A food-grade high corrosion-resistant martensitic stainless steel, its preparation method and application

Abstract

This invention discloses a food-grade, highly corrosion-resistant martensitic stainless steel, its preparation, and its applications, belonging to the field of stainless steel material technology. The stainless steel comprises, by weight percentage: 0.17% to 0.30% carbon, 0.05% to 0.20% titanium, and 13.00% to 13.90% chromium, with the balance being iron and unavoidable impurities. By reducing the carbon content and precisely adding trace amounts of titanium, titanium preferentially combines with carbon to form stable titanium carbide, effectively preventing chromium carbide precipitation along grain boundaries, eliminating chromium-depleted grain boundary regions, and significantly improving resistance to pitting corrosion and intergranular corrosion. Its preparation method includes annealing, quenching, tempering, and surface treatment; the heat treatment process window is fully compatible with existing SUS420 series materials.

Description

Technical Field

[0001] This invention relates to the field of stainless steel materials technology, specifically to a food-grade high corrosion-resistant martensitic stainless steel, its preparation method, and its application. Background Technology

[0002] In the fields of catering utensils, kitchen knives and food processing equipment such as blender blades, meat grinder blades, and food pulverizer blades, materials need to simultaneously meet good mechanical properties, excellent corrosion resistance, and strict food-grade safety requirements; martensitic stainless steel has become the preferred material for blade products because it can achieve high hardness through heat treatment.

[0003] The most widely used stainless steel is the Japanese Industrial Standard SUS420J2, which has a typical chemical composition of 0.26% to 0.40% carbon and 12.0% to 14.0% chromium, and does not contain titanium. This material can achieve a hardness of HRC 45 to 52 through heat treatment, but it has a significant problem of insufficient corrosion resistance.

[0004] Due to the high carbon content, a large amount of M will be formed during the heat treatment process. 23 C6-type chromium carbide precipitates along grain boundaries, creating chromium-depleted zones near the grain boundaries. In acidic or chlorine-containing media such as vinegar, salt water, and fruit and vegetable juices, these chromium-depleted zones preferentially experience pitting and crevice corrosion, leading to rust spots or even perforation on the surface. In terms of food safety testing, when subjected to a 4% acetic acid immersion test according to GB 4806.9-2016 standard, the chromium and nickel precipitation of SUS420J2 often approaches or even exceeds the limit, posing a compliance risk. Furthermore, even after fine polishing or passivation treatment, the durability of its surface passivation film remains poor, resulting in a high rust recurrence rate under the high-frequency cleaning environment of a dishwasher. Summary of the Invention

[0005] This invention aims to at least partially solve one of the technical problems in the related art. To this end, one object of this invention is to provide a food-grade, highly corrosion-resistant martensitic stainless steel, whose chemical composition, by weight percentage, consists of the following elements:

[0006] Carbon 0.17% to 0.30%, titanium 0.05% to 0.20%, chromium 13.00% to 13.90%, nickel ≤0.60%, manganese ≤1.00%, silicon ≤1.00%, phosphorus ≤0.040% and sulfur ≤0.030%, with the balance being iron and unavoidable impurities.

[0007] Preferably, the hardness of the stainless steel is HRC45 to 55.

[0008] A method for preparing food-grade high corrosion-resistant martensitic stainless steel, applicable to the aforementioned food-grade high corrosion-resistant martensitic stainless steel, includes the following steps:

[0009] Annealing treatment involves heating the stainless steel to 750°C to 800°C, holding it at that temperature for 2 to 4 hours, then slowly cooling it in the furnace to below 500°C, and finally air cooling it to room temperature.

[0010] Quenching treatment involves heating the annealed stainless steel to 1000℃ to 1050℃, holding it at that temperature, and then cooling it with air.

[0011] Tempering treatment involves heating the quenched stainless steel to 200°C to 350°C, holding it at that temperature for 2 hours, and then air cooling it to room temperature.

[0012] Preferably, the holding time for the quenching treatment is calculated as 1.5 to 2.0 minutes per millimeter of the effective thickness of the workpiece.

[0013] Preferably, the heating temperature for the quenching treatment is 1020°C.

[0014] Preferably, the heating temperature for the tempering treatment is 250°C to 300°C.

