Acid mist corrosion resistant low cost Cr-Ni system high hardness austenitic stainless steel and its preparation method

Abstract

This invention discloses a low-cost, high-hardness austenitic stainless steel with an acid mist corrosion resistance Cr-Ni system and its preparation method. The method includes: S1, distributing the materials according to the designed composition by mass percentage, wherein C: <0.01%; Si: 3.4~4.6%; Mn: <2.0%; S <0.005%; P: <0.007%; Cr: 16.2~18.8%; Ni: 14.3~16.2%; Cu: <0.02%; Mo: <0.1%; O: <0.003%; N: <0.05%; with the balance being Fe; S2, double vacuum melting; S3, homogenization treatment; S4, ingot forging; S5, steel bar heat treatment; S6, room temperature rotary forging; S7, low-temperature pre-oxidation. This invention combines the advantages of low cost, high hardness, and acid mist corrosion resistance, and can meet the requirements of key component materials such as shafts in nuclear equipment under harsh service environments.

Description

Technical Field

[0001] This invention belongs to the field of nuclear industry technology, specifically relating to a low-cost Cr-Ni system high-hardness austenitic stainless steel resistant to acid mist corrosion and its preparation method. Background Technology

[0002] Key components of nuclear equipment, such as shafts, taking drive shafts as an example, need to withstand high concentrations of acid mist corrosion, while also needing to withstand higher strength or hardness. In other words, they not only need to be resistant to acid mist corrosion, but also require higher strength / hardness.

[0003] Currently, the selection criteria for materials used in such components are based on strength / hardness, with the aim of improving resistance to acid mist corrosion. For example, martensitic precipitation-hardening stainless steel is used, which has high strength. Surface treatment is then applied to improve its corrosion resistance. However, while this improves resistance to acid mist corrosion, intergranular corrosion is still unavoidable. Existing high-corrosion-resistant austenitic stainless steels rely on high Cr and Ni contents (e.g., Cr ≥ 20%, Ni ≥ 18%), resulting in high costs and potential overperformance in moderate acid mist environments. Low-Cr-Ni stainless steels, while inexpensive, lack sufficient corrosion resistance, especially prone to pitting and intergranular corrosion in nitric acid mist environments.

[0004] Furthermore, traditional processes mainly rely on high-temperature pre-oxidation to improve corrosion resistance, which has drawbacks such as high energy consumption, poor adhesion between the oxide film and the substrate, and inability to achieve high hardness. Moreover, existing pre-oxidation technologies are mostly used for ferritic stainless steels, and when used for austenitic steels, they tend to result in a porous surface, making it difficult to balance hardness and corrosion resistance. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a low-cost Cr-Ni system high-hardness austenitic stainless steel with acid mist corrosion resistance and its preparation method, which addresses the above-mentioned shortcomings of the existing technology. It has the advantages of low cost, high hardness and acid mist corrosion resistance, and can meet the requirements of key component materials such as shafts in nuclear equipment in harsh service environments.

[0006] The technical solution of the present invention to solve the above-mentioned technical problems is:

[0007] According to a first aspect of the present invention, a method for preparing a low-cost, acid-mist-resistant, high-hardness austenitic stainless steel based on a Cr-Ni system is provided, comprising:

[0008] S1, Raw material preparation: By mass percentage, according to the designed composition, where C: <0.01%; Si: 3.4~4.6%; Mn: <2.0%; S <0.005%; P: <0.007%; Cr: 16.2~18.8%; Ni: 14.3~16.2%; Cu: <0.02%; Mo: <0.1%; O: <0.003%; N: <0.05%; balance Fe;

[0009] S2, Dual Vacuum Melting: The raw materials prepared in step S1 are melted using a dual vacuum melting process of vacuum induction smelting and vacuum self-consumption smelting to obtain ingots.

[0010] S3, Homogenization treatment: The ingot obtained in step S2 is subjected to high-temperature homogenization treatment;

[0011] S4, Ingot forging: Hot forging is performed on the homogenized ingot to obtain steel bars;

[0012] S5, Heat treatment of steel bar: The steel bar obtained in step S4 is subjected to solution treatment.

[0013] S6, Room temperature rotary forging: The steel bar after solution treatment is rotary forged at room temperature, and the rotary forging ratio is not less than 3.1;

[0014] S7, Low-temperature pre-oxidation: The steel bar after room temperature rotary forging is subjected to low-temperature pre-oxidation treatment at 360~380℃, and then air-cooled to room temperature, so that a pre-oxidation layer of 200~500nm is formed on the surface of the steel bar.

[0015] Optionally, Cr / Ni = 1.03~1.17.

[0016] Optionally, the high-temperature homogenization treatment is performed at a temperature of 1200~1500℃ for a time of not less than 4 hours.

[0017] Optionally, the hot forging process includes: first heating the ingot to 1130~1160℃ and holding it at that temperature for no less than 4 hours; then starting forging, with an initial temperature of 1080~1120℃ and a final forging temperature of 850~950℃.

