A medium-carbon martensitic stainless steel with refined structure and method of manufacture

Abstract

The application discloses a medium-carbon martensitic stainless steel with refined structure and a manufacturing method thereof. The steel has a more uniform structure and better corrosion resistance. The chromium-nickel equivalent ratio of the material is less than or equal to 1.50, the residual austenite phase is greater than or equal to 3%, the hardness after dispersoid carbide precipitation is greater than or equal to 52.5HRC, the hardness difference of the same plate is less than or equal to 1.0HRC, and the pitting point is greater than or equal to 105mV, which is 100% higher than that of 4Cr13. The material can be used in the fields of compressor valve plates and medical devices.

Description

Technical Field

[0001] This invention relates to stainless steel for compressor valve plates and cutting tools, especially surgical instruments in the field of medical devices, and methods for manufacturing the same. Specifically, it relates to a medium-carbon martensitic stainless steel with refined microstructure and methods for manufacturing the same. Background Technology

[0002] Martensitic stainless steel is a chromium-based stainless steel, characterized by high hardness and wear resistance, and is widely used in knives, scissors, surgical instruments, measuring tools, and turbine blades. In addition to high hardness and wear resistance, these applications also require certain toughness and corrosion resistance. Common martensitic stainless steel grades include 2Cr13, 3Cr13, 4Cr13, and 6Cr13, with Cr content generally ranging from 12.5% ​​to 13.5% and C content ranging from ~0.2% to 0.6%. As the carbon content increases, the hardness after quenching or tempering also increases significantly. However, while increasing carbon content and hardness, the precipitation of chromium carbide significantly reduces the corrosion resistance of martensitic stainless steel. Taking pitting corrosion and acidic environment corrosion, which account for the highest proportion of corrosion, as examples, the pitting voltage of 2Cr13 martensitic stainless steel can reach 120mV, while that of 3Cr13 decreases to around 60-80mV, and that of 4Cr13 decreases to around 40-60mV. For compressor valve plates, and for cutting tools, especially medical devices, knives, and razor blades, which require long-term and repeated use and come into contact with the human body, insufficient corrosion resistance is a significant factor affecting their application.

[0003] To improve the corrosion resistance of medium-carbon martensitic stainless steel, microalloying with nitrogen (N) and v (V), along with the addition of a certain amount of molybdenum (Mo), can effectively improve the material's microstructure and corrosion resistance. Among the strengthening alloying elements for martensitic stainless steel, carbon (C) and nitrogen (N) are the most effective elements for increasing strength. However, the addition of carbon often leads to the formation of carbides and the segregation of coarse carbide particles during rolling and heat treatment, reducing the corrosion resistance of martensitic stainless steel. Adding nitrogen (N) is another effective method to improve the mechanical properties of martensitic stainless steel. Adding alloying elements such as molybdenum (Mo) and v (V) is another effective measure to improve corrosion resistance. Mo is a ferrite-forming element, and adding Mo can significantly improve pitting corrosion resistance. The commonly used formula for calculating the pitting corrosion resistance equivalent (PREN) of stainless steel is: PREN = %Cr + 3.3(%Mo + 0.5%W) + 16%N. This shows that the improvement coefficient of mo on pitting corrosion resistance is 3.3 times that of Cr, meaning that adding 1% moly has an improvement in pitting corrosion resistance equivalent to adding 3.3% Cr. As mentioned earlier, nitrogen is also an important element for improving pitting corrosion resistance. The PREN empirical formula also shows that the addition of nitrogen can improve pitting corrosion resistance. Chinese patent CN101195895A reduces the carbon content to 0.13-0.18% to reduce carbide precipitation during air quenching and increases nitrogen by 0.03-0.06% to improve quenching hardness and corrosion resistance. The specific role of nitrogen is to increase quenching hardness, strengthen the passivation film, and inhibit the precipitation of chromium carbides. Patent CN1624182, based on 2Cr13 steel, reduces the carbon content to 0.12-0.17% to reduce carbide precipitation, while adding 0.06-0.10% nitrogen to compensate for the hardness loss caused by carbon reduction. Nitrogen, like carbon, can improve hardness. The nitrogen content needs to be controlled below 0.10% to avoid pinhole defects caused by nitrogen precipitation.

