An ultra-high-strength corrosion-resistant maraging stainless steel, a preparation method and application thereof

Abstract

The application discloses a kind of ultra-high-strength corrosion-resistant martensitic age-hardening stainless steel and its preparation method and application, it is related to metal material technical field.The chemical composition is as follows according to percentage by mass: C 0.25%~0.35%, Si 0.08%~0.12%, Mn 0.25%~0.35%, Cr 6.5%~7.5%, Ni 6.5%~7.5%, Mo 0.8%~1.2%, V 0.40%~0.80%, Al 2.0%~3.0%, the balance is Fe and impurity.Preparation method: smelting casting, homogenization and forging, hot rolling, and quenching and 470 DEG C~520 DEG C aging.The application solves the contradiction that high strength and high corrosion resistance are difficult to be considered in martensitic stainless steel, so that the tensile strength of material is greater than or equal to 2000 MPa, and the pitting potential is greater than or equal to-200 mV / SCE, which can meet the stringent requirements of deep-sea equipment, ocean engineering and other extreme environments for key components.

Description

Technical Field

[0001] This invention relates to the field of metallic materials technology, specifically to an ultra-high strength corrosion-resistant martensitic aging stainless steel, its preparation method, and its applications. Background Technology

[0002] Deep-sea equipment operates in extreme marine environments, needing to withstand the combined effects of high-concentration chloride ion corrosion, high water pressure, and weak oxidation. This places stringent demands on the comprehensive performance of steel materials. On the one hand, deep-sea engineering structural steel must possess extremely high strength and hardness to withstand enormous loads and impacts, generally requiring a tensile strength exceeding 2000 MPa. On the other hand, it must maintain excellent pitting corrosion resistance under high-chlorination conditions. Pitting potential is a key indicator for measuring the initiation of pitting corrosion in steel in a chloride-containing environment; deep-sea steel must have a high pitting potential to ensure the stability of the passivation film and structural safety. Therefore, developing a material that combines ultra-high strength with excellent corrosion resistance has become a critical problem urgently needing to be solved in this field.

[0003] Currently, traditional martensitic stainless steels (such as 17-4PH and H13) can achieve high hardness and strength by adjusting the carbon, chromium, and nickel content and undergoing quenching and tempering treatment. However, this approach suffers from the drawback of not being able to simultaneously achieve high strength, toughness, and corrosion resistance. For example, typical bearing steels such as 100Cr6 (AISI 52100) and M50, which are high-carbon martensitic steels, easily precipitate coarse carbides during high-temperature tempering, leading to a decrease in material strength. Simultaneously, the large amount of Cr and Mo carbides formed consumes dissolved chromium in the matrix, significantly reducing corrosion resistance. While reducing the carbon content can improve corrosion resistance, it sacrifices strengthening effects, making it difficult to achieve the ultimate strength required for deep-sea materials.

[0004] To balance strength and corrosion resistance, existing research often employs methods such as increasing the content of alloying elements like Cr, Ni, Co, and Mo, or using special processing techniques to partially balance performance. However, this often leads to increased costs or complex processes. There is an urgent need in this field to develop a novel "hybrid steel" that combines the advantages of bearing steel, high-alloy steel, and tool steel, achieving a comprehensive performance of high strength, high hardness, and high corrosion resistance to meet the demands of deep-sea steel applications. Summary of the Invention

[0005] To address the problem of balancing strength and corrosion resistance in existing martensitic stainless steels, this invention provides an ultra-high strength and corrosion-resistant martensitic aging stainless steel. It aims to combine ultra-high strength with excellent pitting corrosion resistance, making it suitable for manufacturing key components in deep-sea equipment, marine engineering, and extreme corrosive and abrasive environments where material performance requirements are extremely high.

[0006] In a first aspect, the present invention provides an ultra-high strength corrosion-resistant martensitic aging stainless steel, the chemical composition of which, by mass percentage, consists of the following elements: C: 0.25%~0.35%, Si: 0.08%~0.12%, Mn: 0.25%~0.35%, Cr: 6.5%~7.5%, Ni: 6.5%~7.5%, Mo: 0.8%~1.2%, V: 0.40%~0.80%, Al: 2.0%~3.0%, with the balance being Fe and unavoidable impurities.

[0007] Furthermore, the ultra-high strength corrosion-resistant martensitic aging stainless steel has a tensile strength ≥2000MPa, a hardness ≥58HRC, an elongation ≥4.5%, and a pitting potential ≥-200mV / SCE.