[0015] Preferably, before the annealing treatment, the stainless steel is further subjected to a smelting, forging or hot rolling step; after the tempering treatment, the stainless steel is further subjected to a surface treatment step.

[0016] Preferably, the surface treatment includes the following steps:

[0017] Mechanical grinding or polishing is performed to make the surface roughness Ra of the stainless steel ≤ 0.2 μm;

[0018] Electrolytic passivation treatment involves immersing the sample in a nitric acid solution at a temperature of 20°C to 40°C and a concentration of 20% to 30% for 30 to 60 minutes, followed by rinsing with pure water.

[0019] Preferably, it is used in the preparation of tableware or food processing blades.

[0020] Preferably, the catering utensils include knives and kitchen utensils, and the food processing blades include blender blades, meat grinder blades, or food pulverizer blades.

[0021] The above-described solution of the present invention has at least the following beneficial effects:

[0022] High food safety: Under the stringent conditions of boiling in 4% acetic acid for 30 minutes and soaking at room temperature for 24 hours, the migration of elements such as chromium, nickel, iron, and titanium in this material is far below the national standard limit. It has successfully passed the GB 4806.9-2016 food-grade test and is suitable for catering utensils and food processing blades that come into direct contact with food.

[0023] Excellent corrosion resistance: Because titanium has a much higher affinity for carbon than chromium, titanium preferentially combines with carbon to form extremely stable titanium carbide, effectively preventing the precipitation of chromium carbide along grain boundaries, completely eliminating chromium-depleted areas at grain boundaries, and maintaining a uniform chromium distribution in the matrix, significantly improving resistance to pitting and intergranular corrosion; after alternating immersion tests in citric acid, lactic acid, and sodium chloride solutions, no rust spots were generated on the surface of this material, demonstrating outstanding resistance to acidic foods; after 100 cycles of high-temperature rinsing with detergent in a dishwasher, the surface remains bright with no obvious discoloration or rust spots;

[0024] Good mechanical properties are maintained: The generated nano-sized titanium carbide particles are evenly distributed, which can pin the grain boundaries and refine the grains, so that the material can achieve high hardness while effectively improving fracture toughness. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0026] Figure 1 This is a flowchart illustrating a food-grade, highly corrosion-resistant martensitic stainless steel, its preparation method, and its application, as provided in this invention.

[0027] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0028] Embodiments of the present invention are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0029] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "circumferential," and "radial," etc., indicating the orientation or positional relationship, are based on the orientation or positional relationship shown in the accompanying drawings and are only for the convenience of describing this invention and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention.

[0030] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of this invention, "a plurality of" means two or more, unless otherwise explicitly specified.

[0031] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0032] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can include direct contact between the first and second features, or contact between the first and second features through another feature between them. Furthermore, "above," "over," and "on top" of the second feature includes the first feature directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature includes the first feature directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature.

[0033] The following describes in detail, with reference to the accompanying drawings, an embodiment of the present invention: a food-grade, highly corrosion-resistant martensitic stainless steel, its preparation method, and its application.

[0034] Example 1

[0035] The stainless steel material of this invention is formulated according to the following chemical composition and weight percentages: 0.20% carbon, 0.10% titanium, 13.50% chromium, 0.30% nickel, 0.50% manganese, 0.40% silicon, 0.025% phosphorus, 0.015% sulfur, with the balance being iron and unavoidable impurities. The formulated raw materials are smelted in a medium-frequency induction furnace, cast into steel ingots, then forged into billets, hot-rolled to the required thickness, and then pickled to remove oxide scale, yielding hot-rolled sheets.

[0036] The above-mentioned sheet material is subjected to annealing and softening treatment: heated to 780℃, held for 3 hours, and then slowly cooled in the furnace to below 500℃ before being taken out and air-cooled to room temperature; after annealing, the hardness is HB 205, which can be smoothly processed by stamping, cutting and other machining to produce the blank of the wall-breaking machine blade.

[0037] The shaped cutting blade is then quenched: heated to 1020℃, and held for 1.8 minutes per millimeter based on the effective thickness of the blade, followed by oil cooling; the microstructure after quenching consists of lath martensite, a small amount of retained austenite, and uniformly distributed titanium carbide particles.

