Ferritic stainless steel for bipolar plate of fuel cell and preparation method of ferritic stainless steel

Abstract

The invention provides ferritic stainless steel for a fuel cell bipolar plate and a preparation method of the ferritic stainless steel, and relates to the technical field of ferritic stainless steel. The thickness of the ferritic stainless steel for the fuel cell bipolar plate is 0.02-0.3 mm, and the ferritic stainless steel comprises the following components in percentage by mass: less than or equal to 0.01% of C, less than 0.005% of N, 0.15-0.30% of Si, 0.10-0.40% of Mn, 20-40% of Cr, less than 0.005% of P, less than 0.005% of S, 0.15-0.5% of Nb, 0.03-0.2% of Ti and the balance of Fe. And the balance of Fe and inevitable impurities. The preparation method comprises the steps of raw material weighing, smelting, forging, hot rolling, homogenizing annealing, cold rolling after acid pickling and cold rolling annealing. The method synergistically improves plasticity, corrosion resistance and contact resistance, is simple and easy to operate, green and environment-friendly, low in cost, short in process, high in efficiency, wide in application range and beneficial to industrial large-scale production and popularization.

Description

Technical Field

[0001] The present invention relates to the technical field of ferritic stainless steel, and in particular to ferritic stainless steel for a fuel cell bipolar plate and a preparation method thereof. Background Art

[0002] Stainless steel bipolar plates play a vital role in fuel cells, especially in proton exchange membrane fuel cells (PEMFCs). The performance of bipolar plates directly affects the efficiency, cost and life of fuel cells, accounting for 60-80% of the total weight of the fuel cell system and 25-40% of the cost. As the conductive connection between battery cells, they are not only responsible for providing current, but also need to have good mechanical strength and corrosion resistance to cope with the harsh environment generated during battery operation. With the continuous pursuit of high-efficiency and more environmentally friendly energy solutions, the application scope of fuel cells is expanding, especially in the fields of transportation and renewable energy.

[0003] In recent years, the market demand for stainless steel bipolar plates has gradually increased. This trend is mainly due to its lower production cost, better mechanical strength and conductivity, which have obvious advantages over traditional graphite materials.

[0004] Market trends also show that the integration and miniaturization of fuel cell systems will continue to promote the development of stainless steel bipolar plates. With the advancement of technology, bipolar plate materials with both high performance and good corrosion resistance will be widely used in the new generation of fuel cells, thereby promoting the development and application of clean energy. At present, bipolar plates are mostly made of expensive materials such as austenitic stainless steel and titanium steel.

[0005] Chinese patent CN115029625A discloses a ferritic stainless steel for fuel cell bipolar plates and a preparation method thereof, which improves the conductivity of the passivation film on the surface of the stainless steel, controls the thickness of the passivation film, and reduces the surface contact resistance by composite addition of a large number of alloy elements; however, there are many alloy elements to choose from, the cost is high, the operation process is complicated, and the operation is difficult.

[0006] Chinese patent CN117431457A discloses an uncoated stainless steel bipolar plate for a fuel cell and a method for manufacturing the same, wherein more alloy elements are selected, and high-cost alloy elements account for a large proportion; and the formed carbides have complex compositions and uneven distribution. The corrosion resistance is improved by alloy composition design, but the plasticity of the prepared material is reduced.

[0007] Chinese patent CN110289428A discloses a method for preparing and surface modifying stainless steel bipolar plates for fuel cells. The conductivity and corrosion resistance of the bipolar plates are improved by spraying a nanocarbon / PI / PF mixed coating with a carbon content of 10-30% and a nanocarbon / graphite / PI / PF mixed coating with a carbon content of 40-70% through nitrogen atomization. Obviously, although this method adds an adhesive to increase the viscosity between the coating and the bipolar plate, the uniformity and density of the coating are difficult to control, and the adhesion is also poor.

[0008] Chinese patent CN115275251A discloses a composite bipolar plate for fuel cells, and a preparation method and application thereof, which mainly treats the surface of the bipolar plate to form an alloy layer. However, since the preparation of this alloy layer requires embedding the stainless steel bipolar plate in a surface treatment agent under a vacuum environment, a part of the surface treatment agent is wasted; and the surface treatment agent mainly contains chromium, and the prepared chromium-containing passivation film has technical defects such as uneven thickness, local missing, and poor adhesion. Summary of the invention

[0009] In order to solve the technical problems caused by the fact that the existing fuel cell stainless steel bipolar plates are mostly made of expensive materials such as austenitic stainless steel and titanium steel; or adding high-cost multiple alloy elements to improve conductivity and corrosion resistance, or treating the surface of the bipolar plate to form an alloy layer, or spraying to prepare an alloy coating, or adjusting the process to improve the conductivity and corrosion resistance of the bipolar plate surface, these methods more or less have the technical problems of difficult operation, high cost, low efficiency, and conductivity and corrosion resistance cannot be synergistically improved under the condition of high mechanical properties. The present invention proposes a ferritic stainless steel for fuel cell bipolar plates and a preparation method thereof that can solve the aforementioned problems. The technical scheme is as follows:

[0010] A ferritic stainless steel for a fuel cell bipolar plate, wherein the thickness of the ferritic stainless steel for a fuel cell bipolar plate is 0.02-0.3 mm, and the composition contents are as follows by mass percentage: C≤0.01%, N<0.005%, Si 0.15-0.30%, Mn0.10-0.40%, Cr 20-40%, P<0.005%, S<0.005%, Nb 0.15-0.5%, Ti 0.03-0.2%; the rest are Fe and unavoidable impurities.

