Preparation method of high-nitrogen austenitic stainless steel for hydrogen fuel cell bipolar plate

Abstract

The application provides a preparation method of high-nitrogen austenitic stainless steel for hydrogen fuel cell bipolar plates, and studies the influence of N+Mo on the mechanical properties and corrosion resistance of high-nitrogen austenitic stainless steel. The preparation method comprises the following steps: vacuum induction furnace smelting, casting, rolling, solid solution treatment, warm rolling deformation, isothermal annealing and other treatments, so that high-nitrogen austenitic stainless steel with single austenite structure at room temperature is obtained. In order to ensure the mechanical properties of high-nitrogen austenitic stainless steel, the tensile strength, impact toughness, yield strength and other mechanical property parameters are detected to evaluate the strength and durability. The impact toughness is required to be 150-350 J / cm² at-40 ℃. The open-circuit voltage, corrosion current density and charge transfer resistance of the bipolar plate are detected by electrochemical impedance spectroscopy experiment to determine the corrosion performance. After the salt spray experiment, the surface corrosion pit point number, corrosion pit depth and size of the bipolar plate are detected by metallographic observation to evaluate the corrosion performance. And the temperature stability is detected to evaluate the temperature stability performance of the bipolar plate in the working environment.

Description

Technical Field

[0001] This invention belongs to the field of austenitic stainless steel sheet manufacturing, specifically relating to a method for preparing high-nitrogen austenitic stainless steel for hydrogen fuel cell bipolar plates. Background Technology

[0002] Bipolar plates are a key component in fuel cells. Common bipolar plate materials include graphite, composite materials, and metal-based artificial graphite. Machining bipolar plates offer flexible design and short iteration cycles, but have certain limitations. Composite material bipolar plates can be divided into metal-based and carbon-based types. Metal-based composite bipolar plates combine the advantages of graphite and metal, featuring light weight, high strength, corrosion resistance, and thinness. However, their multi-layered structure and complex manufacturing process result in higher processing costs. Carbon-based composite bipolar plates use carbon materials as the matrix and resin as the binder. They possess the corrosion resistance, excellent electrical and thermal conductivity of carbon materials, and can be molded in a single step, reducing production costs and making them suitable for mass production. However, manufacturing these components requires balancing the relationship between material bulk resistivity, contact resistance, airtightness, and mechanical properties. Metal materials possess high mechanical strength, excellent bulk electrical and thermal conductivity, and are easily fabricated into thin sheets and stamped, meeting the numerous requirements of fuel cells for high volumetric power density bipolar plates. Stainless steel is a material with good corrosion resistance, possessing characteristics such as oxidation resistance, high temperature resistance, and acid and alkali resistance. Therefore, stainless steel bipolar plates exhibit good corrosion resistance. High-nitrogen austenitic stainless steel bipolar plates are a type of austenitic stainless steel material with a high nitrogen content, characterized by high strength, high hardness, and excellent corrosion resistance. The addition of nitrogen can significantly improve the strength and hardness of stainless steel, while also increasing the material's corrosion resistance, especially in chloride environments.

[0003] The preparation of high-nitrogen austenitic stainless steel is a key technology for producing high-nitrogen austenitic stainless steel bipolar plates. High-nitrogen austenitic stainless steel is a special type of stainless steel material, primarily prepared by adding a certain amount of nitrogen to austenitic stainless steel. It possesses good corrosion resistance, high strength, good plasticity and machinability, and excellent wear resistance. However, high-nitrogen austenitic stainless steel also has some drawbacks. It easily forms nitrides, which reduces the material's toughness and impact resistance. Furthermore, it is more expensive, as the preparation cost is higher than that of traditional austenitic stainless steel due to the large amount of nitrogen required.

[0004] To address the aforementioned issues, domestic enterprises have conducted extensive research. For example, Chinese patent application CN202210444390 proposes a method for preparing ultra-high strength, high corrosion resistance, and high-nitrogen austenitic stainless steel. The isothermal annealing temperature is relatively low, and the holding time is short, which not only effectively reduces energy consumption and saves production costs but also avoids the formation of intermetallic compounds and precipitates, eliminating the impact of second-phase precipitation on reduced corrosion resistance. This method can control the precipitated phases of high-nitrogen austenitic stainless steel, improving mechanical properties without reducing its corrosivity. Another example is Chinese patent application CN202310092331, which proposes a method for preparing high-strength, high-toughness, and high-nitrogen austenitic stainless steel by introducing... Appropriate defects can regulate the precipitation behavior of Cr2N in high-nitrogen austenitic stainless steel. This eliminates the reduction in corrosion resistance caused by cold deformation defects and second-phase precipitation, while significantly improving its mechanical strength through the composite strengthening of defects and second phase. The process is simple, easy to implement, and does not require special equipment or technology. It also eliminates the impact of second-phase precipitation on corrosion resistance. For example, Chinese Patent Application No. CN202211405456 proposes a method for manufacturing high-purity high-nitrogen austenitic stainless steel. By controlling the placement of raw material components, the melting temperature, melting time, and casting temperature, the nitrogen content of the high-nitrogen steel can be controlled more precisely. This method can also produce high-nitrogen steel with low oxygen content and more precisely control the nitrogen content of the high-nitrogen steel.

[0005] Existing high-nitrogen austenitic stainless steels also face some technical challenges in their preparation. For example, the uniformity of alloying elements in the steel ingot cannot be controlled after smelting. Most existing technologies involve relatively complex manufacturing processes and are costly. There are no specific requirements for the surface flatness of the produced steel plates, resulting in significant deterioration of plasticity and a predominantly deformed recovery microstructure. The final rolling temperature has a significant impact on the final microstructure, easily leading to the formation of incompletely recrystallized austenitic structures, which affects the final microstructure of the high-nitrogen steel. Meanwhile, the improvement of metal bipolar plates is constantly being enhanced, with increasingly higher requirements for materials and continuous updates and developments in their technology. Therefore, in order to meet the urgent requirements for metal bipolar plates in fuel cells, developing a good bipolar plate substrate is one of the key technical problems that need to be solved today. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a method for preparing high-nitrogen austenitic stainless steel for hydrogen fuel cell bipolar plates, thus solving the existing technical problems.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a method for preparing high-nitrogen austenitic stainless steel for hydrogen fuel cell bipolar plates, wherein the chemical composition of the high-nitrogen austenitic stainless steel is as follows (weight percentage): C: 0.026–0.031%, Si: 0.062–0.063%, Mn: 7.19–8.07%, Cr: 16.0–18.0%, Ni: 1.09–1.11%, Mo: 1.94–1.97%, N: 0.45–0.93%, with the remainder being Fe and unavoidable impurity elements;

[0008] Furthermore, high-nitrogen austenitic stainless steel is a special type of stainless steel in which alloying elements play a crucial role. The following is a detailed description of the functions of each alloying element:

[0009] Carbon (C): Enhances the strength and hardness of high-nitrogen austenitic stainless steel, improving its wear resistance and tensile strength. It promotes nitrogen dissolution, improving the corrosion resistance and wear resistance of stainless steel. It improves machinability, particularly for high-nitrogen austenitic stainless steel, enhancing its cutting performance and plastic deformation capacity. Carbon can form stable carbides with chromium, such as Cr23C6, increasing the hardness and wear resistance of stainless steel. It also helps to dissolve and stabilize nitrogen, further improving the corrosion resistance of stainless steel. The C content should be controlled between 0.026% and 0.031%.