[0038] After quenching, the blade undergoes tempering treatment: heated to 280℃, held for 2 hours, and then air-cooled to room temperature; the blade was tested and found to have a hardness of HRC 54, good toughness, and no cracking or deformation.

[0039] After tempering, surface treatment is performed: first, fine grinding is carried out step by step with 400-mesh to 800-mesh abrasive belt to achieve a surface roughness Ra of 0.15μm. Then, electrolytic passivation treatment is performed by immersing the blade in 25% nitric acid solution at 25℃ for 45 minutes. After removal, the blade is thoroughly cleaned with pure water and dried. After this treatment, a dense chromium-rich passivation film is formed on the blade surface.

[0040] The blades prepared in this embodiment were subjected to performance tests. A 4% acetic acid immersion test was conducted according to GB 4806.9-2016 standard. Under conditions of boiling for 30 minutes and continued immersion at room temperature for 24 hours, the chromium migration was 0.3 mg / kg, the nickel migration was 0.08 mg / kg, and the iron migration was 1.2 mg / kg, all far below the national standard limits. In the neutral salt spray test, under continuous spraying conditions of 20℃ and 5% sodium chloride solution, no red rust appeared on the surface after 12 hours. In the dishwasher cleaning simulation test, after continuous rinsing 120 times at 70℃ using commercial dishwasher detergent, the blade surface remained bright, with no obvious discoloration or rust spots. In the accelerated corrosion test of alternating immersion in citric acid, lactic acid, and sodium chloride, no pitting corrosion occurred after 48 hours.

[0041] Example 2

[0042] The stainless steel material of the present invention is formulated according to the following chemical composition and weight percentages: 0.25% carbon, 0.15% titanium, 13.30% chromium, 0.25% nickel, 0.60% manganese, 0.35% silicon, 0.020% phosphorus, 0.018% sulfur, with the balance being iron and unavoidable impurities. The smelting, forging, and hot rolling processes are the same as in Example 1.

[0043] Annealing treatment: Heat to 800℃, hold for 2.5 hours, slowly cool in the furnace to 480℃ and then air cool. The hardness after annealing is HB212.

[0044] The machined meat grinder blades were quenched: heated to 1050℃, held for 1.5 minutes per millimeter of effective thickness, and cooled by high-pressure air; after quenching, tempering was performed: heated to 300℃, held for 2 hours, and then air-cooled; the blades were tested and found to have a hardness of HRC 55, and the titanium carbide particles in the microstructure were 80nm to 180nm in size, uniformly distributed within the grains and at the grain boundaries.

[0045] Surface treatment: electrochemical polishing to make the surface bright, then soaking in 30% nitric acid solution at 30℃ for 40 minutes, rinsing with pure water and drying.

[0046] The blade prepared in this embodiment was subjected to performance tests. In the 4% acetic acid immersion test, the chromium migration was 0.25 mg / kg and the nickel migration was 0.06 mg / kg. No red rust was observed after 72 hours of neutral salt spray test. No surface change was observed after 100 cycles of dishwasher cleaning. No pitting corrosion was observed after 240 hours of alternating immersion corrosion test.

[0047] Example 3

[0048] The stainless steel material of the present invention is formulated according to the following chemical composition and weight percentage: 0.17% carbon, 0.20% titanium, 13.90% chromium, 0.20% nickel, 0.45% manganese, 0.50% silicon, 0.022% phosphorus, 0.012% sulfur, with the balance being iron and unavoidable impurities; the smelting and forming process is the same as in Example 1.

[0049] Annealing treatment: Heat to 750℃, hold for 4 hours, slowly cool in the furnace to 490℃ and then air cool. The hardness after annealing is HB195.

[0050] Quenching: Heat to 1000℃, hold for 2.0 minutes per millimeter of effective thickness, then oil cool. Tempering: Heat to 250℃, hold for 2 hours, then air cool to obtain a hardness of HRC 52.

[0051] The surface treatment is the same as in Example 1.

