Food-grade high-strength corrosion-resistant C30RE ultrahigh-nitrogen martensitic stainless steel and preparation method thereof

Abstract

The invention belongs to the field of stainless steel material technology and cutter manufacturing, and particularly relates to food-grade high-strength corrosion-resistant C30RE ultrahigh-nitrogen martensitic stainless steel and a preparation method thereof. The steel comprises the following chemical components in percentage by weight: 0.20 to 0.50 percent of C, 0.20 to 0.80 percent of N, 13.0 to 16.0 percent of Cr, 0.50 to 1.50 percent of Mo, 0.10 to 0.60 percent of Mn, 0.30 to 0.80 percent of Si, 0.002 to 0.030 percent of RE, less than or equal to 0.20 percent of Ni, less than or equal to 0.20 percent of Cu, less than or equal to 0.006 percent of Ti, less than or equal to 0.0020 percent of O, less than or equal to 0.0002 percent of H, less than or equal to 0.015 percent of P, less than or equal to 0.005 percent of S, less than or equal to 0.0080 percent of As + Sn + Sb + Bi + Pb and the balance of Fe. The contents of elements such as Ni and Cu which may cause potential injury to a human body are strictly limited in the chemical components, and the contents of impurity elements, residual elements, control elements and gas elements are strictly controlled by adopting technologies such as ultra-pure raw materials, rare earth treatment and vacuum pressurization N alloying, so that the food-grade and even medical-grade use requirements are met. And the food-grade high-strength corrosion-resistant C30RE ultrahigh-nitrogen martensitic stainless steel is obtained through the processes of forging cogging, plate hot rolling, performance heat treatment and the like.

Description

Technical Field

[0001] This invention belongs to the fields of stainless steel material technology and knife manufacturing, specifically relating to a food-grade high-strength corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel and its preparation method. It is suitable for food contact knives, kitchen utensils, food processing equipment, etc., and has the characteristics of food-grade safety, ultra-high strength, excellent corrosion resistance and rare earth antibacterial function. Background Technology

[0002] my country is the world's largest consumer of cutting tools in the food industry, with a huge demand for cutting tool steel. However, due to a lack of systematic research on cutting tool steel and a lack of high-end cutting tool processing technology, domestic cutting tool products are currently mainly concentrated in the low-to-mid-end market, while the high-end cutting tool market is almost monopolized by well-known foreign brands.

[0003] The primary function of knives in the food industry is food cutting. Besides efficiently cutting various ingredients, they must also withstand the corrosion caused by food cutting. Therefore, food industry knives require materials with high hardness, high strength, good wear resistance, corrosion resistance, and appropriate toughness to ensure sharpness and durability during use. Currently, domestic and international food industry knife materials focus more on achieving sharpness, durability, and a certain degree of corrosion resistance, such as 1.4116 from Germany, VG10 from Japan, and 154CM from the United States. Domestic knife materials mainly consist of high-carbon, high-chromium martensitic stainless steel, such as 7Cr17MoV and 9Cr18MoV, whose hardness generally exceeds 58 HRC. However, existing food industry knife materials rarely consider ensuring health during long-term food cutting. During prolonged contact with food, due to corrosion caused by food cutting, trace amounts of ions will inevitably leach from the knife material. This necessitates that the knife material must be free of elements that could potentially harm the human body. In addition, it is necessary to strictly control the content of impurity elements and harmful elements in the chemical composition to ensure that the knife is non-toxic and harmless during long-term use.

[0004] Patent CN108441783A discloses a food-grade high-wear-resistant martensitic stainless steel material and its preparation method. Although it is food-grade stainless steel, its antibacterial properties are achieved solely through copper. The antibacterial effect of copper relies on a high addition amount of 1.5–2.2%, which not only increases costs but also poses a potential risk of copper ion migration. While the material's hardness reaches over 56 HRC, its toughness is insufficient, making it prone to chipping. Patent CN102134688A discloses a super-high-nitrogen martensitic stainless steel and its preparation method. With nickel and copper contents of 3.5–4.5%, the high proportion of these two elements leads to excessive ion migration during food contact, completely lacking the basis for food-grade applications. The nitrogen content is 0.15–0.25%, higher than ordinary stainless steel, but it still cannot achieve a balance between high strength and corrosion resistance for food-grade materials through nitrogen solid solution strengthening. The patent with publication number CN112410674A proposes a rare earth-containing copper-rich precipitate-strengthened martensitic stainless steel and its preparation method. However, due to the low nitrogen content of 0.05-0.16%, it is far from achieving the solid solution strengthening effect of ultra-high nitrogen, and the strength improvement is limited. The role of rare earth is only to promote the precipitation of copper-rich phase, rather than directly achieving antibacterial function. Although rare earth and copper-rich phase are combined, it still cannot meet the requirements of food-grade ultra-high nitrogen martensitic stainless steel and cannot achieve the dual effect of strengthening and antibacterial function.

