A medium temperature tissue stable stainless steel, fuel cell interconnect plate and method of making

Abstract

The application provides a medium-temperature tissue stable stainless steel, a fuel cell interconnection plate and a preparation method, and belongs to the alloy field. The stainless steel comprises the following chemical components: Cr: 21-30%, Mo: 5.1-7.5%, Zr: 0.05-0.5%, B: 0.015-0.45%, Nb: 0.02-2%, Si: 0.45-1%, Mn: 0.1-2.5%, C: less than 0.008%, T.O: less than or equal to 0.002%, N: less than or equal to 0.001% and Fe. At room temperature to working temperature, the alloy tissue is single ferrite tissue, the finished product grain size is kept at 4.5-7 levels, and the grain level is not increased under the working condition of 900 DEG C. Therefore, the medium-temperature tissue stability of the fuel cell interconnection plate is improved.

Description

Technical Field

[0001] This application relates to the field of alloy technology, and in particular to a medium-temperature stable stainless steel, a fuel cell interconnect plate, and a preparation method thereof. Background Technology

[0002] High-temperature solid oxide fuel cells (SOFCs) are highly efficient energy conversion devices that directly convert the chemical energy of fuel into electrical energy without a chemical reaction. They can directly use carbon-containing gases such as methane, natural gas, and water gas as fuel. In recent years, improvements in material manufacturing processes and design technologies have enabled SOFCs to operate at temperatures ranging from 1000℃ to a mid-temperature range of 600-900℃, making it possible to use metal interconnects. Metal interconnects can meet the basic requirements for electrical conductivity, coefficient of thermal expansion, thermal conductivity, and oxidation resistance at operating temperatures.

[0003] The coefficients of thermal expansion of the various components of a solid oxide fuel cell are (10.5–12.5) × 10⁻⁶. -6 Therefore, the coefficient of thermal expansion (TEC) of the metal connector material must be close to this value to avoid excessive thermal stress that would cause rapid degradation of the fuel cell performance. Generally, austenitic stainless steel and nickel-based alloys have relatively high TEC values, typically (16–19) × 10⁻⁶. -6 / K. To date, due to limitations imposed by the coefficient of thermal expansion at high temperatures, achieving the same coefficient of thermal expansion as solid electrolytes (9–11) × 10⁻⁶ has been a challenge. -6 Currently, the most common approach is to use iron-based alloys. Four relatively refined ferritic stainless steel integrators are Crofer 22APU, SUS430, X10CrAl18, and ZMG232. Representative ferritic Fe-Cr alloys include ZMG232 developed by Hitachi Metals and the Crofer22 APU alloy announced by ThyssenKrupp VDM in 2003. Later optimizations led to the development of ZMG232L and Crofer22H. Existing technologies generally use ferritic stainless steel; however, the ferrite growth temperature is around 650℃, meaning that under operating conditions of 600℃~900℃, the ferrite microstructure is difficult to maintain stability, and continuous growth significantly impacts stability. Furthermore, the formation of the Laves phase in the matrix also affects microstructural stability; the Laves phase also acts as a channel for crack nucleation and propagation, reducing high-temperature creep strength and room-temperature plasticity. Summary of the Invention

[0004] This application provides a medium-temperature structurally stable stainless steel, a fuel cell interconnect plate, and a preparation method thereof to solve the following technical problem: how to improve the medium-temperature structural stability of the fuel cell interconnect plate.

[0005] In a first aspect, embodiments of this application provide a medium-temperature stable stainless steel, which, by mass fraction, comprises the following chemical composition: Cr: 21%–30%, Mo: 5.1%–7.5%, Zr: 0.05%–0.5%, B: 0.015%–0.45%, Nb: 0.02%–2%, Si: 0.45%–1%, Mn: 0.1%–2.5%, C<0.008%, TO≤0.002%, N≤0.001%, and Fe.

[0006] Optionally, the stainless steel further includes the following chemical composition: rare earth elements: 0.005% to 0.2%, wherein the rare earth elements include one or more of La, Ce, Sc and Y.

[0007] Optionally, the chemical components satisfy the following relationship:

[0008] 6%≤[Mo]+[Nb]+3×[Si]≤13%

[0009] In the formula, [Mo] represents the mass fraction of Mo, [Nb] represents the mass fraction of Nb, and [Si] represents the mass fraction of Si. Optionally, the chemical composition satisfies the following relationship:

[0010] 0.01≤([Zr]+1.1[B]) / ([Mo]+[Nb])≤0.2

[0011] In the formula, [Zr] represents the mass fraction of Zr, [B] represents the mass fraction of B, [Mo] represents the mass fraction of Mo, and [Nb] represents the mass fraction of Nb.

[0012] Secondly, this application provides a method for preparing stainless steel according to any embodiment of the first aspect, the method comprising:

[0013] The molten steel is vacuum refined until the TO content of the molten steel is ≤0.002%. Then, an alloy containing rare earth elements is added to the molten steel to obtain alloyed molten steel.

[0014] The alloyed steel liquid is poured to obtain the stainless steel.

