary of the Invention

[0007] To address the issue of improving the strength and ductility of martensitic stainless steel in existing technologies, a combination of compositional improvement, hot working, and heat treatment is generally employed. However, this approach suffers from several drawbacks, including high alloy costs, alteration of the casting microstructure by hot working resulting in the loss of the advantages of cast microstructure, unsuitable and difficult-to-control selection of quenching fluid composition and cryogenic treatment temperature during heat treatment, and the inability or limited effectiveness of subsequent aging, annealing, and tempering treatments in synergistically improving strength and ductility. This invention proposes a method for designing high-strength martensitic stainless steel based on phase transformation toughening, which overcomes these shortcomings. The technical solution is as follows:

[0008] A method for designing high-strength martensitic stainless steel based on phase transformation toughening, comprising the following steps:

[0009] S1. Raw material weighing and smelting: The composition of high-strength martensitic stainless steel is designed, the raw materials are weighed according to the chemical composition, and then the weighed raw materials are added to a vacuum induction furnace for smelting. The molten steel obtained from smelting is then cast into ingots.

[0010] S2. Homogenization heat treatment - oil quenching: After heating the S1 ingot, homogenization heat treatment is performed, followed by oil quenching to obtain a steel ingot with a uniform structure.

[0011] S3, Solution treatment-oil quenching: The S2 uniformly structured steel ingot is heated for solution treatment, and then oil quenched to obtain a solution-treated steel ingot.

[0012] S4. Temperature-controlled cryogenic treatment: S3 solution-treated steel ingots are subjected to liquid nitrogen cryogenic treatment to obtain cryogenic steel ingots;

[0013] S5. Aging treatment to adjust the content of reverse austenite: The S4 cryogenic steel ingot is aged to adjust the content of reverse austenite, and then air-cooled to room temperature to obtain high-strength and ductile martensitic stainless steel.

[0014] Optionally, the composition of the high-strength martensitic stainless steel in S1, by mass percentage, is: Cr 10-12%, Ni 7-9%, Co 4-6%, Mo 2-4%, and also contains Mn, V, Si, C and other elements, with the remainder being Fe and unavoidable impurities.

[0015] Optionally, the total content of Mn, V, Si, C and other elements in S1 is 0.6-0.8%.

[0016] Optionally, the heating rate for homogenization heat treatment in S2 is 3-5℃ / min, the temperature is 950-1050℃, and the holding time is 60min; the oil temperature for oil quenching treatment is 50-80℃, and the oil quenching time is 3-5min.

[0017] Optionally, the heating rate for solution treatment in S3 is 3-5℃ / min, the temperature is 750-800℃, and the holding time is 60min; the oil temperature for oil quenching is 50-80℃, and the oil quenching time is 2-3min.

[0018] Optionally, the temperature for cryogenic treatment with liquid nitrogen in S4 is -73°C, and the treatment time is 100-140 min.

[0019] Optionally, the heating rate for aging treatment in S5 is 3-5℃ / min, the temperature is 300-600℃, and the holding time is 2-8h; the high-strength and ductile martensitic stainless steel is in block shape with dimensions of 40-44×6-10×6-9mm; room temperature properties: hardness 370.5HV-419.1HV, tensile strength 1227.3-1363.0MPa, yield strength 802.6-1242.1MPa, yield ratio 0.746-0.911, elongation at break 20.1-27.6%, reduction of area 68.9-84.9%, and strength-ductility product 25.220-33.874GPa·s.

[0020] Optionally, the volume percentage of reverse-transformed austenite in S5 high-strength and high-ductility martensitic stainless steel is 30.9-53.5%, and the volume percentage of reverse-transformed austenite is 46.5-69.1%.

[0021] Optionally, when the content of precipitated reverse-transformed austenite in S5 is <53.5%, the strength will increase with the increase of austenite content; when the content of precipitated reverse-transformed austenite is >53.5%, the strength will decrease with the increase of reverse-transformed austenite content.

[0022] Optionally, when S5 is heat-treated at 300℃ for 8 hours, the reverse-transformed austenite content is 23.2%, and the room temperature properties are: hardness 386.9 HV, tensile strength 1254.7 MPa, yield strength 936.2 MPa, yield ratio 0.746, elongation at break 20.1%, reduction of area 68.9%, and strength-ductility product 25.220 GPa·s.

[0023] Optionally, when S5 is heat-treated at 400℃ for 4 hours, the reverse-transformed austenite content is 30.9%, and the room temperature properties are: hardness 419.1 HV, tensile strength 1358.3 MPa, yield strength 1195.4 MPa, yield ratio 0.880, elongation at break 20.8%, reduction of area 74.3%, and strength-ductility product 28.253 GPa·s.

[0024] Optionally, when S5 is heat-treated at 500℃ for 4 hours, the reverse-transformed austenite content is 53.5%, and the room temperature properties are: hardness 418.7 HV, tensile strength 1363.0 MPa, yield strength 1242.1 MPa, yield ratio 0.911, elongation at break 21.7%, reduction of area 76.2%, and strength-ductility product 29.578 GPa·s.

[0025] Optionally, S5, after heat treatment at 500℃ for 6 hours, exhibits the following properties at room temperature: hardness 414.1 HV, tensile strength 1317.2 MPa, yield strength 1169.4 MPa, yield-to-tensile ratio 0.888, elongation at break 20.5%, reduction of area 72.1%, and strength-ductility product 27.003 GPa·%.

[0026] Optionally, when S5 is heat-treated at 600℃ for 2 hours, the reverse-transformed austenite content is 92.2%, and the room temperature properties are: hardness 370.5 HV, tensile strength 1127.3 MPa, yield strength 802.6 MPa, yield ratio 0.806, elongation at break 27.6%, reduction of area 84.9%, and strength-ductility product 33.874 GPa·s.