[0010] Nitrogen (N): High-nitrogen austenitic stainless steel has a high nitrogen content. The presence of nitrogen can significantly improve the strength and hardness of the steel, while also improving its corrosion resistance. Nitrogen atoms combine with chromium atoms to form chromium nitride, a compound that can prevent intergranular corrosion and localized corrosion, thus improving the steel's corrosion resistance. The N content is controlled between 0.45% and 0.93%.

[0011] Chromium (Cr): Chromium is one of the most important alloying elements in stainless steel, typically comprising 16-20% of its composition. Chromium reacts with oxygen to form a dense chromium oxide layer, preventing further oxidation of the steel and thus improving its corrosion resistance. High chromium content also increases the strength and hardness of the steel. The Cr content should be controlled between 16.0% and 18.0%.

[0012] Nickel (Ni): Nickel's main function is to improve the toughness and impact resistance of steel. It also reduces the hardness and strength of steel, improving its machinability. Furthermore, nickel improves the corrosion resistance of steel, particularly against strong acids and alkalis. The Ni content should be controlled between 1.09% and 1.11%.

[0013] Molybdenum (Mo): The addition of molybdenum can improve the corrosion resistance of high-nitrogen austenitic stainless steel and effectively inhibit the precipitation of nitrogen during the process. Molybdenum can form stable molybdates with chromium, preventing further corrosion by chloride ions, thereby improving the steel's resistance to chloride ion corrosion. In addition, molybdenum can also improve the strength and hardness of the steel. The Mo content should be controlled at 1.94–1.97%.

[0014] Silicon (Si): Enhances corrosion resistance, particularly for high-nitrogen austenitic stainless steel, showing significant improvement in resistance to strong acids such as sulfuric acid and hydrochloric acid. It forms a dense silicon oxide layer, effectively preventing further erosion by corrosive media. It also increases strength and hardness, enhancing tensile strength and wear resistance. It promotes the formation of more hard phases, such as carbides and nitrides, in high-nitrogen austenitic stainless steel. Furthermore, it improves machinability, reducing tool wear and cutting forces. It forms a lubricating film, reducing friction during cutting. The Si content should be controlled between 0.062% and 0.063%.

[0015] Manganese (Mn): Improves the strength and hardness of steel. It can increase steel strength through solid solution strengthening and precipitation strengthening. Manganese's solid solution strengthening effect increases the strength and hardness of steel by forming a solid solution. Manganese can also form a strengthening phase with nitrogen, further improving the strength and hardness of steel. Manganese can also improve the heat resistance and corrosion resistance of high-nitrogen austenitic stainless steel. The Mn content should be controlled between 7.19% and 8.07%.

[0016] The production process of the high-nitrogen austenitic stainless steel specifically includes the following steps:

[0017] The required raw materials are smelted to obtain alloy ingots;

[0018] The alloy ingot is subjected to electroslag remelting to generate a remelted liquid.

[0019] The remelted liquid is cast to obtain a high-nitrogen austenitic stainless steel billet;

[0020] The high-nitrogen austenitic stainless steel billet undergoes a first solution treatment.

[0021] The high-nitrogen austenitic stainless steel billet was then continuously rolled at 1000-1100℃ to obtain a plate with a thickness of 3-5 mm.

[0022] The plate is treated with iron oxide scale using a combination of mechanical and chemical methods, employing an acid pickling process.

[0023] Furthermore, a combination of mechanical and chemical methods is used in the pickling treatment of iron oxide scale. The mechanical method first employs a twin-drum cleaning machine to remove the oxide scale adhering to the surface of the hot-rolled wire rod, loosening the remaining iron oxide scale structure. This facilitates a faster chemical reaction rate during the chemical treatment, ensuring thorough removal of the iron oxide scale from the wire rod surface. The equipment is as follows: Figure 1 The chemical method employs a combined alkali boiling and pickling process. After the wire rod is rolled and cooled to room temperature, it is placed in a treatment tank containing a mixture of sodium hydroxide and sodium nitrate alkali and heated for 1-2 hours. Alkali boiling loosens the oxide scale, which is then removed by pickling. After alkali boiling, the steel should be immediately immersed in a water tank to remove the oxide scale. Then, the immersed wire rod is placed in a treatment tank containing nitric acid and hydrochloric acid for 0.5-1 hour for further pickling to remove the oxide scale.

[0024] Furthermore, due to the high strength of high-nitrogen austenitic stainless steel, its flatness needs to be controlled during processing. This invention controls the grain size of high-nitrogen austenitic stainless steel through solution treatment and annealing processes. By controlling the holding temperature, holding time and cooling rate during the first solution treatment, the second solution treatment and isothermal annealing, a single austenitic structure is obtained and its grain size is reduced, making it more uniform and thus improving flatness.

[0025] Furthermore, the equipment used in the pickling process, such as Figure 1 The device includes a conveying device and two rotating drums for removing iron oxide scale. The first drum is a horizontal hexagonal drum, and the second is a vertical cylindrical drum. An impurity collection device is installed outside each drum. The specific steps are as follows: The rolled sheet is conveyed to the first drum via the conveying device for vertical rotation at a speed of 20-35 rad / s. After 30 minutes of operation, the sheet is removed and transported to the second drum via a transmission device for horizontal rotation at a speed of 30-40 rad / s. After 30 minutes of operation, the sheet is removed by the conveying device.

[0026] The sheet material is then subjected to a second solution treatment to obtain a cold-rolled sheet.

[0027] The cold-rolled sheet is then subjected to warm rolling;

[0028] The cold-rolled sheet is subjected to isothermal annealing to complete the preparation of the high-nitrogen austenitic stainless steel;

[0029] The high-nitrogen austenitic stainless steel sheet is prepared and evaluated.

[0030] Preferably, the process of smelting the required raw materials to obtain alloy ingots includes the following steps:

[0031] The required raw materials are surface-treated with an ethanol solution with a mass fraction of more than 70% to remove surface dust, impurities, particles, and surface defects.

[0032] The processed raw materials are added into the furnace and heated using a medium-frequency vacuum induction heating furnace with a power between 60Kw and 90Kw.

[0033] Add the required element N according to the composition requirements, including nitrogen-containing compounds such as iron nitride and chromium nitride;

[0034] Turn on the vacuum system and continuously evacuate while heating the system to 1450-1600°C at a heating rate of 5-15°C / min. Then, ventilate the device by introducing protective nitrogen gas to ensure a nitrogen atmosphere of 0.1 MPa.

[0035] Once the temperature is reached, turn on the electromagnetic stirrer at a speed of 100-120 rad / min. Add the required element Mo according to the composition requirements, including Mo-containing compounds, ferromolybdenum alloys, etc.

[0036] Ventilation is performed inside the device, and the electromagnetic stirring speed is adjusted to 150-300 rad / min. At the same time, ultrasonic vibration is turned on. After the temperature reaches 1400-1500℃, it is kept at that temperature for 2-4 hours to obtain the alloy ingot.