[0018] Optionally, in the hot forging process, the forging includes repeatedly forging the ingot in the longitudinal-transverse-longitudinal directions with a large reduction, repeating the forging at least 6 times, with a forging ratio >10, and air cooling to room temperature after the final forging is completed.

[0019] Optionally, the solution treatment includes: holding the steel bar at a temperature of 1050~1150℃ for 1~2 hours, and then water cooling it to room temperature.

[0020] Optionally, during the room temperature rotary forging process, the deformation of the steel bar in each pass is not less than 9%, and no annealing is performed between passes or after rotary forging.

[0021] Optionally, the low-temperature pre-oxidation treatment time is 6~10 hours.

[0022] Optionally, the hardness value of the steel bar after the low-temperature pre-oxidation treatment is not less than 370HV from the surface to the core.

[0023] According to a second aspect of the present invention, a low-cost Cr-Ni system high-hardness austenitic stainless steel resistant to acid mist corrosion is also provided, which is prepared by the method described above.

[0024] Beneficial effects:

[0025] 1) The chemical composition of stainless steel was improved, reducing the Cr and Ni content and imposing stricter limits on impurity elements such as Cu and Mo to prevent synergistic effects with other harmful elements (such as S and P). Strictly limiting Cu (<0.02%) and Mo (<0.1%) prevents impurities in low-Cr steel from exacerbating intergranular corrosion. In terms of the manufacturing process, a low-temperature pre-oxidation step was added, and process parameters such as solution treatment temperature, final forging temperature, and room-temperature rotary forging deformation were optimized. Furthermore, the new composition and process combination allows the stainless steel to exhibit better corrosion resistance in specific acid mist environments (such as a mixed environment of low-concentration oxalic acid and nitric acid) while maintaining hardness. The hardness value of the stainless steel bar is no less than 370 HV from the surface to the core, thus overcoming the technical barrier of the incompatibility between high strength / hardness and corrosion resistance in shaft materials for spent fuel turbine melters. This resulted in stainless steel and its manufacturing process that simultaneously achieve high strength / hardness and resistance to acid mist corrosion.

[0026] 2) By combining low Cr and Ni content with low-temperature pre-oxidation treatment, a Cr-rich oxide layer can be formed, which improves the surface properties of stainless steel and makes up for the lack of corrosion resistance caused by low Cr. While reducing costs, it maintains high corrosion resistance. It can withstand corrosion for 14 days in a constant temperature environment of 97℃, a 6mol / L nitric acid mist environment and a mixed acid mist environment of 0.03mol / L oxalic acid + 3.5mol / L nitric acid without weight loss, and no uniform corrosion or intergranular corrosion occurs. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the preparation method of low-cost Cr-Ni system high-hardness austenitic stainless steel for transformers resistant to acid mist corrosion in an embodiment of the present invention.

[0028] Figure 2 The macroscopic morphology of the stainless steel samples obtained in Example 2 of this invention before and after corrosion is shown.

[0029] Figure 3 This is a high-magnification morphology of the surface of the stainless steel obtained in Example 2 of the present invention after corrosion in a nitric acid mist environment;

[0030] Figure 4 The high-magnification morphology of the stainless steel prepared in Comparative Example 2 after corrosion in a nitric acid mist environment;

[0031] Figure 5 The high-magnification morphology of the stainless steel prepared in Comparative Example 5 after corrosion in a nitric acid mist environment is shown. Detailed Implementation

[0032] To enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present invention.

[0033] It is understood that the specific embodiments and accompanying drawings described herein are merely for explaining the invention and are not intended to limit the invention.

[0034] It is understood that, without conflict, the various embodiments and features in the embodiments of the present invention can be combined with each other.

[0035] It is understood that, for ease of description, only the parts related to the present invention are shown in the accompanying drawings, while the parts unrelated to the present invention are not shown in the drawings.

[0036] To address the issues of existing technologies where key components for nuclear equipment cannot simultaneously possess high hardness, acid mist corrosion resistance, and low cost, this invention provides a method for preparing a low-cost, acid mist-resistant, Cr-Ni system high-hardness austenitic stainless steel, comprising:

[0037] S1, Raw material preparation: By mass percentage, the raw materials are distributed according to the designed composition, wherein C: <0.01%; Si: 3.4~4.6%; Mn: <2.0%; S <0.005%; P: <0.007%; Cr: 16.2~18.8%; Ni: 14.3~16.2%; Cu: <0.02%; Mo: <0.1%; O: <0.003%; N: <0.05%; the balance is Fe.

[0038] Specifically, conventional stainless steel primarily exhibits intergranular corrosion in nitric acid environments. In austenitic stainless steel, carbon (C) readily combines with chromium (Cr) to form Cr-containing carbides. The precipitation of these carbides at grain boundaries creates surrounding Cr-depleted zones, making the steel more susceptible to intergranular corrosion. To avoid the formation of Cr-containing carbides, the C content in the steel should be kept as low as possible. However, considering the level of smelting, the C content in this invention is limited to below 0.01%.