[0004] Invention patent CN201310145690.5 proposes a tin-containing martensitic stainless steel with excellent performance. The weight percentage of its chemical composition is as follows: C: 0.15~0.40%, Si≤1.0%, Mn≤2.0%, P≤0.040%, S≤0.010%, Cr=12.0~14.0%, N: 0.05~0.12%, Sn: 0.10~0.30%, with the remainder being Fe and unavoidable impurities. After quenching at 1050℃, this tin-containing martensitic stainless steel has a tensile strength ≥1500MPa, Rockwell hardness ≥48HRC, and pitting potential ≥105mV, which is superior to the similar 2Cr13 martensitic stainless steel.

[0005] Chinese invention patent CN201180063203.X discloses a medium-to-high carbon martensitic stainless steel with improved corrosion resistance, primarily used for high-quality razor blades. The material composition is 0.45%–0.60% carbon, 0.02%–0.08% nitrogen, 0.2%–0.4% silicon, 0.3%–0.6% manganese, 12%–15% chromium, 0.1%–1.5% molybdenum, 0.1%–1.5% tungsten, and the balance being Fe and other unavoidable impurities. The corrosion resistance of the 0.45–0.60% carbon stainless steel is improved by adding alloying elements such as Mo and N. Chinese patent CN1145644A discloses a martensitic stainless steel with 0.15–0.40% C and 11.00–15.00% Cr, in which 0.12–4.0% Cu and 1.00–3.00% Mo are added. The addition of precious metals, especially molybdenum, improves corrosion resistance but significantly increases manufacturing costs.

[0006] Adding a certain amount of W to stainless steel can also improve its corrosion resistance and mechanical properties. In fact, tungsten is an important alloying element in steel, which can enhance the strength of steel, especially the high-temperature strength, hardness and wear resistance. The main tungsten-containing steels include high-speed tool steels, hot-work die steels, steels for eddy current blades, etc. The existing forms of tungsten include solid solution or dispersion precipitation. The invention patent CN201110250792.4 discloses a duplex tungsten stainless steel alloy material, the components and their mass percentages are as follows: 0.030% < C ≤ 0.05%, 0.0023% ≤ S ≤ 0.01%, 0.463% ≤ Si ≤ 1.0%, 0.010% ≤ P ≤ 0.015%, 20.0% ≤ Cr ≤ 23.0%, 0.5% ≤ W ≤ 1.5%, 1.0% ≤ Ni ≤ 3.0%, 0.20% ≤ N ≤ 0.30%, 0.001% ≤ B ≤ 0.01%, rare earth Ce or Y: 0.005% - 0.20%, and the rest is iron. It has a ferritic-austenitic duplex structure in terms of microstructure, and it is a duplex stainless steel. The tungsten in it enhances the strength of the steel, strengthens the refinement of the cross-sectional microstructure of the steel, and also has an obvious impact on the material microstructure and corrosion resistance, effectively improving the crevice corrosion resistance of the duplex stainless steel. The invention patent CN201611074806.0 proposes a tungsten-containing austenitic stainless steel seamless tube, which can be used for the superheater and reheater of a boiler in a 650°C - 700°C supercritical thermal power unit. The main components of this tungsten-containing austenitic heat-resistant steel are 0.03% - 0.08% C, less than 0.5% Si, less than 0.5% Mn, 18% - 25% Cr, 21.5% - 31% Ni, 2 - 4% Cu, 0.10 - 0.35% N, 0.30 - 0.65% Nb, 1.0 - 5.0% W, 0.1 - 0.4% Mo, 1.0 - 4.0% Co, 0.003 - 0.009% B, and the balance is Fe. The added W element in this alloy is an important element for solid solution strengthening and precipitation phase strengthening of the material, which can significantly improve the high-temperature creep strength of the material. In fact, for the mature and mass-produced standard steel grade 1Cr12WMoV, its components are C: 0.12 - 0.18, Si: ≤ 0.50, Mn: 0.50 - 0.90, Cr: 11.00 - 13.00, Ni: 0.40 - 0.80, V: 0.18 - 0.30, Mo: 0.50 - 0.70, W: 0.70 - 1.10. Its microstructure is martensite, and it is used in the quenched and tempered state, with high thermal strength, good shock absorption and microstructure stability. It is widely used in turbine blades, fasteners, rotors and discs, etc., and is often used to manufacture components that work at high temperatures in industrial departments such as boilers, steam turbines, power machinery, industrial furnaces, aviation, petrochemicals, etc. Summary of the Invention