[0037] The nitrogen content and impurity content of the alloy ingot were determined again to ensure that the composition contained C: 0.026-0.031%, Si: 0.062-0.063%, Mn: 7.19-8.07%, Cr: 16.0-18.0%, Ni: 1.09-1.11%, Mo: 1.94-1.97%, N: 0.45-0.93%, with the remainder being Fe and unavoidable impurity elements;

[0038] Once the requirements are met, preparations for casting can begin;

[0039] Furthermore, during the smelting process, the heating process begins by purging the sealed chamber with a protective gas mixture of argon and nitrogen, where argon comprises 20% by volume and nitrogen comprises 80% by volume. Air is replaced with this protective gas. Simultaneously, the smelting furnace begins heating. Once the protective gas volume in the sealed chamber is at least 90%, the pressure in the sealed chamber is adjusted to 0.1 MPa to 0.2 MPa, and the protective gas is continuously introduced until the protective gas volume in the sealed chamber is at least 90%.

[0040] Furthermore, during the smelting process, after the alloy ingot is placed in the smelting furnace, a Mo-containing alloy, such as ferromolybdenum alloy or copper-molybdenum alloy, needs to be added. The addition of Mo increases the solid solubility of nitrogen (N) in the austenitic region, ensuring the N content reaches the desired level. It also improves the resistance of high-nitrogen austenitic stainless steel to chloride ion corrosion, especially under high temperature and high chloride ion concentration conditions. Molybdenum can form stable molybdates with chromium, preventing further chloride ion corrosion and thus enhancing the steel's resistance to chloride ion corrosion. In addition, Molybdenum can also increase the strength and hardness of the steel.

[0041] Preferably, the process of electroslag remelting the alloy ingot to generate a remelted liquid includes the following steps:

[0042] Using a three-phase electroslag remelting furnace, the alloy ingot is placed in the electroslag remelting furnace as a consumable electrode. The furnace voltage is set to 660V and the furnace frequency is 50-60Hz.

[0043] To reduce nitrogen loss after electroslag treatment, a 100% N2 protective atmosphere was selected.

[0044] When current flows through the molten slag, the slag has a relatively large resistance in the power supply circuit, which accounts for most of the voltage drop in the circuit. This generates a large amount of Joule heat in the slag pool, causing its temperature to rise continuously. The furnace temperature is controlled at 1500℃~1800℃.

[0045] When the temperature of the molten slag exceeds the melting point of the consumable electrode, the end of the consumable electrode begins to melt, and the molten metal droplets drip from the end of the electrode, pass through the slag pool and enter the molten metal pool.

[0046] Preferably, casting the remelted liquid to obtain a high-nitrogen austenitic stainless steel billet includes the following steps:

[0047] The remelted liquid in the molten metal pool is poured into a steel ingot mold or casting mold at a pouring temperature of 1450℃~1500℃. During the pouring process, a protective gas atmosphere with a pressure of 0.1Mpa~0.2Mpa is maintained in the closed chamber.

[0048] Finally, the steel ingot is gradually solidified under the forced cooling of the water-cooled crystallizer. After casting, the sealed chamber is opened, the ingot mold is removed, and the ingot is removed after it has completely solidified in the ingot mold to obtain a high-nitrogen austenitic stainless steel billet.

[0049] Preferably, the first solution treatment of high-nitrogen austenitic stainless steel billet includes the following steps:

[0050] The high-nitrogen austenitic stainless steel billet was held at 1200℃ for 0.5 to 1 hour;

[0051] The austenitic structure was then cooled to room temperature at 25-40℃ / s to obtain a fully recrystallized single austenitic structure.

[0052] The average grain size of the fully recrystallized single austenitic structure obtained was 40–50 μm.

[0053] Preferably, the sheet material is then subjected to a second solution treatment to obtain a cold-rolled sheet, comprising the following steps:

[0054] The obtained board material was solution treated in a furnace at 1100℃ for 6–8 hours;

[0055] Subsequently, it is water-cooled at 35-50℃ / s to rapidly reduce its temperature to room temperature, thus obtaining a single-phase high-nitrogen austenitic stainless steel sheet with a single austenitic structure, namely cold-rolled sheet.

[0056] The average grain size of the fully recrystallized single austenitic structure obtained was 15–20 μm.

[0057] Preferably, the warm rolling of the cold-rolled sheet includes the following steps:

[0058] After the second solution treatment, the single-phase high-nitrogen austenitic stainless steel sheet is warm rolled.

[0059] First, place the high-nitrogen austenitic stainless steel sheet in a heating furnace at 300℃ and hold for 10-15 minutes to remove the internal stress caused by work hardening in the high-nitrogen austenitic stainless steel.

[0060] Then, before each rolling pass, it is placed in a 300℃ heating furnace and held for 5-8 minutes, and continuously rolled to a thickness of 2-3 mm;

[0061] Finally, the iron oxide scale is treated again using the mechanical equipment in the aforementioned mechanical method.

[0062] Preferably, the cold-rolled sheet undergoes isothermal annealing to complete the preparation of the high-nitrogen austenitic stainless steel for the hydrogen fuel cell bipolar plate, which includes the following steps:

[0063] The cold-rolled sheet is subjected to short-time isothermal annealing at 800-1000℃ for 1-5 minutes.

[0064] Then, it is water-cooled to room temperature at 60-80℃ / s to obtain high-strength and high-toughness high-nitrogen austenitic stainless steel.

[0065] The microstructure of high-strength and high-toughness high-nitrogen austenitic stainless steel consists of fully recrystallized, uniform, equiaxed austenitic grains with an average grain size of 2–4 μm.

[0066] Preferably, the process of fabricating the high-nitrogen austenitic stainless steel sheet and evaluating the high-nitrogen austenitic stainless steel sheet includes the following steps:

[0067] The bipolar plate made of the high-nitrogen austenitic stainless steel was cut into 300mm×300mm plates, and the plates were placed in an environmental container simulating a hydrogen fuel cell.

[0068] The electrolyte was set to 0.5 mol / L H2SO4 + 2×10 -6 For mol / L HF solution, maintain the ambient temperature between 60 and 80°C and the pressure between 2 and 3 atmospheres.

[0069] Hydrogen is supplied to the anode side of the plate, with a flow rate typically between 0.3 and 0.6 L / min. The hydrogen interacts with the catalyst on the anode side, undergoing an oxidation reaction and releasing electrons.

[0070] Oxygen is supplied to the cathode side of the plate, with a flow rate typically between 0.3 and 0.6 L / min. The oxygen interacts with the catalyst on the cathode side, undergoing a reduction reaction and accepting electrons.

[0071] Between the anode and cathode of the plate, hydrogen and oxygen undergo an electrochemical reaction. The oxidation reaction of hydrogen and the reduction reaction of oxygen together generate an electron flow, which drives the electrical equipment in the external circuit to work.

[0072] Electrochemical impedance spectroscopy was performed on the samples using an Autolab electrochemical workstation.