[0011] Optionally, the composition of the ferritic stainless steel for fuel cell bipolar plates further includes one or a combination of several alloy elements selected from the group consisting of 0.005-0.5% Sn, 0.001-0.1% Sb, 0.05-0.3% Ru and Ni≤5.0%.

[0012] Optionally, the organizational structure of the ferritic stainless steel for the fuel cell bipolar plate is that the matrix organization is a ferrite organization, the shape of the ferrite organization is equiaxed, the average grain size is 10-50 μm, and C+N is less than 150 ppm.

[0013] Optionally, the ferritic stainless steel for the fuel cell bipolar plate has a tensile strength of 450-650 MPa, a yield strength of 300-600 MPa, and an elongation of not less than 20%. 2 SO 4 +0.1mg / L HF solution with a scan rate of 1mV / s and a corrosion current density of ≤10μA / cm at an open circuit potential of 0.44V-1.6V 2 , at 140N / cm 2 The contact resistance is ≤25mΩ·cm under the contact pressure 2 .

[0014] Optionally, the fuel cell bipolar plate is made of ferritic stainless steel with a vacuum ion plating Ti-C film or Ti-N film on the surface, the film thickness is 5-25 μm, the overall tensile strength is 450-650 MPa, the yield strength is 300-600 MPa, and the elongation is 20-40%. The stainless steel is 0.5 mM H 2 SO 4 +0.1mg / L HF solution with a scan rate of 1mV / s and a corrosion current density of ≤4μA / cm at an open circuit potential of 0.44V-1.6V 2 , at 140N / cm 2 The contact resistance is ≤1.25mΩ·cm under the contact pressure 2 After coating, the corrosion current density is reduced by 60-90%, and the contact resistance is reduced by more than 90%.

[0015] Optionally, the fuel cell bipolar plate is made of ferritic stainless steel with a vacuum ion-plated Ti-C film on the surface, the film thickness is 5-25 μm, the overall tensile strength is 450-650 MPa, the yield strength is 300-600 MPa, the elongation is 20-40%, and the stainless steel is 0.5 mM H 2 SO 4 In +0.1mg / L HF solution with a scan rate of 1mV / s, the corrosion current density at the open circuit potential of 0.44V-1.6V is reduced by more than 65% and the contact resistance is reduced by more than 90% compared with before the Ti-C film is added. The minimum corrosion current density can reach 0.3 and the minimum contact resistance can reach 3.25.

[0016] Optionally, the fuel cell bipolar plate is made of ferritic stainless steel with a vacuum ion plating Ti-N film on the surface, the film thickness is 5-25 μm, the overall tensile strength is 450-650 MPa, the yield strength is 300-600 MPa, the elongation is 20-40%, and the stainless steel is 0.5 mM H 2 SO 4 In +0.1mg / L HF solution with a scan rate of 1mV / s, the corrosion current density at an open circuit potential of 0.44V-1.6V is reduced by more than 80% and the contact resistance is reduced by more than 90% compared with before the addition of Ti-N film. The minimum corrosion current density can reach 0.2 and the minimum contact resistance can reach 3.4.

[0017] A method for preparing ferritic stainless steel for a fuel cell bipolar plate is provided. The method for preparing ferritic stainless steel for a fuel cell bipolar plate comprises the following steps:

[0018] S1. Weighing and smelting raw materials: weighing raw materials according to the chemical composition content of ferritic stainless steel for fuel cell bipolar plates, melting and casting the weighed raw materials to obtain stainless steel ingots;

[0019] S2, forging: forging the S1 stainless steel ingot to obtain a stainless steel forging blank;

[0020] S3, hot rolling: remove the surface oxide scale of the S2 stainless steel forging billet, and then hot roll it to obtain a hot-rolled slab;

[0021] S4, homogenization annealing: homogenization annealing is performed on the hot-rolled slab in S3 to obtain a hot-rolled plate with uniform structure after annealing;

[0022] S5, cold rolling after pickling: pickling the hot-rolled plate with uniform structure after annealing in S4, then grinding the surface, and finally cold rolling to obtain a cold-rolled thin steel plate;

[0023] S6, cold rolling annealing: The S5 cold rolled thin steel is annealed at high temperature for a short time to obtain a ferritic stainless steel sheet.

[0024] Optionally, the preparation method also includes surface treatment S7, which is to mechanically polish the ferritic stainless steel sheet of S6, clean and dry the mechanically polished sample, and then electrolytically polish it in an electrolytic solution or perform vacuum ion plating of any one of Ti-C and Ti-N, and the Ti layer and the N layer are alternately and evenly plated on the substrate.

[0025] Optionally, the ingot size of S1 stainless steel is 350×250×75-290×190×60mm, the forging size of S2 stainless steel is 150×130×40-110×100×35mm, the size of S3 hot-rolled plate is 1000×150×1-2000×200×3mm, and the size of S5 cold-rolled thin steel plate and S6 ferritic stainless steel thin plate is 1000×50×0.02-1500×65×0.3mm.

[0026] Optionally, the heating rate of S3 hot rolling is 20-35°C / min, the number of hot rolling passes is 7-10, the reduction in each pass is 25-35%, the total reduction is 85-95%, and the thickness after hot rolling is 1-3 mm.

[0027] Optionally, the temperature of S4 homogenization annealing is 900-1050° C., and the annealing time is 1-60 min.