[0008] Preferably, the chemical composition of the ultra-high strength corrosion-resistant martensitic aging stainless steel is composed of the following elements by mass percentage: C: 0.30%~0.35%, Si: 0.08%~0.12%, Mn: 0.25%~0.35%, Cr: 6.5%~7.0%, Ni: 6.5%~7.5%, Mo: 0.8%~1.2%, V: 0.40%~0.80%, Al: 2.0%~3.0%, with the balance being Fe and unavoidable impurities; the ultra-high strength corrosion-resistant martensitic aging stainless steel has a tensile strength ≥2270MPa, a hardness ≥59HRC, an elongation ≥4.5%, and a pitting potential ≥-200mV / SCE.

[0009] Preferably, the chemical composition of the ultra-high strength corrosion-resistant martensitic aging stainless steel is composed of the following elements by mass percentage: C: 0.25%~0.30%, Si: 0.08%~0.12%, Mn: 0.25%~0.35%, Cr: 7.0%~7.5%, Ni: 6.5%~7.5%, Mo: 0.8%~1.2%, V: 0.40%~0.80%, Al: 2.0%~3.0%, with the balance being Fe and unavoidable impurities; the ultra-high strength corrosion-resistant martensitic aging stainless steel has a tensile strength ≥2200MPa, a hardness ≥58HRC, an elongation ≥5.0%, and a pitting potential ≥-160mV / SCE.

[0010] A second aspect of the present invention provides a method for preparing ultra-high strength corrosion-resistant martensitic aging stainless steel, comprising the following steps: S1. Smelting and casting: The alloying elements are vacuum smelted according to the chemical composition described in any one of claims 1-4, and then cast into ingots. S2. Homogenization and forging: The ingot is subjected to solution treatment, and then multiple forgings are performed in the austenitic single-phase region to obtain a slab. S3. Hot rolling: The slab is hot rolled to obtain a hot-rolled sheet; S4. Quenching and aging treatment: The hot-rolled sheet is quenched, then aged at a temperature range of 470℃~520℃ for 1~48 hours, and then air-cooled to room temperature.

[0011] Furthermore, in step S1, the vacuum melting is performed no less than four times, the melting temperature is 1550℃~1650℃, and the melting time is 20min~30min.

[0012] Furthermore, in step S2, the solution treatment temperature is 1150℃±10℃, the solution treatment time is 10h~14h, the initial forging temperature is 1150℃, the forging ratio is 7~9, and the final forging temperature is 900℃~950℃.

[0013] Further, in step S3, the slab is heated to 1150℃±10℃ and held for 1.5h~2.5h, and then hot rolled for no less than 3 passes, with a cumulative reduction of ≥70% and a final rolling temperature of ≥900℃.

[0014] Furthermore, in step S4, the aging treatment temperature is 470℃~490℃, and the aging treatment time is 19h~21h.

[0015] A third aspect of the present invention provides the application of the above-mentioned ultra-high strength corrosion-resistant martensitic aging stainless steel in deep-sea equipment.

[0016] Compared with the prior art, the present invention has at least the following beneficial effects: The ultra-high strength and corrosion-resistant martensitic aging stainless steel provided by this invention successfully solves the long-standing contradiction between high strength and high corrosion resistance in martensitic stainless steel. Through a unique medium-carbon composition system and synergistic design of nano-scale B2-NiAl phase reinforcement, the material achieves ultra-high strength (tensile strength ≥ 2000 MPa) while possessing excellent pitting corrosion resistance (pitting potential ≥ -200 mV / SCE), significantly surpassing the comprehensive performance of traditional high-carbon martensitic steel or low-carbon NiAl-reinforced steel. Furthermore, the material retains a certain degree of plasticity while maintaining ultra-high strength, achieving a good balance between strength and toughness. Regarding the preparation method, the thermomechanical treatment employed in this invention has good industrial repeatability; in particular, the optimized aging process can achieve peak performance in a short time, significantly reducing energy consumption and production cycle. In terms of cost control, the reasonable composition design reduces dependence on expensive alloying elements, optimizing costs while ensuring high performance. The material's superior comprehensive performance enables it to meet the stringent requirements of critical components in extreme environments such as deep-sea equipment and marine engineering, possessing significant engineering application value and broad market prospects. Attached Figure Description

[0017] 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 recorded in the embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings.