[0027] Technical principle of the invention:

[0028] This invention relates to a method for improving the strength and plasticity of FeCrNiCoMo martensitic stainless steel by controlling the reversible austenite content during the phase transformation behavior of martensitic stainless steel during tensile testing. The method investigates the influence of different reversible austenite contents on the tensile strength and elongation at break of FeCrNiCoMo. With increasing reversible austenite content, the tensile strength initially increases and then decreases, while the elongation at break steadily increases. Based on this finding, the strength and plasticity of FeCrNiCoMo martensitic stainless steel can be designed by altering the reversible austenite content through heat treatment process design. This method is applicable to heat-treatable martensitic stainless steels with a main composition of approximately 11% Cr, 8% Ni, 5% Co, and 3% Mo, and also contains alloying elements such as Mn, V, Si, and C. According to this method, adjusting the heat treatment process to change the reversible austenite content in martensitic stainless steel provides a reasonable reference for the strengthening and toughening design of martensitic stainless steel.

[0029] The process flow adopted by the present invention to achieve the above objectives is as follows: heat treatment of the sample → regulation of the precipitation of reverse austenite in martensitic stainless steel → enhancement and plasticization through phase transformation.

[0030] Heat treatment of the samples: FeCrNiCoMo martensitic stainless steel is a steel material that can be strengthened by aging treatment. Different aging temperatures are selected: above 700℃, it belongs to the chemically stable austenite state, rather than the reverse-transformed austenite. In addition, in this composition system, the reverse-transformed austenite in martensitic stainless steel begins to precipitate at around 400℃. Therefore, the selected temperature range is 300-600℃, and the selected aging time range is 2-8h. This method is applicable to martensitic stainless steels that can be strengthened by phase transformation in the Fe11Cr8Ni5Co3Mo composition system.

[0031] Controlling the precipitation of reverse-transformed austenite in martensitic stainless steel: This method first homogenizes the martensitic stainless steel at 1000℃ for 1 hour, followed by oil quenching to promote complete diffusion of alloying elements. Then, it performs solution treatment at 760℃ for 1 hour, followed by oil quenching to increase the dislocation density in the reverse-transformed austenite. Next, it undergoes deep cryogenic treatment at -73℃ for 2 hours to promote complete martensitic transformation. Finally, it is aged at 300-600℃ for 2-8 hours and then air-cooled to obtain reverse-transformed austenite with a volume fraction of approximately 23.2-92.2%. This prepares the microstructure for the TRIP (transformation-induced plasticity) effect of reverse-transformed austenite during tensile testing.

[0032] Enhanced Plasticity Through Phase Transformation: This method uses tensile strength as the strength metric for FeCrNiCoMo martensitic stainless steel and elongation at break as the plasticity metric. For strength control, the reverse austenite-to-martensite transformation during stretching compresses the nearby martensite phase, increasing dislocation density and thus strength. For plasticity control, the reverse austenite-to-martensite phase transformation during stretching absorbs energy, thereby improving plasticity. In summary, the strong-plasticity design leverages the TRIP effect of austenite during stretching to enhance both strength and plasticity.

[0033] The theoretical basis of this invention is that as-cast martensitic stainless steel was first prepared, and then FeCrNiCoMo martensitic stainless steels after different aging processes were designed. The phase compositions under each process are as follows: 300℃ heat treatment for 8h: martensitic stainless steel containing 23.2% reverse austenitic phase; 400℃ heat treatment for 4h: martensitic stainless steel containing 30.9% reverse austenitic phase; 500℃ heat treatment for 4h: martensitic stainless steel containing 53.5% reverse austenitic phase; 500℃ heat treatment for 6h: martensitic stainless steel containing 62.2% reverse austenitic phase; 600℃ heat treatment for 2h: martensitic stainless steel containing 92.2% reverse austenitic phase. Because the reverse austenitic phase after solution treatment further inherits the high dislocation density of martensite, and stress-induced martensitic phase transformation occurs during tensile testing, causing lattice distortion and introducing a large number of dislocations, the material strength is significantly improved. Solid solution strengthening has a limited contribution to martensitic stainless steel due to the relatively small degree of lattice distortion caused by substitutional solid solution strengthening and the low carbon content. Furthermore, this heat treatment process focuses on increasing dislocation density through phase transformation to generate martensite; therefore, this steel grade mainly relies on phase transformation strengthening and dislocation strengthening mechanisms. Because there is a certain degree of mutual exclusion between strength and plasticity in high-strength steel, higher plasticity hinders the improvement of strength. According to experimental results, more reverse-transformed austenite precipitates with increasing aging temperature and aging time. At 300℃, the majority of the phase is martensite, resulting in high strength but poor plasticity. At 600℃ for 2 hours, the soft austenite content is the highest, and the plasticity is also the highest, but the strength is lower compared to the predominantly martensitic structure at 300℃. After aging treatment at 400℃ and 500℃, some martensite underwent a phase transformation into austenite. While the plasticity was improved, the strength was also improved by the TRIP effect of the reverse austenite during the tensile process. Moreover, the final strength was higher than that of the sample at 300℃, which was mostly martensitic. This indicates that the reverse austenite under this process maximizes the TRIP effect through stress-induced martensitic phase transformation. It does not limit the influence of the phase transformation on mechanical properties due to the small volume fraction of austenite, nor does it limit the high strength due to the large amount of soft austenite remaining after the phase transformation.

[0034] The key feature of this invention is that after heat treatment of FeCrNiCoMo martensitic stainless steel, it was found that reverse austenite undergoes a phase transformation during tensile testing. With increasing reverse austenite content, the tensile strength initially increases and then decreases, while the elongation at break shows a steady increasing trend. In FeCrNiCoMo martensitic stainless steel, strength and plasticity exhibit a certain degree of mutual exclusion. At an aging temperature of 600℃, the austenite content reaches over 90%, and the elongation exceeds 76%, but the strength decreases by approximately 100 MPa. When aged at 400℃ for 4 hours and 500℃ for 4 hours, 30.9% and 53.5% reverse austenite are generated, respectively. These reverse austenite undergo a phase transformation and the TRIP effect during tensile testing, increasing both the strength and plasticity of the martensitic stainless steel. Therefore, it exhibits high strength and good plasticity, with tensile strengths reaching 1358.3 MPa and 1363.0 MPa, and elongation at break reaching 20.8% and 21.7%, respectively.

[0035] Brief description of the present invention:

[0036] 1) Heat treatment of the sample. First, FeCrNiCoMo martensitic stainless steel was prepared. In order to obtain higher strength and hardness, it was necessary to perform heat treatment. First, it was homogenized at 1000℃ for 1 hour and then oil quenched to ensure complete recrystallization of austenite and promote the full diffusion of alloying elements. Then, it was solution treated at 760℃ for 1 hour and then oil quenched to increase the dislocation density of reverse austenite. After deep cryogenic treatment at -73℃ for 2 hours, the complete transformation of martensite was promoted, which prepared the microstructure for the subsequent precipitation process of reverse austenite.