[0015] Thirdly, this application provides a fuel cell interconnect, wherein the fuel cell interconnect plate is made of stainless steel as described in any embodiment of the first aspect, and the alloy structure of the fuel cell interconnect is a single ferrite structure at room temperature to operating temperature, and the grain grade of the ferrite structure is 4.5 to 7.

[0016] Optionally, fine Laves phase particles are dispersed in the matrix of the fuel cell interconnect, and the particle size of the Laves phase particles is <2μm, and the total amount of precipitated phase in the matrix of the fuel cell interconnect is <2.5%.

[0017] Optionally, the fuel cell interconnect plate satisfies at least one of the following performance characteristics:

[0018] Under an atmosphere of 20% O2-80% H2O, oxidation at 900℃ for 30,000 hours resulted in a total oxide film thickness of <10 μm.

[0019] Tensile strength Rm ≥ 260 MPa at 900℃;

[0020] The yield strength Rp0.2 at 900℃ is ≥210MPa.

[0021] Fourthly, this application provides a method for preparing a fuel cell interconnect plate according to any embodiment of the third aspect, the method comprising:

[0022] The stainless steel described in any one of the embodiments of the first aspect is subjected to hot rolling, cold rolling, stamping, solution treatment and water cooling in sequence to obtain a fuel cell interconnect plate.

[0023] Optionally, the solution treatment includes the following parameters: heating temperature of 1000℃~1100℃, holding time of 30min~45min, and hydrogen injection rate ≥20m³. 3 / h.

[0024] The technical solutions provided in this application have the following advantages compared with the prior art:

[0025] This application provides a medium-temperature stable stainless steel, which, by mass fraction, comprises the following chemical composition: Cr: 21%–30%, Mo: 5.1%–7.5%, Zr: 0.05%–0.5%, B: 0.015%–0.45%, Nb: 0.02%–2%, Si: 0.45%–1%, Mn: 0.1%–2.5%, C<0.008%, TO≤0.002%, N≤0.001%, and Fe. The Fe-Cr-Mo-Nb-Zr-B ferritic stainless steel is used, with Cr-Mo-Nb solid solution strengthening. The reasonable matching of Cr, Mo, and Nb achieves the mechanical performance requirements of solid oxide fuel cells at medium temperatures. Mo and Nb are Laves phase forming elements; during solidification, a B2A structured Laves brittle phase, namely (Fe,Cr)2(Mo,Nb), precipitates. Boron (B) segregates at grain boundaries, improving grain boundary bonding. Appropriate addition can mitigate phase brittleness. Zr improves both the room-temperature and mid-temperature plasticity of the Laves phase. The combined effect of B and Zr improves the overall mid-temperature performance of the alloy from 600℃ to 900℃, enhancing its machinability. By controlling the composition of Mo and Nb and the solid solution process parameters, the proportion and size of the Laves phase can be controlled, reducing the impact of Laves phase precipitation on the stability of the ferrite microstructure at mid-temperature conditions. This ensures the alloy maintains a single ferrite microstructure, with the precipitated Laves phase proportion not exceeding 2.5% and a particle size less than 2μm, thus stabilizing thermal expansion and mechanical properties. Simultaneously, Zr, B, and the Laves phase pinnate grain boundaries, eliminating the continuous growth of ferrite under mid-temperature operating conditions. At mid-temperature temperatures of 600℃ to 900℃, the ferrite grain size remains stable at grade 4.5 to 7, ensuring microstructure stability. This improves the mid-temperature microstructure stability of the fuel cell interconnect plate. Attached Figure Description

[0026] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0027] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 This is a schematic flowchart illustrating a method for preparing stainless steel, as provided in an embodiment of this application. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0030] Various embodiments of this application may exist in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be construed as a hard limitation on the scope of this application; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within that range. For example, it should be considered that the range description from 1 to 6 has specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Furthermore, whenever a numerical range is referred to herein, it means including any referenced number (fraction or integer) within the range referred to.

[0031] Furthermore, in the description of this application, the terms "comprising," "including," etc., mean "including but not limited to." In this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. In this document, "and / or" describes the relationship between related objects, indicating that three relationships can exist; for example, A and / or B can represent: A alone, A and B simultaneously, or B alone. A and B can be singular or plural. In this document, "at least one" means one or more, and "more than" means two or more. "At least one," "at least one of the following," or similar expressions refer to any combination of these items, including any combination of single or plural items. For example, "at least one of a, b, or c" or "at least one of a, b, and c" can both represent: a, b, c, ab (i.e., a and b), ac, bc, or abc, where a, b, and c can be single or multiple. "Parts representation," such as parts by weight or parts by mass, indicates the proportional relationship between components. In the proportional relationships discussed in this article, parameters described by proportion should be understood as the first term of the proportion in the order of description, while the proportion figure should be understood as the second term. For example, if the mass ratio of substance A, substance B, and substance C is 1:2:3, then substances A, B, and C should correspond one-to-one with the proportion figure in the proportion in the order of description, i.e., the mass of substance A : the mass of substance B : the mass of substance C = 1:2:3.

[0032] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods.