[0073] Among them, the oxygen supplied to the bipolar plates of the hydrogen fuel cell must have a purity of over 99.5%;

[0074] Further, electrochemical impedance spectroscopy detection:

[0075] After the sample has been polished, a wire is connected to the pre-drilled hole at the top and wrapped with epoxy resin, leaving a 1cm gap. 2 ×1cm 2 The working surface was then polished and soaked. A three-electrode system was used, with a saturated calomel electrode (SCE) as the reference electrode, a platinum sheet electrode as the auxiliary electrode, and the sample as the experimental electrode. The open-circuit potential of the sample was measured using an Autolab electrochemical workstation, and after the open-circuit potential stabilized, AC impedance and dynamic potential polarization were tested. The AC impedance frequency range was 10²–10⁵ Hz, the sinusoidal potential perturbation was 5 mV, and AC impedance was fitted using ZsimpWin. The polarization potential scan range was the corrosion potential -1 to 1 mV. The experimental equivalent circuit diagram is shown below. Figure 2 Electrochemical analysis circuit diagram as follows Figure 3 .

[0076] Furthermore, the evaluation process involves three steps:

[0077] Step 1, Mechanical performance testing:

[0078] The tensile strength, impact toughness, and yield strength of bipolar plates are measured using a universal testing machine to evaluate their strength and durability. The tensile strength requirement is 1280–1400 MPa. The yield strength requirement is 950–1050 MPa. The impact toughness requirement at -40℃ is 150–350 J / cm². 2 .

[0079] The second step is corrosion performance testing.

[0080] The bipolar plate was subjected to a 5% NaCl salt spray test, and then metallographic observation was performed to evaluate the number, depth and size of corrosion pits on its surface.

[0081] Step 3, Temperature Stability Testing:

[0082] Its coefficient of thermal expansion and thermal conductivity were measured using a thermal dilatometer and a thermal cycling test chamber to evaluate its temperature stability under working conditions. The coefficient of thermal expansion is 14–16 × 10⁻⁶. -6 / ℃; thermal conductivity is 18~24W / (m·K).

[0083] Preferably, the device consists of a motor housing, a conveying device, a wire rod transport box, a horizontal hexagonal roller, a vertical cylindrical roller, an impurity collection device, a horizontal connecting shaft, a vertical connecting shaft, and a conveyor belt;

[0084] The upper part of the motor housing is connected to the transmission device, and the transmission belt is installed on the transmission device. The wire rod transport box sequentially transmits the wire rod to the horizontal hexagonal roller and the vertical cylindrical roller for processing through the transport action of the transmission device and the transmission belt. Due to the different rotation modes of the rollers, the horizontal hexagonal roller is connected to the motor housing through the horizontal connecting shaft, and the vertical cylindrical roller is connected to the motor housing through the vertical connecting shaft. The bottom of both the vertical connecting shaft and the vertical cylindrical roller is equipped with an impurity collection device.

[0085] Furthermore, after smelting, an electroslag remelting process is carried out. After the molten steel is washed with slag, the inclusions are absorbed, improving the purity of the steel ingot; reducing chemical composition segregation and making the composition of the steel uniform; the high nitrogen content in the steel is very easy to precipitate nitrogen bubbles at the end of solidification under normal pressure, forming pores or escaping. Electroslag remelting relies on strong water cooling, which can suppress the precipitation of nitrogen and control the nitrogen content in the steel again.

[0086] Compared to other austenitic stainless steel compositions where Mo is added as a trace element, this invention requires the addition of a Mo-containing alloy, such as ferromolybdenum alloy or copper-molybdenum alloy, to the furnace after the alloy ingot is placed in the furnace during the smelting process. The addition of Mo can increase the solid solution content of N in the austenitic region, effectively suppressing the precipitation of N during the process, thereby ensuring that the N content reaches the expected level.

[0087] The grain size of high-nitrogen austenitic stainless steel is controlled by solution treatment and annealing processes. By controlling the holding temperature, holding time and cooling rate during the first solution treatment, the second solution treatment and isothermal annealing, a single austenitic structure is obtained, reducing the average grain size from 40-50 μm to 15-20 μm and then further reducing it by 2-4 μm, making it more uniform and thus improving flatness.

[0088] The produced steel, used in metal bipolar plates, has a tensile strength between 800 and 1200 MPa; a yield strength between 1400 and 1700 MPa; and an impact toughness between 200 and 600 J / cm². 2 Between; the coefficient of thermal expansion is 14 to 16 × 10⁻⁶. -6 / ℃; thermal conductivity is 18~24W / (m·K).

[0089] The high-nitrogen austenitic stainless steel produced, through its "N+Mo" compositional mechanism design, exhibits high electrode plate stability and significantly improved corrosion resistance, as detected by salt spray testing and electrochemical impedance spectroscopy. This meets the performance requirements for metal bipolar plates, thus eliminating the need for subsequent hot-dip galvanizing or other coating treatments.

[0090] Beneficial effects

[0091] This invention provides a method for preparing high-nitrogen austenitic stainless steel for hydrogen fuel cell bipolar plates. It offers the following advantages: This method involves adding the alloying element Mo during the smelting process. Compared to traditional stainless steel, the high-nitrogen austenitic stainless steel of this invention exhibits higher corrosion resistance. The metallurgical and processing processes achieve synergistic optimization and control of the metastability of stainless steel through the combination of N and Mo, thereby improving the steel's resistance to localized corrosion while ensuring its excellent high strength and toughness.

[0092] The pickling process employs physical and chemical methods to loosen the remaining iron oxide scale, which facilitates a faster chemical reaction during pickling and thoroughly removes the iron oxide scale from the wire rod surface. Mechanical methods are then used again during the hot-dip galvanizing pretreatment to remove impurities, preparing the surface for subsequent processing.

[0093] The manufacturing method of the invention does not require high-end equipment. The existing vacuum degassing induction melting furnace can be slightly modified into a non-vacuum atmospheric pressure protective gas induction melting furnace. The heating process involves first purging the sealed chamber, and the protective gas is selected from a mixture of argon and nitrogen. Heating, melting and casting are carried out in a fully enclosed environment, thus implementing the manufacturing method of high-nitrogen austenitic stainless steel of the present invention.

[0094] Through special processing methods, the Mo content in the final high-nitrogen austenitic stainless steel can be controlled at 1.94-1.97%, and the N content at 0.45-0.93%. Attached Figure Description

[0095] Figure 1 This is a schematic diagram of the iron oxide scale treatment device of the present invention.

[0096] Figure 2 This is a schematic diagram of the equivalent circuit for electrochemical impedance spectroscopy detection according to the present invention.

[0097] Figure 3 This is a schematic diagram of the electrochemical analysis circuit of the present invention.

[0098] Figure 4 This is a schematic diagram of the AC impedance curve of the sample in Example 1 of the present invention.

[0099] Figure 5 This is a schematic diagram of the dynamic point polarization curve of Embodiment 1 of the present invention.

[0100] Figure 6 This is a schematic diagram of the AC impedance curve of the sample in Example 2 of the present invention.

[0101] Figure 7 This is a schematic diagram of the dynamic point polarization curve of Embodiment 2 of the present invention.

[0102] Figure 8 This is a schematic diagram of the AC impedance curve of the sample in Example 3 of the present invention.

[0103] Figure 9 This is a schematic diagram of the dynamic point polarization curve in Embodiment 3 of the present invention.