[0037] 2) Controlling the precipitation of reverse-transformed austenite in martensitic stainless steel. Martensitic stainless steels with varying contents of reverse-transformed austenite were precipitated through aging processes at 300-600℃ for 2-8 hours. During tensile testing, these reverse-transformed austenites, due to their different stability, undergo phase transformation after applying a certain level of stress, altering the phase distribution in the steel and ultimately affecting the performance of the martensitic stainless steel. By summarizing the influence of reverse-transformed austenite content on the strength and plasticity of martensitic stainless steel, the control of the strength and plasticity of FeCrNiCoMo martensitic stainless steel was achieved.

[0038] 3) Enhancement and plasticity through phase transformation. To design FeCrNiCoMo martensitic stainless steel with high strength and plasticity, the aging process can be controlled to achieve a reversible austenite precipitation rate within the range of 30.9%-53.5%. To design martensitic stainless steel with high plasticity, the aging process can be controlled to achieve a reversible austenite precipitation rate of over 90%.

[0039] The above technical solution has at least the following advantages compared with the existing technology:

[0040] The above-mentioned solution proposes a method for designing high-strength martensitic stainless steel based on phase transformation toughening. This method can solve the problems in the prior art where the strength and plasticity of martensitic stainless steel are generally improved by a combination of composition improvement, hot working and heat treatment. However, this method has technical defects such as high alloy cost, hot working will change the casting structure and lose the advantages of the casting structure, unsuitable selection of quenching liquid composition and deep cryogenic treatment temperature during heat treatment, difficulty in control, and subsequent aging treatment, annealing and tempering treatment cannot achieve synergistic improvement or the improvement is not significant.

[0041] This invention obtains steel ingots through raw material design, weighing, and smelting, ensuring good metallurgical quality and providing a uniform chemical composition, which helps enhance the stability of phase transformations during subsequent heat treatment. It avoids deviations in the content of alloying elements, which could adversely affect strength and plasticity, ensuring the consistency of the steel ingot's composition and structure, and guaranteeing the reliability and repeatability of large-scale production processes.

[0042] This invention utilizes homogenization heat treatment followed by oil quenching to ensure sufficient diffusion of alloying elements, reduce segregation, and achieve fully recrystallized austenite. This guarantees uniform phase transformation in the steel ingot and prevents differences in strength and hardness caused by localized structural inhomogeneities during subsequent oil quenching. Compared to water quenching, oil quenching has a slower cooling rate, reducing potential cracks and deformations that may occur during rapid cooling. Oil quenching also makes the cooling rate and transformation behavior more controllable, improving the consistency and stability of the production process.

[0043] This invention utilizes solution treatment followed by oil quenching to allow the austenite to inherit part of the dislocation density of the martensite matrix and the grain orientation of the original austenite, thus preparing the microstructure for aging treatment. Compared to water quenching, oil quenching is gentler, reducing internal stress and deformation caused by excessive temperature gradients, improving the overall stability of the steel ingot, preserving some of the microstructure's toughness, and significantly reducing performance fluctuations caused by water quenching. This ensures consistent performance across batches during mass production.

[0044] This invention promotes martensitic transformation at low temperatures through controlled cryogenic treatment, thereby strengthening the microstructure of the steel ingot and improving its low-temperature performance. After cryogenic treatment, the steel ingot's microstructure becomes more stable, and its performance does not degrade after long-term service, ensuring consistent performance over extended periods.

[0045] This invention, through aging treatment that adjusts the content of reverse-transformed austenite, enables austenite to transform into martensite under stress-induced phase transformation, thus optimizing the phase transformation process of the steel ingot. The precipitated reverse-transformed austenite can exert the TRIP (transformation-induced plasticity) effect, absorbing the energy required for the austenite-to-martensite phase transformation, increasing the toughness of the steel ingot, while delaying necking, improving strain uniformity, and enhancing the plasticity of the steel ingot.

[0046] The high-strength and ductile martensitic stainless steel of this invention is in block shape with dimensions of 40-44×6-10×6-9mm; its room temperature properties are as follows: hardness 370.5HV-419.1HV, tensile strength 1227.3-1363.0MPa, yield strength 802.6-1242.1MPa, yield ratio 0.746-0.911, elongation at break 20.1-27.6%, reduction of area 68.9-84.9%, and strength-ductility product 25.220-33.874GPa·s.

[0047] In summary, compared with other traditional methods, the method of this invention prepares high-strength and ductile martensitic stainless steel through raw material design, weighing and smelting, homogenization heat treatment-oil quenching, solution treatment-oil quenching, deep cryogenic treatment with controlled temperature, and aging treatment to adjust the content of reverse austenite. This method is simple to operate, environmentally friendly, low in cost, short in process, and highly efficient, which is conducive to large-scale industrial production and promotion. Attached Figure Description

[0048] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0049] Figure 1 These are X-ray diffraction comparison images of high-strength martensitic stainless steel based on phase transformation toughening design in Examples 1-5 of this invention after heat treatment with different processes for 2-8 hours.

[0050] Figure 2 This is the electron backscatter diffraction pattern of the sample of high-strength martensitic stainless steel based on phase transformation toughening design in Embodiment 1 of the present invention after heat treatment at 300℃ for 8 hours.

[0051] Figure 3 This is the electron backscatter diffraction pattern of a sample of high-strength martensitic stainless steel based on phase transformation toughening design after heat treatment at 400℃ for 4 hours in Embodiment 2 of the present invention.

[0052] Figure 4This is the electron backscatter diffraction pattern of the sample of high-strength martensitic stainless steel based on phase transformation toughening design in Embodiment 3 of the present invention after heat treatment at 500℃ for 4 hours.

[0053] Figure 5 This is the electron backscatter diffraction pattern of the sample of high-strength martensitic stainless steel based on phase transformation toughening design in Example 4 of the present invention after heat treatment at 500℃ for 6 hours.

