Abstract: A stainless steel sheet used for a separator of a polymer electrolyte fuel cell, wherein the steel sheet has a volume ratio of M2B boride-based metal precipitates of 8% or higher and a volumetric-basis particle-size frequency distribution of M2B boride-based metal precipitates having sphere-equivalent diameters of 6.72 ?m or less of 99.5% or higher, in a surface layer which has a depth of 0.2 t from at least one of surfaces (t is an overall thickness of the steel sheet), and the steel sheet has an apparent density of 0.995 or higher, assuming a real density to be 1. A polymer electrolyte fuel cell in which the stainless steel sheet is used for a separator is given an excellent cell performance without high-cost surface treatment such as expensive gold plating for the reduction of the contact resistance of a surface.

Abstract: A ferritic stainless steel material contains, by mass %, C: 0.02 to 0.15%, Si: 0.01 to 1.5%, Mn: 0.01 to 1.5%, P: 0.035% or less, S: 0.01% or less, Cr: 22.5 to 35.0%, Mo: 0.01 to 6.0%, Ni: 0.01 to 6.0%, Cu: 0.01 to 1.0%, N: 0.035% or less, V: 0.01 to 0.35%, B: 0.5 to 1.0%, Al: 0.001 to 6.0%, rare earth metal: 0 to 0.10%, Sn: 0 to 2.50%, and the balance: Fe and impurities, and a value calculated in mass % as {Cr+3×Mo?2.5×B?17×C} ranges from 20 to 45%. The ferritic stainless steel material has a parent phase comprising only a ferritic phase. At least composite metallic precipitates including M23C6 carbide-based metallic precipitates precipitated on surfaces and at peripheries of M2B boride-based metallic precipitates serving as precipitation nuclei are dispersed and exposed on a parent phase surface.

Abstract: A method for producing a stainless steel three-ply clad sheet for a fuel cell separator comprises heating a source blank and then performing rough rolling, hot rolling and cold rolling thereto, whereby the clad steel sheet for the fuel cell separator or a solid polymer type fuel cell separator thereby is obtained. The source blank for the clad steel sheet is composed of a stainless steel utilizing 0 to 0.3% B as a core component and a stainless steel containing 0.3 to 2.5% B as face component. The source blank is made using face components, a core component, tabs and protectors and performing steps including bonding of the tabs and protectors to the face components, removing portions of the tabs and protectors, piling of the face components to the core component, and further bonding of the face and core components together to form the source blank.

Abstract: Provided is a first method for producing a source blank for a clad steel sheet composed of a stainless steel containing 0 to 0.3% B as a core component and a stainless steel containing 0.3 to 2.

Abstract: A stainless steel slab containing B, wherein a protecting material is joined onto at least two faces across the bloom from each other in a stainless steel bloom containing B of 0.3–2.5 mass %, being integrated into one-piece by forming a weld metal comprising a stainless steel with chemical composition that satisfies the relationship expressed by following formulas (1)–(4), and a method to produce a steel product by rolling said slab. Further, it is preferable to interpose an insert material between above stainless steel bloom and above protecting material in bonding process. 15?Cr eq?30,??(1) 4?Cr eq?Ni eq?17,??(2) Cr eq=Cr+1.5 Si+Mo?5 B,??(3) Ni eq=Ni+30 (C+N)+0.5 Mn??(4) Herein, each symbol of a chemical element designates the content (mass %) of relevant chemical element contained in steel.

Abstract: A stainless steel slab containing B, wherein a protecting material is joined onto at least two faces across the bloom from each other in a stainless steel bloom containing B of 0.3-2.5 mass %, being integrated into one-piece by forming a weld metal comprising a stainless steel with chemical composition that satisfies the relationship expressed by following formulas (1)-(4), and a method to produce a steel product by rolling said slab. Further, it is preferable to interpose an insert material between above stainless steel bloom and above protecting material in bonding process. 15?Cr eq?30 ??(1), 4?Cr eq?Ni eq?17 ??(2), Cr eq=Cr+1.5 Si+Mo?5 B ??(3), Ni eq=Ni+30 (C+N)+0.5 Mn ??(4) Herein, each symbol of a chemical element designates the content (mass %) of relevant chemical element contained in steel.