[0018] Figure 1 The polarization curves of stainless steel with different aging times in artificial seawater solution for Examples 1 and 2 of the present invention are shown. Figure 2 The images are transmission electron microscope images of the ultra-high strength corrosion-resistant martensitic aging stainless steel in Example 1 of the present invention; (a) bright field image, showing lath martensitic matrix; (b) selected area electron diffraction pattern, labeled as B2-NiAl phase; (c) dark field image, showing the diffuse distribution of nanoscale B2-NiAl phase; (d) high resolution image. Figure 3 The images show the surface morphology after electrochemical testing in artificial seawater in Example 2 of this invention; where (a) 480℃ / 1h, (b) 480℃ / 4h, (c) 480℃ / 20h, and (d) 480℃ / 48h. Detailed Implementation

[0019] To better understand the above technical solutions, the technical solutions of the embodiments of this application will be described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the embodiments of this application and the specific features in the embodiments are detailed descriptions of the technical solutions of the embodiments of this application, rather than limitations on the technical solutions of this application. In the absence of conflict, the embodiments of this application and the technical features in the embodiments can be combined with each other.

[0020] In a first aspect, the present invention provides an ultra-high strength corrosion-resistant martensitic aging stainless steel, the chemical composition of which, by mass percentage, consists of the following elements: C: 0.25%~0.35%, Si: 0.08%~0.12%, Mn: 0.25%~0.35%, Cr: 6.5%~7.5%, Ni: 6.5%~7.5%, Mo: 0.8%~1.2%, V: 0.40%~0.80%, Al: 2.0%~3.0%, with the balance being Fe and unavoidable impurities.

[0021] The embodiments of the present invention, based on the chemical composition design obtained from a large number of experiments and optimizations, achieve ultra-high strength and excellent corrosion resistance of steel through the synergistic effect of alloying elements such as C, Si, Mn, Cr, Ni, Mo, V, and Al. The carbon content is precisely controlled between 0.25% and 0.35% (forming fine carbides with Mo and V to assist in strengthening and avoid segregation and decreased corrosion resistance caused by high carbon content); Si (0.08%-0.12%) and Mn (0.25%-0.35%) act as deoxidizers and solid solution strengthening elements to improve purity and mechanical properties; Cr (6.5%-7.5%), Ni (6.5%-7.5%) and Mo (0.8%-1.2%) ensure excellent corrosion resistance in high chlorination environments (Cr forms a dense oxide film, Ni improves hardenability and toughness, and Mo further enhances corrosion resistance); V (0.40%-0.80%) forms fine carbides, and Al (2.0%-3.0%) forms a nanoscale coherent B2-NiAl phase with Ni as the main strengthening source. This composition system achieves an optimized ratio of B2 phase strengthening, carbide strengthening, and solid solution strengthening, effectively solving the problem of balancing high strength and corrosion resistance. It enables the material to have a tensile strength ≥2000MPa, hardness ≥58HRC, elongation ≥4.5%, and pitting potential ≥-200mV / SCE.

[0022] The microstructure of the martensitic aging stainless steel in this embodiment of the invention is a lath martensitic matrix with a B2-NiAl intermetallic compound phase of average size of 1-7 nm dispersed on the matrix. This B2-NiAl phase is coherent with the martensitic matrix. This microstructure was characterized by transmission electron microscopy (TEM). The steel exhibits optimal overall performance when the average size of the B2-NiAl intermetallic compound phase is ~3 nm. The uniform distribution of the nanoscale B2-NiAl phase on the lath martensitic matrix effectively hinders dislocation movement, significantly improving the strength and hardness of the steel. Simultaneously, its coherent relationship with the matrix reduces interfacial stress, which is beneficial for maintaining the steel's toughness. Furthermore, the appropriate amount of carbide precipitation further enhances the steel's strength without significantly negatively impacting its corrosion resistance.

[0023] Preferably, the chemical composition of the ultra-high strength corrosion-resistant martensitic aging stainless steel is composed of the following elements by mass percentage: C: 0.30%~0.35%, Si: 0.08%~0.12%, Mn: 0.25%~0.35%, Cr: 6.5%~7.0%, Ni: 6.5%~7.5%, Mo: 0.8%~1.2%, V: 0.40%~0.80%, Al: 2.0%~3.0%, with the balance being Fe and unavoidable impurities; its tensile strength is ≥2270MPa, hardness is ≥59HRC, elongation is ≥4.5%, and pitting potential is ≥-200mV / SCE. It can be used in deep-sea load-bearing components and high-strength fasteners that bear extremely high loads but have relatively relaxed requirements for the corrosive environment.