[0039] Si is a strong oxide-forming element. Adding Si allows for the formation of a Si-rich oxide film on the surface of steel in oxidizing media. This oxide film exhibits strong resistance to nitric acid corrosion and effectively isolates the substrate from nitric acid corrosion. Therefore, Si is a key element in this invention. Generally, the higher the Si content, the more uniform and dense the Si-rich oxide film formed, and the stronger its corrosion resistance. However, high Si content not only affects the hot working properties of the steel but also easily leads to the formation of ferrite structures. Considering all factors, the Si content in this invention is controlled at 3.4% to 4.6%.

[0040] Cr can promote passivation of alloys and maintain them in a stable passive state. This effect of Cr results in a continuous and dense Cr2O3 passivation film on the alloy surface, which can hinder acid dissolution and thus improve the corrosion resistance of the alloy. Existing high-corrosion-resistant austenitic stainless steels rely on high Cr and Ni content (e.g., Cr ≥ 20%, Ni ≥ 18%). Through extensive experimentation, the inventors discovered that by adding Si, utilizing the aforementioned effect of Cr to enhance the synergistic effect with Si, the resistance to nitric acid corrosion can be improved. This allows the Cr content to be reduced to 16.2~18.8% while still forming a basic Cr2O3 passivation film, reducing costs by 20%~30%, although the corrosion resistance is weaker than high-Cr steel. Furthermore, low-temperature pre-oxidation treatment at 360~380℃ can promote Cr diffusion to the surface, forming a Cr-rich oxide layer (Cr2O3-Fe2O3 composite layer), significantly improving surface corrosion resistance, preventing acid mist from directly contacting the substrate, compensating for the passivation film stability defects caused by insufficient overall Cr and Ni content, and avoiding the processing difficulties associated with high Cr content. Taking all factors into consideration, the Cr content in this invention is controlled at 16.2-18.8%.

[0041] Ni's primary role is to form and stabilize austenite, resulting in a fully austenitic microstructure and improved thermodynamic stability. Simultaneously, Ni enhances the flexibility of the austenitic matrix, mitigating the embrittlement caused by high Si content. Extensive experimentation revealed that reducing the Ni content to 14.3–16.2% maintains sufficient austenitic stability (combined with Cr: 16.2–18.8%). Compared to existing high-corrosion-resistant austenitic stainless steels that rely on high Cr and Ni contents (e.g., Cr ≥ 20%, Ni ≥ 18%), this significantly reduces precious metal costs (by 30%–40%).

[0042] Phosphorus (P) can significantly reduce the resistance of Cr-Ni austenitic stainless steel to corrosion by nitric acid of various concentrations in both solution-treated and sensitized states. This effect is mainly related to the segregation of P at grain boundaries. Therefore, for nitric acid-resistant stainless steel, the lower the P content, the better. Considering smelting costs, the P content in this invention is controlled below 0.007%.

[0043] S2, Dual Vacuum Melting: The raw materials prepared in step S1 are melted using a dual vacuum melting process of vacuum induction smelting and vacuum self-consumption smelting to obtain ingots.

[0044] S3, Homogenization treatment: The ingot obtained in step S2 is subjected to high-temperature homogenization treatment.

[0045] S4, Ingot forging: The homogenized ingot is hot-forged to obtain a steel bar.

[0046] S5, Heat treatment of steel bar: The steel bar obtained in step S4 is subjected to solution treatment.

[0047] S6, Room temperature rotary forging: The steel bar after solution treatment is rotary forged at room temperature, and the rotary forging ratio is not less than 3.1.

[0048] S7, Low-temperature pre-oxidation: The steel bar after room temperature rotary forging is subjected to low-temperature pre-oxidation treatment at 360~380℃, and then air-cooled to room temperature, so that a pre-oxidation layer of 200~500nm is formed on the surface of the steel bar.

[0049] Specifically, there is an inherent contradiction between high strength / hardness and acid corrosion resistance (especially stress corrosion resistance): the higher the strength / hardness, the more sensitive the steel is to acid corrosion. This invention starts with the corrosion resistance of steel, shifting from "corrosion resistance" to "high hardness." The design concept for "corrosion resistance" involves adding a high content of Si to austenitic stainless steel. In an oxygen environment, Si forms a Si-rich oxide film (SiO2) on the steel surface to inhibit acid corrosion of the substrate. The goal of "high hardness" is achieved through cold deformation of austenitic stainless steel (i.e., room temperature rotary forging). This transforms the stainless steel from "corrosion-resistant" to a stainless steel that is both "corrosion-resistant" and "highly hard," thus obtaining shaft materials for spent fuel concentrated nitric acid dissolvers that combine corrosion resistance and high hardness.