[0104] In the diagram: 1. Motor housing; 2. Conveying device; 3. Wire rod transport box; 4. Horizontal hexagonal roller; 5. Vertical cylindrical roller; 6. Impurity collection device; 7. Horizontal connecting shaft; 8. Vertical connecting shaft; 9. Conveyor belt. Detailed Implementation

[0105] The technical solutions of the embodiments 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, and 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 scope of protection of the present invention.

[0106] Those skilled in the art should connect all electrical components and their compatible power supplies in this case via wires, and should select appropriate controllers according to actual conditions to meet control requirements. The specific connection and control sequence should refer to the working principle described below, where the electrical components are connected in sequence. The detailed connection methods are well-known in the art. The following mainly introduces the working principle and process, without explaining the electrical control.

[0107] Please see Figure 1-9 The present invention provides a technical solution: within the composition range of the present invention, three furnaces of steel were smelted and the above-mentioned process was carried out to prepare the required high-density austenitic stainless steel sheets, namely Example 1, Example 2, Example 3 and Example 4.

[0108] Example 1:

[0109] The raw materials are surface-treated to remove defects and impurities using an ethanol solution with a mass fraction higher than 70% to remove surface dust and impurity particles. The treated alloy ingots are then added to a furnace and heated using a medium-frequency vacuum induction heating furnace with a power of 65 kW, adjustable as needed. The required element N, such as iron nitride and chromium nitride, is added according to the composition requirements. The vacuum system is then activated, and while continuously evacuating, the system is heated to 1500°C at a rate of 10°C / min. Gas is introduced into the apparatus. Once the temperature is reached, electromagnetic stirring is activated at a speed of 100 rad / min. The required element Mo is added, such as Mo-containing molybdenum-iron alloys, and the apparatus is ventilated. Simultaneously, the electromagnetic stirring speed is adjusted to 150 rad / min, and ultrasonic vibration is activated. After the temperature reaches 1400℃, it is held at that temperature for 2.5 hours. The nitrogen content and impurity content are then measured again to ensure the composition contains: C: 0.026–0.031%, Si: 0.062–0.063%, Mn: 7.19–8.07%, Cr: 16.0–18.0%, Ni: 1.09–1.11%, Mo: 1.94–1.97%, N: 0.45–0.93%, with the remainder being Fe and unavoidable impurity elements. Using a three-phase electroslag remelting furnace, the smelted steel ingot was placed as a consumable electrode into the furnace. The furnace voltage was set to 660V, the furnace frequency to 50Hz, and the furnace temperature was controlled at 1600℃ under a 100% N2 protective atmosphere. The elemental composition of the final steel in Example 1 is shown in Table 1. The remelted liquid from the molten metal pool was poured into an ingot mold or casting mold at a pouring temperature of 1450℃. During the pouring process, a protective gas atmosphere of 0.1 MPa was maintained in the sealed chamber. After pouring, the sealed chamber was opened, the ingot mold was removed, and the ingot was removed after complete solidification in the ingot mold. The high-nitrogen austenitic stainless steel billet was held at 1200℃ for 1 hour, and then water-cooled to room temperature to obtain a fully recrystallized single austenitic structure. The average grain size of the obtained fully recrystallized single austenitic structure was 40–50 μm. Subsequently, the plate was continuously rolled at 1000℃ to obtain a thickness of 3mm; the pickling process used a combination of mechanical and chemical methods to treat the iron oxide scale; the second solution treatment was carried out by treating the obtained plate in a heating furnace at 1150℃ for 6 hours, followed by water cooling to rapidly reduce the temperature to room temperature to obtain a single-phase high-nitrogen austenitic stainless steel plate with a single austenitic structure; after the second solution treatment, the above-mentioned single-phase high-nitrogen austenitic stainless steel plate was warm rolled.First, the high-nitrogen austenitic stainless steel sheet is placed in a heating furnace at 300℃ and held for 10 minutes to remove the internal stress caused by work hardening. Then, before each rolling pass, it is placed in a heating furnace at 300℃ and held for 5 minutes, and continuously rolled to a thickness of 2 mm. After that, the cold-rolled sheet is subjected to iron oxide scale treatment again using mechanical equipment and then short-time isothermal annealing at 850℃ for 3 minutes. It is then water-cooled to room temperature to obtain high-strength and high-toughness high-nitrogen austenitic stainless steel. The microstructure of the high-strength and high-toughness high-nitrogen austenitic stainless steel consists of fully recrystallized, uniform, equiaxed austenitic grains with an average grain size of 2-4 μm.

[0110] Table 1. Chemical elemental composition (mass percentage) of stainless steel in Example 1

[0111]

[0112] Stainless steel bipolar plates were placed in an environmental container simulating a hydrogen fuel cell. The electrolyte was 0.5 mol / L H₂SO₄ + 2×10⁻⁶. -6 A 1 mol / L HF solution is used to control the ambient temperature at 60°C and the pressure at 2 atmospheres. Hydrogen gas is supplied to the anode side of the stainless steel bipolar plate. The hydrogen flow rate is typically 0.3 L / min. On the anode side, the hydrogen interacts with the catalyst, undergoing an oxidation reaction and releasing electrons. Oxygen is supplied to the cathode side of the stainless steel bipolar plate. The oxygen flow rate is also typically 0.3 L / min. On the cathode side, the oxygen interacts with the catalyst, undergoing a reduction reaction and accepting electrons. Between the anode and cathode of the stainless steel bipolar plate, hydrogen and oxygen undergo an electrochemical reaction. The oxidation reaction of hydrogen and the reduction reaction of oxygen together generate an electron flow, driving the electrical equipment in the external circuit. The oxygen supplied to the hydrogen fuel cell bipolar plate has a purity of 99.5%.

[0113] Step 1, Mechanical performance testing:

[0114] The tensile strength, impact toughness, yield strength, and other mechanical properties of the bipolar plate were measured using a universal testing machine to evaluate its strength and durability. The test results are shown in Table 2.

[0115] The second step is corrosion performance testing.

[0116] The bipolar plate was subjected to a 5% NaCl salt spray test, and then the number, depth and size of corrosion pits on its surface were evaluated by metallographic observation. The test results are shown in Table 3.

[0117] Step 3, Temperature Stability Testing:

[0118] Its coefficient of thermal expansion and thermal conductivity were measured using a thermal expansion tester and a thermal cycling test chamber to evaluate its temperature stability under working conditions. The test results are shown in Table 4.

[0119] Step 4, Electrochemical Impedance Spectroscopy (EIS) Detection:

[0120] The open-circuit potential of the sample was determined using an Autolab electrochemical workstation. After the open-circuit potential stabilized, the AC impedance and dynamic potential polarization were measured. The test results are as follows: Figure 4 Based on electrochemical analysis, the open circuit potential, corrosion current density, and charge transfer resistance were measured, and the results are shown in Table 5.