[0007] The purpose of this invention is to further improve the microstructure uniformity of medium-carbon martensitic stainless steel, thereby improving hardness uniformity and corrosion resistance. In particular, it addresses the problem of a significant decrease in corrosion resistance with increasing hardness, resolving the contradiction of increasing hardness by increasing carbon content but decreasing corrosion resistance. Through W and N alloying and V microalloying, combined with optimized manufacturing methods, the distribution of carbides is improved, resulting in medium-carbon martensitic stainless steel with superior corrosion resistance and its preparation method, thus solving the problem of matching hardness and corrosion resistance. The material is more economical compared to Mo-containing alloys and can be used in compressor valves, medical devices, cutting tools, and other fields.

[0008] The purpose of this invention is to provide a medium-carbon martensitic stainless steel with refined microstructure and its manufacturing method. Compared with the similar 4Cr13, the material exhibits significantly improved corrosion resistance by more than 100% while achieving the same hardness (≥52.0HRC). Specific measures include: (1) N alloying: by reducing the C content in the alloy to below 0.35%, adding N and ensuring that C+N≥0.35% compensates for the effect of reduced C content on austenitization and hardness, ensuring that the hardness is comparable to that of 4Cr13 with 0.35~0.45% C content, while using N to improve corrosion resistance; (2) W and V alloying: the addition of W can further improve corrosion resistance, and its function is similar to that of Mo. Experiments have found that W also has a similar function to V, refining the structure and avoiding the presence of coarse carbide particles, thereby improving the uniformity of the structure and the corrosion resistance of the material; (3) controlling 1.28≤(Cr+0.7W+1.5Si) / (20C+20N+Mn)≤1.49, the residual austenite content after quenching is ≥3%, which can make the excess C in the material dissolve in the face-centered cubic austenite phase, and at the same time obtain better toughness.

[0009] Based on the above composition design, the manufacturing process matching the composition characteristics specifically includes: (1) controlling 0.021(Cr+0.9Mn)-0.204≥N to ensure that nitrogen will not escape and form pinholes; (2) using directional forging or hot rolling to make steel plates or strips, specifically after the billet is forged or rolled, especially after rolling 20% ​​or 30%, the steel is turned 90 degrees and forged or hot rolled again, thereby improving the difference between the longitudinal and transverse properties of the material, obtaining uniform structure and properties, with a hardness difference of ≤1.0HRC from the plate; pitting points ≥105mV, which is 100% higher than 4Cr13.

[0010] To achieve the above objectives, the technical solution of the present invention is as follows:

[0011] According to a first aspect of the present invention, a medium-carbon martensitic stainless steel with refined microstructure is provided, having the following composition (weight percentage):

[0012] C = 0.21–0.35%, Si = 0.07–0.35%, Mn = 2.2–2.9%, P ≤ 0.040%, S ≤ 0.005%, Cr = 12.8–14.0%, N = 0.10–0.15%, V = 0.05–0.15%, W = 0.35–0.65%, Nb = 0.001–0.05%, Ti = 0.001–0.05%, B = 0.0005–0.003%, with the remainder being unavoidable impurities and Fe.