[0005] Therefore, there is an urgent need to develop new knife materials that, while achieving the functions of sharpness, durability, and corrosion resistance of knives used in the food industry, fully consider the potential damage to the human body caused by the internal chemical components of the knife materials, and ensure that the knife materials remain non-toxic and harmless during long-term use. Summary of the Invention

[0006] The purpose of this invention is to provide a food-grade high-strength and corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel and its preparation method. The food-grade C30RE ultra-high nitrogen martensitic stainless steel has high strength and hardness, and excellent corrosion resistance, ensuring that high-end knives made of food-grade C30RE ultra-high nitrogen martensitic stainless steel have functions such as sharpness, durability, and corrosion resistance. It solves the problems of insufficient strength, corrosion resistance, and antibacterial properties of existing food-grade stainless steel, as well as the problems of high nickel and copper content and uncontrolled harmful impurities in non-food-grade martensitic stainless steel, which cannot meet the requirements of food contact safety.

[0007] The technical solution of this invention is:

[0008] A food-grade, high-strength, corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel, with the following chemical composition by weight percentage: C: 0.20–0.50, N: 0.20–0.80, Cr: 13.0–16.0, Mo: 0.50–1.50, Mn: 0.10–0.60, Si: 0.30–0.80, RE: 0.002–0.030, Ni≤0.20, Cu≤0.20, Ti≤0.006, O≤0.0020, H≤0.0002, P≤0.015, S≤0.005, As+Sn+Sb+Bi+Pb≤0.0080, with the balance being Fe.

[0009] The food-grade high-strength corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel, preferably, is alloyed with N by weight percentage, N = 0.20~0.60; and micro-alloyed with RE by weight percentage, RE = 0.002~0.020.

[0010] The food-grade high-strength corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel, preferably, has a chemical composition in which the content of elements Ni and Cu that may cause potential harm to the human body is strictly limited, with Ni ≤ 0.10 and Cu ≤ 0.10 by weight percentage.

[0011] The food-grade high-strength corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel, preferably, has the following chemical composition by weight percentage: strictly controlled content of impurity elements, residual elements, controlled elements, and gaseous elements: Ti≤0.005, O≤0.0015, H≤0.0001, P≤0.010, S≤0.003, As+Sn+Sb+Bi+Pb≤0.0050.

[0012] The food-grade high-strength corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel, preferably, has the following composition: C: 0.30-0.45, Cr: 14.0-15.5, Mo: 0.80-1.20.

[0013] The method for preparing food-grade high-strength and corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel uses ultra-high purity iron and high-purity metals as raw materials. Ingots are formed through vacuum pressure induction melting for nitrogen alloying, rare earth treatment, and protective casting. Then, through homogeneous forging, slab hot rolling, and performance heat treatment processes, food-grade high-strength and corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel material with a uniform, dense, fine, and dispersed microstructure is finally obtained. The specific steps include:

[0014] 1) Raw materials: Ultra-high purity iron and high purity metals with very low content of elements such as O, S, As, Sn, Sb, Bi, Pb and Ti are used as smelting raw materials;

[0015] 2) Vacuum pressure melting: Vacuum pressure induction furnace is used for melting. A reasonable material feeding method and addition sequence are selected. High-purity carbon, ultra-high-purity iron and high-purity metals that are not easily burned are added first, followed by high-purity metals of easily burned elements. The content of impurity elements and gaseous elements is strictly controlled.

[0016] 3) Nitrogen alloying: High-purity nitrogen is used for pressurized nitrogen alloying. N2 is continuously introduced during the melting process, and the N2 pressure range is 2 atm to 30 atm to prevent nitrogen from escaping as gas.

[0017] 4) Rare earth treatment: Before rare earth treatment, the molten steel is fully deoxidized and rare earth micro-alloying is carried out using high-purity rare earth metals; the high-purity rare earth metals are crushed into small pieces, wrapped in aluminum foil, and added from the top of the molten steel; during the casting process, the N2 pressure range is 0.2atm to 30atm until the ingot is completely solidified.

[0018] 5) Isothermal annealing: After casting, the ingot is demolded at high temperature and isothermal annealed at 600-700℃ for 2-10 hours to reduce internal stress, eliminate microstructure inheritance, and improve the initial microstructure.

[0019] 6) Homogeneous forging: The ingot after isothermal annealing is subjected to high-temperature diffusion annealing and then formed into slabs or bars using conventional forging methods;

[0020] 7) Hot rolling of plates: Using conventional hot rolling methods, slabs or bars are rolled into plates with a thickness of 1 to 7 mm;

[0021] 8) Performance heat treatment: The hot-rolled sheet is subjected to performance heat treatment using a quenching + cryogenic + tempering process. The specific process is as follows: quenching at 980~1050℃ for 3~5h, cryogenic holding at -80~-196℃ for 1~3h, tempering at 200~500℃ for 2~4h, and air cooling to room temperature after tempering. Finally, food-grade high-strength corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel material with uniform, dense, fine and dispersed microstructure is obtained.