[0050] Furthermore, another important design concept of this invention is the low-temperature (360~380℃) pre-oxidation treatment of steel bars after cold working (i.e., room temperature rotary forging), which is a crucial preparation process. The prerequisite for high-Si austenitic stainless steel to possess resistance to nitric acid corrosion is the presence of highly oxidizing concentrated nitric acid. In a highly oxidizing environment, the Si in the high-Si austenitic steel reacts with the oxygen in the nitric acid environment to form a Si-rich layer (SiO2) that blocks further corrosion. However, the concentration of nitric acid in the service environment is only 6 mol / L, which is insufficient to activate the role of Si in the steel. This invention, through low-temperature pre-oxidation treatment, first forms a Si-rich oxide film on the surface of the steel. Importantly, the low-temperature pre-oxidation treatment does not reduce the hardness of the steel after cold deformation.

[0051] Furthermore, the present invention also provides a low-cost Cr-Ni system high-hardness austenitic stainless steel resistant to acid mist corrosion, which is prepared by the method described above.

[0052] The method for preparing low-cost, acid-fog-resistant, high-hardness austenitic stainless steel based on the Cr-Ni system in this invention improves the overall design of the chemical composition formulation, cold deformation, and low-temperature pre-oxidation treatment of the Cr-Ni system austenitic stainless steel. This allows the final low-cost, acid-fog-resistant, high-hardness austenitic stainless steel to combine the advantages of low cost, high hardness, and acid-fog corrosion resistance, thus achieving a balance between low cost, high hardness, and acid-fog corrosion resistance. This method can meet the requirements of key components such as shafts in nuclear equipment for use in harsh service environments.

[0053] Example 1

[0054] like Figure 1 As shown, this embodiment discloses a method for preparing a low-cost Cr-Ni system high-hardness austenitic stainless steel resistant to acid mist corrosion, comprising:

[0055] S1, Raw material preparation: By mass percentage, according to the designed composition, where C: <0.01%; Si: 3.4~4.6%; Mn: <2.0%; S <0.005%; P: <0.007%; Cr: 16.2~18.8%; Ni: 14.3~16.2%; Cu: <0.02%; Mo: <0.1%; O: <0.003%; N: <0.05%; balance Fe;

[0056] S2, Dual Vacuum Melting: The raw materials prepared in step S1 are melted using a dual vacuum melting process of vacuum induction smelting and vacuum self-consumption smelting to obtain ingots.

[0057] S3, Homogenization treatment: The ingot obtained in step S2 is subjected to high-temperature homogenization treatment;

[0058] S4, Ingot forging: Hot forging is performed on the homogenized ingot to obtain steel bars;

[0059] S5, Heat treatment of steel bar: The steel bar obtained in step S4 is subjected to solution treatment.

[0060] S6, Room temperature rotary forging: The steel bar after solution treatment is rotary forged at room temperature, and the rotary forging ratio is not less than 3.1;

[0061] S7, Low-temperature pre-oxidation: The steel bar after room temperature rotary forging is subjected to low-temperature pre-oxidation treatment at 360~380℃, and then air-cooled to room temperature, so that a pre-oxidation layer of 200~500nm is formed on the surface of the steel bar.

[0062] The low-temperature pre-oxidation treatment takes 6 to 10 hours.

[0063] In some implementations, the Cr / Ni ratio is 1.03 to 1.17 by mass percentage. By precisely controlling the Cr / Ni ratio and combining it with an ultra-low carbon design (C < 0.01%), it is possible to ensure that no deformed martensite occurs during cold working (such as room temperature rotary forging) and maintain the toughness of the material.

[0064] In some implementations, the high-temperature homogenization treatment is carried out at a temperature of 1200~1500℃ for a time of not less than 4 hours.

[0065] In some embodiments, hot forging includes: first heating the ingot to 1130~1160°C and holding it at that temperature for at least 4 hours; then starting forging with an initial temperature of 1080~1120°C and a final forging temperature of 850~950°C.

[0066] In some embodiments, during the hot forging process, forging includes repeatedly forging the ingot in three directions (longitudinal-transverse-longitudinal) with large reduction, repeating the forging at least 6 times, with a forging ratio >10, and then air-cooling to room temperature after the final forging is completed.

[0067] In some embodiments, solution treatment includes:

[0068] The steel bar is kept at 1050~1150℃ for 1~2 hours, and then cooled to room temperature by water.

[0069] In some embodiments, during room temperature rotary forging, the deformation of the steel bar in each pass is not less than 9%, and no annealing is performed between passes or after rotary forging.

[0070] In some embodiments, the hardness value of the steel bar after low-temperature pre-oxidation treatment is not less than 370HV from the surface to the core.

[0071] Furthermore, this embodiment also discloses a low-cost Cr-Ni system high-hardness austenitic stainless steel resistant to acid mist corrosion, which is prepared by the preparation method described above.