[0013] Preferably, the refined medium-carbon martensitic stainless steel has a chemical composition weight percentage that satisfies at least one of the following conditions: C+N≥0.35%, Cr+1.65W+16N≥15.3%, 1.25≤(Cr+0.7W+1.5Si) / (20C+20N+Mn)≤1.50, or 0.021(Cr+0.9Mn)-0.204≥N; more preferably, the refined medium-carbon martensitic stainless steel has a chemical composition weight percentage that satisfies the following conditions: C+N≥0.35%, Cr+1.65W+16N≥15.3%, 1.28≤(Cr+0.7W+1.5Si) / (20C+20N+Mn)≤1.49, and 0.021(Cr+0.9Mn)-0.204≥N. The martensitic stainless steel described in this invention is ultimately a multiphase structure consisting of dispersed V and W carbides and nitrides in martensite plus a small amount of retained austenite, resulting in a finer and more uniform material structure.

[0014] The medium-carbon martensitic stainless steel with refined microstructure described in this invention has a lower C content than 4Cr13, and its hardness after quenching is comparable to that of 4Cr13. The specific measures are to add 0.10-0.15% N, control C+N≥0.35%, reduce the precipitation of coarse carbides and increase the precipitation of fine nitrides, and improve the hardness by controlling V=0.05-0.15% and W=0.35-0.65%, with a hardness ≥52.5HRC.

[0015] According to a second aspect of the present invention, a method for manufacturing medium-carbon martensitic stainless steel with refined microstructure is provided, comprising the following steps:

[0016] (1) According to the designed composition, steel ingots are cast by electric furnace and ladle refining;

[0017] (2) Steel plates or strips are produced by directional forging or hot rolling, with a heating temperature of 1100-1200℃, a holding time of 2-5 hours, and a final rolling (forging) temperature of 850-930℃.

[0018] (3) The solution temperature is 1000-1100℃, the solution holding time is 15-30min, and the solution is cooled with water and tempered.

[0019] Preferably, the specific process of reversing forging or hot rolling in step (2) is to first reduce the deformation of the billet by 20%-30%, then rotate it 90 degrees, and continue forging or rolling to the required thickness to complete the remaining 80%-70% deformation.

[0020] This invention relates to a medium-carbon martensitic stainless steel with refined microstructure and its manufacturing method. Compared with the similar 4Cr13, the material exhibits significantly improved corrosion resistance by more than 50% while achieving the same hardness (≥52.0 HRC). This is achieved by reducing the carbon content in the alloy to below 0.35%, adding nitrogen (N) and ensuring C+N ≥ 0.35% to compensate for the impact of reduced carbon content on austenitization and hardness. This ensures near-perfect hardness while improving corrosion resistance with nitrogen. The addition of titanium (W) further enhances corrosion resistance, refines the microstructure, and avoids the presence of coarse carbide particles, thereby improving microstructure uniformity and material corrosion resistance. The steel is produced by directional forging or hot rolling, specifically by forging or rolling the billet by 20% or 30%, followed by a 90-degree rotation for further forging or hot rolling. This improves the longitudinal and transverse performance differences, resulting in a uniform microstructure and properties. The hardness difference between plates is ≤1.0 HRC, and the pitting corrosion sites are ≥105 mV, 100% higher than 4Cr13.

[0021] In the component design of this invention:

[0022] Carbon is an important austenitizing element; a certain carbon content ensures a fully austenitic structure at high temperatures. It is also crucial for maintaining hardness after heat treatment, acting as both a solid solution strengthening element and a precipitation strengthening element. It stabilizes the retained austenite structure. However, excessively high carbon content leads to the precipitation of large amounts of chromium carbide, severely impacting corrosion resistance. To achieve the desired effect, a carbon content of 0.21–0.35% is required, lower than that of 4Cr13 with the same hardness.