[0024] Preferably, the chemical composition of the ultra-high strength corrosion-resistant martensitic aging stainless steel is composed of the following elements by mass percentage: C: 0.25%~0.30%, Si: 0.08%~0.12%, Mn: 0.25%~0.35%, Cr: 7.0%~7.5%, Ni: 6.5%~7.5%, Mo: 0.8%~1.2%, V: 0.40%~0.80%, Al: 2.0%~3.0%, with the balance being Fe and unavoidable impurities; its tensile strength is ≥2200MPa, hardness is ≥58HRC, elongation is ≥5.0%, and pitting potential is ≥-160mV / SCE. It can be used for key components in deep-sea equipment, seawater pumps and valves, offshore platforms, etc., which are subjected to long-term high chloride ion corrosion environments.

[0025] A second aspect of this invention provides a method for preparing ultra-high strength corrosion-resistant martensitic aging stainless steel, comprising the following steps: S1. Melting and casting: The alloying elements are vacuum melted according to the above chemical composition, and then cast into ingots. S2. Homogenization and forging: The ingot is subjected to solution treatment, and then multiple forgings are performed in the austenitic single-phase region to obtain a slab. S3. Hot rolling: The slab is hot rolled to obtain a hot-rolled sheet; S4. Quenching and aging treatment: The hot-rolled sheet is quenched, then aged at a temperature range of 470℃~520℃ for 1~48 hours, and then air-cooled to room temperature.

[0026] This invention provides a method for preparing ultra-high strength, corrosion-resistant martensitic aged stainless steel. This method employs thermomechanical treatment and exhibits good industrial repeatability. Its core lies in using a medium-temperature aging process of 470℃ to 520℃. This temperature range is the optimal window for the large-scale precipitation of the B2-NiAl phase while maintaining its nanoscale size. This avoids excessive precipitation of harmful phases while promoting the formation of ultra-hard carbides such as VC. Furthermore, this aging process can reach peak performance in a short time, significantly reducing energy consumption and shortening the production cycle.

[0027] In some embodiments, in step S1, the vacuum melting is performed at least four times, the melting temperature is 1550℃~1650℃, and the melting time is 15min~30min.

[0028] Specifically, each alloying element is placed in a vacuum arc furnace for at least four vacuum melting processes to ensure compositional homogeneity. During the melting process, the melting temperature and time are strictly controlled to avoid element loss and segregation. After melting, the alloy is cast into ingots.

[0029] In some embodiments, in step S2, the solution treatment temperature is 1150℃±10℃, the solution treatment time is 10h~14h, the initial forging temperature is 1150℃, the forging ratio is 7~9, and the final forging temperature is 900℃~950℃.

[0030] Specifically, this embodiment of the invention employs a specific thermomechanical treatment process, involving homogenization solution treatment at 1150℃±10℃ for 10h~14h to eliminate compositional segregation and residual stress in the as-cast microstructure. Subsequently, multi-pass forging is performed in the austenite single-phase region (temperature greater than 1000℃), with an initial forging temperature of 1150℃, a forging ratio of 7~9, and a final forging temperature controlled within the range of 900℃~950℃, to obtain a slab of predetermined dimensions. During the forging process, the deformation amount and rate are strictly controlled to avoid crack formation. By controlling the forging process, the original austenite grains are refined, providing uniform nucleation sites for subsequent phase transformation.

[0031] In some embodiments, in step S3, the slab is heated to 1150℃±10℃ and held for 1.5h~2.5h, and then hot-rolled for no less than 3 passes, with a cumulative reduction of ≥70% and a final rolling temperature of ≥900℃.

[0032] Specifically, the slab is heated to 1150℃±10℃ and held for 2±0.5 hours to ensure uniform slab temperature. Then, at least three passes of hot rolling are performed, with a cumulative reduction of ≥70% and a final rolling temperature of ≥900℃, to refine the original austenite grains and provide uniform nucleation sites for subsequent phase transformation. Optionally, the hot rolling process is four passes: initial thickness 50mm→38mm→29mm→21mm→15mm, initial rolling temperature 1150℃, and final rolling temperature approximately 910℃. During hot rolling, the rolling speed and rolling force are strictly controlled to ensure slab shape and dimensional accuracy.

[0033] In step S4, the hot-rolled sheet is held at 1050℃±10℃ for 20-40 minutes, then water-quenched to room temperature. Preferably, the aging treatment temperature is 470℃~490℃, and the aging treatment time is 19h~21h, followed by air cooling to room temperature. More preferably, the aging treatment temperature is 480℃, and the aging treatment time is 20h.

[0034] A third aspect of the present invention provides the application of the above-mentioned ultra-high strength corrosion-resistant martensitic aging stainless steel in deep-sea equipment.

[0035] The ultra-high strength, corrosion-resistant martensitic aging stainless steel of this invention effectively suppresses the problems of precipitate segregation and grain boundary corrosion commonly found in traditional high-strength steels, while maintaining a stable tempered microstructure and good fracture toughness. It is suitable for manufacturing key components in deep-sea equipment, marine engineering, and extreme corrosive and abrasive environments, and has significant engineering application value and promising prospects for widespread application.