[0072] The low-cost, acid-mist-resistant, high-hardness austenitic stainless steel based on the Cr-Ni system and its preparation method described in this embodiment have the following advantages:

[0073] 1) The chemical composition of stainless steel was improved, reducing the Cr and Ni content and imposing stricter limits on impurity elements such as Cu and Mo to prevent synergistic effects with other harmful elements (such as S and P). Strictly limiting Cu (<0.02%) and Mo (<0.1%) prevents impurities in low-Cr steel from exacerbating intergranular corrosion. In terms of the manufacturing process, a low-temperature pre-oxidation step was added, and process parameters such as solution treatment temperature, final forging temperature, and room-temperature rotary forging deformation were optimized. Furthermore, the new composition and process combination allows the stainless steel to exhibit better corrosion resistance in specific acid mist environments (such as a mixed environment of low-concentration oxalic acid and nitric acid) while maintaining hardness. The hardness value of the stainless steel bar is no less than 370 HV from the surface to the core, thus overcoming the technical barrier of the incompatibility between high strength / hardness and corrosion resistance in shaft materials for spent fuel turbine melters. This resulted in stainless steel and its manufacturing process that simultaneously achieve high strength / hardness and resistance to acid mist corrosion.

[0074] 2) By combining low Cr and Ni content with low-temperature pre-oxidation treatment, a Cr-rich oxide layer can be formed, which improves the surface properties of stainless steel and makes up for the lack of corrosion resistance caused by low Cr. While reducing costs, it maintains high corrosion resistance. It can withstand corrosion for 14 days in a constant temperature environment of 97℃, a 6mol / L nitric acid mist environment and a mixed acid mist environment of 0.03mol / L oxalic acid + 3.5mol / L nitric acid without weight loss, and no uniform corrosion or intergranular corrosion occurs.

[0075] Example 2

[0076] This embodiment discloses a method for preparing a low-cost, acid-mist-resistant, high-hardness austenitic stainless steel based on a Cr-Ni system, comprising:

[0077] S1, Raw material preparation: By mass percentage, according to the designed composition, C: 0.005%; Si: 3.6%; Mn: 1.48%; S: 0.002%; P: 0.006%; Cr: 16.4%; Ni: 15.9%; Cu: 0.01%; Mo: 0.08%; O: 0.002%; N: 0.03%; balance Fe;

[0078] S2, Dual Vacuum Melting: The raw materials prepared in step S1 are melted using a dual vacuum melting process of vacuum induction smelting and vacuum self-consumption smelting to obtain ingots.

[0079] S3, Homogenization treatment: The ingot obtained in step S2 is subjected to high-temperature homogenization treatment and then air-cooled to room temperature. The temperature of the high-temperature homogenization treatment is 1220℃ and the time is 4h.

[0080] S4, Ingot Forging: The homogenized ingot is hot-forged to obtain a steel bar. The hot forging process specifically includes: first, the ingot is heated to 1160℃ in the furnace and held for 5 hours; then, forging begins at 1120℃, with drawing-upsetting-drawing repeated 6 times, forging ratio 12.3, and forging ends at 890℃, followed by air cooling to room temperature;

[0081] S5, Heat treatment of steel bar: The steel bar obtained in step S4 is subjected to solution treatment at a temperature of 1060℃ for 1 hour, and then cooled to room temperature by water.

[0082] S6, Room temperature rotary forging: The solution-treated steel bar is machined to obtain a steel bar with an outer diameter of 37mm, and then subjected to room temperature rotary forging, deformed in successive passes, with deformation amounts of 10.8%, 9.1%, 10.0%, 11.1%, and 12.5% ​​for each pass, until a steel bar with a diameter of 21mm is obtained. The rotary forging ratio is 3.1, and no annealing is performed between passes or after rotary forging.

[0083] S7, Low-temperature pre-oxidation: The steel bar after room temperature rotary forging is held at 380℃ for 8.5h for low-temperature pre-oxidation treatment, and then air-cooled to room temperature to obtain a 300nm pre-oxidation layer on the surface of the steel bar.

[0084] Example 3

[0085] This embodiment discloses a method for preparing a low-cost Cr-Ni system high-hardness austenitic stainless steel resistant to acid mist corrosion. The difference between this method and Example 2 lies in steps S1 and S7.

[0086] In this embodiment, S1, raw material preparation: by mass percentage, the raw materials are prepared according to the designed composition, wherein, C: 0.006%; Si: 4.4%; Mn: 1.88%; S: 0.002%; P: 0.005%; Cr: 16.3%; Ni: 16.0%; Cu: 0.01%; Mo: 0.07%; O: 0.002%; N: 0.03%; the balance is Fe.

[0087] In this embodiment, S7, low-temperature pre-oxidation: The steel bar, after being forged at room temperature, is held at 380°C for 8.5 hours for low-temperature pre-oxidation treatment, and then air-cooled to room temperature to obtain a 450nm pre-oxidized layer on the surface of the steel bar. The reason for this is that, under the same pre-oxidation process, a higher Si content can provide more sufficient reactive materials for the growth of the pre-oxidized layer, accelerating the growth and densification of the Si-rich oxide film (SiO2).