[0023] Nitrogen: Like carbon, nitrogen is an austenitizing element and can exist as interstitial atoms. It has a solid solution strengthening effect. Nitrogen has a higher solubility in austenite than carbon, resulting in fewer nitrogen precipitates during heat treatment. Furthermore, nitrogen dissolved in the matrix can improve the corrosion resistance of stainless steel. Therefore, nitrogen is an element that can improve both the strength and corrosion resistance of martensitic stainless steel. However, nitrogen content exceeding its solubility will lead to subcutaneous pinhole formation, deteriorating corrosion resistance and hardness. This invention adds a certain amount of Mn to increase nitrogen solubility, controlling N to 0.10–0.15% and ensuring C+N ≥ 0.35% to guarantee a material hardness of not less than 4Cr13. To avoid nitrogen precipitation and porosity, 0.021(Cr+0.9Mn)-0.204 ≥ N is controlled.

[0024] Chromium: Chromium is an element that improves the corrosion resistance of stainless steel, but chromium is a strong ferrite-forming element. When the content is high, it will make it difficult for low-carbon martensitic steel to austenitize. The content of Cr should be controlled at 12.8-14.0%, and Cr+1.65W+16N≥15.3% to ensure excellent corrosion resistance.

[0025] Tungsten: Tungsten is an important element in the alloy of this invention. The addition of tungsten can further improve corrosion resistance, and its function is similar to that of Mo. Experiments have shown that W also has a similar function to V, refining the microstructure and avoiding the presence of coarse carbide particles, thereby improving the microstructure uniformity and material corrosion resistance. However, excessive tungsten content leads to severe casting segregation until the formation of the Laves phase. Therefore, W is controlled to be 0.35-0.65%.

[0026] Vanadium: Vanadium is a strong nitride-forming element. During hot working or heat treatment, it readily forms fine nitrides and other precipitates with nitrogen, thereby improving the uniformity of the microstructure. This invention specifies V = 0.05 to 0.15%.

[0027] Niobium and titanium are both strong carbide-forming elements. The addition of niobium and titanium to martensitic stainless steel can improve the grain structure and carbide morphology of annealed steel sheets. Niobium can refine the microstructure of annealed steel sheets and increase their toughness. However, since niobium and titanium are strong ferrite-forming elements, high content can lead to decreased hardenability of martensitic stainless steel, increasing the difficulty of subsequent quenching heat treatment. This invention controls Nb to be 0.001-0.05% and Ti to be 0.001-0.05%.

[0028] Manganese: Manganese can stabilize the austenite phase. By comprehensively adjusting the ratio of ferrite and austenite forming elements, a certain amount of residual austenite phase can be guaranteed. In this invention, Mn is also an important element to improve the solubility of N and avoid the precipitation of N content to form pinhole defects. However, too high Mn content leads to a decrease in corrosion resistance and an excessively high residual austenite content. Mn is controlled to be 2.2-2.9% and 0.021(Cr+0.9Mn)-0.204≥N.

[0029] Synergistic control of carbon, nitrogen, chromium, tungsten, and manganese: C+N≥0.35%, Cr+1.65W+16N≥15.3%, 1.25≤(Cr+0.7W+1.5Si) / (20C+20N+Mn)≤1.50 and 0.021(Cr+0.9Mn)-0.204≥N, in order to avoid nitrogen porosity defects and obtain the designed hardness and corrosion resistance.

[0030] Silicon: Primarily added to steel as a deoxidizer, it plays a role in solid solution strengthening and also significantly improves resistance to high-temperature oxidation. However, high silicon content in steel leads to decreased ductility and toughness; therefore, this invention controls the Si content to be between 0.07% and 0.35%.

[0031] Phosphorus: Phosphorus is a harmful element, so it should be reduced as much as possible according to the production control level to ensure that P≤0.040%.

[0032] Sulfur: Sulfur is also a harmful element. The sulfides formed not only cause hot brittleness but also reduce corrosion resistance. Sulfurization is carried out through ladle refining to control S≤0.005% to avoid the harmful effects of sulfur.

[0033] Boron: Boron can improve grain boundary bonding, which is beneficial for improving hot working performance; a certain amount of boron can also purify molten steel. Excessive boron content leads to network borides and brittleness, therefore boron content should be controlled at 0.0005-0.003%.