[0033] This application provides a medium-temperature stable stainless steel, which, by mass fraction, comprises the following chemical composition: Cr: 21%–30%, Mo: 5.1%–7.5%, Zr: 0.05%–0.5%, B: 0.015%–0.45%, Nb: 0.02%–2%, Si: 0.45%–1%, Mn: 0.1%–2.5%, C<0.008%, TO≤0.002%, N≤0.001%, and Fe.

[0034] The functions of each element are as follows:

[0035] Cr: A fundamental element ensuring the oxidation resistance and thermal expansion properties of stainless steel. A content below 21% cannot achieve the strong oxidizing atmosphere resistance required in this invention. Due to the solid solution effect of Cr, a content above 30% will lead to an excessively large coefficient of expansion, resulting in a significant difference from the coefficient of expansion of the solid electrolyte, deteriorating conductivity, and reducing battery life. For example, the Cr content can be 21%, 23%, 25%, 27%, 29%, 30%, etc.

[0036] Mo: The addition of Mo has three main effects. First, it significantly reduces the coefficient of thermal expansion of nickel-based alloys. Second, it acts as a solid solution strengthening element, improving the mechanical properties of the alloy, increasing the initial temperature of ferrite growth, and enhancing microstructural stability. Third, it readily forms the Laves phase during solidification. The optimal Mo content, considering these three synergistic effects, is 5.1%–7.5%. For example, the Mo content can be 5.1%, 5.5%, 6%, 6.5%, 7%, 7.5%, etc.

[0037] Si: A deoxidizer and a strong ferrite-forming element. Si can improve the high-temperature performance of steel and its corrosion resistance in strong oxidizing media (such as fuming nitric acid). It can also improve casting properties. For example, the Si content can be 0.45%, 0.55%, 0.65%, 0.75%, 0.85%, 0.95%, 1%, etc.

[0038] Nitrogen (Nb): A solid solution element and a ferrite-forming element. In stainless steel, Nb acts as a carbon stabilizer, preferentially combining with carbon to form stable carbides that are uniformly distributed in the matrix, preventing the formation of chromium carbides and thus preventing intergranular corrosion. It is also a constituent element of alumina (A) in the Laves phase B₂A. For example, the Nb content can be 0.02%, 0.1%, 0.3%, 0.5%, 0.8%, 1%, 1.5%, 1.8%, 2%, etc.

[0039] C: Significantly reduces resistance to intergranular corrosion. Therefore, reducing carbon content is the most effective measure to prevent intergranular corrosion. For example, the C content can be 0.007%, 0.006%, 0.005%, 0.004%, etc.

[0040] N: Significantly reduces resistance to intergranular corrosion. For example, the content of N can be 0.001%, 0.0009%, 0.0008%, 0.0007%, 0.0006%, etc.

[0041] Mn is an austenite-expanding and stabilizing element. Often, N and Mn are used together as a primary material to replace and conserve Ni. Mn can improve strength and increase the solubility of N in steel; however, Mn can promote the precipitation of the β phase, causing brittleness in the steel and negatively impacting its low-temperature toughness and weldability. In this invention, the Mn content is 0.1% to 2.5% to form a single ferrite structure with a low coefficient of thermal expansion, avoiding the formation of austenite structures with high coefficients of thermal expansion. For example, the Mn content can be 0.1%, 0.5%, 0.8%, 1.0%, 1.5%, 2.0%, 2.5%, etc.

[0042] Zr: The addition of Zr introduces numerous phase interfaces between the ferrite grains and Fe2Zr, significantly increasing the dislocation density in the ferrite grains. Dislocations propagate towards and interact with the phase boundaries. The phase boundaries continuously absorb these dislocations until they can no longer accommodate them. Therefore, interstitial spaces are created along the phase boundaries. Finally, Fe atoms gradually diffuse from the ferrite layers into these interstitial spaces, leading to grain growth and a gradual contraction of the interlayer spacing. Zr alloys achieve the effect of hindering grain boundary migration through a pinning effect and generate numerous low-stress concentration regions by increasing the proportion of phase boundaries, thereby reducing the likelihood of creep fracture. For example, the Zr content can be 0.05%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, etc.

[0043] Boron (B): Reduces alloy density and acts as a grain boundary strengthening element, increasing alloy plasticity and facilitating coordinated deformation of grain boundaries during hot working. It also improves the alloy's oxidation and creep resistance. Excessive B content leads to large-sized B precipitates at grain boundaries, deteriorating performance. For example, the B content can be 0.015%, 0.05%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, etc.

[0044] In some embodiments, the stainless steel further includes the following chemical composition: rare earth elements: 0.005% to 0.2%, wherein the rare earth elements include one or more of La, Ce, Sc and Y.