[0088] Example 4

[0089] This embodiment discloses a method for preparing a low-cost Cr-Ni system high-hardness austenitic stainless steel resistant to acid mist corrosion. Compared with Embodiment 2, the difference lies in steps S1 and S7, as follows:

[0090] S1, Raw material preparation: By mass percentage, according to the designed composition, C: 0.002%; Si: 4.5%; Mn: 1.87%; S: 0.002%; P: 0.005%; Cr: 17.3%; Ni: 16.0%; Cu: 0.01%; Mo: 0.07%; O: 0.002%; N: 0.033%; balance Fe.

[0091] S7, Low-temperature pre-oxidation: The steel bar after room temperature rotary forging is held at 360℃ for 9.5h for low-temperature pre-oxidation treatment, and then air-cooled to room temperature to obtain a 400nm pre-oxidation layer on the surface of the steel bar.

[0092] Example 5

[0093] This embodiment discloses a method for preparing a low-cost, acid-mist-resistant, high-hardness austenitic stainless steel based on a Cr-Ni system, comprising:

[0094] S1, Raw material preparation: By mass percentage, according to the designed composition, C: 0.009%; Si: 4.6%; Mn: 1.9%; S: 0.0045%; P: 0.0065%; Cr: 16.2%; Ni: 14.3%; Cu: 0.01%; Mo: 0.01%; O: 0.001%; ​​N: 0.04%; balance Fe;

[0095] S2, Dual Vacuum Melting: The raw materials prepared in step S1 are melted using a dual vacuum melting process of vacuum induction smelting and vacuum self-consumption smelting to obtain ingots.

[0096] S3, Homogenization treatment: The ingot obtained in step S2 is subjected to high-temperature homogenization treatment and then air-cooled to room temperature. The temperature of the high-temperature homogenization treatment is 1200℃ and the time is 6h.

[0097] S4, Ingot Forging: The homogenized ingot is hot-forged to obtain a steel bar. The hot forging process specifically includes: first, the ingot is heated to 1130℃ in the furnace and held for 6 hours; then, forging begins at 1080℃, with drawing-upsetting-drawing repeated 8 times, forging ratio of 10, and forging ends at 850℃, followed by air cooling to room temperature;

[0098] S5, Heat treatment of steel bar: The steel bar obtained in step S4 is subjected to solution treatment at a temperature of 1050℃ for 2 hours, and then cooled to room temperature by water.

[0099] S6, Room temperature rotary forging: The solution-treated steel bar is machined to obtain a steel bar with an outer diameter of 37mm, and then subjected to room temperature rotary forging, deformed in successive passes, with each pass having a deformation amount of 9.0%, 9.0%, 9.0%, 9.0%, 9.0%, until a steel bar with a diameter of 21mm is obtained. The rotary forging ratio is 5.2, and no annealing is performed between passes or after rotary forging.

[0100] S7, Low-temperature pre-oxidation: The steel bar after room temperature rotary forging is kept at 360℃ for 10 hours for low-temperature pre-oxidation treatment, and then air-cooled to room temperature to obtain a 500nm pre-oxidation layer on the surface of the steel bar.

[0101] Example 6

[0102] This embodiment discloses a method for preparing a low-cost, acid-mist-resistant, high-hardness austenitic stainless steel based on a Cr-Ni system, comprising:

[0103] S1, Raw material preparation: By mass percentage, according to the designed composition, C: 0.002%; Si: 3.4%; Mn: 0.1%; S: 0.001%; ​​P: 0.001%; ​​Cr: 18.8%; Ni: 16.2%; Cu: 0.01%; Mo: 0.05%; O: 0.001%; ​​N: 0.01%; balance Fe;

[0104] S2, Dual Vacuum Melting: The raw materials prepared in step S1 are melted using a dual vacuum melting process of vacuum induction smelting and vacuum self-consumption smelting to obtain ingots.

[0105] S3, Homogenization treatment: The ingot obtained in step S2 is subjected to high-temperature homogenization treatment and then air-cooled to room temperature. The temperature of the high-temperature homogenization treatment is 1500℃ and the time is 5h.

[0106] S4, Ingot Forging: The homogenized ingot is hot-forged to obtain a steel bar. The hot forging process specifically includes: first, the ingot is heated to 1150℃ in the furnace and held for 4 hours; then, forging begins at 1100℃, with drawing-upsetting-drawing repeated 7 times, the forging ratio being 11, and forging ends at 950℃, followed by air cooling to room temperature;

[0107] S5, Heat treatment of steel bar: The steel bar obtained in step S4 is subjected to solution treatment at a temperature of 1150℃ for 1 hour, and then cooled to room temperature by water.

[0108] S6, Room temperature rotary forging: The solution-treated steel bar is machined to obtain a steel bar with an outer diameter of 37mm, and then subjected to room temperature rotary forging, deformed in successive passes, with each pass having a deformation amount of 9.0%, 9.0%, 9.0%, 9.0%, 9.0%, until a steel bar with a diameter of 21mm is obtained. The rotary forging ratio is 4.0, and no annealing is performed between passes or after rotary forging.