[0022] The method for preparing food-grade high-strength corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel, in step 1), the purity of ultra-high purity iron is ≥99.995wt%, the purity of high-purity metal is ≥99.95wt%, the mass percentage of each element As, Sn, Sb, Bi and Pb in the raw materials is controlled to be ≤0.0020%, and the preheating temperature of the smelting raw materials is 400~800℃.

[0023] In the preparation method of the food-grade high-strength corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel, in step 4), the high-purity rare earth metal used for rare earth treatment is lanthanum, cerium or a lanthanum-cerium mixture with a total oxygen content TO≤300ppm, and the rare earth element content is above 99wt%. The total oxygen content TO in the molten steel before rare earth treatment is ≤30ppm.

[0024] The preparation method of the food-grade high-strength corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel, in step 6), the forging process is to heat the ingot to 1180~1260℃ before forging and perform high-temperature diffusion annealing treatment, and the holding time is 1h / 100mm~2.5h / 100mm; the billet is formed by conventional forging method, the final forging temperature is not lower than 950℃, and the forging is air-cooled to room temperature.

[0025] The preparation method of the food-grade high-strength corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel, in step 7), involves heating the forged billet to 1180-1220℃ and holding it for 1h / 100mm to 2.5h / 100mm; rolling it into a billet of a set thickness through multiple passes, with a final rolling temperature of 900-950℃; annealing it at 750-850℃ for 3-5h after rolling; and then furnace cooling to 300-350℃ followed by air cooling to room temperature.

[0026] The design concept of this invention is:

[0027] 1. Material alloy composition design

[0028] This invention strictly limits the content of elements such as Ni and Cu in the chemical composition that may cause potential harm to the human body, controlling the nickel content to ≤0.20wt% and the copper content to ≤0.20wt%. It employs ultra-high purity raw materials, rare earth treatment, and vacuum pressure N alloying technologies to strictly control the content of impurity elements, residual elements, control elements, and gaseous elements. The total amount of harmful impurities (arsenic + tin + antimony + bismuth + lead) is controlled to ≤0.0080wt%, and the oxygen content is ≤0.0020wt%, ensuring the purity of the material and guaranteeing that it meets the requirements for food-grade and even medical-grade use. This ensures the health of high-end knives made of food-grade C30RE ultra-high nitrogen martensitic stainless steel during long-term use.

[0029] Among them, nitrogen (N), as a strong austenitizing element, can significantly replace traditional austenitizing elements such as carbon (C), nickel (Ni), and copper (Cu), ensuring a high-strength, corrosion-resistant martensitic structure during rapid cooling. Furthermore, considering the role and solubility of N in martensitic stainless steel, N alloying is achieved under pressure. Based on the strong interstitial solid solution strengthening effect of N, N interacts with alloying elements such as chromium (Cr) and molybdenum (Mo), promoting the precipitation of fine, dispersed secondary phases during tempering, thus improving the stability of the tempered structure and enhancing the material's strength, toughness, and corrosion resistance. Preferably, in the food-grade high-strength, corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel, N = 0.30–0.50 wt%. In addition, the addition of 0.002–0.030 wt% of rare earth elements not only refines inclusions and improves the microstructure through the metamorphic effects of rare earths but also utilizes their antibacterial properties to achieve an antibacterial effect distinct from copper.

[0030] 2. Preparation method

[0031] This invention utilizes a vacuum pressure induction furnace for smelting. Raw materials include ultra-high purity iron and high-purity metals, which undergo preheating treatment. A reasonable material distribution method and addition sequence are selected to control the content of impurities such as O and S in the steel. While ensuring the total oxygen content (TO) in the steel is ≤20ppm, a high nitrogen partial pressure is created in the furnace by adjusting the flow rate of high-purity nitrogen (N2 pressure range of 2atm to 30atm) for nitrogen alloying. The alloying process is maintained for 3-5 minutes to ensure nitrogen recovery. Furthermore, high-purity rare earth metals are added by wrapping them in aluminum foil to reduce the loss of rare earth elements at high temperatures. The use of high-purity rare earth metals deeply purifies the molten steel, significantly reducing the oxygen content and non-metallic inclusions. Moreover, the addition of rare earth elements has a certain antibacterial effect. Subsequently, during the steel pouring process, the nitrogen partial pressure inside the vacuum furnace was maintained continuously, the pouring speed was strictly controlled, and the steel was rapidly cooled by bottom cooling circulating water to prevent nitrogen bubbles from escaping during solidification, thereby obtaining a dense food-grade C30RE ultra-high nitrogen martensitic stainless steel ingot.