[0028] Optionally, the S5 cold rolling is performed for 10-15 passes, the reduction in each pass is 30-35%, the total reduction is not less than 90%, and the thickness after cold rolling is 0.02-0.3 mm.

[0029] Optionally, the annealing temperature of S6 high temperature short time annealing is 900-1050° C., and the annealing time is 1-10 min.

[0030] Optionally, the S7 electrolytic solution is a solution with a volume percentage of phosphoric acid: sulfuric acid: deionized water = 23:11:6, the roughness after electrolytic polishing Ra≤0.1μm, the Ti layer thickness of Ti-C is 2-15μm, and the C layer thickness is 5-20μm; the Ti layer thickness of Ti-N is 2-15μm, and the N layer thickness is 5-20μm, and the Ti layer and the N layer are plated alternately.

[0031] Optionally, after S7 electrolytic polishing, the corrosion current density is reduced by more than 80% compared with the ferritic stainless steel sheet that has not passed through S6, and the contact resistance is reduced by more than 70% compared with the ferritic stainless steel sheet that has not passed through S6; the corrosion current density after S7 vacuum ion plating Ti-C film is reduced by more than 65% compared with the ferritic stainless steel sheet that has not passed through S6, and the contact resistance is reduced by more than 90% compared with the ferritic stainless steel sheet that has not passed through S6; the corrosion current density after S7 vacuum ion plating Ti-N film is reduced by more than 80% compared with the ferritic stainless steel sheet that has not passed through S6, and the contact resistance is reduced by more than 90% compared with the ferritic stainless steel sheet that has not passed through S6.

[0032] The technical principle of the composition selection of the present invention:

[0033] C: Carbon is an impurity element in steel. Solid solution in stainless steel can reduce the plasticity of the steel and reduce corrosion resistance. Considering the refining cost, carbon cannot be completely removed. The carbon content of the present invention is controlled at C≤0.01%.

[0034] N: Nitrogen in stainless steel can increase the strength of steel, but is not conducive to formability, so the nitrogen content is controlled at N < 0.005%.

[0035] Si: Silicon can play a role of deoxidation in ferritic stainless steel. At the same time, as an alloying element, it increases the strength of the steel to a certain extent and reduces the processability of the material. In the present invention, the silicon content is controlled at 0.15-0.30%.

[0036] Mn: Manganese has the function of fixing sulfur and improving hot workability. Too high a manganese content will impair corrosion resistance. In the present invention, the manganese content is controlled to be 0.10-0.40%.

[0037] P and S: Sulfur and phosphorus are considered harmful elements in stainless steel and should be controlled as low as possible.

[0038] Cr: Chromium is the most important alloying element in stainless steel. The higher the chromium content, the better the corrosion resistance. 2 O 3 The dense oxide film improves the corrosion resistance of the steel. In ferritic stainless steel, when the Cr content exceeds 40%, it is difficult to mass produce. In the present invention, the chromium content is 20-40%.

[0039] Nb and Ti: exist in ferritic stainless steel in the form of solid solution and precipitate, especially in combination with C and N, they can reduce C and N, and also improve the processability and mechanical properties of steel. In the present invention, the niobium content is controlled at 0.15-0.5%, and the titanium content is controlled at 0.03-0.1%.

[0040] Sn: Tin is enriched in the passive film on the surface of stainless steel in the form of metallic tin or tin oxide, which can improve the repassivation ability of the passive film, but excessive addition inhibits its hot workability. In the present invention, the tin content is controlled at 0.005-0.5%.

[0041] Sb: Antimony helps improve the compactness of the passivation film in a high-oxygen environment, and can improve the corrosion resistance in a reducing environment through a synergistic effect with tin. Considering the high volatility of antimony, it is easy to be lost during the smelting process, resulting in increased costs. In the present invention, the antimony content is controlled at 0.001-0.1%.

[0042] Ni: Continuous casting of stainless steel with high chromium content often has poor plasticity, so adding a proper amount of nickel to stainless steel can reduce the tough-to-brittle transition temperature of the casting to avoid cracking of the casting. Considering the cost increase caused by Ni, the nickel content is controlled below 5.0%.

[0043] Ru: Ruthenium reduces the stacking fault energy of super high Cr ferrite stainless steel in stainless steel, can increase the uniformity of deformation, thereby improving its plastic deformation ability, and at the same time, ruthenium can improve the stability of the passivation film and reduce the interface contact resistance. Considering the high cost, the ruthenium content is controlled at 0.1-0.3%.

[0044] Preferably, the addition amount of Nb and Ti satisfies: w(Ti+Nb)≥0.2+4×w(C+N)≤0.8. The addition of Nb and Ti can stabilize carbon and nitrogen elements, and improve the uniformity and formability of stainless steel structure through the dispersion and precipitation of carbonitrides. During the high-temperature cooling process of stainless steel, carbon and nitrogen combine with Nb and Ti as much as possible to form carbon and nitrogen compounds, reducing the formation of Cr by combining C with Cr. 23 C 6 type precipitation tendency, thus avoiding the precipitation of Cr at the grain boundary 23 C 6 The addition of these alloying elements ensures that the steel has good corrosion resistance and high processability at the same time.