[0052] Comparative Example

[0053] Using commercially available SUS420J2 stainless steel as raw material, the main chemical composition is 0.35% carbon and 13.2% chromium, without titanium; the same blade forming process and heat treatment regime are adopted, quenching at 1020℃ with oil cooling, tempering at 280℃ for 2 hours with air cooling, and the hardness is HRC 50. After surface polishing and passivation treatment under the same conditions, comparative tests were carried out: in the 4% acetic acid immersion test, the chromium migration was 2.5mg / kg, exceeding the national standard limit; obvious red rust appeared after 36 hours in the neutral salt spray test; after 60 cycles of dishwasher washing, visible rust spots appeared on the surface; and multiple pitting corrosions appeared within 72 hours in the alternating immersion test.

[0054] As can be seen from the comparison of the results of the above embodiments and comparative examples, the present invention achieves a significant improvement in corrosion resistance and food safety performance by precisely adding trace amounts of titanium elements within a specific composition range and appropriately controlling the carbon content. At the same time, it maintains a heat treatment process window that is fully compatible with existing SUS420 series materials and excellent mechanical properties, making it particularly suitable for catering utensils and food processing blades with high requirements for food safety and corrosion resistance.

[0055] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0056] The above description is merely a preferred embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural transformations made using the contents of the present invention's specification and drawings under the inventive concept of the present invention, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

Claims

1. A food-grade, highly corrosion-resistant martensitic stainless steel, characterized in that, Its chemical composition, by weight percentage, consists of the following elements: Carbon 0.17% to 0.30%, titanium 0.05% to 0.20%, chromium 13.00% to 13.90%, nickel ≤0.60%, manganese ≤1.00%, silicon ≤1.00%, phosphorus ≤0.040% and sulfur ≤0.030%, with the balance being iron and unavoidable impurities.

2. The food-grade, highly corrosion-resistant martensitic stainless steel according to claim 1, characterized in that, The stainless steel has a hardness of HRC45 to 55.

3. A method for preparing food-grade high corrosion-resistant martensitic stainless steel, applied to the food-grade high corrosion-resistant martensitic stainless steel described in claims 1 to 2, characterized in that, Includes the following steps: Annealing treatment involves heating the stainless steel to 750°C to 800°C, holding it at that temperature for 2 to 4 hours, then slowly cooling it in the furnace to below 500°C, and finally air cooling it to room temperature. Quenching treatment involves heating the annealed stainless steel to 1000℃ to 1050℃, holding it at that temperature, and then cooling it with air. Tempering treatment involves heating the quenched stainless steel to 200°C to 350°C, holding it at that temperature for 2 hours, and then air cooling it to room temperature.

4. The method for preparing food-grade high corrosion-resistant martensitic stainless steel according to claim 3, characterized in that, The holding time for the quenching treatment is calculated as 1.5 to 2.0 minutes per millimeter of the effective thickness of the workpiece.

5. The method for preparing food-grade high corrosion-resistant martensitic stainless steel according to claim 3, characterized in that, The quenching process is performed at a heating temperature of 1020℃.

6. The method for preparing food-grade high corrosion-resistant martensitic stainless steel according to claim 3, characterized in that, The heating temperature for the tempering process is 250°C to 300°C.

7. The method for preparing food-grade high corrosion-resistant martensitic stainless steel according to claim 3, characterized in that, Before the annealing treatment, the stainless steel is further subjected to a smelting, forging, or hot rolling step; after the tempering treatment, the stainless steel is further subjected to a surface treatment step.

8. The method for preparing food-grade high corrosion-resistant martensitic stainless steel according to claim 3, characterized in that, The surface treatment includes the following steps: Mechanical grinding or polishing is performed to make the surface roughness Ra of the stainless steel ≤ 0.2 μm; Electrolytic passivation treatment involves immersing the sample in a nitric acid solution at a temperature of 20°C to 40°C and a concentration of 20% to 30% for 30 to 60 minutes, followed by rinsing with pure water.

9. The application of a food-grade, highly corrosion-resistant martensitic stainless steel according to claim 2, characterized in that, It is used in the preparation of tableware or food processing blades.

10. A food-grade, highly corrosion-resistant martensitic stainless steel according to claim 9, characterized in that, The catering utensils include knives and kitchen utensils, and the food processing blades include blender blades, meat grinder blades, or food pulverizer blades.

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