[0045] The addition of rare earth elements serves several purposes: first, it acts as a purifying agent, reducing the harmful effects of oxygen and sulfur at grain boundaries; second, it acts as a microalloying element, segregating at grain boundaries to strengthen them; and third, during operation at temperatures between 600 and 900°C, rare earth elements such as La segregated at grain boundaries inhibit the diffusion of chromium into the steel substrate, reducing the formation rate of the Cr2O3 film. Simultaneously, La, Y, and Sc segregated at grain boundaries accelerate the diffusion of oxygen into the steel substrate, thus forming a nail-wedge structure between the Cr2O3 oxide film and the substrate. This improves the adhesion between the Cr2O3 oxide film and the substrate, making the Cr2O3 oxide film less prone to detachment and thereby enhancing the material's oxidation resistance. In other words, the metallic compounds located between the Cr2O3 oxide film and the substrate, as well as elements such as La segregating at the grain boundaries, inhibit the diffusion of chromium out of the steel substrate, reducing the growth rate of the Cr2O3 oxide film. The metallic compounds, along with La and Sc, form a nail-wedge structure between the Cr2O3 oxide film and the substrate, improving the adhesion between the Cr2O3 oxide film and the substrate, making it less prone to detachment, thereby enhancing the oxidation resistance of the interconnect board. For example, the content of rare earth elements can be 0.005%, 0.01%, 0.05%, 0.1%, 0.15%, 0.2%, etc.

[0046] It should be noted that TO represents the total oxygen content.

[0047] Fe is the matrix element. The specific content / range of Fe can be obtained by the formula for the upper and lower limits of the components. That is, the sum of the percentage content of each component in a composition should be equal to 100%. The content range of several components should meet the following conditions: the upper limit of a certain component + the lower limit of other components ≤ 100; the lower limit of a certain component + the upper limit of other components ≥ 100.

[0048] In some embodiments, the chemical components satisfy the following relationship:

[0049] 6%≤[Mo]+[Nb]+3×[Si]≤13%

[0050] In the formula, [Mo] represents the mass fraction of Mo, [Nb] represents the mass fraction of Nb, and [Si] represents the mass fraction of Si.

[0051] A [Mo]+[Nb]+3×[Si] ratio below 6% is unfavorable for Laves phase nucleation and makes it difficult for the phase to effectively pin ferrite grain boundaries. A [Mo]+[Nb]+3×[Si] ratio above 13% easily leads to the formation of coarse Laves phases. Mo and Nb are the forming elements of the Laves phase, and the addition of Si can increase the precipitation temperature of the Laves phase. By matching the proportions of Mo, Nb, and Si elements, the precipitation temperature of the Laves phase can be raised to above 1000℃, thereby controlling the proportion of the Laves phase to be less than 2.5% and the maximum size to be less than 2μm. For example, the value of [Mo]+[Nb]+3×[Si] can be 6%, 7%, 8%, 9%, 11%, 13%, etc.

[0052] In some embodiments, the chemical components satisfy the following relationship:

[0053] 0.01≤([Zr]+1.1[B]) / ([Mo]+[Nb])≤0.2

[0054] In the formula, [Zr] represents the mass fraction of Zr, [B] represents the mass fraction of B, [Mo] represents the mass fraction of Mo, and [Nb] represents the mass fraction of Nb.

[0055] A ratio of ([Zr]+1.1[B]) / ([Mo]+[Nb]) less than 0.01 indicates ineffective action; a ratio greater than 0.2 suggests the formation of brittle borides. Mo and Nb are Laves phase-forming elements. Zr, B, and the Laves phase act as grain boundary pinning elements, eliminating the continuous growth of ferrite under intermediate temperature conditions. At intermediate temperatures of 600℃ to 900℃, grain boundary growth is controlled by adjusting the content of elements with grain boundary pinning effects, resulting in stable ferrite grain size at 4.5–7 levels, ensuring the stability of the microstructure. For example, the value of ([Zr]+1.1[B]) / ([Mo]+[Nb]) can be 0.01, 0.03, 0.05, 0.08, 0.1, 0.12, 0.15, 0.2, etc.

[0056] In summary, the embodiments of this application employ Fe-Cr-Mo-Nb-Zr-B ferritic stainless steel, with Cr-Mo-Nb solid solution strengthening. The reasonable matching of Cr, Mo, and Nb achieves the mechanical performance requirements of solid oxide fuel cells at intermediate temperatures. Mo and Nb are Laves phase-forming elements; during solidification, a B2A structure Laves brittle phase, namely (Fe,Cr)2(Mo,Nb), precipitates. B segregates at grain boundaries, improving grain boundary bonding strength; its appropriate addition can improve phase brittleness. Zr can improve both the room-temperature plasticity of the Laves phase and the intermediate-temperature plasticity of the alloy. The combined effect of B and Zr can improve the overall performance of the alloy at intermediate temperatures (600℃-900℃) and enhance its processing properties. By controlling the composition of Mo, Nb, Al, and W, as well as the solid solution process parameters, the proportion and size of the Laves phase are controlled, thus weakening the influence of Laves phase precipitation on the stability of the ferrite microstructure at intermediate temperatures. This ensures the alloy maintains a single ferrite microstructure, and the proportion of precipitated Laves phase does not exceed a certain limit, thereby stabilizing thermal expansion and mechanical properties. Simultaneously, Zr, B, and the Laves phase act as grain boundary pinning agents, eliminating the continuous growth of ferrite microstructure under intermediate temperature conditions. At intermediate temperatures of 600℃–900℃, the ferrite grain size remains stable at 4.5–7, ensuring the stability of the microstructure.