[0054] Figure 6 This is the electron backscatter diffraction pattern of the sample of high-strength martensitic stainless steel based on phase transformation toughening design in Example 5 of the present invention after heat treatment at 600℃ for 2 hours.

[0055] Figure 7 This is a comparison chart of austenite content and heat treatment process curves of high-strength martensitic stainless steel samples based on phase transformation toughening design after 2-8 hours of heat treatment at 300-600℃.

[0056] Figure 8 These are comparison diagrams of the true stress-strain curves of high-strength martensitic stainless steel samples based on phase transformation toughening design after heat treatment at different temperatures for 2-8 hours, according to Examples 1-5 of this invention.

[0057] Figure 9 This is a statistical chart comparing the austenite content and mechanical properties of samples of high-strength martensitic stainless steel based on phase transformation toughening design after heat treatment at different temperatures for 2-8 hours, according to Examples 1-5 of this invention. Detailed Implementation

[0058] The technical solution of the present invention will now be described with reference to the accompanying drawings.

[0059] In embodiments of the present invention, words such as "exemplarily," "for example," etc., are used to indicate that something is an example, illustration, or description. Any embodiment or design described as "exemplary" in the present invention should not be construed as being more preferred or advantageous than other embodiments or designs. Specifically, the use of the word "exemplary" is intended to present the concept in a concrete manner. Furthermore, in embodiments of the present invention, the meaning expressed by "and / or" can be both, or either one.

[0060] In the embodiments of the present invention, the terms "image" and "picture" may sometimes be used interchangeably. It should be noted that when the distinction is not emphasized, their intended meanings are consistent.

[0061] In this embodiment of the invention, sometimes a subscript such as W1 may be written in a non-subscript form such as W1. When the difference is not emphasized, the meaning they express is the same.

[0062] To make the technical problems, technical solutions and advantages of the present invention clearer, a detailed description will be given below in conjunction with the accompanying drawings and specific embodiments.

[0063] A method for designing high-strength martensitic stainless steel based on phase transformation toughening, comprising the following steps:

[0064] S1. Raw material weighing and smelting: The composition of high-strength martensitic stainless steel is designed, the raw materials are weighed according to the chemical composition, and then the weighed raw materials are added to a vacuum induction furnace for smelting. The molten steel obtained from smelting is then cast into ingots.

[0065] S2. Homogenization heat treatment - oil quenching: After heating the S1 ingot, homogenization heat treatment is performed, followed by oil quenching to obtain a steel ingot with a uniform structure.

[0066] S3, Solution treatment-oil quenching: The S2 uniformly structured steel ingot is heated for solution treatment, and then oil quenched to obtain a solution-treated steel ingot.

[0067] S4. Temperature-controlled cryogenic treatment: S3 solution-treated steel ingots are subjected to liquid nitrogen cryogenic treatment to obtain cryogenic steel ingots;

[0068] S5. Aging treatment to adjust the content of reverse austenite: The S4 cryogenic steel ingot is aged to adjust the content of reverse austenite, and then air-cooled to room temperature to obtain high-strength and ductile martensitic stainless steel.

[0069] Specifically, the composition of high-strength martensitic stainless steel in S1, by mass percentage, is: Cr 10-12%, Ni 7-9%, Co 4-6%, Mo 2-4%, and also contains Mn, V, Si, C and other elements, with the remainder being Fe and unavoidable impurities.

[0070] Specifically, the total content of Mn, V, Si, C and other elements in S1 is 0.6-0.8%.

[0071] Specifically, in S2, the heating rate for homogenization heat treatment is 3-5℃ / min, the temperature is 950-1050℃, and the holding time is 60min; the oil temperature for oil quenching treatment is 50-80℃, and the oil quenching time is 3-5min.

[0072] Specifically, in S3, the heating rate for solution treatment is 3-5℃ / min, the temperature is 750-800℃, and the holding time is 60min; the oil temperature for oil quenching is 50-80℃, and the oil quenching time is 2-3min.

[0073] Specifically, the temperature for cryogenic treatment with liquid nitrogen in S4 is -73°C, and the treatment time is 100-140 minutes.

[0074] Specifically, the heating rate for aging treatment in S5 is 3-5℃ / min, the temperature is 300-600℃, and the holding time is 2-8h; the high-strength and ductile martensitic stainless steel is in block shape with dimensions of 40-44×6-10×6-9mm; room temperature properties: hardness 370.5HV-419.1HV, tensile strength 1227.3-1363.0MPa, yield strength 802.6-1242.1MPa, yield ratio 0.746-0.911, elongation at break 20.1-27.6%, reduction of area 68.9-84.9%, and strength-ductility product 25.220-33.874GPa·s.

[0075] Specifically, the volume percentage of reverse-transformed austenite in S5 high-strength and high-ductility martensitic stainless steel is 30.9-53.5%, and the volume percentage of reverse-transformed austenite is 46.5-69.1%.

[0076] Specifically, when the content of precipitated reverse-transformed austenite in S5 is <53.5%, the strength increases with the increase of austenite content; when the content of precipitated reverse-transformed austenite is >53.5%, the strength decreases with the increase of reverse-transformed austenite content.

[0077] Specifically, S5, after heat treatment at 500℃ for 4 hours, exhibits a reverse-transformed austenite content of 53.5%, and its room temperature properties are as follows: hardness 418.7 HV, tensile strength 1363.0 MPa, yield strength 1242.1 MPa, yield-to-tensile ratio 0.911, elongation at break 21.7%, reduction of area 76.2%, and strength-ductility product 29.578 GPa·s.

[0078] Specifically, S5, after heat treatment at 300℃ for 8 hours, exhibits a reverse-transformed austenite content of 23.2%, and its room temperature properties are as follows: hardness 386.9 HV, tensile strength 1254.7 MPa, yield strength 936.2 MPa, yield-to-tensile ratio 0.746, elongation at break 20.1%, reduction of area 68.9%, and strength-ductility product 25.220 GPa·s.

[0079] Specifically, in S5, after heat treatment at 400℃ for 4 hours, the reverse-transformed austenite content is 30.9%, and the room temperature properties are as follows: hardness 419.1 HV, tensile strength 1358.3 MPa, yield strength 1195.4 MPa, yield ratio 0.880, elongation at break 20.8%, reduction of area 74.3%, and strength-ductility product 28.253 GPa·s.