[0036] Example 1: An ultra-high strength corrosion-resistant martensitic aging stainless steel (I) Preparation method Includes the following steps: Batching and Smelting: Weigh 20 kg of high-purity raw materials strictly according to the following mass percentages: C: 0.28%, Si: 0.1%, Mn: 0.3%, Cr: 7.0%, Ni: 7.0%, Mo: 1.0%, V: 0.5%, Al: 2.5%, with the balance being Fe. Smelt and cast into ingots using a vacuum induction melting furnace to reduce gas content and impurities.

[0037] Homogenization and Forging: The ingot is placed in a heating furnace and subjected to a solution treatment at 1150℃ for up to 12 hours to ensure thorough homogenization of the composition. Subsequently, forging is carried out in the austenitic single-phase region (temperature > 1000℃), with an initial forging temperature of 1150℃. The ingot is forged in multiple passes to form a 50mm × 100mm slab with a forging ratio of 8. The final forging temperature is strictly controlled at around 950℃, followed by air cooling.

[0038] Hot rolling: The forging billet is reheated to 1150℃ and held for 2 hours, then hot rolled. The rolling process consists of 4 passes: 50mm → 38mm → 29mm → 21mm → 15mm. The initial rolling temperature is 1150℃, the final rolling temperature is approximately 910℃, and the cumulative reduction is 70%.

[0039] Quenching and aging treatment: Quenching temperature is 1050℃, held for 30 minutes and then water quenched to room temperature; aging is held at 480℃ for 20 hours, and then taken out and air cooled to room temperature.

[0040] (ii) Microstructural characterization and performance testing The mechanical property test results of this embodiment show that its tensile strength reaches 2478 MPa, its yield strength is 1925 MPa, and its elongation after fracture is 5%, demonstrating a good balance between high strength and a certain degree of plasticity. Electrochemical corrosion testing was conducted in artificial seawater solution, with a saturated calomel electrode (SCE) as the reference electrode. The results show that the material's self-corrosion potential is -323.00 mV / SCE, and its pitting potential is -151.66 mV / SCE. Figure 1 As shown, it exhibits obvious passivation characteristics and excellent resistance to pitting corrosion; no pitting corrosion was observed on the surface after testing. Transmission electron microscopy (TEM) characterization results show that the matrix structure is a typical lath martensite morphology, containing a large number of nanoscale precipitates coherent with the matrix, such as... Figure 2 As shown in the figure. High-resolution observation shows that the average size of the precipitated phase is about 3 nm, and energy dispersive spectroscopy analysis confirmed that it is a B2-NiAl phase, indicating that the precipitated phase not only strengthens the alloy but also helps to stabilize the microstructure and improve corrosion resistance.

[0041] Example 2: The Impact of Different Time Limitation Systems (I) Preparation method Example 2, under the same chemical composition and heat treatment conditions as Example 1, differed from Example 1 in the aging regime. The effect of different aging times at an aging temperature of 480℃ on the microstructure and properties of the test steel was investigated. Specifically, the samples were held at 480℃ for 1 hour, 4 hours, and 48 hours, respectively, and then air-cooled to room temperature.

[0042] (ii) Microstructural characterization and performance testing Microstructural observation revealed that all samples under the four aging conditions exhibited lath martensite. After 1 hour of aging, the precipitate was scarce and approximately 1-2 nm in size; after 4 hours, the precipitate size was approximately 2 nm; in Example 1, after 20 hours of aging, the precipitate density significantly increased, with a size of approximately 3 nm. After 48 hours, the precipitate grew to approximately 5 nm and showed coarsening. Energy dispersive spectroscopy (EDS) analysis showed that the precipitate was of the B2–NiAl type, and its volume fraction initially increased and then decreased over time, indicating the presence of a peak aging stage.