[0045] Compared with the prior art, the above technical solution has at least the following beneficial effects:

[0046] The above scheme, the present invention proposes a ferritic stainless steel for fuel cell bipolar plates and a preparation method based on the above, which can solve the technical problems caused by the fact that the fuel cell stainless steel bipolar plates in the prior art are mostly made of expensive materials such as austenitic stainless steel and titanium steel to make bipolar plates; or adding high-cost multiple alloy elements to improve conductivity and corrosion resistance, or treating the surface of the bipolar plate to form an alloy layer, or spraying to prepare an alloy coating, or adjusting the process to improve the conductivity and corrosion resistance of the bipolar plate surface. These methods more or less have the technical problems of difficult operation, high cost, low efficiency, and the inability to synergistically improve conductivity and corrosion resistance under high mechanical properties.

[0047] The present invention adds simple alloy elements and adjusts the content ratio. During the high-temperature cooling process of stainless steel, carbon and nitrogen are combined with Nb and Ti as much as possible to form carbon and nitrogen compounds, and the formation of Cr by combining C with Cr is reduced. 23 C 6 type precipitation tendency, thus avoiding the precipitation of Cr at the grain boundary 23 C 6 Type compounds lead to a decrease in corrosion resistance due to chromium depletion at the grain boundaries.

[0048] The present invention adds Sn, Sb, Ru and Ni, so that the stability, corrosion resistance and contact resistance of the passivation film can be effectively controlled and reach or even exceed the expected goals, thereby being able to better synergistically improve the conductivity and corrosion resistance of the bipolar plate.

[0049] The ferritic stainless steel for the fuel cell bipolar plate prepared by the present invention has a tensile strength of 450-650 MPa, a yield strength of 300-600 MPa, and an elongation of not less than 20%. 2 SO 4 +0.1mg / L HF solution with a scan rate of 1mV / s and a corrosion current density of ≤10μA / cm at an open circuit potential of 0.44V-1.6V 2 , at 140N / cm 2 The contact resistance is ≤25mΩ·cm under the contact pressure 2 .

[0050] The ferrite stainless steel for fuel cell bipolar plates prepared by the present invention has a Ti-C film or a Ti-N film vacuum ion-plated on the surface, the film thickness is 5-25 μm, the overall tensile strength is 450-650 MPa, the yield strength is 300-600 MPa, the elongation is 20-40%, and the stainless steel has a high thermal conductivity at 0.5 mM H 2 SO 4 +0.1mg / L HF solution with a scan rate of 1mV / s and a corrosion current density of ≤4μA / cm at an open circuit potential of 0.44V-1.6V 2 , at 140N / cm 2 The contact resistance is ≤1.25mΩ·cm under the contact pressure 2 After coating, the corrosion current density is reduced by 60-90%, and the contact resistance is reduced by more than 90%.

[0051] The Ti-C film is vacuum ion plated on the surface of the ferrite stainless steel for the fuel cell bipolar plate prepared by the present invention, the film thickness is 5-25 μm, the overall tensile strength is 450-650 MPa, the yield strength is 300-600 MPa, the elongation is 20-40%, and the stainless steel is 0.5 mM H 2 SO 4 In +0.1mg / L HF solution with a scan rate of 1mV / s, the corrosion current density at the open circuit potential of 0.44V-1.6V is reduced by more than 65% and the contact resistance is reduced by more than 90% compared with before the Ti-C film is added. The minimum corrosion current density can reach 0.3 and the minimum contact resistance can reach 3.25.

[0052] The Ti-N film prepared by the present invention on the surface of the ferrite stainless steel for the fuel cell bipolar plate has a thickness of 5-25 μm, an overall tensile strength of 450-650 MPa, a yield strength of 300-600 MPa, and an elongation of 20-40%. The stainless steel has a high thermal conductivity and a high thermal conductivity. 2 SO 4 In +0.1mg / L HF solution with a scan rate of 1mV / s, the corrosion current density at an open circuit potential of 0.44V-1.6V is reduced by more than 80% and the contact resistance is reduced by more than 90% compared with before the addition of Ti-N film. The minimum corrosion current density can reach 0.2 and the minimum contact resistance can reach 3.4.

[0053] In summary, compared with other traditional methods, the method of the present invention can improve the plasticity, corrosion resistance and contact resistance of ferritic stainless steel by controlling alloy composition, rolling process and surface treatment process; the method is simple and easy to operate, green and environmentally friendly, low in cost, short in process, high in efficiency, wide in application range, and conducive to large-scale industrial production and promotion. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0055] Figure 1 This is a microstructure diagram of the ferritic stainless steel for the fuel cell bipolar plate according to Example 1 of the present invention;

[0056] Figure 2 It is a comparison diagram of engineering stress-strain curves of the ferritic stainless steel for fuel cell bipolar plates according to Examples 1-6 of the present invention. DETAILED DESCRIPTION

[0057] The technical solution of the present invention is described below in conjunction with the accompanying drawings.

[0058] In the embodiments of the present invention, words such as "exemplarily" and "for example" are used to indicate examples, illustrations or explanations. Any embodiment or design described as "example" in the present invention should not be interpreted as being more preferred or more advantageous than other embodiments or designs. Specifically, the use of the word "example" is intended to present the concept in a specific way. In addition, in the embodiments of the present invention, the meaning expressed by "and / or" can be both, or it can be either of the two.

[0059] In the embodiments of the present invention, "image" and "picture" may sometimes be used interchangeably. It should be noted that when the distinction between them is not emphasized, the meanings they intend to express are consistent.

[0060] In the embodiments of the present invention, sometimes the subscripts such as W 1 It may be written in non-subscript form such as W1. When the difference is not emphasized, the meaning is the same.

[0061] In order to make the technical problems, technical solutions and advantages to be solved by the present invention more clear, a detailed description will be given below with reference to the accompanying drawings and specific embodiments.