[0121] Table 2 Mechanical properties of metal samples from Example 1

[0122]

[0123] Table 3 Corrosion performance parameters of the sample in Example 1

[0124]

[0125] Table 4 Temperature stability parameters of the sample in Example 1

[0126]

[0127]

[0128] Table 5 Electrochemical Analysis Results of Example 1

[0129]

[0130] Example 2

[0131] The raw materials are surface-treated to remove defects and impurities. An ethanol solution with a mass fraction higher than 70% is used to remove surface dust and impurity particles. The treated alloy ingots are then added to a furnace and heated using a medium-frequency vacuum induction heating furnace with a power of 70 kW, adjustable as needed. The required element N, such as iron nitride and chromium nitride, is added according to the composition requirements. The vacuum system is then activated, and while continuously evacuating, the system is heated to 1550°C at a rate of 13°C / min. Gas is introduced into the apparatus. Once the temperature is reached, electromagnetic stirring is activated at a speed of 120 rad / min. The required element Mo is added to the composition, such as Mo-containing molybdenum-iron alloys. The apparatus is then ventilated, and the electromagnetic stirring speed is adjusted to 180 rad / min. Ultrasonic vibration is activated, and the temperature is maintained at 1380℃ for 3 hours. The nitrogen content and impurity content are then measured again to ensure the composition contains: C: 0.026-0.031%, Si: 0.062-0.063%, Mn: 7.19-8.07%, Cr: 16.0-18.0%, Ni: 1.09-1.11%, Mo: 1.94-1.97%, N: 0.45-0.93%, with the remainder being Fe and unavoidable impurity elements. Using a three-phase electroslag remelting furnace, the smelted steel ingot was placed in the furnace as a consumable electrode. The furnace voltage was set to 660V, the furnace frequency to 60Hz, and the furnace temperature was controlled at 1550℃ under a 100% N2 protective atmosphere. The elemental composition of the final steel in Example 2 is shown in Table 6. The remelted liquid in the molten metal pool was poured into an ingot mold or casting mold at a pouring temperature of 1500℃. During the pouring process, a protective gas atmosphere of 0.2 MPa was maintained in the sealed chamber. After pouring, the sealed chamber was opened, the ingot mold was removed, and the ingot was removed after complete solidification in the ingot mold. The high-nitrogen austenitic stainless steel billet was held at 1170℃ for 1.5h, and then water-cooled to room temperature to obtain a fully recrystallized single austenitic structure. The average grain size of the obtained fully recrystallized single austenitic structure was 40-50 μm. Subsequently, the plate was continuously rolled at 980℃ to obtain a thickness of 3mm; the pickling process used a combination of mechanical and chemical methods to treat the iron oxide scale; the second solution treatment was carried out by treating the obtained plate in a heating furnace at 1150℃ for 5 hours, followed by water cooling to rapidly reduce the temperature to room temperature to obtain a single-phase high-nitrogen austenitic stainless steel plate with a single austenitic structure; after the second solution treatment, the above-mentioned single-phase high-nitrogen austenitic stainless steel plate was warm rolled.First, the high-nitrogen austenitic stainless steel sheet is placed in a heating furnace at 300℃ and held for 15 minutes to remove the internal stress caused by work hardening. Then, before each rolling pass, it is placed in a heating furnace at 300℃ and held for 5 minutes, and continuously rolled to a thickness of 2 mm. After that, the cold-rolled sheet is subjected to iron oxide scale treatment again using mechanical equipment and then short-time isothermal annealing at 850℃ for 3 minutes. It is then water-cooled to room temperature to obtain high-strength and high-toughness high-nitrogen austenitic stainless steel. The microstructure of the high-strength and high-toughness high-nitrogen austenitic stainless steel consists of fully recrystallized, uniform, equiaxed austenitic grains with an average grain size of 2-4 μm.

[0132] Table 6 Chemical elemental composition (mass percentage) of stainless steel in Example 2

[0133]

[0134] Stainless steel bipolar plates were placed in an environmental container simulating a hydrogen fuel cell. The electrolyte was 0.5 mol / L H₂SO₄ + 2×10⁻⁶. -6 A 1 mol / L HF solution is used to control the ambient temperature at 70°C and the pressure at 2 atmospheres. Hydrogen gas is supplied to the anode side of the stainless steel bipolar plate. The hydrogen flow rate is typically 0.45 L / min. On the anode side, the hydrogen interacts with the catalyst, undergoing an oxidation reaction and releasing electrons. Oxygen is supplied to the cathode side of the stainless steel bipolar plate. The oxygen flow rate is also typically 0.45 L / min. On the cathode side, the oxygen interacts with the catalyst, undergoing a reduction reaction and accepting electrons. Between the anode and cathode of the stainless steel bipolar plate, hydrogen and oxygen undergo an electrochemical reaction. The oxidation reaction of hydrogen and the reduction reaction of oxygen together generate an electron flow, driving the electrical equipment in the external circuit. The oxygen supplied to the hydrogen fuel cell bipolar plate has a purity of 99.5%.

[0135] Step 1, Mechanical performance testing:

[0136] The tensile strength, impact toughness, yield strength, and other mechanical properties of the bipolar plates were measured using a universal testing machine to evaluate their strength and durability. The test results are shown in Table 7.

[0137] The second step is corrosion performance testing.

[0138] The bipolar plate was subjected to a 5% NaCl salt spray test, and then the number, depth and size of corrosion pits on its surface were evaluated by metallographic observation; the test results are shown in Table 8.

[0139] Step 3, Temperature Stability Testing:

[0140] Its coefficient of thermal expansion and thermal conductivity were measured using a thermal expansion tester and a thermal cycling test chamber to evaluate its temperature stability under working conditions. The test results are shown in Table 9.

[0141] Step 4, Electrochemical Impedance Spectroscopy (EIS) Detection:

[0142] The open-circuit potential of the sample was determined using an Autolab electrochemical workstation. After the open-circuit potential stabilized, the AC impedance and dynamic potential polarization were measured. The test results are as follows: Figure 5 Based on electrochemical analysis, the open circuit potential, corrosion current density, and charge transfer resistance were measured, and the results are shown in Table 10.

[0143] Table 7 Mechanical properties of metal samples from Example 2

[0144]

[0145] Table 8 Corrosion performance parameters of the sample in Example 2

[0146]

[0147] Table 9 Temperature stability parameters of the sample in Example 2

[0148]

[0149] Table 10 Electrochemical Analysis Results of Example 2

[0150]