[0043] The test results of martensitic aged stainless steel samples after four different aging times are shown in Table 1. The mechanical property results show that both tensile strength and hardness reach their peak values ​​at 20 hours. After aging for 1 hour, the tensile strength is 1947 MPa and the hardness is 53.0 HRC; after aging for 4 hours, the tensile strength is 2020 MPa and the hardness is 55.5 HRC; after aging for 20 hours, the tensile strength is 2478 MPa and the hardness is 58.5 HRC; and after aging for 48 hours, the tensile strength is 2428 MPa and the hardness is 58.3 HRC. Electrochemical corrosion tests were conducted in artificial seawater solution. Figure 3 The surface morphology of samples after electrochemical testing in artificial seawater at different aging times is shown. The results indicate that only scattered micro-pitting pits exist on the sample surface after 1 hour of aging; the number of pits increases after 4 hours; the pit density further increases after 20 hours, but the pits remain relatively small; after 48 hours, the pits show significant coarsening and merging, with a significant increase in both local corrosion depth and width, indicating that over-aging treatment deteriorates the material's pitting corrosion resistance. The pitting potential is -37.36 mV / SCE after 1 hour of aging, -115.53 mV / SCE after 4 hours, slightly increasing to -151.66 mV / SCE after 20 hours, and -188.25 mV / SCE after 48 hours. In summary, the steel of this invention exhibits superior mechanical properties and corrosion resistance after aging at 480℃ for 20 hours. At this temperature, the alloy microstructure is refined, precipitation strengthening is most significant, and it combines high strength, high hardness, and excellent corrosion resistance.

[0044] Table 1 Performance after processing at different efficiency times

[0045] Example 3: Influence of Key Alloy Components Based on the hot working and heat treatment process of Example 1, only the content of C and Cr elements was adjusted, while the composition of other elements and the subsequent preparation method remained the same as in Example 1, in order to study the influence of the change of key alloy composition on the microstructure and properties of the steel of the present invention.

[0046] Specifically, three different compositions of martensitic aging stainless steel samples were prepared: Scheme A: C: 0.32%, Si: 0.1%, Mn: 0.3%, Cr: 7.0%, Ni: 7.0%, Mo: 1.0%, V: 0.5%, Al: 2.5%, balance Fe.

[0047] Scheme B: C: 0.35%, Si: 0.1%, Mn: 0.3%, Cr: 7.0%, Ni: 7.0%, Mo: 1.0%, V: 0.5%, Al: 2.5%, balance Fe.

[0048] Scheme C: C: 0.28%, Si: 0.1%, Mn: 0.3%, Cr: 7.5%, Ni: 7.0%, Mo: 1.0%, V: 0.5%, Al: 2.5%, balance Fe.

[0049] The test results of three martensitic aging stainless steel samples with different compositions are shown in Table 2. The matrix structure of all three schemes is lath martensite containing nanoscale B2-NiAl precipitates, but their properties show a regular difference. Scheme A (C 0.32%) has a hardness, yield strength, and tensile strength of 59.2 HRC, 1849 MPa, and 2501 MPa, respectively, with an elongation of 4.8% and a pitting potential of -186.23 mV / SCE. Scheme B (C 0.35%) shows a slight increase in strength and hardness to 59.5 HRC, 1925 MPa, and 2528 MPa, but the elongation decreases slightly to 4.5%, and the pitting potential shifts slightly negatively to -195.12 mV / SCE. In comparison, Scheme C (Cr 7.5%) exhibits a hardness of 58.6 HRC, a yield strength of 1830 MPa, and a tensile strength of 2444 MPa, with an elongation of 5.1%, which is essentially equivalent to Example 1. However, its pitting potential shifts significantly to -140.28 mV / SCE, demonstrating the superior resistance to pitting corrosion. In summary, increasing the C content primarily enhances the solid solution strengthening effect, improving strength and hardness, but at the cost of slight loss in plasticity and pitting corrosion resistance. Conversely, increasing the Cr content significantly improves the material's corrosion resistance while maintaining its excellent balance between high strength and plasticity. This demonstrates that by adjusting the C and Cr content, the overall performance of the steel of this invention can be effectively optimized.

[0050] Table 2 Performance of different component formulations after aging treatment at 480℃ for 20h

[0051] Comparative Example 1 (I) Preparation method Includes the following steps: Batching and Smelting: Weigh 20 kg of high-purity raw materials strictly according to the following mass percentages: C: 0.38%, Si: 0.3%, Mn: 0.5%, Cr: 8.0%, Ni: 8.0%, Mo: 1.0%, V: 0.5%, Al: 3.0%, with the balance being Fe. Smelt and cast into ingots using a vacuum induction melting furnace to reduce gas content and impurities.

[0052] Homogenization and forging: Same as in Example 1.

[0053] Hot rolling: Same as in Example 1.

[0054] Quenching and aging treatment: The quenching regime is the same as in Example 1; the aging regime is 480℃~610℃, held for 1h, 4h, 20h and 48h respectively, and then taken out and air-cooled to room temperature.