[0080] Specifically, S5, after heat treatment at 500℃ for 6 hours, exhibits a reverse-transformed austenite content of 62.2%, and its room temperature properties are as follows: hardness 414.1 HV, tensile strength 1317.2 MPa, yield strength 1169.4 MPa, yield-to-tensile ratio 0.888, elongation at break 20.5%, reduction of area 72.1%, and strength-ductility product 27.003 Pa·s.

[0081] Specifically, S5, after heat treatment at 600℃ for 2 hours, exhibits a reverse-transformed austenite content of 92.2%, and its room temperature properties are as follows: hardness 370.5 HV, tensile strength 1227.3 MPa, yield strength 802.6 MPa, yield-to-tensile ratio 0.806, elongation at break 27.6%, reduction of area 84.9%, and strength-ductility product 33.874 GPa·s.

[0082] Example 1

[0083] A method for designing high-strength martensitic stainless steel based on phase transformation toughening, comprising the following steps:

[0084] S1. Raw material weighing and smelting: The composition of the high-strength martensitic stainless steel is designed. In this embodiment, the composition of the high-strength martensitic stainless steel by mass percentage is: Cr 10.9%, Ni 8.18%, Co 5.37%, Mo 2.67%, C 0.018%, with the remainder being Fe and unavoidable impurities. The raw materials are weighed according to the chemical composition, and then the weighed raw materials are added to a vacuum induction furnace for smelting. The molten steel obtained from smelting is then cast into ingots.

[0085] S2. Homogenization heat treatment - oil quenching: After heating the S1 ingot, homogenization heat treatment is performed at a heating rate of 5℃ / min, a temperature of 1000℃, and a holding time of 60min. Then, oil quenching is performed at an oil temperature of 60℃ for 4min to obtain a steel ingot with a uniform structure.

[0086] S3, Solution Treatment-Oil Quenching: The S2 uniformly structured steel ingot is heated for solution treatment at a heating rate of 5℃ / min, a temperature of 760℃, and a holding time of 60min; then it is subjected to oil quenching at an oil temperature of 60℃ for 2min to obtain a solution-treated steel ingot.

[0087] S4. Temperature-controlled cryogenic treatment: S3 solution-treated steel ingots are subjected to liquid nitrogen cryogenic treatment at a temperature of -73°C for 120 minutes to obtain cryogenic steel ingots.

[0088] S5. Aging treatment to adjust the reverse austenite content: The S4 cryogenic steel ingot is aged to adjust the reverse austenite content. The heating rate of the aging treatment is 5℃ / min, the temperature is 300℃, and the holding time is 8h. It is then air-cooled to room temperature to obtain high-strength and ductile martensitic stainless steel.

[0089] The high-strength and ductile martensitic stainless steel prepared in this embodiment is in block shape with dimensions of 40×6×9mm; its room temperature properties are as follows: hardness 386.9HV, tensile strength 1254.7MPa, yield strength 936.2MPa, yield ratio 0.746, elongation at break 20.1%, reduction of area 68.9%, and strength-ductility product 25.220GPa·s.

[0090] Example 2

[0091] A method for designing high-strength martensitic stainless steel based on phase transformation toughening, comprising the following steps:

[0092] S1. Raw material weighing and smelting: The composition of the high-strength martensitic stainless steel is designed. In this embodiment, the composition of the high-strength martensitic stainless steel by mass percentage is: Cr 10.9%, Ni 8.18%, Co 5.37%, Mo 2.67%, C 0.018%, with the remainder being Fe and unavoidable impurities. The raw materials are weighed according to the chemical composition, and then the weighed raw materials are added to a vacuum induction furnace for smelting. The molten steel obtained from smelting is then cast into ingots.

[0093] S2. Homogenization heat treatment - oil quenching: After heating the S1 ingot, homogenization heat treatment is performed at a heating rate of 3℃ / min, a temperature of 1000℃, and a holding time of 60min. Then, oil quenching is performed at an oil temperature of 60℃ for 3min to obtain a steel ingot with a uniform structure.

[0094] S3, Solution Treatment-Oil Quenching: The S2 uniformly structured steel ingot is heated for solution treatment at a heating rate of 5℃ / min, a temperature of 760℃, and a holding time of 60min; then it is subjected to oil quenching at an oil temperature of 60℃ for 2min to obtain a solution-treated steel ingot.

[0095] S4. Temperature-controlled cryogenic treatment: S3 solution-treated steel ingots are subjected to liquid nitrogen cryogenic treatment at a temperature of -73°C for 120 minutes to obtain cryogenic steel ingots.

[0096] S5. Aging treatment to adjust the reverse austenite content: The S4 cryogenic steel ingot is aged to adjust the reverse austenite content. The heating rate of the aging treatment is 5℃ / min, the temperature is 400℃, and the holding time is 4h. It is then air-cooled to room temperature to obtain high-strength and ductile martensitic stainless steel.

[0097] The high-strength and ductile martensitic stainless steel prepared in this embodiment is in block shape with dimensions of 42×8×7.5mm; its room temperature properties are as follows: hardness 419.1HV, tensile strength 1358.3MPa, yield strength 1195.4MPa, yield ratio 0.880, elongation at break 20.8%, reduction of area 74.3%, and strength-ductility product 28.253GPa·s.

[0098] Example 3

[0099] A method for designing high-strength martensitic stainless steel based on phase transformation toughening, comprising the following steps:

[0100] S1. Raw material weighing and smelting: The composition of the high-strength martensitic stainless steel is designed. In this embodiment, the composition of the high-strength martensitic stainless steel by mass percentage is: Cr 10.9%, Ni 8.18%, Co 5.37%, Mo 2.67%, C 0.018%, with the remainder being Fe and unavoidable impurities. The raw materials are weighed according to the chemical composition, and then the weighed raw materials are added to a vacuum induction furnace for smelting. The molten steel obtained from smelting is then cast into ingots.

[0101] S2. Homogenization heat treatment - oil quenching: After heating the S1 ingot, homogenization heat treatment is performed at a heating rate of 5℃ / min, a temperature of 1000℃, and a holding time of 60min. Then, oil quenching is performed at an oil temperature of 60℃ for 3min to obtain a steel ingot with a uniform structure.