[0151] Example 3

[0152] The raw materials are surface-treated to remove defects and impurities. An ethanol solution with a mass fraction higher than 70% is used to remove surface dust and impurity particles. The treated alloy ingots are then added to a furnace and heated using a medium-frequency vacuum induction heating furnace with a power of 90 kW, adjustable according to specific needs. The required element N, such as iron nitride and chromium nitride, is added according to the composition requirements. The vacuum system is then activated, and while continuously evacuating, the system is heated to 1600°C at a rate of 15°C / min. Gas is introduced into the apparatus. Once the temperature is reached, electromagnetic stirring is activated at a speed of 150 rad / min. The required element Mo is added to the composition, such as Mo-containing molybdenum-iron alloys. The apparatus is then ventilated, and the electromagnetic stirring speed is adjusted to 200 rad / min. Ultrasonic vibration is activated, and the temperature is maintained at 1400℃ for 3 hours. The nitrogen content and impurity content are then measured again to ensure the composition contains: C: 0.026–0.031%, Si: 0.062–0.063%, Mn: 7.19–8.07%, Cr: 16.0–18.0%, Ni: 1.09–1.11%, Mo: 1.94–1.97%, N: 0.45–0.93%, with the remainder being Fe and unavoidable impurity elements. Using a three-phase electroslag remelting furnace, the smelted steel ingot was placed in the furnace as a consumable electrode. The furnace voltage was set to 660V, the furnace frequency to 50Hz, and the furnace temperature was controlled at 1650℃ under a 100% N2 protective atmosphere. The elemental composition of the final steel in Example 3 is shown in Table 11. The remelted liquid in the molten metal pool was poured into an ingot mold or casting mold at a pouring temperature of 1500℃. During the pouring process, a protective gas atmosphere with a pressure of 0.2 MPa was maintained in the sealed chamber. After pouring, the sealed chamber was opened, the ingot mold was removed, and the ingot was removed after it had completely solidified in the ingot mold. The high-nitrogen austenitic stainless steel billet was held at 1170℃ for 1.5h, and then water-cooled to room temperature to obtain a fully recrystallized single austenitic structure. The average grain size of the obtained fully recrystallized single austenitic structure was 40-50 μm. Subsequently, the plate was continuously rolled at 1000℃ to obtain a thickness of 3mm; the pickling process used a combination of mechanical and chemical methods to treat the iron oxide scale; the second solution treatment was carried out by treating the obtained plate in a heating furnace at 1150℃ for 5 hours, followed by water cooling to rapidly reduce the temperature to room temperature to obtain a single-phase high-nitrogen austenitic stainless steel plate with a single austenitic structure; after the second solution treatment, the above-mentioned single-phase high-nitrogen austenitic stainless steel plate was warm rolled.First, the high-nitrogen austenitic stainless steel sheet is placed in a heating furnace at 300℃ and held for 15 minutes to remove the internal stress caused by work hardening. Then, before each rolling pass, it is placed in a heating furnace at 300℃ and held for 5 minutes, and continuously rolled to a thickness of 2 mm. After that, the cold-rolled sheet is subjected to iron oxide scale treatment again using mechanical equipment and then short-time isothermal annealing at 850℃ for 3 minutes. It is then water-cooled to room temperature to obtain high-strength and high-toughness high-nitrogen austenitic stainless steel. The microstructure of the high-strength and high-toughness high-nitrogen austenitic stainless steel consists of fully recrystallized, uniform, equiaxed austenitic grains with an average grain size of 2-4 μm.

[0153] Table 11 Chemical elemental composition (mass percentage) of stainless steel in Example 3

[0154]

[0155] Stainless steel bipolar plates were placed in an environmental container simulating a hydrogen fuel cell. The electrolyte was 0.5 mol / L H₂SO₄ + 2×10⁻⁶. -6 A 1 mol / L HF solution is used to control the ambient temperature at 80°C and the pressure at 2 atmospheres. Hydrogen gas is supplied to the anode side of the stainless steel bipolar plate. The hydrogen flow rate is typically 0.6 L / min. On the anode side, the hydrogen interacts with the catalyst, undergoing an oxidation reaction and releasing electrons. Oxygen is supplied to the cathode side of the stainless steel bipolar plate. The oxygen flow rate is also typically 0.6 L / min. On the cathode side, the oxygen interacts with the catalyst, undergoing a reduction reaction and accepting electrons. Between the anode and cathode of the stainless steel bipolar plate, hydrogen and oxygen undergo an electrochemical reaction. The oxidation reaction of hydrogen and the reduction reaction of oxygen together generate an electron flow, driving the electrical equipment in the external circuit. The oxygen supplied to the hydrogen fuel cell bipolar plate has a purity of 99.5%.

[0156] Step 1, Mechanical performance testing:

[0157] The tensile strength, impact toughness, yield strength, and other mechanical properties of bipolar plates were measured using a universal testing machine to evaluate their strength and durability. The test results are shown in Table 12.

[0158] The second step is corrosion performance testing.

[0159] The bipolar plate was subjected to a 5% NaCl salt spray test, and then the number, depth and size of corrosion pits on its surface were evaluated by metallographic observation; the test results are shown in Table 13.

[0160] Step 3, Temperature Stability Testing:

[0161] Its coefficient of thermal expansion and thermal conductivity were measured using a thermal expansion tester and a thermal cycling test chamber to evaluate its temperature stability under working conditions. The test results are shown in Table 14.

[0162] Step 4, Electrochemical Impedance Spectroscopy (EIS) Detection:

[0163] The open-circuit potential of the sample was determined using an Autolab electrochemical workstation. After the open-circuit potential stabilized, the AC impedance and dynamic potential polarization were measured. The test results are as follows: Figure 6 Based on electrochemical analysis, the open circuit potential, corrosion current density, and charge transfer resistance were measured, and the results are shown in Table 15.

[0164] Table 12 Mechanical properties of metal samples from Example 3

[0165]

[0166] Table 13 Corrosion performance parameters of the sample in Example 3

[0167]

[0168] Table 14 Temperature stability parameters of sample in Example 3

[0169]

[0170] Table 15 Electrochemical Analysis Results of Example 3

[0171]

[0172] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, the phrase "comprising an element defined as..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0173] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing high-nitrogen austenitic stainless steel for bipolar plates of hydrogen fuel cells, characterized in that, The chemical composition of the high-nitrogen austenitic stainless steel is as follows (weight percentage): C: 0.026~0.031%, Si: 0.062~0.063%, Mn: 7.19~8.07%, Cr: 16.0~18.0%, Ni: 1.09~1.11%, Mo: 1.94~1.97%, N: 0.45~0.93%, with the remainder being Fe and unavoidable impurity elements; The production process of the high-nitrogen austenitic stainless steel specifically includes the following steps: The required raw materials are smelted to obtain alloy ingots; The alloy ingot is subjected to electroslag remelting to generate a remelted liquid. The remelted liquid is cast to obtain a high-nitrogen austenitic stainless steel billet; The high-nitrogen austenitic stainless steel billet undergoes a first solution treatment. The high-nitrogen austenitic stainless steel billet was then continuously rolled at 1000-1100℃ to obtain a plate with a thickness of 3-5mm. The plate is treated with iron oxide scale using a combination of mechanical and chemical methods, employing an acid pickling process. The sheet material is then subjected to a second solution treatment to obtain a cold-rolled sheet. The cold-rolled sheet is then subjected to warm rolling; The cold-rolled sheet is subjected to isothermal annealing to complete the preparation of the high-nitrogen austenitic stainless steel; Prepare steel sheets of the high-nitrogen austenitic stainless steel and evaluate the steel sheets of the high-nitrogen austenitic stainless steel. The first solution treatment of high-nitrogen austenitic stainless steel billets includes the following steps: The high-nitrogen austenitic stainless steel billet was held at 1200℃ for 0.5~1h; The austenitic structure was then cooled to room temperature at 25-40℃ / s to obtain a fully recrystallized single austenitic structure. The average grain size of the fully recrystallized single austenitic structure obtained was 40~50 μm; The sheet material is then subjected to a second solution treatment to obtain a cold-rolled sheet, comprising the following steps: The obtained board was solution treated in a furnace at 1100℃ for 6-8 hours; Subsequently, it is water-cooled at 35~50℃ / s to rapidly reduce its temperature to room temperature, thus obtaining a single-phase high-nitrogen austenitic stainless steel sheet with a single austenitic structure, namely cold-rolled sheet. The average grain size of the fully recrystallized single austenitic structure obtained was 15~20 μm; The warm rolling process for the cold-rolled sheet includes the following steps: After the second solution treatment, the single-phase high-nitrogen austenitic stainless steel sheet is warm rolled. First, place the high-nitrogen austenitic stainless steel sheet in a heating furnace at 300℃ and hold for 10-15 minutes to remove the internal stress caused by work hardening in the high-nitrogen austenitic stainless steel. Then, before each rolling pass, it is placed in a 300℃ heating furnace and held for 5-8 minutes, and continuously rolled to a thickness of 2-3 mm; Finally, the iron oxide scale is treated again using mechanical equipment in the mechanical method. The cold-rolled sheet undergoes isothermal annealing to complete the preparation of the high-nitrogen austenitic stainless steel for the hydrogen fuel cell bipolar plate, which includes the following steps: The cold-rolled sheet is subjected to short-time isothermal annealing at 800~1000℃ for 1~5 minutes. Then, it is water-cooled to room temperature at 60~80℃ / s to obtain high-strength and high-toughness high-nitrogen austenitic stainless steel. The microstructure of high-strength and high-toughness high-nitrogen austenitic stainless steel consists of fully recrystallized, uniform, equiaxed austenitic grains with an average grain size of 2~4μm.