[0062] A ferritic stainless steel for a fuel cell bipolar plate, wherein the thickness of the ferritic stainless steel for a fuel cell bipolar plate is 0.02-0.3 mm, and the composition contents are as follows by mass percentage: C≤0.01%, N<0.005%, Si 0.15-0.30%, Mn0.10-0.40%, Cr 20-40%, P<0.005%, S<0.005%, Nb 0.15-0.5%, Ti 0.03-0.2%; the rest are Fe and unavoidable impurities.

[0063] In particular, the composition of the ferritic stainless steel for fuel cell bipolar plates further includes one or a combination of several alloy elements selected from the group consisting of 0.005-0.5% Sn, 0.001-0.1% Sb, 0.05-0.3% Ru and Ni≤5.0%.

[0064] Particularly, the organizational structure of the ferritic stainless steel for the fuel cell bipolar plate is that the matrix organization is a ferrite organization, the shape of the ferrite organization is equiaxed, the average grain size is 10-50 μm, and C+N is less than 150 ppm.

[0065] In particular, the ferritic stainless steel for the fuel cell bipolar plate has a tensile strength of 450-650 MPa, a yield strength of 300-600 MPa, and an elongation of not less than 20%. 2 SO 4 +0.1mg / L HF solution with a scan rate of 1mV / s and a corrosion current density of ≤10μA / cm at an open circuit potential of 0.44V-1.6V 2 , at 140N / cm 2 The contact resistance is ≤25mΩ·cm under the contact pressure 2 .

[0066] In particular, the surface of the ferritic stainless steel for the fuel cell bipolar plate is vacuum ion plated with a Ti-C film or a Ti-N film, the film thickness is 5-25 μm, the overall tensile strength is 450-650 MPa, the yield strength is 300-600 MPa, the elongation is 20-40%, and the stainless steel is 0.5 mM H 2 SO 4 +0.1mg / L HF solution with a scan rate of 1mV / s and a corrosion current density of ≤4μA / cm at an open circuit potential of 0.44V-1.6V 2 , at 140N / cm 2 The contact resistance is ≤1.25mΩ·cm under the contact pressure 2 After coating, the corrosion current density is reduced by 60-90%, and the contact resistance is reduced by more than 90%.

[0067] In particular, the fuel cell bipolar plate is provided with a Ti-C film on the surface of a ferritic stainless steel vacuum ion-plated, the film layer thickness is 5-25 μm, the overall tensile strength is 450-650 MPa, the yield strength is 300-600 MPa, the elongation is 20-40%, and the stainless steel is resistant to oxidative stress at 0.5 mM H 2 SO 4 In +0.1mg / L HF solution with a scan rate of 1mV / s, the corrosion current density at the open circuit potential of 0.44V-1.6V is reduced by more than 65% and the contact resistance is reduced by more than 90% compared with before the Ti-C film is added. The minimum corrosion current density can reach 0.3 and the minimum contact resistance can reach 3.25.

[0068] In particular, the fuel cell bipolar plate is provided with a Ti-N film on the surface of a ferritic stainless steel vacuum ion-plated, the film thickness is 5-25 μm, the overall tensile strength is 450-650 MPa, the yield strength is 300-600 MPa, the elongation is 20-40%, and the stainless steel is resistant to oxidative stress at 0.5 mM H 2 SO 4 In +0.1mg / L HF solution with a scan rate of 1mV / s, the corrosion current density at an open circuit potential of 0.44V-1.6V is reduced by more than 80% and the contact resistance is reduced by more than 90% compared with before the addition of Ti-N film. The minimum corrosion current density can reach 0.2 and the minimum contact resistance can reach 3.4.

[0069] A method for preparing ferritic stainless steel for a fuel cell bipolar plate is provided. The method for preparing ferritic stainless steel for a fuel cell bipolar plate comprises the following steps:

[0070] S1. Weighing and smelting raw materials: weighing raw materials according to the chemical composition content of ferritic stainless steel for fuel cell bipolar plates, melting and casting the weighed raw materials to obtain stainless steel ingots;

[0071] S2, forging: forging the S1 stainless steel ingot to obtain a stainless steel forging blank;

[0072] S3, hot rolling: remove the surface oxide scale of the S2 stainless steel forging billet, and then hot roll it to obtain a hot-rolled slab;

[0073] S4, homogenization annealing: homogenization annealing is performed on the hot-rolled slab in S3 to obtain a hot-rolled plate with uniform structure after annealing;

[0074] S5, cold rolling after pickling: pickling the hot-rolled plate with uniform structure after annealing in S4, then grinding the surface, and finally cold rolling to obtain a cold-rolled thin steel plate;

[0075] S6, cold rolling annealing: The S5 cold rolled thin steel is annealed at high temperature for a short time to obtain a ferritic stainless steel sheet.

[0076] In particular, the preparation method also includes surface treatment of S7, which is to mechanically polish the ferritic stainless steel sheet of S6, clean and dry the mechanically polished sample, and then electrolytically polish it in an electrolytic solution or perform vacuum ion plating of any one of Ti-C and Ti-N, and the Ti layer and the N layer are alternately and evenly plated on the substrate.

[0077] In particular, the ingot size of S1 stainless steel is 350×250×75-290×190×60mm, the forging size of S2 stainless steel is 150×130×40-110×100×35mm, the size of S3 hot-rolled plate is 1000×150×1-2000×200×3mm, and the size of S5 cold-rolled thin steel plate and S6 ferritic stainless steel thin plate is 1000×50×0.02-1500×65×0.3mm.