[0034] In terms of manufacturing process, steel billets or continuously cast billets with specified compositions are hot-rolled into hot-rolled steel plates or strips at a heating temperature between 1100℃ and 1200℃. The steel plates or strips are produced by reversible forging or hot rolling. Specifically, after the cast billet has been forged or rolled to a deformation of 20% or 30%, it is rotated 90 degrees and forged or hot-rolled again. This improves the difference in longitudinal and transverse properties of the material, resulting in a uniform microstructure and properties. The hardness difference between the two plates is ≤1.0 HRC; the pitting corrosion sites are ≥105 mV, 100% higher than 4Cr13.

[0035] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0036] The martensitic stainless steel with excellent corrosion resistance described in this invention, compared with the similar 4Cr13, has a corrosion resistance that is significantly improved by more than 100% while achieving the same hardness (≥52.0HRC).

[0037] By reducing the C content in the alloy to below 0.35%, adding N and ensuring that C+N≥0.35% compensates for the impact of the reduced C content on austenitization and hardness, ensuring that near-hardness is obtained while improving corrosion resistance with N;

[0038] The addition of W can further improve corrosion resistance, refine the microstructure, and avoid the presence of coarse carbide particles, thereby improving the microstructure uniformity and material corrosion resistance.

[0039] Steel plates or strips are produced by reverse forging or hot rolling. Specifically, after the billet is forged or rolled to deform by 20% or 30%, it is turned 90 degrees and forged or hot rolled again. This improves the difference between the longitudinal and transverse properties of the material, resulting in a uniform microstructure and properties. The hardness difference between the two plates is ≤1.0HRC; the pitting corrosion sites are ≥105mV, which is 100% higher than 4Cr13. Attached Figure Description

[0040] Figure 1 This is the metallographic structure of Comparative Example G of the present invention.

[0041] Figure 2The metallographic structure is shown in Embodiment C of the present invention. Detailed Implementation

[0042] Medium-carbon martensitic stainless steel with refined microstructure is prepared according to the following steps:

[0043] (1) According to the designed composition, steel ingots are cast by electric furnace and ladle refining;

[0044] (2) Steel plates or strips are produced by directional forging or hot rolling, with a heating temperature of 1100-1200℃, a holding time of 2-5 hours, and a final rolling (forging) temperature of 850-930℃.

[0045] (3) The solution temperature is 1000-1100℃, the solution holding time is 15-30min, and the solution is cooled with water and tempered.

[0046] The specific process of reversing forging or hot rolling in step (2) is to first reduce the deformation of the billet by 20% or 30%, then rotate it 90 degrees and continue forging or rolling to the required thickness to complete the remaining 80% or 70% deformation.

[0047] The present invention will be further described below with reference to the embodiments and accompanying drawings. Table 1 shows the composition of the steel in the embodiments and comparative examples of the present invention, and Table 2 shows the composition, process and performance of the embodiments. Figure 1 The microstructure of Comparative Example G of this invention is shown below. Comparative Example G is a conventional 4Cr13 martensitic stainless steel, whose quenching (tempering) hardness can reach 52.0 HRC. The microstructure is a fully martensitic structure. Due to the high carbon content, there are more carbide precipitations, and relatively coarse carbides are prone to appear. Figure 1 The coarser carbide particles and segregation bands in conventional 4Cr13 can be clearly seen. These structures and segregation will lead to: (1) uneven mechanical properties, with large differences in mechanical properties in the longitudinal and transverse (perpendicular to the rolling direction) of the same steel plate, and the hardness difference at different locations of the same steel plate can reach HRC2.0; (2) excessive carbide precipitation and segregation will also lead to deterioration of corrosion resistance, so the pitting potential of 4Cr13 is about 40 to 60 mV. Figure 2 The image shows the metallographic structure of Embodiment C of the present invention. In terms of composition, the carbon content is controlled at 0.35%, and 0.11% nitrogen is added. At the same time, W and V alloying is adopted, and the material hardness reaches HRC53.0. Microalloying promotes a finer and more uniform microstructure and eliminates coarse carbides. Meanwhile, the difference between transverse and longitudinal microstructure is reduced by directional rolling. The hardness difference of different positions of the steel plate is HRC1.0, which is significantly better than conventional 4Cr13.