[0102] S3, Solution treatment-oil quenching: The S2 uniformly structured steel ingot is heated for solution treatment at a heating rate of 4℃ / min, a temperature of 760℃, and a holding time of 60min; then it is quenched in oil at a temperature of 60℃ for 2min to obtain a solution-treated steel ingot.

[0103] S4. Temperature-controlled cryogenic treatment: S3 solution-treated steel ingots are subjected to liquid nitrogen cryogenic treatment at a temperature of -73°C for 120 minutes to obtain cryogenic steel ingots.

[0104] S5. Aging treatment to adjust the reverse austenite content: The S4 cryogenic steel ingot is aged to adjust the reverse austenite content. The heating rate of the aging treatment is 3℃ / min, the temperature is 500℃, and the holding time is 4h. It is then air-cooled to room temperature to obtain high-strength and ductile martensitic stainless steel.

[0105] The high-strength and ductile martensitic stainless steel prepared in this embodiment is in block shape with dimensions of 43×9×6mm; its room temperature properties are as follows: hardness 418.7HV, tensile strength 1363.0MPa, yield strength 1242.1MPa, yield ratio 0.911, elongation at break 21.7%, reduction of area 76.2%, and strength-ductility product 29.578 GPa•.

[0106] Example 4

[0107] A method for designing high-strength martensitic stainless steel based on phase transformation toughening, comprising the following steps:

[0108] S1. Raw material weighing and smelting: The composition of the high-strength martensitic stainless steel is designed. In this embodiment, the composition of the high-strength martensitic stainless steel by mass percentage is: Cr 10.9%, Ni 8.18%, Co 5.37%, Mo 2.67%, C 0.018%, with the remainder being Fe and unavoidable impurities. The raw materials are weighed according to the chemical composition, and then the weighed raw materials are added to a vacuum induction furnace for smelting. The molten steel obtained from smelting is then cast into ingots.

[0109] S2. Homogenization heat treatment - oil quenching: After heating the S1 ingot, homogenization heat treatment is performed at a heating rate of 3℃ / min, a temperature of 1000℃, and a holding time of 60min. Then, oil quenching is performed at an oil temperature of 60℃ for 3min to obtain a steel ingot with a uniform structure.

[0110] S3, Solution Treatment-Oil Quenching: The S2 uniformly structured steel ingot is heated for solution treatment at a heating rate of 5℃ / min, a temperature of 760℃, and a holding time of 60min; then it is subjected to oil quenching at an oil temperature of 60℃ for 2min to obtain a solution-treated steel ingot.

[0111] S4. Temperature-controlled cryogenic treatment: S3 solution-treated steel ingots are subjected to liquid nitrogen cryogenic treatment at a temperature of -73°C for 120 minutes to obtain cryogenic steel ingots.

[0112] S5. Aging treatment to adjust the reverse austenite content: The S4 cryogenic steel ingot is aged to adjust the reverse austenite content. The heating rate of the aging treatment is 4℃ / min, the temperature is 500℃, and the holding time is 6h. It is then air-cooled to room temperature to obtain high-strength and ductile martensitic stainless steel.

[0113] The high-strength and ductile martensitic stainless steel prepared in this embodiment is in block shape with dimensions of 44×7×8mm; its room temperature properties are as follows: hardness 424.1HV, tensile strength 1317.2MPa, yield strength 1169.4MPa, yield ratio 0.888, elongation at break 20.5%, reduction of area 72.1%, and strength-ductility product 27.003GPa·s.

[0114] Example 5

[0115] A method for designing high-strength martensitic stainless steel based on phase transformation toughening, comprising the following steps:

[0116] S1. Raw material weighing and smelting: The composition of the high-strength martensitic stainless steel is designed. In this embodiment, the composition of the high-strength martensitic stainless steel by mass percentage is: Cr 10.9%, Ni 8.18%, Co 5.37%, Mo 2.67%, C 0.018%, with the remainder being Fe and unavoidable impurities. The raw materials are weighed according to the chemical composition, and then the weighed raw materials are added to a vacuum induction furnace for smelting. The molten steel obtained from smelting is then cast into ingots.

[0117] S2. Homogenization heat treatment - oil quenching: After heating the S1 ingot, homogenization heat treatment is performed at a heating rate of 5℃ / min, a temperature of 1000℃, and a holding time of 60min. Then, oil quenching is performed at an oil temperature of 60℃ for 3min to obtain a steel ingot with a uniform structure.

[0118] S3, Solution Treatment-Oil Quenching: The S2 uniformly structured steel ingot is heated for solution treatment at a heating rate of 5℃ / min, a temperature of 760℃, and a holding time of 60min; then it is subjected to oil quenching at an oil temperature of 60℃ for 2min to obtain a solution-treated steel ingot.

[0119] S4. Temperature-controlled cryogenic treatment: S3 solution-treated steel ingots are subjected to liquid nitrogen cryogenic treatment at a temperature of -73°C for 120 minutes to obtain cryogenic steel ingots.

[0120] S5. Aging treatment to adjust the reverse austenite content: The S4 cryogenic steel ingot is aged to adjust the reverse austenite content. The heating rate of the aging treatment is 5℃ / min, the temperature is 600℃, and the holding time is 2h. It is then air-cooled to room temperature to obtain high-strength and ductile martensitic stainless steel.

[0121] The high-strength and ductile martensitic stainless steel prepared in this embodiment is in block shape with dimensions of 43×10×6mm; its room temperature properties are as follows: hardness 370.5HV, tensile strength 1227.3MPa, yield strength 802.6MPa, yield ratio 0.654, elongation at break 27.6%, reduction of area 84.9%, and strength-ductility product 33.874GPa·s.

[0122] Table 1. Mechanical property data (room temperature) of martensitic stainless steel obtained from Examples 1 and 2 and Comparative Examples 1 and 2.