[0078] In particular, the heating rate of S3 hot rolling is 20-35°C / min, the number of hot rolling passes is 7-10, the reduction in each pass is 25-35%, the total reduction is 85-95%, and the thickness after hot rolling is 1-3 mm.

[0079] In particular, the temperature of the S4 homogenization annealing is 900-1050° C., and the annealing time is 1-60 min.

[0080] In particular, the number of passes of S5 cold rolling is 10-15, the reduction in each pass is 30-35%, the total reduction is not less than 90%, and the thickness after cold rolling is 0.02-0.3mm.

[0081] In particular, the annealing temperature of S6 high temperature short time annealing is 900-1050°C, and the annealing time is 1-10min.

[0082] In particular, the S7 electrolytic solution is a solution with a volume percentage of phosphoric acid: sulfuric acid: deionized water = 23:11:6, the roughness after electrolytic polishing Ra≤0.1μm, the Ti layer thickness of Ti-C is 2-15μm, and the C layer thickness is 5-20μm; the Ti layer thickness of Ti-N is 2-15μm, and the N layer thickness is 5-20μm, and the Ti layer and the N layer are plated alternately.

[0083] In particular, after S7 electrolytic polishing, the corrosion current density was reduced by more than 80% compared with the ferritic stainless steel sheet that had not passed S6, and the contact resistance was reduced by more than 70% compared with the ferritic stainless steel sheet that had not passed S6; the corrosion current density of S7 after vacuum ion plating Ti-C film was reduced by more than 65% compared with the ferritic stainless steel sheet that had not passed S6, and the contact resistance was reduced by more than 90% compared with the ferritic stainless steel sheet that had not passed S6; the corrosion current density of S7 after vacuum ion plating Ti-N film was reduced by more than 80% compared with the ferritic stainless steel sheet that had not passed S6, and the contact resistance was reduced by more than 90% compared with the ferritic stainless steel sheet that had not passed S6.

[0084] Examples 1-6

[0085] The composition of the ferritic stainless steel for the fuel cell bipolar plates of Examples 1-6 is shown in Table 1 below:

[0086] Table 1

[0087]

[0088] A method for preparing ferritic stainless steel for fuel cell bipolar plates comprises the following steps:

[0089] S1. Weighing and smelting raw materials: weighing raw materials according to the chemical composition content of ferritic stainless steel for fuel cell bipolar plates, melting and casting the weighed raw materials to obtain 50 kg stainless steel ingots;

[0090] S2, forging: the 50kg stainless steel ingot in S1 is kept at 1200°C for 2h and then forged, and then cut into 35×100×110mm billets after forging to obtain stainless steel forging billets;

[0091] S3, hot rolling: remove the surface oxide scale of the S2 stainless steel forging billet, and then perform hot rolling, the heating rate of hot rolling is 20-35℃ / min, the number of hot rolling passes is 7-10, the reduction amount of each pass is 25-35%, the total reduction amount is 85-95%, the thickness after hot rolling is 1-3mm, and the hot rolled slab is obtained;

[0092] S4, homogenization annealing: homogenization annealing is performed on the hot-rolled slab in S3, the annealing temperature is 900-1050°C, the annealing time is 1-60min, and a hot-rolled plate with uniform structure is obtained after annealing;

[0093] S5, cold rolling after pickling: pickling the hot-rolled plate with uniform structure after S4 annealing, then grinding the surface, and finally cold rolling, the number of cold rolling passes is 10-15, the reduction amount of each pass is 30-35%, the total reduction amount is not less than 90%, the thickness after cold rolling is 0.02-0.3mm, and the cold-rolled thin steel plate is obtained;

[0094] S6, cold rolling annealing: the S5 cold rolled thin steel is subjected to high temperature short time annealing, the annealing temperature is 900-1050°C, the annealing time is 1-10min, and a ferritic stainless steel thin plate is obtained;

[0095] S7. Surface treatment: Mechanically polish the ferritic stainless steel sheet of S6. After cleaning and drying the mechanically polished sample, electrolytic polishing is performed in an electrolytic solution or vacuum ion plating of either Ti-C or Ti-N is performed. Ti layers and N layers are alternately and evenly plated on the substrate.

[0096] The process parameters of the preparation methods of Examples 1-6 are specifically selected as shown in Table 2 below. The material structure of Example 1 is as follows: Figure 1 As shown:

[0097] Table 2

[0098]

[0099] In Examples 2 and 3, the S7 electrolytic solution is a solution with a volume percentage of phosphoric acid: sulfuric acid: deionized water = 23:11:6, the roughness after electrolytic polishing Ra≤0.1μm, the Ti layer thickness of Ti-C is 2-15μm, and the C layer thickness is 5-20μm; the Ti layer thickness of Ti-N is 2-15μm, and the N layer thickness is 5-20μm, and the Ti layer and the N layer are plated alternately.

[0100] The properties of the bipolar plates prepared in Examples 1-6 are shown in Table 3 below, where the mechanical properties are compared. Figure 2 As shown:

[0101] Table 3

[0102]

[0103] In Table 3, the corrosion current density is the corrosion current density of the stainless steel in 0.5 mM H 2 SO 4 +0.1mg / L HF solution with a scan rate of 1mV / s and a current density at an open circuit potential of 0.44V-1.6V; the contact resistance is 140N / cm2 The contact resistance obtained under the contact pressure.