[0057] Figure 1 This is a schematic flowchart illustrating a method for preparing stainless steel, as provided in an embodiment of this application.

[0058] like Figure 1 As shown, this application provides a method for preparing stainless steel according to any one of the above embodiments, the method comprising:

[0059] S1. Vacuum refine the molten steel until the TO content of the molten steel is ≤0.002%, then add an alloy containing rare earth elements to the molten steel to obtain alloyed molten steel.

[0060] S2. The alloyed steel liquid is poured to obtain the stainless steel.

[0061] In step S1, when the alloy is added, the TO in the molten steel is ≤20ppm. At this time, the molten steel has been completely deoxidized, which can reduce the risk of the alloy being oxidized and improve the alloy yield.

[0062] In some embodiments, when the vacuum refining time is 0.6×T to 0.8×T, an alloy containing rare earth elements is added to the molten steel, specifically before tapping the steel in the later stages of refining. This can improve the yield of expensive rare earth elements. Here, T represents the vacuum refining cycle. If the alloy is added too early, the yield of the alloying elements will be reduced to some extent; if the alloy is added too late, it will lead to alloy inhomogeneity, resulting in uneven conductivity and oxidation resistance in the product.

[0063] The product prepared by the method of preparing stainless steel is the stainless steel described above. The chemical composition and structure of the stainless steel prepared by the method of preparing stainless steel can be referred to the above embodiments. Since the method of preparing stainless steel adopts some or all of the technical solutions of the stainless steel embodiments, it has at least all the beneficial effects brought about by the technical solutions of the stainless steel embodiments, which will not be elaborated here.

[0064] Based on a general inventive concept, this application provides a fuel cell interconnect, wherein the fuel cell interconnect plate is made of stainless steel as described in any of the above embodiments, and the alloy structure of the fuel cell interconnect is a single ferrite structure at room temperature to operating temperature, and the grain grade of the ferrite structure is 4.5 to 7.

[0065] Using Fe-Cr-Mo-Nb-Zr-B ferritic stainless steel as the base material and strengthening it with Cr-Mo-Nb solid solution, the alloy maintains the required mechanical properties at intermediate temperatures. This compositional design encourages the formation of a ferritic structure during solidification and subsequent processing. Simultaneously, Mo and Nb, as Laves phase-forming elements, precipitate brittle Laves phases with a B2A structure during solidification. However, by rationally controlling the composition of Mo, Nb, Al, and W, as well as the solid solution process parameters, the proportion and size of the Laves phase can be precisely controlled. This control not only weakens the impact of Laves phase precipitation on the stability of the ferritic structure but also ensures that the alloy maintains a single ferritic structure. The addition of Zr and B significantly affects the alloy's properties. Zr improves the room-temperature and intermediate-temperature plasticity of the Laves phase, while B segregates at grain boundaries, increasing the grain boundary bonding strength. The combined effect of these two elements not only improves the overall performance of the alloy at intermediate temperatures but also enhances its machinability. Furthermore, Zr, B, and the Laves phase have a grain boundary pinning effect, which helps to eliminate the continuous growth of ferrite under intermediate temperature operating conditions. This pinning effect ensures the stability of the ferrite grain size, allowing the ferrite grain size to remain stable at level 4.5–7 within the intermediate temperature range of 600℃–900℃. In summary, through reasonable alloy composition design, precise control of the Laves phase, the addition of Zr and B, and the grain boundary pinning effect, the alloy microstructure of the fuel cell interconnect is maintained as a single ferrite structure, and the ferrite grain size is stable at level 4.5–7, thereby ensuring the stability of the microstructure and excellent mechanical properties. For example, the ferrite grain size can be 4.5, 5, 5.5, 6, 6.5, 7, etc.

[0066] In some embodiments, fine Laves phase particles are dispersed in the matrix of the fuel cell interconnect, and the particle size of the Laves phase particles is <2μm, and the total amount of precipitated phase in the matrix of the fuel cell interconnect is <2.5%.

[0067] Finely dispersed Laves phase particles can act as a reinforcing phase, effectively hindering dislocation movement and thus improving the material's strength. Simultaneously, the fine particles help disperse crack propagation energy, improving the material's toughness and preventing brittle fracture. Furthermore, due to their small size and dispersed distribution, they have minimal impact on the material's plasticity, maintaining good ductility and processing properties. The total amount of precipitated phases refers to the sum of the volume fractions of all precipitated phases (including Laves phase, etc.) in the material. The matrix properties dominate the overall material properties, and the precipitated phases primarily function to strengthen and refine the grains. For example, the particle size of Laves phase particles can be 1.9 μm, 1.8 μm, 1.7 μm, 1.6 μm, 1.5 μm, etc., and the total amount of precipitated phases in a stainless steel matrix can be 2.4%, 2.3%, 2.2%, 2.1%, 2.0%, 1.9%, 1.8%, 1.7%, etc.

[0068] In some embodiments, the fuel cell interconnect plate satisfies at least one of the following performance characteristics:

[0069] Under an atmosphere of 20% O2-80% H2O, oxidation at 900℃ for 30,000 hours resulted in a total oxide film thickness of <10 μm.