[0032] In particular, the ingot is demolded at high temperature after casting and isothermally annealed at 600–700℃ to reduce internal stress, eliminate inherited microstructure, and improve the initial microstructure. After rolling, it is annealed at 750–850℃, followed by furnace cooling to 300–350℃ and air cooling to eliminate forging defects and improve the post-forging microstructure. Subsequently, a performance heat treatment process of quenching, deep cryogenic treatment, and tempering is adopted. The specific process is quenching at 980–1050℃ + deep cryogenic treatment at (-80 to -196℃) + tempering at 200–500℃ to refine the lath martensite microstructure, promote the dispersed precipitation of nitrides, and further improve strength and toughness. Finally, a microstructure with uniform composition, dense structure, and fine dispersion is obtained, ensuring that the food-grade C30RE ultra-high nitrogen martensitic stainless steel rolled material obtains high strength, high hardness, good wear resistance, corrosion resistance, and appropriate impact toughness.

[0033] The advantages and beneficial effects of this invention are:

[0034] 1. This invention strictly limits the content of elements such as Ni and Cu in the chemical composition that may cause potential harm to the human body. It uses ultra-high purity raw materials, rare earth treatment and vacuum pressure N alloying technology to strictly control the content of impurity elements, residual elements, control elements and gaseous elements. This ensures that the requirements for food-grade and even medical-grade use are met, and ensures the health of high-end knives made of food-grade C30RE ultra-high nitrogen martensitic stainless steel during long-term use.

[0035] 2. By utilizing processes such as forging, hot rolling of plates, and performance heat treatment, this invention enables food-grade C30RE ultra-high nitrogen martensitic stainless steel to obtain a uniform, dense, fine, and dispersed microstructure, exhibiting high strength and hardness, good wear resistance, corrosion resistance, and appropriate impact toughness.

[0036] 3. This invention fully considers the solubility of nitrogen (N) in martensitic stainless steel and its alloying effect. It employs a vacuum pressure induction furnace for melting and casting under nitrogen protection. By simply adjusting the nitrogen partial pressure, martensitic stainless steels with different nitrogen contents can be prepared. This ensures the solubility of nitrogen in ultra-high nitrogen martensitic stainless steel while avoiding defects such as bubbles during solidification. The vacuum pressure melting method is simple, low-cost, and universally applicable, suitable for the preparation of nitrogen-containing martensitic and austenitic stainless steels.

[0037] 4. This invention, by adding an appropriate amount of rare earth elements, fully leverages the purifying, impurity-modifying, and micro-alloying effects of rare earth elements in molten steel, further enhancing the purity and comprehensive mechanical properties of the material. Furthermore, the addition of rare earth elements exhibits a certain antibacterial effect; through the direct antibacterial action of rare earth elements, the inhibition rate against Escherichia coli and Staphylococcus aureus is ≥75%, and the antibacterial performance remains stable over a long period.

[0038] 5. This invention, through the combination of the above-mentioned alloy composition design, material preparation, and heat treatment process, yields a food-grade C30RE ultra-high nitrogen martensitic stainless steel with a room temperature microstructure dominated by martensite, containing a small amount of thin-film retained austenite and fine dispersed precipitates. It possesses extremely high strength and hardness, good wear resistance, corrosion resistance, and appropriate impact toughness. Room temperature tensile strength ≥1990MPa (up to 2100MPa), yield strength ≥1660MPa (up to 1800MPa), and hardness ≥58HRC (up to 62HRC). Furthermore, this food-grade C30RE ultra-high nitrogen martensitic stainless steel exhibits high tempering stability and good corrosion resistance, and can also be used in aerospace, weaponry, shipbuilding, military cutting tools, medical devices, and other fields. Attached Figure Description

[0039] Figure 1 The microstructure of food-grade high-strength corrosion-resistant ultra-high nitrogen martensitic stainless steel after tempering at 200°C is shown in Example 1.

[0040] Figure 2 The microstructure of food-grade high-strength corrosion-resistant ultra-high nitrogen martensitic stainless steel after tempering at 500℃ in Example 1 is shown.

[0041] Figure 3 Example 1 illustrates the antibacterial effect of food-grade high-strength corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel. Detailed Implementation

[0042] In specific implementation, the food-grade high-strength corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel of the present invention, by weight percentage, has the following chemical composition range: C: 0.20~0.50, N: 0.20~0.80, Cr: 13.0~16.0, Mo: 0.50~1.50, Mn: 0.10~0.60, Si: 0.30~0.80, RE: 0.002~0.030, Ni≤0.20, Cu≤0.20, Ti≤0.006, O≤0.0020, H≤0.0002, P≤0.015, S≤0.005, As+Sn+Sb+Bi+Pb≤0.0080, with the balance being Fe.