2. The method for preparing high-nitrogen austenitic stainless steel for hydrogen fuel cell bipolar plates according to claim 1, characterized in that... The process of smelting the required raw materials to obtain alloy ingots includes the following steps: The required raw materials are surface-treated with an ethanol solution with a mass fraction of more than 70% to remove surface dust, impurities, particles, and surface defects. The processed raw materials are added into the furnace and heated using a medium-frequency vacuum induction heating furnace with a power between 60kW and 90kW. Add the required element N according to the composition requirements, including nitrogen-containing iron nitride and chromium nitride; Turn on the vacuum system and continuously evacuate while heating the system to 1450~1600℃ at a heating rate of 5~15℃ / min. Then, ventilate the device by introducing protective nitrogen gas to ensure that the nitrogen atmosphere is 0.1MPa. Once the temperature is reached, turn on the electromagnetic stirrer at a speed of 100-120 rad / min. Add the required element Mo according to the composition requirements, including Mo-containing compounds and ferromolybdenum alloys. Ventilation is performed inside the device, and the electromagnetic stirring speed is adjusted to 150~300 rad / min. At the same time, ultrasonic vibration is turned on. After the temperature reaches 1400~1500℃, it is held for 2~4 hours to obtain alloy ingots. The nitrogen content and impurity content of the alloy ingot were determined again to ensure that the composition contained C: 0.026~0.031%, Si: 0.062~0.063%, Mn: 7.19~8.07%, Cr: 16.0~18.0%, Ni: 1.09~1.11%, Mo: 1.94~1.97%, N: 0.45~0.93%, with the remainder being Fe and unavoidable impurity elements; Once the requirements are met, preparations for casting can begin.

3. The method for preparing high-nitrogen austenitic stainless steel for hydrogen fuel cell bipolar plates according to claim 1, characterized in that... The process of electroslag remelting the alloy ingot to generate a remelted liquid includes the following steps: Using a three-phase electroslag remelting furnace, the alloy ingot is placed in the electroslag remelting furnace as a consumable electrode, the furnace voltage is set to 660V, and the furnace frequency is 50~60Hz. To reduce nitrogen loss after electroslag treatment, a 100% N2 protective atmosphere was selected. When current flows through the molten slag, the slag has a relatively large resistance in the power supply circuit, which accounts for most of the voltage drop in the circuit. This generates a large amount of Joule heat in the slag pool, causing its temperature to rise continuously. The furnace temperature is controlled at 1500°C~1800°C. When the temperature of the molten slag exceeds the melting point of the consumable electrode, the end of the consumable electrode begins to melt. The molten metal droplets, known as remelted liquid, drip from the end of the electrode, pass through the slag pool, and enter the molten metal pool.

4. The method for preparing high-nitrogen austenitic stainless steel for hydrogen fuel cell bipolar plates according to claim 3, characterized in that... The process of casting the remelted liquid to obtain a high-nitrogen austenitic stainless steel billet includes the following steps: The remelted liquid in the molten metal pool is poured into a steel ingot mold or casting mold at a pouring temperature of 1450℃~1500℃. During the pouring process, a protective gas atmosphere with a pressure of 0.1MPa~0.2MPa is maintained in the closed chamber. Finally, the steel ingot is gradually solidified under the forced cooling of the water-cooled crystallizer. After casting, the sealed chamber is opened, the ingot mold is removed, and the ingot is removed after it has completely solidified in the ingot mold to obtain a high-nitrogen austenitic stainless steel billet.

5. The method for preparing high-nitrogen austenitic stainless steel for hydrogen fuel cell bipolar plates according to claim 1, characterized in that... The process of fabricating and evaluating the high-nitrogen austenitic stainless steel sheets includes the following steps: The bipolar plate made of the high-nitrogen austenitic stainless steel was cut into 300mm×300mm plates, and the plates were placed in an environmental container simulating a hydrogen fuel cell. The electrolyte was set to 0.5 mol / L H2SO4 + 2×10 -6 For mol / L HF solution, maintain the ambient temperature between 60 and 80°C and the pressure between 2 and 3 atmospheres. Hydrogen is supplied to the anode side of the plate at a flow rate typically between 0.3 and 0.6 L / min. The hydrogen interacts with the catalyst on the anode side, undergoing an oxidation reaction and releasing electrons. Oxygen is supplied to the cathode side of the plate, with a flow rate typically between 0.3 and 0.6 L / min. The oxygen interacts with the catalyst on the cathode side, undergoing a reduction reaction and accepting electrons. Between the anode and cathode of the plate, hydrogen and oxygen undergo an electrochemical reaction. The oxidation reaction of hydrogen and the reduction reaction of oxygen together generate an electron flow, which drives the electrical equipment in the external circuit to work. Electrochemical impedance spectroscopy was performed on the samples using an Autolab electrochemical workstation. Among them, the oxygen supplied to the bipolar plates of the hydrogen fuel cell must have a purity of over 99.5%.

6. The method for preparing high-nitrogen austenitic stainless steel for hydrogen fuel cell bipolar plates according to claim 2, characterized in that, The iron oxide scale treatment device consists of a motor box, a conveying device, a wire rod transport box, a horizontal hexagonal roller, a vertical cylindrical roller, an impurity collection device, a horizontal connecting shaft, a vertical connecting shaft, and a conveyor belt. The upper part of the motor housing is connected to the conveying device, and the conveyor belt is installed on the conveying device. The wire rod transport box conveys the wire rod sequentially to the horizontal hexagonal roller and the vertical cylindrical roller for processing through the conveying action of the conveying device and the conveyor belt. Due to the different rotation modes of the rollers, the horizontal hexagonal roller is connected to the motor housing through the horizontal connecting shaft, and the vertical cylindrical roller is connected to the motor housing through the vertical connecting shaft. The bottom of both the vertical connecting shaft and the vertical cylindrical roller is equipped with an impurity collection device.

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