[0055] (ii) Microstructural characterization and performance testing The hardness test results of the comparative steel at different aging temperatures (holding times of 1h, 4h, 20h, and 48h) are shown in Table 3. The data indicate that within the aging temperature range of 480℃ to 610℃, the hardness of the comparative steel is generally low, and no significant age-hardening peak is formed. Taking aging at 480℃ as an example, the hardness at each holding time is only 24.0~26.0 HRC, far lower than the 58.5 HRC of Example 1 after aging at 480℃ for 20h. Even when the aging temperature is increased to 610℃, the hardness after 20h aging is only 49.0 HRC, still significantly lower than that of Example 1.

[0056] Table 3 Hardness (HRC) of Comparative Example 1 after different aging temperatures and aging times

[0057] Electrochemical corrosion testing showed that the comparative steel did not exhibit a clear passivation zone in artificial seawater, the self-corrosion current density increased significantly, and the pitting potential could not be effectively measured. Surface morphology observation revealed large-area uniform corrosion and dense pitting on the sample surface, indicating severe corrosion. This was due to the excessively high C content (0.38%) leading to coarse carbides that precipitated continuously along grain boundaries, forming a severely chromium-depleted zone; simultaneously, the excessively high Al content (3.0%) promoted the formation of coarse aluminum-rich phases, disrupting the continuity of the matrix and the integrity of the passivation film; while excessively high Cr (8.0%) and Ni (8.0%) content, although beneficial to corrosion resistance, actually exacerbated the precipitation of harmful phases in this composition system. In addition, excessively high Si (0.3%) and Mn (0.5%) content also reduced the purity of the matrix.

[0058] This comparative example illustrates that the content of each alloying element must be synergistically proportioned within the limits defined by this invention to achieve a balance between effective precipitation strengthening of the B2-NiAl phase (hardness ≥ 58 HRC) and excellent corrosion resistance (significant passivation range). Composition designs exceeding the scope of this invention not only fail to achieve age hardening effects but also severely impair the material's resistance to seawater corrosion.

[0059] Comparative Example 2 This comparative example uses the same chemical composition as Example 1. The difference in preparation method lies in the quenching and aging treatment steps: the quenching temperature is 1030℃, and after holding for 30 minutes, it is water quenched to room temperature; the aging regime is to hold at 460℃ for 20 hours, and then air-cooled to room temperature.

[0060] Hardness tests were conducted on the steel used in this comparative example. The results showed that after quenching at 1030℃ and aging at 460℃ for 20 hours, the hardness of the three parallel samples were 53.6 HRC, 54.1 HRC, and 51.7 HRC, respectively, with an average hardness of approximately 53.1 HRC, which is significantly lower than the 58.5 HRC of Example 1. Mechanical property tests indicated that its tensile strength was only about 1850 MPa, its yield strength was about 1420 MPa, and its elongation was about 6.5%.

[0061] Microstructural characterization revealed that due to the low quenching temperature, austenitization was insufficient, and some carbides failed to dissolve, resulting in insufficient solid solubility of alloying elements in the martensitic matrix after quenching. Simultaneously, the aging temperature of 460℃ was too low, significantly slowing the precipitation kinetics of the B2-NiAl phase. The number density and volume fraction of nanoscale precipitates were far lower than in Example 1, failing to achieve the peak age-strength effect. Furthermore, the failure of some carbides to dissolve resulted in a large number of Cr-depleted zones, and the ineffective precipitation of the NiAl strengthening phase led to low matrix hardness. In corrosion tests, the mechanical stability of the passivation film decreased, with a pitting potential of only -210 mV / SCE, inferior to Example 1. This comparative example verifies the necessity of controlling the quenching temperature within 1050℃±10℃ and the aging temperature within the range of 470℃~520℃. Deviating from this process window significantly weakens the precipitation strengthening effect, making it impossible to obtain ultra-high strength.

[0062] In summary, this invention successfully developed a martensitic aging stainless steel with high strength, good plasticity, and excellent corrosion resistance through optimized composition design and heat treatment process. Its properties can be precisely adjusted by controlling the aging regime and the content of key alloying elements. At 480℃, the properties improve with increasing aging time, reaching a peak at 20 hours, at which point the nano-sized B2-NiAl phase is fully precipitated, achieving the best precipitation strengthening effect. Under a fixed aging process, increasing the C content (0.28%→0.35%) further enhances solid solution strengthening, significantly improving strength and hardness, but slightly sacrifices plasticity and pitting corrosion resistance; while increasing the Cr content (7.0%→7.5%) significantly enhances the stability of the passivation film, causing a substantial positive shift in the pitting potential by approximately 50mV, significantly improving corrosion resistance while maintaining basic mechanical properties. This indicates that the material system of the present invention has a high degree of tunability in composition and process, and can achieve customized performance design for different application scenarios (such as pursuing extreme strength or harsh corrosive environments) by balancing the content of C and Cr elements and aging parameters.