[0123] Example 1 386.9 936.2 1254.7 20.1 25.220 23.2 Example 2 419.1 1195.4 1358.3 20.8 28.253 30.9 Example 3 418.7 1242.1 1363.0 21.7 29.578 53.5 Example 4 414.1 1169.4 1317.2 20.5 27.003 62.2 Example 5 370.5 802.6 1227.3 27.6 33.874 92.2

[0124] From Table 1 and Figure 2 , Figure 3 , Figure 7 , Figure 9 It can be seen that the volume fraction of austenite in the martensitic stainless steel obtained in Example 1 is 23.2%, while the volume fraction of austenite in the martensitic stainless steel obtained in Example 2 is 30.9%. The tensile strength of Example 1 is 1254.7 MPa, and the tensile strength of Example 2 is 1358.3 MPa. Comparatively, Example 1 has a higher hard martensitic matrix than Example 2, but the strength of Example 2 is greater than that of Example 1. This is because the reverse austenite undergoes stress-induced martensitic transformation during stretching, dispersing stress distribution and improving deformation uniformity, thus increasing plasticity. The austenite transformation into martensite also increases dislocation density, thus increasing both plasticity and strength. The comparison of strength-plasticity products between Example 1 and Example 2 in Table 1 also shows that the overall performance of Example 2 is superior to that of Example 1. Therefore, Example 2 of this invention has a better strength-plasticity match than Example 1. Further increasing the heating temperature to 500℃ based on Example 2, Example 3 shows improved strength and plasticity compared to Example 2, with little change in hardness. This is similar to the fact that both the strength and plasticity of Example 2 were improved compared to Example 1.

[0125] Compared to Example 5, Example 4 of the present invention has a martensitic stainless steel with an austenite volume fraction of 62.2%, while Example 5 has a martensitic stainless steel with a reverse-transformed austenite volume fraction of 92.2%. It can be observed that in Example 5, the majority of the phases are softer austenitic phases. Although the plasticity increased from 20.5% in Example 4 to 27.6%, the excessive soft austenite phase led to a decrease in strength. The high austenite volume fraction in Example 5 not only resulted in lower strength but also worse performance under high stress conditions. In contrast, Examples 3 and 4 showed more outstanding performance in terms of strength and strength-ductility product, indicating that they maintained good plasticity while achieving high strength. This allows the material to maintain good deformation capacity under high load conditions, increasing its adaptability under impact loads. It is suitable for applications requiring high strength and high deformation resistance, thus having a wider range of applications.

[0126] like Figure 1-7 As shown in the comparison of Examples 1-5, it was found that with the increase of heat treatment temperature and holding time, the content of reverse-transformed austenite increases, the elongation increases with the increase of reverse-transformed austenite content, and the tensile strength first increases and then decreases with the increase of reverse-transformed austenite content. The TRIP effect of reverse-transformed austenite during the tensile process in heat treatment improves strength and plasticity, and has a strengthening effect on martensitic stainless steel. Therefore, as... Figure 2 , Figure 3 , Figure 9 As shown, the strength of Example 2 increased from 1254.7 MPa to 1358.3 MPa compared to Example 1. Figure 3 , Figure 4 , Figure 8 As shown, Example 3, while maintaining improved strength compared to Example 2, also increased plasticity by 21.7%, achieving a strength-ductility product of 29.578 GPa·%, thus achieving a better balance between strength and toughness. This allows the material to withstand high loads while maintaining a certain degree of deformation capacity, effectively alleviating stress concentration under external forces, preventing failure, and ensuring the safety and reliability of the material during use. Figure 4 , Figure 5 , Figure 6As shown, Example 4 further increased the holding time to 6 hours, and Example 5 further increased the heat treatment temperature to 600℃ with a holding time of 2 hours. Although the plasticity reached 27.6% at this point, increasing the material's deformation capacity under stress and preventing fracture under large deformation, the strength of the martensitic stainless steel decreased from 1363.0 MPa in Example 3 to 1227.3 MPa, even lower than the strength of Example 1 (which had lower austenitic strength due to reversal). This resulted in insufficient resistance to external forces, especially with a yield strength decrease of 300 MPa, potentially leading to premature yielding, reduced fatigue life, and even failure under high loads, thus limiting its application areas. The reason is that the increased temperature provided a greater driving force for the reversal of austenite, and the nucleation and growth process of the reversed austenite was relatively complete, with the austenite volume fraction reaching 92.2%.

[0127] Only an appropriate austenite volume fraction helps maintain good martensitic properties, thereby enhancing hardness and resistance to deformation, while ensuring structural safety during actual use, making martensitic stainless steel perform better in high-stress, high-deformation applications.

[0128] The above-mentioned solution proposes a method for designing high-strength martensitic stainless steel based on phase transformation toughening. This method can solve the problems in the prior art where the strength and plasticity of martensitic stainless steel are generally improved by a combination of composition improvement, hot working and heat treatment. However, this method has technical defects such as high alloy cost, hot working will change the casting structure and lose the advantages of the casting structure, unsuitable selection of quenching liquid composition and deep cryogenic treatment temperature during heat treatment, difficulty in control, and subsequent aging treatment, annealing and tempering treatment cannot achieve synergistic improvement or the improvement is not significant.

[0129] This invention obtains steel ingots through raw material design, weighing, and smelting, ensuring good metallurgical quality and providing a uniform chemical composition, which helps enhance the stability of phase transformations during subsequent heat treatment. It avoids deviations in the content of alloying elements, which could adversely affect strength and plasticity, ensuring the consistency of the steel ingot's composition and structure, and guaranteeing the reliability and repeatability of large-scale production processes.

[0130] This invention utilizes homogenization heat treatment followed by oil quenching to ensure sufficient diffusion of alloying elements, reduce segregation, and achieve fully recrystallized austenite. This guarantees uniform phase transformation in the steel ingot and prevents differences in strength and hardness caused by localized structural inhomogeneities during subsequent oil quenching. Compared to water quenching, oil quenching has a slower cooling rate, reducing potential cracks and deformations that may occur during rapid cooling. Oil quenching also makes the cooling rate and transformation behavior more controllable, improving the consistency and stability of the production process.

[0131] This invention utilizes solution treatment followed by oil quenching to allow the austenite to inherit part of the dislocation density of the martensite matrix and the grain orientation of the original austenite, thus preparing the microstructure for aging treatment. Compared to water quenching, oil quenching is gentler, reducing internal stress and deformation caused by excessive temperature gradients, improving the overall stability of the steel ingot, preserving some of the microstructure's toughness, and significantly reducing performance fluctuations caused by water quenching. This ensures consistent performance across batches during mass production.