[0070] Tensile strength Rm ≥ 260 MPa at 900℃;

[0071] The yield strength Rp0.2 at 900℃ is ≥210MPa.

[0072] The interconnect plate prepared in this embodiment exhibits good resistance to hydrogen embrittlement and oxidation during fuel cell operation, thereby improving the service life of the fuel cell.

[0073] Oxidation at 900℃ for 30,000 hours in an atmosphere of 20% O2-80% H2O resulted in a total oxide film thickness of <10μm. This long-term high-temperature oxidation test simulates the harsh conditions that fuel cells may encounter in actual operation, ensuring that the interconnect plate maintains stable performance under such conditions. Furthermore, the total oxide film thickness of <10μm further limits the oxidation rate of the interconnect plate during the high-temperature oxidation process, thus guaranteeing its long-term stability and reliability.

[0074] At 900℃, the tensile strength Rm ≥ 260MPa. Tensile strength is the maximum stress that a material can withstand during tensile testing and is one of the important indicators for evaluating the mechanical properties of a material. At high temperatures, the mechanical properties of materials often change; therefore, the interconnect plate must maintain sufficient tensile strength at high temperatures to ensure its stability and safety during fuel cell operation.

[0075] At 900℃, the yield strength Rp0.2 ≥ 210 MPa. Yield strength is the stress value at which a material begins to undergo plastic deformation during tensile testing. It reflects the material's ability to resist plastic deformation. The yield strength of a material also changes at high temperatures. Maintaining sufficient yield strength at high temperatures ensures that the interconnect board is not easily subjected to plastic deformation under external forces, thus preserving its structural stability and integrity.

[0076] Based on a general inventive concept, this application provides a method for preparing a fuel cell interconnect plate according to any of the above embodiments, the method comprising:

[0077] The stainless steel described in any of the above embodiments is subjected to hot rolling, cold rolling, stamping, solution treatment and water cooling in sequence to obtain a fuel cell interconnect plate.

[0078] Stainless steel materials are heated and rolled at high temperatures to achieve the desired shape and size. Rolling is then performed at or near room temperature to further refine the material's shape and size, and to improve its hardness and strength. The stainless steel material is then stamped into the desired shape and size using a die to prepare the initial form of the interconnect plate. Heating and holding at high temperatures allow the alloying elements in the stainless steel to dissolve into the matrix, forming a homogeneous solid solution. Simultaneously, the morphology and quantity of precipitated phases can be controlled by adjusting the cooling method (e.g., water cooling). Immediately after solution treatment, water cooling is performed to rapidly cool the material, prevent the growth of precipitated phases, and retain dispersed, fine second-phase particles.

[0079] In some embodiments, the solution treatment includes the following parameters: heating temperature of 1000℃~1100℃, holding time of 30min~45min, and hydrogen injection rate ≥20m³. 3 / h.

[0080] A heating temperature of 1000℃~1100℃ ensures that the alloying elements in the stainless steel fully dissolve into the matrix, forming a homogeneous solid solution. A holding time of 30min~45min ensures complete dissolution of the alloying elements while preventing excessive growth of precipitates. Water cooling is performed immediately after holding to rapidly cool the material. This prevents further growth of the precipitates, ensuring that Laves phase particles do not have time to grow, forming dispersed, fine second-phase particles. These fine particles improve the strength and toughness of the material. The amount of precipitates can be controlled by adjusting the parameters of the solution treatment and water cooling. A total precipitate content of less than 2.5% ensures good overall material performance and avoids excessive hardening and embrittlement. Furthermore, hydrogen spraying is employed during the solution treatment process, with a hydrogen spraying volume ≥20m³. 3 / h. Hydrogen can act as a reducing gas, preventing stainless steel from oxidizing during heat treatment. Simultaneously, hydrogen can promote the dissolution and uniform distribution of alloying elements. For example, the solution treatment temperature can be 1000℃, 1020℃, 1040℃, 1060℃, 1080℃, 1100℃, etc., the holding time can be 30min, 32min, 35min, 40min, 42min, 45min, etc., and the hydrogen injection rate can be 20m³. 3 / h、22m 3 / h, 25m 3 / h、28m 3 / h, 30m 3 / h etc.

[0081] The product prepared by the method of preparing the fuel cell interconnect plate is the aforementioned fuel cell interconnect plate. The chemical composition and microstructure of the fuel cell interconnect plate prepared by the method of preparing the fuel cell interconnect plate can be referred to the above embodiments. Since the method of preparing the fuel cell interconnect plate adopts some or all of the technical solutions of the fuel cell interconnect plate embodiments, it has at least all the beneficial effects brought about by the technical solutions of the fuel cell interconnect plate embodiments, which will not be elaborated here.

[0082] The advantages of the embodiments of this application are mainly reflected in the following aspects:

[0083] (1) Excellent oxidation resistance: With a reasonable ratio of Cr, Mo and Nb elements, stainless steel is oxidized at 900℃ for 30,000 hours in an atmosphere of 20% O2-80% H2O, and the total thickness of the oxide film is less than 10μm, showing excellent oxidation resistance and ensuring stability during long-term operation of fuel cells.