[0063] It will be readily understood by those skilled in the art that the above-described advantageous methods can be freely combined and superimposed without conflict. The above are merely preferred embodiments of this application and are not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application. The above are merely preferred embodiments of this application. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of this application, and these improvements and modifications should also be considered within the protection scope of this application.

Claims

1. A high-strength, corrosion-resistant, martensitic aging stainless steel, characterized in that, Its chemical composition, by mass percentage, consists of the following elements: C: 0.25%~0.35%, Si: 0.08%~0.12%, Mn: 0.25%~0.35%, Cr: 6.5%~7.5%, Ni: 6.5%~7.5%, Mo: 0.8%~1.2%, V: 0.40%~0.80%, Al: 2.0%~3.0%, with the balance being Fe and unavoidable impurities.

2. The ultra-high strength corrosion-resistant martensitic aging stainless steel according to claim 1, characterized in that, The ultra-high strength corrosion-resistant martensitic aging stainless steel has a tensile strength ≥2000MPa, hardness ≥58HRC, elongation ≥4.5%, and pitting potential ≥-200mV / SCE.

3. The ultra-high strength corrosion-resistant martensitic aging stainless steel according to claim 1, characterized in that, The chemical composition of the ultra-high strength corrosion-resistant martensitic aging stainless steel, by mass percentage, consists of the following elements: C: 0.30%~0.35%, Si: 0.08%~0.12%, Mn: 0.25%~0.35%, Cr: 6.5%~7.0%, Ni: 6.5%~7.5%, Mo: 0.8%~1.2%, V: 0.40%~0.80%, Al: 2.0%~3.0%, with the balance being Fe and unavoidable impurities; the ultra-high strength corrosion-resistant martensitic aging stainless steel has a tensile strength ≥2270MPa, a hardness ≥59HRC, an elongation ≥4.5%, and a pitting potential ≥-200mV / SCE.

4. The ultra-high strength corrosion-resistant martensitic aging stainless steel according to claim 3, characterized in that, The chemical composition of the ultra-high strength corrosion-resistant martensitic aging stainless steel is composed of the following elements by mass percentage: C: 0.25%~0.30%, Si: 0.08%~0.12%, Mn: 0.25%~0.35%, Cr: 7.0%~7.5%, Ni: 6.5%~7.5%, Mo: 0.8%~1.2%, V: 0.40%~0.80%, Al: 2.0%~3.0%, with the balance being Fe and unavoidable impurities; the ultra-high strength corrosion-resistant martensitic aging stainless steel has a tensile strength ≥2200MPa, a hardness ≥58HRC, an elongation ≥5.0%, and a pitting potential ≥-160mV / SCE.

5. A method for preparing ultra-high strength corrosion-resistant martensitic aging stainless steel, characterized in that, Includes the following steps: S1. Smelting and casting: The alloying elements are vacuum smelted according to the chemical composition described in any one of claims 1-4, and then cast into ingots. S2. Homogenization and forging: The ingot is subjected to solution treatment, and then forged in multiple passes in the austenitic single-phase region to obtain a slab; S3. Hot rolling: The slab is hot rolled to obtain a hot-rolled sheet; S4. Quenching and aging treatment: The hot-rolled sheet is quenched, then aged at a temperature range of 470℃~520℃ for 1~48 hours, and then air-cooled to room temperature.

6. The preparation method according to claim 5, characterized in that, In step S1, the vacuum melting is performed no less than four times, the melting temperature is 1550℃~1650℃, and the melting time is 20min~30min.

7. The preparation method according to claim 5, characterized in that, In step S2, the solution treatment temperature is 1150℃±10℃, the solution treatment time is 10h~14h, the initial forging temperature is 1150℃, the forging ratio is 7~9, and the final forging temperature is 900℃~950℃.

8. The preparation method according to claim 5, characterized in that, In step S3, the slab is heated to 1150℃±10℃ and held for 1.5h~2.5h, and then hot rolling is performed in no less than 3 passes, with a cumulative reduction of ≥70% and a final rolling temperature of ≥900℃.

9. The preparation method according to claim 5, characterized in that, In step S4, the aging treatment temperature is 470℃~490℃, and the aging treatment time is 19h~21h.

10. The application of the ultra-high strength corrosion-resistant martensitic aging stainless steel according to any one of claims 1-4 or the ultra-high strength corrosion-resistant martensitic aging stainless steel prepared by the preparation method according to any one of claims 5-9 in deep-sea equipment.

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