[0043] The food-grade high-strength and corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel of this invention is prepared through a process involving vacuum pressure induction melting, nitrogen alloying, high-purity rare earth metal treatment, gas-protected casting, homogeneous forging, slab rolling, and performance heat treatment. This process ultimately yields a uniform, dense, fine, and dispersed microstructure, resulting in a food-grade high-strength and corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel with high strength, high hardness, good wear resistance and corrosion resistance, and appropriate impact toughness. The specific steps include:

[0044] 1) Raw materials: Ultra-high purity iron and high purity metals with very low content of elements such as O, S, As, Sn, Sb, Bi, Pb and Ti are used as smelting raw materials. The preheating temperature of the smelting raw materials is 400-800℃.

[0045] 2) Vacuum pressure induction melting: During the melting process, select a reasonable material feeding method and addition order, prioritize the addition of ultra-high purity iron and high purity metals that are not easily burned, and then add high purity metals of easily burned elements, strictly control the content of O and S impurity elements.

[0046] 3) Nitrogen alloying: High-purity nitrogen (volume purity greater than 99.99%) is used as the nitrogen source for nitrogen alloying. N2 is continuously introduced during the pressurization process, with an N2 pressure of 2 to 10 atm. The ingot is poured under the protection of high-purity nitrogen to prevent nitrogen from escaping as gas, and finally the ingot is formed.

[0047] 4) Rare earth treatment: High-purity rare earth metals are used for rare earth treatment. Before treatment, the molten steel must be fully deoxidized. The rare earth metals are crushed into small pieces, wrapped in aluminum foil, and added from the top of the molten steel.

[0048] 5) Post-cast annealing: After casting, the ingot is demolded at high temperature and isothermal annealed at 600-700℃ for 2-10 hours. Then it is cooled to room temperature to reduce internal stress and improve the initial structure.

[0049] 6) Homogeneous forging: The ingot is heated to 1180-1260℃ for high-temperature diffusion annealing, and the holding time is 1h / 100mm to 2.5h / 100mm (thickness). After that, conventional forging methods are used to open the billet, with the initial forging temperature being 1080-1150℃ and the final forging temperature not lower than 950℃.

[0050] 7) Hot rolling of sheet metal: The forged billet is heated to 1180–1220℃ and held for 1 h / 100 mm to 2.5 h / 100 mm (thickness). Then, it is rolled in multiple passes into slabs or bars of 1–7 mm thickness, with a final rolling temperature of 900–950℃. After rolling, it is annealed at 750–850℃ for 3–5 h, followed by furnace cooling to 300–350℃ and air cooling to room temperature.

[0051] 8) Performance heat treatment: The hot-rolled sheet is subjected to performance heat treatment using a process of quenching + deep cryogenic + tempering. The specific process is as follows: quenching at 980~1050℃ for 3~5h, deep cryogenic holding at -80~-196℃ for 1~3h, tempering at 200~500℃ for 2~4h, and air cooling to room temperature after tempering. Finally, food-grade high-strength corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel material with uniform, dense, fine and dispersed microstructure is obtained.

[0052] To make the technical solutions and advantages of the present invention clearer, a detailed description is provided below in conjunction with specific embodiments and accompanying drawings.

[0053] Example 1:

[0054] In this embodiment, the food-grade high-strength corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel has the following specific composition by weight percentage: C 0.35%, N 0.50%, Cr 14.50%, Mo 1.20%, Mn 0.50%, Si 0.35%, RE (La-Ce = 1:1) 0.015%, Ni 0.05%, Cu 0.02%, Ti 0.003%, O 0.0012%, H 0.0001%, P 0.0060%, S 0.0008%, As + Sn + Sb + Bi + Pb = 0.0035%, with the balance being Fe.

[0055] In this embodiment, the preparation process of food-grade high-strength and corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel material is as follows:

[0056] The alloy was smelted under pressure using a vacuum induction furnace. The alloy composition consisted of high-purity carbon (99.99 wt%), ultra-high-purity iron (99.995 wt%), metallic chromium, metallic manganese, metallic silicon, and metallic molybdenum (chromium, manganese, silicon, and molybdenum all 99.99 wt%), preheated at 600°C. First, the high-purity carbon and ultra-high-purity iron were placed in a crucible, followed by the addition of metallic chromium, manganese, silicon, and molybdenum. Then, high-purity rare earth metals (a lanthanum-cerium mixed metal with a total oxygen content of 230 ppm and a lanthanum-cerium element content of 99.5 wt%) were added for rare earth treatment. After the alloy was completely melted, the vacuum was increased to a pressure of 2 × 10⁻⁶. -3 The ingot was refined for 10 minutes, with strict control over the content of impurity elements such as O and S. Then, high-purity nitrogen gas (99.995% volume purity) at 8 atm was introduced for nitrogen alloying. N2 was continuously introduced during the pressurization process, and the ingot was poured under the protection of high-purity nitrogen gas (N2 pressure 1.2 atm) to prevent nitrogen from escaping as gas. After pouring, the ingot was demolded at high temperature and placed in a heat treatment furnace at 680℃ for isothermal annealing, held for 6 hours, and then furnace cooled to room temperature to relieve stress and improve the initial microstructure. After removing the riser and ingot tail, the ingot underwent high-temperature diffusion annealing, heated to 1200℃ and held for 5 hours before being forged. The forging temperature range was 1150–950℃, and the forged ingot was air-cooled to room temperature. It was then heated to 1190℃ and held for 2.5 hours before hot rolling, with a hot rolling temperature range of 1150–950℃, into a 5mm thick slab. After rolling, the material is held at 800℃ for 4 hours, furnace cooled to 320℃, and then air-cooled to room temperature to eliminate the post-rolling microstructure. Subsequent performance heat treatment processes involve quenching at 980℃ for 5 hours, followed by deep cryogenic treatment at -80℃ for 2 hours, and then tempering at 200℃, 300℃, 400℃, and 500℃ for 3 hours. After tempering, the material is air-cooled to room temperature.

[0057] In this embodiment, samples were cut from hot-rolled food-grade high-strength and corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel sheets after performance heat treatment. Metallographic structure, room temperature tensile strength, and Rockwell hardness were tested to evaluate the strength and microstructure uniformity of the food-grade high-strength and corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel. The tensile properties and hardness test results are shown in Table 1.

[0058] Table 1 Mechanical properties of food-grade high-strength and corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel after tempering at different temperatures

[0059] Tempering temperature (°C) 200 300 400 500 Yield strength (MPa) 1661 1668 1666 1660 Tensile strength (MPa) 2070 1999 1996 1993 Hardness (HRC) 60.5 60.1 59.7 59.8

[0060] like Figure 1 and Figure 2 As shown, this embodiment illustrates the matrix microstructure and precipitated phase morphology of food-grade high-strength corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel after tempering at 200℃ and 500℃. Figure 1It can be seen that the microstructure after tempering at 200℃ is tempered martensite, with a large number of fine, uniform, and dispersed second phases precipitated. From... Figure 2 It can be seen that the microstructure after tempering at 500℃ still retains the lath martensite morphology, and the secondary precipitates are still finely and dispersedly distributed.

[0061] like Figure 3 The image shows the antibacterial effect of food-grade high-strength and corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel after the addition of rare earth elements. Using the Japanese Industrial Standard JIS Z 2801 test method, *E. coli* was activated to the fourth generation; after 24 hours, 100 μL was plated and the number of *E. coli* colonies was tested. The number of *E. coli* colonies on the food-grade high-strength and corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel with added rare earth elements was 133, a reduction of 77.9% compared to the 604 colonies without added rare earth elements.

[0062] Example 2

[0063] In this embodiment, the food-grade high-strength corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel has the following specific composition by weight percentage: C 0.26%, N 0.43%, Cr 13.60%, Mo 0.65%, Mn 0.27%, Si 0.52%, RE(La) 0.012%, Ni 0.03%, Cu 0.09%, Ti 0.004%, O 0.0008%, H 0.0001%, P 0.0093%, S 0.0018%, As+Sn+Sb+Bi+Pb=0.0024%, with the balance being Fe.

[0064] In this embodiment, the preparation process of food-grade high-strength and corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel material is as follows:

[0065] The alloy was smelted under pressure using a vacuum induction furnace. The alloy composition consisted of high-purity carbon, ultra-high-purity iron, metallic chromium, metallic manganese, metallic silicon, and metallic molybdenum, preheated at 600℃. First, high-purity carbon and ultra-high-purity iron were placed in a crucible, followed by metallic chromium, metallic manganese, metallic silicon, and metallic molybdenum. Then, high-purity rare earth metals (lanthanum with a total oxygen content of 160 ppm and a lanthanum content of 99.5 wt%) were added for rare earth treatment. After the alloy was completely melted, the vacuum was increased to a pressure of 2 × 10⁻⁶. -3The ingot was refined for 10 minutes, with strict control over the content of impurity elements such as O and S. Then, 6 atm of high-purity nitrogen was introduced for nitrogen alloying. During the pressurization process, N2 was continuously introduced, and the ingot was poured under the protection of high-purity nitrogen (N2 pressure 1.0 atm) to prevent nitrogen from escaping as gas. After pouring, the ingot was demolded at high temperature and placed in a heat treatment furnace at 680℃ for isothermal annealing, held for 8 hours, and then furnace cooled to room temperature to relieve stress and improve the initial microstructure. After removing the riser and ingot tail, the ingot underwent high-temperature diffusion annealing, heated to 1230℃ and held for 4 hours before being forged. The forging temperature range was 1150–950℃, and the forged ingot was air-cooled to room temperature. It was then heated to 1210℃ and held for 2 hours before hot rolling, with a hot rolling temperature range of 1150–950℃, to a 3mm thick slab. After rolling, the material is held at 780℃ for 3 hours, furnace cooled to 300℃, and then air-cooled to room temperature to eliminate the post-rolling microstructure. The subsequent performance heat treatment process consists of quenching at 1000℃ for 5 hours, followed by cryogenic treatment at -120℃ for 3 hours, and tempering at 200℃ for 4 hours.