[0109] S7, Low-temperature pre-oxidation: The steel bar after room temperature rotary forging is held at 380℃ for 6 hours for low-temperature pre-oxidation treatment, and then air-cooled to room temperature to obtain a 350nm pre-oxidation layer on the surface of the steel bar.

[0110] Comparative Example 1

[0111] The difference between Comparative Example 1 and Example 2 lies in step S6, as follows:

[0112] S6, Room temperature rotary forging: The solution-treated steel bar is machined to obtain a steel bar with an outer diameter of 37mm, and then subjected to room temperature rotary forging, deformed in successive passes, with deformation amounts of 5.4%, 5.7%, 6.1%, 6.5%, 6.9%, 7.4%, 8.0%, and 8.7% for each pass, until a steel bar with a diameter of 21mm is obtained. The rotary forging ratio is not less than 3.1, and no annealing is performed between passes or after rotary forging.

[0113] Comparative Example 2

[0114] The difference between Comparative Example 2 and Example 2 is that step S7 is not performed.

[0115] Comparative Example 3

[0116] The difference between Comparative Example 3 and Example 2 lies in step S7, as follows:

[0117] S7, Low-temperature pre-oxidation: The steel bar after room temperature rotary forging is pre-oxidized by holding it at 400℃ for 5 hours, and then air-cooled to room temperature to obtain a 350nm pre-oxidized layer on the surface of the steel bar.

[0118] Comparative Example 4

[0119] Comparative Example 4 is a 17-4 martensitic precipitation hardening stainless steel bar with the following chemical composition: C: 0.05%; Si: 0.3%; Mn: 0.5%; S: 0.01%; P: 0.02%; Cr: 15.8%; Ni: 3.7%; Nb: 0.3%; Cu: 4.2%; balance Fe.

[0120] Furthermore, the steel bar was solution treated at 1040℃ for 1 hour, then water-cooled to room temperature, and then tempered at 550℃ for 4 hours before being air-cooled to room temperature.

[0121] Comparative Example 5

[0122] Comparative Example 5 is a 310 austenitic stainless steel bar with high Cr and high Ni content. Its chemical composition is: C: 0.02%; Si: 0.6%; Mn: 0.8%; S: 0.01%; P: 0.02%; Cr: 25.0%; Ni: 19.4%; Nb: 0.03%; balance Fe.

[0123] Furthermore, the steel bar was solution treated at 1080℃ for 1 hour, and then water-cooled to room temperature.

[0124] 1) Take the entire cross section of the stainless steel finally obtained in Examples 2-6 and Comparative Examples 1-5 above, and perform Vickers hardness measurement from the surface to the core. The load is 500g and the holding time is 15s. The results are shown in Table 1.

[0125] Table 1. Vickers hardness measurement results

[0126]

[0127] 2) The 10mm thick stainless steel discs obtained in Examples 2-6 and Comparative Examples 1-5 were cut into six equal parts, mechanically ground, and then subjected to corrosion experiments in an acid mist corrosion apparatus. Specifically:

[0128] Prepare 1 L each of two acids: 6 mol / L nitric acid and 0.03 mol / L oxalic acid + 3.5 mol / L nitric acid.

[0129] The sample to be tested was hung about 5 cm above the two acid solutions mentioned above, and heated at a constant temperature of 97°C until the acid boiled. After 14 days, the sample was taken out and its weight loss (weight of sample before corrosion - weight of sample after corrosion) was measured. The surface corrosion was observed. The results are shown in Table 2.

[0130] Table 2 Corrosion test results

[0131]

[0132] Note: "-" indicates that no weight loss occurred after corrosion.

[0133] Comparing Tables 1 and 2, it can be seen that the stainless steel bars obtained in Examples 2-5 not only achieve Vickers hardness values ​​above 370 HV, but also do not experience weight loss under both acid mist corrosion environments. More intuitively, as... Figure 2 As shown (left image is before corrosion, right image is after corrosion), after 14 days of acid mist corrosion, the surface is still not significantly different from before corrosion, and there is no intergranular corrosion on the surface morphology after corrosion (e.g., Figure 3 (As shown).

[0134] Comparing Example 2 and Comparative Example 1, it can be found that if the amount of deformation per cycle during room temperature rotary forging is insufficient, even if the acid mist corrosion resistance is good, the hardness value cannot reach a higher level.

[0135] Comparing Example 2 and Comparative Example 2, it can be found that without low-temperature pre-oxidation treatment, a certain degree of intergranular corrosion (such as...) will occur. Figure 4 As shown in the figure, the role of Si in the steel is not fully utilized, thus failing to achieve optimal corrosion resistance. However, low-temperature pre-oxidation treatment does not reduce the hardness of the steel after cold deformation.

[0136] Comparing Examples 2 and 3, it can be found that the higher Si content results in a thicker pre-oxidized layer and no weight loss after corrosion.

[0137] Comparing Example 2 and Comparative Example 5, it can be found that if the added Si content is too low, the resistance to acid mist corrosion is poor (e.g., Figure 5 (As shown).