[0104] After S7 electrolytic polishing in Examples 2 and 3, the corrosion current density was reduced by more than 80% compared with the ferritic stainless steel sheet that had not passed through S6, and the contact resistance was reduced by more than 70% compared with the ferritic stainless steel sheet that had not passed through S6; the corrosion current density after S7 vacuum ion plating Ti-C film was reduced by more than 65% compared with the ferritic stainless steel sheet that had not passed through S6, and the contact resistance was reduced by more than 90% compared with the ferritic stainless steel sheet that had not passed through S6; the corrosion current density after S7 vacuum ion plating Ti-N film was reduced by more than 80% compared with the ferritic stainless steel sheet that had not passed through S6, and the contact resistance was reduced by more than 90% compared with the ferritic stainless steel sheet that had not passed through S6.

[0105] The above scheme, the present invention proposes a ferritic stainless steel for fuel cell bipolar plates and a preparation method based on the above, which can solve the technical problems caused by the fact that the fuel cell stainless steel bipolar plates in the prior art are mostly made of expensive materials such as austenitic stainless steel and titanium steel to make bipolar plates; or adding high-cost multiple alloy elements to improve conductivity and corrosion resistance, or treating the surface of the bipolar plate to form an alloy layer, or spraying to prepare an alloy coating, or adjusting the process to improve the conductivity and corrosion resistance of the bipolar plate surface. These methods more or less have the technical problems of difficult operation, high cost, low efficiency, and the inability to synergistically improve conductivity and corrosion resistance under high mechanical properties.

[0106] The present invention adds simple alloy elements and adjusts the content ratio. During the high-temperature cooling process of stainless steel, carbon and nitrogen are combined with Nb and Ti as much as possible to form carbon and nitrogen compounds, and the formation of Cr by combining C with Cr is reduced. 23 C 6 type precipitation tendency, thus avoiding the precipitation of Cr at the grain boundary 23 C 6 Type compounds lead to a decrease in corrosion resistance due to chromium depletion at the grain boundaries.

[0107] The present invention adds Sn, Sb, Ru and Ni, so that the stability, corrosion resistance and contact resistance of the passivation film can be effectively controlled and reach or even exceed the expected goals, thereby being able to better synergistically improve the conductivity and corrosion resistance of the bipolar plate.

[0108] The ferritic stainless steel for the fuel cell bipolar plate prepared by the present invention has a tensile strength of 450-650 MPa, a yield strength of 300-600 MPa, and an elongation of not less than 20%. 2 SO 4+0.1mg / L HF solution with a scan rate of 1mV / s and a corrosion current density of ≤10μA / cm at an open circuit potential of 0.44V-1.6V 2 , at 140N / cm 2 The contact resistance is ≤25mΩ·cm under the contact pressure 2 .

[0109] The ferrite stainless steel for fuel cell bipolar plates prepared by the present invention has a Ti-C film or a Ti-N film vacuum ion-plated on the surface, the film thickness is 5-25 μm, the overall tensile strength is 450-650 MPa, the yield strength is 300-600 MPa, the elongation is 20-40%, and the stainless steel has a high thermal conductivity at 0.5 mM H 2 SO 4 +0.1mg / L HF solution with a scan rate of 1mV / s and a corrosion current density of ≤4μA / cm at an open circuit potential of 0.44V-1.6V 2 , at 140N / cm 2 The contact resistance is ≤1.25mΩ·cm under the contact pressure 2 After coating, the corrosion current density is reduced by 60-90%, and the contact resistance is reduced by more than 90%.

[0110] The Ti-C film is vacuum ion plated on the surface of the ferrite stainless steel for the fuel cell bipolar plate prepared by the present invention, the film thickness is 5-25 μm, the overall tensile strength is 450-650 MPa, the yield strength is 300-600 MPa, the elongation is 20-40%, and the stainless steel is 0.5 mM H 2 SO 4 In +0.1mg / L HF solution with a scan rate of 1mV / s, the corrosion current density at the open circuit potential of 0.44V-1.6V is reduced by more than 65% and the contact resistance is reduced by more than 90% compared with before the Ti-C film is added. The minimum corrosion current density can reach 0.3 and the minimum contact resistance can reach 3.25.

[0111] The Ti-N film prepared by the present invention on the surface of the ferrite stainless steel for the fuel cell bipolar plate has a thickness of 5-25 μm, an overall tensile strength of 450-650 MPa, a yield strength of 300-600 MPa, and an elongation of 20-40%. The stainless steel has a high thermal conductivity and a high thermal conductivity. 2 SO 4 In +0.1mg / L HF solution with a scan rate of 1mV / s, the corrosion current density at an open circuit potential of 0.44V-1.6V is reduced by more than 80% and the contact resistance is reduced by more than 90% compared with before the addition of Ti-N film. The minimum corrosion current density can reach 0.2 and the minimum contact resistance can reach 3.4.

[0112] In summary, compared with other traditional methods, the method of the present invention can improve the plasticity, corrosion resistance and contact resistance of ferritic stainless steel by controlling alloy composition, rolling process and surface treatment process; the method is simple and easy to operate, green and environmentally friendly, low in cost, short in process, high in efficiency, wide in application range, and conducive to large-scale industrial production and promotion.

[0113] It should be understood that the term "and / or" in this article is only a description of the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. A and B can be singular or plural. In addition, the character " / " in this article generally indicates that the associated objects before and after are in an "or" relationship, but it may also indicate an "and / or" relationship. Please refer to the context for specific understanding.