[0066] In this embodiment, the mechanical properties of food-grade high-strength corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel are as follows: yield strength 1705MPa, tensile strength 2017MPa, hardness 58.6HRC.

[0067] Example 3

[0068] In this embodiment, the food-grade high-strength corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel has the following alloy composition by weight percentage: C 0.41%, N 0.76%, Cr 15.80%, Mo 1.32%, Mn 0.45%, Si 0.63%, RE(Ce) 0.021%, Ni 0.08%, Cu 0.05%, Ti 0.005%, O 0.0013%, H 0.0001%, P 0.0056%, S 0.0012%, As+Sn+Sb+Bi+Pb=0.0047%, with the balance being Fe.

[0069] In this embodiment, the preparation process of food-grade high-strength and corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel material is as follows:

[0070] The alloy was smelted under pressure using a vacuum induction furnace. The alloy composition consisted of high-purity carbon, ultra-high-purity iron, metallic chromium, metallic manganese, metallic silicon, and metallic molybdenum, preheated at 600℃. First, high-purity carbon and ultra-high-purity iron were placed in a crucible, followed by metallic chromium, metallic manganese, metallic silicon, and metallic molybdenum. Then, high-purity rare earth metals (cerium with a total oxygen content of 180 ppm and a cerium content of 99.5 wt%) were added for rare earth treatment. After the alloy was completely melted, the vacuum level was increased to a pressure of 2 × 10⁻⁶. -3The ingot was refined for 10 minutes, with strict control over the content of impurity elements such as O and S. Then, 4 atm of high-purity nitrogen was introduced for nitrogen alloying. During the pressurization process, N2 was continuously introduced, and the ingot was poured under the protection of high-purity nitrogen (N2 pressure 0.8 atm) to prevent nitrogen from escaping as gas. After pouring, the ingot was demolded at high temperature and placed in a heat treatment furnace at 680℃ for isothermal annealing, held for 4 hours, and then furnace cooled to room temperature to relieve stress and improve the initial microstructure. After removing the riser and ingot tail, the ingot underwent high-temperature diffusion annealing, heated to 1250℃ and held for 3 hours before being forged. The forging temperature range was 1150–950℃, and the forged ingot was air-cooled to room temperature. It was then heated to 1200℃ and held for 1.5 hours before hot rolling, with a hot rolling temperature range of 1150–950℃, to a 7mm thick slab. After rolling, the material is held at 760℃ for 5 hours, furnace cooled to 350℃, and then air-cooled to room temperature to eliminate the post-rolling microstructure. The subsequent performance heat treatment process consists of quenching at 1050℃ for 3 hours, followed by cryogenic treatment at -196℃ for 1 hour, and tempering at 300℃ for 5 hours.

[0071] In this embodiment, the mechanical properties of food-grade high-strength corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel are as follows: yield strength 1762MPa, tensile strength 2093MPa, and hardness 61.5HRC.

[0072] The results of the embodiments show that the present invention, by combining the material composition design of food-grade high-strength and corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel, vacuum pressure melting, forging, hot rolling of plates, and performance heat treatment, produces a uniform and dense microstructure with fine and dispersed precipitates. It has ultra-high strength and hardness, and excellent high-temperature tempering stability, which can meet the material selection requirements of high-end cutting tools in the current food industry. It can be widely used in food processing, medical devices, and other health fields, and has broad market prospects and important application value.

Claims

1. A food-grade, high-strength, corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel, characterized in that, The chemical composition by weight percentage includes: C: 0.20–0.50, N: 0.20–0.80, Cr: 13.0–16.0, Mo: 0.50–1.50, Mn: 0.10–0.60, Si: 0.30–0.80, RE: 0.002–0.030, Ni≤0.20, Cu≤0.20, Ti≤0.006, O≤0.0020, H≤0.0002, P≤0.015, S≤0.005, As+Sn+Sb+Bi+Pb≤0.0080, with the balance being Fe.

2. The food-grade high-strength corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel according to claim 1, characterized in that, Preferably, by weight percentage, N alloying is used, N = 0.20 to 0.60; and RE microalloying is used, RE = 0.002 to 0.

020.

3. The food-grade high-strength corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel according to claim 1, characterized in that, Preferably, the chemical composition strictly limits the content of elements such as Ni and Cu that may cause potential harm to the human body, with Ni ≤ 0.10 and Cu ≤ 0.10 by weight percentage.