[0132] This invention promotes martensitic transformation at low temperatures through controlled cryogenic treatment, thereby strengthening the microstructure of the steel ingot and improving its low-temperature performance. After cryogenic treatment, the steel ingot's microstructure becomes more stable, and its performance does not degrade after long-term service, ensuring consistent performance over extended periods.

[0133] This invention, through aging treatment that adjusts the content of reverse-transformed austenite, enables austenite to transform into martensite under stress-induced phase transformation, thus optimizing the phase transformation process of the steel ingot. The precipitated reverse-transformed austenite can exert the TRIP (transformation-induced plasticity) effect, absorbing the energy required for the austenite-to-martensite phase transformation, increasing the toughness of the steel ingot, while delaying necking, improving strain uniformity, and enhancing the plasticity of the steel ingot.

[0134] The high-strength and ductile martensitic stainless steel of this invention is in block shape with dimensions of 40-44×6-10×6-9mm; its room temperature properties are as follows: hardness 370.5HV-419.1HV, tensile strength 1227.3-1363.0MPa, yield strength 802.6-1242.1MPa, yield ratio 0.746-0.911, elongation at break 20.1-27.6%, reduction of area 68.9-84.9%, and strength-ductility product 25.220-33.874GPa·s.

[0135] In summary, compared with other traditional methods, the method of this invention prepares high-strength and ductile martensitic stainless steel through raw material design, weighing and smelting, homogenization heat treatment-oil quenching, solution treatment-oil quenching, deep cryogenic treatment with controlled temperature, and aging treatment to adjust the content of reverse austenite. This method is simple to operate, environmentally friendly, low in cost, short in process, and highly efficient, which is conducive to large-scale industrial production and promotion.

[0136] It should be understood that the term "and / or" in this article is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A alone, A and B simultaneously, or B alone. A and B can be singular or plural. Additionally, the character " / " in this article generally indicates an "or" relationship between the preceding and following related objects, but it can also represent an "and / or" relationship. Please refer to the context for a more accurate understanding.

[0137] In this invention, "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refer to any combination of these items, including any combination of a single item or a plurality of items. For example, at least one of a, b, or c can represent: a, b, c, ab, ac, bc, or abc, where a, b, and c can be a single item or multiple items.

[0138] It should be understood that, in various embodiments of the present invention, the order of the above-mentioned process numbers does not imply the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present invention.

[0139] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for designing high-strength martensitic stainless steel based on phase transformation toughening, characterized in that, The method is applicable to martensitic stainless steel with phase transformation strengthening in the Fe11Cr8Ni5Co3Mo composition system. The method for designing high-strength martensitic stainless steel based on phase transformation toughening includes the following steps: S1. Raw Material Weighing and Smelting: The composition of high-strength martensitic stainless steel is designed. The composition of high-strength martensitic stainless steel, by mass percentage, is: Cr 10-12%, Ni 7-9%, Co 4-6%, Mo 2-4%, and also contains Mn, V, Si, C and other elements. The total content of Mn, V, Si, C and other elements is 0.6-0.8%, with the remainder being Fe and unavoidable impurities. The raw materials are weighed according to the chemical composition, and then the weighed raw materials are added to a vacuum induction furnace for smelting. The molten steel obtained from smelting is then cast into ingots. S2. Homogenization heat treatment - oil quenching: After heating the S1 ingot, homogenization heat treatment is performed at a heating rate of 3-5℃ / min, a temperature of 950-1050℃, and a holding time of 60min. Then, oil quenching is performed at an oil temperature of 50-80℃ for 3-5min to obtain a steel ingot with a uniform structure. S3, Solution Treatment-Oil Quenching: The S2 uniformly structured steel ingot is heated for solution treatment at a heating rate of 3-5℃ / min, a temperature of 750-800℃, and a holding time of 60min; then it is subjected to oil quenching at an oil temperature of 50-80℃ for 2-3min to obtain a solution-treated steel ingot. S4. Temperature-controlled cryogenic treatment: S3 solution-treated steel ingots are subjected to liquid nitrogen cryogenic treatment at a temperature of -73°C for 100-140 minutes to obtain cryogenic steel ingots. S5. Aging treatment to adjust the content of reverse austenite: The S4 cryogenic steel ingot is aged to adjust the content of reverse austenite. The heating rate of the aging treatment is 3-5℃ / min, the temperature is 300-600℃, the holding time is 2-8h, and it is air-cooled to room temperature to obtain high-strength and ductile martensitic stainless steel. The high-strength and high-ductility martensitic stainless steel is blocky in shape, with dimensions of 40-44×6-10×6-9mm. Its room temperature properties include: hardness of 370.5HV-419.1HV, tensile strength of 1227.3-1363.0MPa, yield strength of 802.6-1242.1MPa, yield-to-tensile ratio of 0.746-0.911, elongation at break of 20.1-27.6%, reduction of area of ​​68.9-84.9%, and strength-ductility product of 25.220-33.874GPa·s.

2. The method for designing high-strength martensitic stainless steel based on phase transformation toughening according to claim 1, characterized in that, When the content of precipitated reverse-transformed austenite in S5 is <53.5%, the strength increases with the increase of austenite content; when the content of precipitated reverse-transformed austenite is >53.5%, the strength decreases with the increase of reverse-transformed austenite content.

3. The method for designing high-strength martensitic stainless steel based on phase transformation toughening according to claim 2, characterized in that, S5, after heat treatment at 500℃ for 4 hours, exhibits a reverse-transformed austenite content of 53.5%, and its room temperature properties are as follows: hardness 418.7 HV, tensile strength 1363.0 MPa, yield strength 1242.1 MPa, yield-to-tensile ratio 0.911, elongation at break 21.7%, reduction of area 76.2%, and strength-ductility product 29.578 GPa·s.

4. The method for designing high-strength martensitic stainless steel based on phase transformation toughening according to claim 2, characterized in that, S5, after heat treatment at 400℃ for 4 hours, exhibits a reverse-transformed austenite content of 30.9%, and its room temperature properties are as follows: hardness 419.1 HV, tensile strength 1358.3 MPa, yield strength 1195.4 MPa, yield-to-tensile ratio 0.880, elongation at break 20.8%, reduction of area 74.3%, and strength-ductility product 28.253 GPa·s.

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