[0138] Comparative Example 4 is a precipitation-hardening martensitic steel. It can also achieve high hardness through solution treatment and low-temperature pre-oxidation treatment, but it has the worst resistance to acid mist corrosion and the most corrosion weight loss.

[0139] In summary, this invention improves the chemical composition of Cr-Ni system austenitic stainless steel and combines room temperature rotary forging and low temperature pre-oxidation treatment to achieve a transformation from corrosion-resistant stainless steel to one that is both resistant to acid mist corrosion and has high hardness. This meets the requirements for use of shaft materials in nuclear equipment in environments with specific concentrations of nitric acid or a mixture of nitric acid and oxalic acid. Furthermore, compared to existing high corrosion-resistant austenitic stainless steels that rely on high Cr and Ni content (e.g., Cr ≥ 20%, Ni ≥ 18%), it also achieves lower costs.

[0140] It is understood that the above embodiments are merely exemplary implementations used to illustrate the principles of the present invention, and the present invention is not limited thereto. For those skilled in the art, various modifications and improvements can be made without departing from the spirit and essence of the present invention, and these modifications and improvements are also considered to be within the scope of protection of the present invention.

Claims

1. A method for preparing a low-cost, acid-mist-resistant, high-hardness austenitic stainless steel based on a Cr-Ni system, characterized in that, include: S1, Raw material preparation: By mass percentage, according to the designed composition, where C: <0.01%; Si: 3.4~4.6%; Mn: <2.0%; S <0.005%; P: <0.007%; Cr: 16.2~18.8%; Ni: 14.3~16.2%; Cu: <0.02%; Mo: <0.1%; O: <0.003%; N: <0.05%; balance Fe; S2, Dual Vacuum Melting: The raw materials prepared in step S1 are melted using a dual vacuum melting process of vacuum induction smelting and vacuum self-consumption smelting to obtain ingots. S3, Homogenization treatment: The ingot obtained in step S2 is subjected to high-temperature homogenization treatment; S4, Ingot forging: Hot forging is performed on the homogenized ingot to obtain steel bars; S5, Heat treatment of steel bar: The steel bar obtained in step S4 is subjected to solution treatment. S6, Room temperature rotary forging: The steel bar after solution treatment is rotary forged at room temperature, and the rotary forging ratio is not less than 3.1; S7, Low-temperature pre-oxidation: The steel bar after room temperature rotary forging is subjected to low-temperature pre-oxidation treatment at 360~380℃, and then air-cooled to room temperature, so that a pre-oxidation layer of 200~500nm is formed on the surface of the steel bar.

2. The method for preparing low-cost Cr-Ni system high-hardness austenitic stainless steel resistant to acid mist corrosion according to claim 1, characterized in that, Cr / Ni = 1.03~1.

17.

3. The method for preparing low-cost Cr-Ni system high-hardness austenitic stainless steel resistant to acid mist corrosion according to claim 1, characterized in that, The high-temperature homogenization treatment is carried out at a temperature of 1200~1500℃ for a time of not less than 4 hours.

4. The method for preparing low-cost Cr-Ni system high-hardness austenitic stainless steel resistant to acid mist corrosion according to claim 1, characterized in that, The hot forging process includes: First, heat the ingot to 1130~1160℃ and keep it at that temperature for no less than 4 hours; Then forging begins, with an initial forging temperature of 1080~1120℃ and a final forging temperature of 850~950℃.

5. The method for preparing low-cost Cr-Ni system high-hardness austenitic stainless steel resistant to acid mist corrosion according to claim 4, characterized in that, During the hot forging process, the forging includes repeatedly forging the ingot in three directions (longitudinal-transverse-longitudinal) with a large reduction, repeating the forging at least 6 times, with a forging ratio >10, and then air-cooling it to room temperature after the final forging is completed.

6. The method for preparing low-cost Cr-Ni system high-hardness austenitic stainless steel resistant to acid mist corrosion according to claim 1, characterized in that, The solution treatment includes: The steel bar is kept at 1050~1150℃ for 1~2 hours, and then cooled to room temperature by water.

7. The method for preparing low-cost Cr-Ni system high-hardness austenitic stainless steel resistant to acid mist corrosion according to claim 1, characterized in that, During the room temperature rotary forging process, the deformation of the steel bar in each pass is not less than 9%, and no annealing is performed between passes or after rotary forging.

8. The method for preparing low-cost Cr-Ni system high-hardness austenitic stainless steel resistant to acid mist corrosion according to claim 1, characterized in that, The low-temperature pre-oxidation treatment takes 6 to 10 hours.

9. The method for preparing low-cost Cr-Ni system high-hardness austenitic stainless steel resistant to acid mist corrosion according to claim 1, characterized in that, The hardness value of the steel bar after low-temperature pre-oxidation treatment is not less than 370HV from the surface to the core.

10. A low-cost, acid-mist-resistant, high-hardness austenitic stainless steel based on a Cr-Ni system, characterized in that, It is prepared by the method described in any one of claims 1 to 9.

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