[0048] Regarding corrosion resistance, Example C, by reducing carbon content, adding N and W elements to improve corrosion resistance, and reducing carbide segregation, achieved a pitting potential PREN = Cr + 1.65W + 16N ≥ 15.3, significantly higher than the 13.2 and 13.4 of conventional 4Cr13. Therefore, physicochemical testing results show that the pitting potential of Example C reaches 130 mV, far superior to the 50 mV of conventional 4Cr13 (Comparative Example G).

[0049] Comparing the composition and properties of the examples and comparative examples in Tables 1 and 2, it can be found that compared with the same type of 4Cr13, the corrosion resistance of the material is significantly improved by more than 100% while achieving the same hardness (≥52.0 HRC). By reducing the C content in the alloy to below 0.35%, adding N and ensuring that C+N≥0.35% compensates for the impact of the reduced C content on austenitization and hardness, ensuring that near-hardness is achieved while using N to improve corrosion resistance; the addition of W can further improve corrosion resistance, refine the microstructure, and avoid the presence of coarse carbide particles, thereby improving the uniformity of the microstructure and the corrosion resistance of the material; the steel plate or strip is produced by directional forging or hot rolling, specifically by forging or rolling the billet by 20% or 30%, then turning the steel 90 degrees to continue forging or hot rolling, thereby improving the difference between longitudinal and transverse properties of the material, obtaining uniform microstructure and properties, with a hardness difference of ≤1.0 HRC between plates; pitting corrosion sites ≥105mV, nearly 50mV higher than 4Cr13.

[0050] Table 1. Chemical composition (wt, %) of the examples and comparative examples

[0051]

[0052]

[0053] Table 2. Components, Process, and Performance of Examples

[0054]

[0055]

[0056] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A medium-carbon martensitic stainless steel with refined microstructure, the chemical composition by weight percentage being: C=0.21~0.35%, Si=0.07~0.35%, Mn=2.2~2.9%, P≤0.040%, S≤0.005%, Cr=12.8~14.0%, N=0.10~0.15%, V=0.05~0.15%, W=0.35~0.65%, Nb=0.001-0.05%, Ti=0.001-0.05%, B=0.0005-0.003%, with the remainder being unavoidable impurities and Fe. Its chemical composition by weight percentage must satisfy at least one of the following conditions: C+N≥0.35%, Cr+1.65W+16N≥15.3%, 1.25≤(Cr+0.7W+1.5Si) / (20C+20N+Mn)≤1.50, and 0.021(Cr+0.9Mn)-0.204≥N; The method for preparing the refined medium-carbon martensitic stainless steel includes the following steps: (1) According to the designed composition, steel ingots are cast into steel ingots by electric furnace and ladle refining; (2) Steel plates or strips are produced by directional forging or hot rolling, with a heating temperature of 1100~1200℃, a holding time of 2~5 hours, and a final rolling temperature of 850~930℃; (3) The solution treatment temperature is 1000~1100℃, the solution treatment holding time is 15~30min, and then water cooling and tempering are performed. The specific process of reversing forging or hot rolling in step (2) is to first press down the billet by 20%-30% of the deformation, then rotate it 90 degrees, and continue forging or rolling to the required thickness to complete the remaining 80%-70% of the deformation.

2. The medium-carbon martensitic stainless steel with refined microstructure according to claim 1, wherein the chemical composition by weight percentage satisfies the following conditions: C+N≥0.35%, Cr+1.65W+16N≥15.3%, 1.28≤(Cr+0.7W+1.5Si) / (20C+20N+Mn)≤1.49 and 0.021(Cr+0.9Mn)-0.204≥N.

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