[0084] (2) High strength and good toughness: The tensile strength Rm≥260MPa and yield strength Rp0.2≥210MPa at 900℃ ensure the mechanical properties of stainless steel at high temperatures and meet the strength requirements of fuel cell interconnection plates. At the same time, the strength and toughness of the material are improved by solid solution strengthening and the strengthening effect of dispersed fine Laves phase particles, preventing brittle fracture.

[0085] (3) Stable thermal expansion performance: The addition of Cr and Mo elements and the reasonable alloy composition design make the thermal expansion coefficient of stainless steel match that of solid electrolyte, avoiding the deterioration of conductivity and reduction of battery life caused by excessive difference in thermal expansion coefficient.

[0086] (4) Good processing performance: The addition of Zr and B improves the processing performance of the alloy, making stainless steel easy to handle in hot rolling, cold rolling, stamping and forming processes, and maintaining good plasticity and toughness.

[0087] (5) Stability of the microstructure: By controlling the composition of Mo, Nb, Al, and W, as well as the solid solution process parameters, the proportion and size of the Laves phase were precisely controlled, which weakened the influence of Laves phase precipitation on the stability of the ferrite microstructure and ensured that the alloy maintained a single ferrite microstructure. At the same time, Zr, B, and the Laves phase had the effect of pinning grain boundaries, eliminating the continuous growth of the ferrite microstructure under medium-temperature working conditions, so that the ferrite grain size was stabilized at 4.5-7, ensuring the stability of the microstructure.

[0088] (6) Long life and high reliability: The excellent oxidation resistance, high strength and good toughness of stainless steel, as well as the stable thermal expansion properties and structural stability, together ensure the long life and high reliability of the fuel cell interconnect plate, and improve the overall performance and service life of the fuel cell.

[0089] (7) Flexibility of preparation methods: Detailed preparation methods are provided, including steps such as vacuum refining, casting, hot rolling, cold rolling, stamping, solution treatment and water cooling, which can be adjusted and optimized according to actual needs to adapt to different production conditions and requirements.

[0090] In summary, this invention achieves high performance and long lifespan of fuel cell interconnects through rational alloy composition design, precise Laves phase control, the addition of Zr and B, and grain boundary pinning, providing strong support for the development of fuel cell technology.

[0091] The present application is further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to industry standards. If there is no corresponding industry standard, then common international standards, conventional conditions, or conditions recommended by the manufacturer are followed.

[0092] This application provides a method for preparing an interconnect board, the specific process of which is as follows:

[0093] Step 1: Add industrial pure iron, Cr alloy, and C to a vacuum induction furnace for vacuum refining. The vacuum induction furnace is loaded with 3 tons of materials, the vacuum refining time is 60 minutes, and the vacuum refining temperature is 1500℃. The alloy containing Ce is added at the 40th minute of vacuum refining, at which point the TO content is 0.0015%.

[0094] Step 2: After the molten steel from Step 1 is poured and crystallized, stainless steel alloy ingots as shown in Table 1 and Table 2 are obtained. The stainless steel alloy ingots are heated at 1100℃, forged, and tempered multiple times to obtain flat billets.

[0095] Step 3: The flat billet is hot-rolled, annealed and softened, and then ground and welded to obtain a hot-rolled strip billet;

[0096] Step 4: The hot-rolled strip is subjected to billet opening, cold rolling, intermediate grinding, edge trimming and solution treatment to obtain strip material;

[0097] The hot rolling temperature is 1100℃. The hot rolling process includes: a first hot rolling pass, a second hot rolling pass, and a third hot rolling pass. The deformation amount in the first hot rolling pass is 40%, in the second hot rolling pass it is 60%, and in the third hot rolling pass it is 70%. The final rolling temperature is 910℃. After rolling, the material is water-cooled to room temperature.

[0098] Cold rolling includes: a first cold rolling, a second cold rolling and a third cold rolling, wherein the deformation amount of the first cold rolling is 50%, the deformation amount of the second cold rolling is 60%, and the deformation amount of the third cold rolling is 70%.

[0099] Step 5, Stamping: Heat treatment to 300℃, hydrogen injection rate 5m3 / h, hold for 20min, and then stamp.

[0100] Step 6: Solution treat the steel sheet formed by stamping in step S5, and then water cool it to room temperature. The parameters for solution treatment are shown in Table 3.

[0101] Table 1. Chemical composition (wt, %) of stainless steel alloy ingots, balance being Fe and unavoidable impurities.