4. The food-grade high-strength corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel according to claim 1, characterized in that, Preferably, the content of impurity elements, residual elements, control elements, and gaseous elements in the chemical composition is strictly controlled by weight percentage: Ti≤0.005, O≤0.0015, H≤0.0001, P≤0.010, S≤0.003, As+Sn+Sb+Bi+Pb≤0.0050.

5. The food-grade high-strength corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel according to claim 1, characterized in that, Preferably, C: 0.30-0.45, Cr: 14.0-15.5, Mo: 0.80-1.

20.

6. A method for preparing food-grade high-strength corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel according to any one of claims 1 to 5, characterized in that, Using ultra-high purity iron and high-purity metals as raw materials, nitrogen alloying, rare earth treatment, and protective casting are carried out through vacuum pressure induction melting to form ingots. Then, through homogeneous forging, slab hot rolling, and performance heat treatment processes, food-grade high-strength and corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel material with uniform, dense, fine, and dispersed microstructure is finally obtained. The specific steps include the following: 1) Raw materials: Ultra-high purity iron and high purity metals with very low content of elements such as O, S, As, Sn, Sb, Bi, Pb and Ti are used as smelting raw materials; 2) Vacuum pressure melting: Vacuum pressure induction furnace is used for melting. A reasonable material feeding method and addition sequence are selected. High-purity carbon, ultra-high-purity iron and high-purity metals that are not easily burned are added first, followed by high-purity metals of easily burned elements. The content of impurity elements and gaseous elements is strictly controlled. 3) Nitrogen alloying: High-purity nitrogen is used for pressurized nitrogen alloying. N2 is continuously introduced during the melting process, and the N2 pressure range is 2 atm to 30 atm to prevent nitrogen from escaping as gas. 4) Rare earth treatment: Before rare earth treatment, the molten steel is fully deoxidized and rare earth micro-alloying is carried out using high-purity rare earth metals; the high-purity rare earth metals are crushed into small pieces, wrapped in aluminum foil, and added from the top of the molten steel; during the casting process, the N2 pressure range is 0.2atm to 30atm until the ingot is completely solidified. 5) Isothermal annealing: After casting, the ingot is demolded at high temperature and isothermal annealed at 600-700℃ for 2-10 hours to reduce internal stress, eliminate microstructure inheritance, and improve the initial microstructure. 6) Homogeneous forging: The ingot after isothermal annealing is subjected to high-temperature diffusion annealing and then formed into slabs or bars using conventional forging methods; 7) Hot rolling of plates: Using conventional hot rolling methods, slabs or bars are rolled into plates with a thickness of 1 to 7 mm; 8) Performance heat treatment: The hot-rolled sheet is subjected to performance heat treatment using a quenching + cryogenic + tempering process. The specific process is as follows: quenching at 980~1050℃ for 3~5h, cryogenic holding at -80~-196℃ for 1~3h, tempering at 200~500℃ for 2~4h, and air cooling to room temperature after tempering. Finally, food-grade high-strength corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel material with uniform, dense, fine and dispersed microstructure is obtained.

7. The method for preparing food-grade high-strength corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel according to claim 6, characterized in that, In step 1), the purity of ultra-high purity iron is ≥99.995wt%, the purity of high purity metal is ≥99.95wt%, the mass percentage of each element As, Sn, Sb, Bi and Pb in the raw materials is controlled to be ≤0.0020%, and the preheating temperature of the smelting raw materials is 400~800℃.

8. The method for preparing food-grade high-strength corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel according to claim 6, characterized in that, In step 4), the high-purity rare earth metal used in the rare earth treatment is lanthanum, cerium or a lanthanum-cerium mixture with a total oxygen content of ≤300ppm and a rare earth element content of ≥99wt%. The total oxygen content of the molten steel before rare earth treatment is ≤30ppm.

9. The method for preparing food-grade high-strength corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel according to claim 6, characterized in that, In step 6), the forging process involves heating the ingot to 1180-1260°C before forging and performing high-temperature diffusion annealing treatment, with a holding time of 1h / 100mm to 2.5h / 100mm; using conventional forging methods to form the billet, with a final forging temperature of not less than 950°C, and air cooling to room temperature after forging.

10. The method for preparing food-grade high-strength corrosion-resistant C30RE ultra-high nitrogen martensitic stainless steel according to claim 6, characterized in that, In step 7), the rolling process involves heating the forged billet to 1180–1220°C and holding it for 1 h / 100 mm to 2.5 h / 100 mm. The billet is rolled into a billet of a set thickness through multiple passes. The final rolling temperature is 900–950°C. After rolling, the billet is annealed at 750–850°C for 3–5 h and then furnace-cooled to 300–350°C and air-cooled to room temperature.

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