[0114] In the present invention, "at least one" means one or more, and "plurality" means two or more. "At least one of the following" or similar expressions refers to any combination of these items, including any combination of single items or plural items. For example, at least one of a, b, or c can mean: a, b, c, ab, ac, bc, or abc, where a, b, and c can be single or multiple.

[0115] It should be understood that in various embodiments of the present invention, the size of the serial numbers of the above-mentioned processes does not mean the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present invention.

[0116] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily think of changes or substitutions within the technical scope disclosed by the present invention, which should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A ferritic stainless steel for a fuel cell bipolar plate, characterized in that: The thickness of the ferritic stainless steel for the fuel cell bipolar plate is 0.02-0.3 mm, and the composition content is calculated by mass percentage as follows: C≤0.01%, N<0.005%, Si0.15-0.30%, Mn 0.10-0.40%, Cr 20-40%, P<0.005%, S<0.005%, Nb 0.15-0.5%, Ti 0.03-0.2%; the rest is Fe and unavoidable impurities.

2. The ferritic stainless steel for fuel cell bipolar plates according to claim 1, characterized in that: The composition of the ferritic stainless steel for fuel cell bipolar plates further includes one or a combination of several alloy elements selected from the group consisting of 0.005-0.5% Sn, 0.001-0.1% Sb, 0.05-0.3% Ru and Ni≤5.0%.

3. The ferritic stainless steel for fuel cell bipolar plates according to claim 1, characterized in that: The organizational structure of the ferritic stainless steel for the fuel cell bipolar plate is that the matrix organization is a ferrite organization, the shape of the ferrite organization is equiaxed, the average grain size is 10-50 μm, and C+N is less than 150 ppm.

4. The ferritic stainless steel for fuel cell bipolar plates according to claim 1, characterized in that: The ferritic stainless steel for the fuel cell bipolar plate has a tensile strength of 450-650 MPa, a yield strength of 300-600 MPa, and an elongation of not less than 20%. The corrosion current density of the stainless steel at an open circuit potential of 0.44 V to 1.6 V at a scanning rate of 1 mV / s in a 0.5 mM H2SO4+0.1 mg / L HF solution is ≤10 μA / cm 2 , at 140N / cm 2 The contact resistance is ≤25mΩ·cm under the contact pressure 2 .

5. The ferritic stainless steel for fuel cell bipolar plates according to claim 1, characterized in that: The surface of the ferritic stainless steel for the fuel cell bipolar plate is vacuum ion plated with a Ti-C film or a Ti-N film, the film thickness is 5-25 μm, the overall tensile strength is 450-650 MPa, the yield strength is 300-600 MPa, the elongation is 20-40%, and the corrosion current density of the stainless steel at an open circuit potential of 0.44V-1.6V at a scanning rate of 1mV / s in a 0.5mM H2SO4+0.1mg / L HF solution is ≤4μA / cm 2 , at 140N / cm 2 The contact resistance is ≤1.25mΩ·cm under the contact pressure 2 After coating, the corrosion current density is reduced by 60-90%, and the contact resistance is reduced by more than 90%.

6. A method for preparing ferritic stainless steel for fuel cell bipolar plates according to any one of claims 1 to 5, characterized in that: The preparation method of the ferritic stainless steel for the fuel cell bipolar plate comprises the following steps: S1. Weighing and smelting raw materials: weighing raw materials according to the chemical composition content of ferritic stainless steel for fuel cell bipolar plates, melting and casting the weighed raw materials to obtain stainless steel ingots; S2, forging: forging the S1 stainless steel ingot to obtain a stainless steel forging blank; S3, hot rolling: remove the surface oxide scale of the S2 stainless steel forging billet, and then hot roll it to obtain a hot-rolled slab; S4, homogenization annealing: homogenization annealing is performed on the hot-rolled slab in S3 to obtain a hot-rolled plate with uniform structure after annealing; S5, cold rolling after pickling: pickling the hot-rolled plate with uniform structure after annealing in S4, then grinding the surface, and finally cold rolling to obtain a cold-rolled thin steel plate; S6, cold rolling annealing: The S5 cold rolled thin steel is annealed at high temperature for a short time to obtain a ferritic stainless steel sheet.

7. The method for preparing ferritic stainless steel for fuel cell bipolar plates according to claim 6, characterized in that: The preparation method also includes surface treatment of S7, which is to mechanically polish the ferritic stainless steel sheet of S6, clean and dry the mechanically polished sample, and then electrolytically polish it in an electrolytic solution or perform vacuum ion plating of either Ti-C or Ti-N, and alternately and evenly plate Ti layers and N layers on the substrate.

8. The method for preparing ferritic stainless steel for fuel cell bipolar plates according to claim 6, characterized in that: The ingot size of S1 stainless steel is 350×250×75-290×190×60mm, the forging size of S2 stainless steel is 150×130×40-110×100×35mm, the hot-rolled plate size of S3 is 1000×150×1-2000×200×3mm, the cold-rolled thin steel plate and S6 ferritic stainless steel thin plate size are 1000×50×0.02-1500×65×0.3mm.

9. The method for preparing ferritic stainless steel for fuel cell bipolar plates according to claim 6, characterized in that: The heating rate of S3 hot rolling is 20-35℃ / min, the number of hot rolling passes is 7-10, the reduction in each pass is 25-35%, the total reduction is 85-95%, and the thickness after hot rolling is 1-3mm.

10. The method for preparing ferritic stainless steel for fuel cell bipolar plates according to claim 6, characterized in that: The temperature of S4 homogenization annealing is 900-1050°C, and the annealing time is 1-60min.

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