[0102] serial number Cr Mo Zr B Nb Si Mn C TO N Example 1 21 5.1 0.05 0.015 0.02 0.45 0.1 0.006 0.002 0.001 Example 2 25 6 0.2 0.2 0.5 0.6 05 0.007 0.0018 0.0009 Example 3 27 6.5 0.3 0.3 1 0.8 1.5 0.005 0.0019 0.0008 Example 4 30 7.5 0.5 0.45 2 1 1 0.006 0.0018 0.0008 Example 5 23 6 0.1 0.1 0.2 0.5 2.5 0.007 0.0019 0.0009 Comparative Example 1 25 4 0.5 0.2 2 0.5 0.1 0.006 0.0020 0.001 Comparative Example 2 25 6 2 0.2 2 0.55 05 0.007 0.0016 0.001 Comparative Example 3 21 6 1 0.2 1 0.55 1.5 0.007 0.0017 0.001 Comparative Example 4 25 6 0.2 0.2 0.5 0.6 1 0.006 0.0018 0.001

[0103] Table 2 Relationship between the chemical compositions of stainless steel alloy ingots

[0104] serial number [Mo] + [Nb] + 3 × [Si] ([Zr]+1.1[B]) / ([Mo]+[Nb]) Example 1 6.47 0.01 Example 2 8.3 0.06 Example 3 9.9 0.08 Example 4 12.5 0.10 Example 5 7.7 0.03 Comparative Example 1 5.52 0.18 Comparative Example 2 9.65 0.28 Comparative Example 3 8.3 0.06 Comparative Example 4 9.9 0.08

[0105] Table 3 Parameters of solution treatment

[0106]

[0107]

[0108] The interconnect boards prepared in Examples 1 to 5 and Comparative Examples 1 to 4 were subjected to performance tests. Oxidation was performed at 900°C for 30,000 hours under a 20% O2-80% H2O atmosphere, and the total thickness of the oxide film was measured. The tensile strength Rm and yield strength Rp0.2 at 900°C were also measured. The results are shown in Table 4.

[0109] Table 4 shows the performance of the interconnect boards prepared in Examples 1-5 and Comparative Examples 1-4.

[0110]

[0111] As shown in Tables 1 to 4, Comparative Example 1 does not meet the requirement of 6% ≤ [Mo] + [Nb] + 3 × [Si] ≤ 13%, Comparative Example 2 does not meet the requirement of 0.01 ≤ ([Zr] + 1.1 [B]) / ([Mo] + [Nb]) ≤ 0.2, Comparative Example 3 does not meet the requirements of heat treatment temperature and time, and Comparative Example 4 does not meet the requirements of hydrogen injection amount for heat treatment. The antioxidant properties and mechanical properties are all lower than those of Examples 1 to 5.

[0112] Furthermore, one or more technical solutions in the embodiments of this application have at least the following technical effects or advantages:

[0113] In the embodiments of this application, from room temperature to operating temperature, the alloy structure is a single ferrite structure, and the finished grain size is maintained at grade 4.5 to 7. Under operating conditions of 900°C, the grain grade does not increase.

[0114] The fuel cell interconnect plate was oxidized at 900℃ for 30,000 hours in an atmosphere of 20% O2-80% H2O, and the total oxide film thickness was <10μm. The tensile strength Rm at 900℃ was ≥260MPa, and the yield strength Rp0.2 at 900℃ was ≥210MPa. The prepared interconnect plate has good resistance to hydrogen embrittlement and oxidation during fuel cell operation, which improves the service life of the fuel cell.

[0115] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A medium temperature tissue stable stainless steel, comprising the following chemical components in mass fraction: Cr: 21% to 30%, Mo: 5.1% to 7.5%, Zr: 0.05% to 0.5%, B: 0.015% to 0.45%, Nb: 0.02% to 2%, Si: 0.45% to 1%, Mn: 0.1% to 2.5%, C <0.008%, T.O ≤0.002%, N ≤0.001%, and Fe; the chemical components satisfy the following relations: 6% ≤ [Mo] + [Nb] + 3 × [Si] ≤ 13%; 0.01 ≤ ([Zr] + 1.1[B]) / ([Mo] + [Nb]) ≤ 0.2; wherein [Mo] represents the mass fraction of Mo, [Nb] represents the mass fraction of Nb, [Si] represents the mass fraction of Si, [Zr] represents the mass fraction of Zr, and [B] represents the mass fraction of B.

2. A fuel cell interconnect plate, which is made of the stainless steel of claim 1, and has a single ferrite structure and a grain size grade of 4.5 to 7 at room temperature to working temperature. Fine Laves phase particles are dispersed in the matrix of the fuel cell interconnect plate, and the size of the Laves phase particles is less than 2 μm, and the total amount of precipitated phases in the matrix of the fuel cell interconnect plate is less than 2.5%. The fuel cell interconnect plate satisfies at least one of the following properties: The total thickness of the oxide film is less than 10 μm after 900 °C oxidation for 30000 h in 20% O2-80% H2O atmosphere; 3. The fuel cell interconnect of claim 2, wherein The tensile strength Rm at 900 °C is greater than or equal to 260 MPa; 4. The fuel cell interconnect of claim 2, wherein The yield strength Rp0.2 at 900 °C is greater than or equal to 210 MPa.

5. A method for preparing the fuel cell interconnect plate of any one of claims 2 to 4, comprising: sequentially subjecting the stainless steel of claim 1 to hot rolling, cold rolling, punch forming, solid solution treatment, and water cooling to obtain the fuel cell interconnect plate. ​ ​ ​ ​ 6. The preparation method according to claim 5, characterized in that, The solid solution treatment comprises the following parameters: heating temperature is 1000-1100℃, holding time is 30-45 min, and hydrogen spraying amount is ≥20 m 3 / h.

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