Abstract: Disclosed is a ferritic stainless steel for hot-water tanks with welded structure, comprising, in terms of % by mass, at most 0.02% of C, from 0.01 to 0.30% of Si, at most 1% of Mn, at most 0.04% of P, at most 0.03% of S, from more than 21 to 26% of Cr, at most 2% of Mo, from 0.05 to 0.6% of Nb, from 0.05 to 0.4% of Ti, at most 0.025% of N, and from 0.02 to 0.3% of Al, and optionally containing at least one of at most 2%, preferably from 0.1 to 2% of Ni and at most 1%, preferably from 0.1 to 1% of Cu, with a balance of Fe and inevitable impurities.

Abstract: A hot water tank having, on the inner surface thereof, a back bead formed by TIG-welding with no back gas sealing of steel members to each other having a composition comprising, % by mass, C: at most 0.025%, Si: at most 1%, Mn: at most 1%, P: at most 0.045%, S: at most 0.01%, Ni: from 0.1 to 1%, Cr: from more than 21 to 25%, Mo: from 0.1 to 2%, Al: from 0.02 to 0.3%, N: at most 0.025%, and Cu: from 0 to 1%, and containing at least one of Ti: from 0.05 to 0.4% and Nb: from 0.05 to 0.5%, with the balance of Fe and inevitable impurities, wherein the Cr concentration in the steel basis material in the area having oxide scale soluble in an aqueous chloride solution, as formed therein in the heat-affected zone on the welded back side thereof, is at least 16% by mass in the depth region of at least 10 nm from the steel basis material/oxide scale interface.

Abstract: Disclosed is a ferritic stainless steel for hot-water tanks with welded structure, comprising, in terms of % by mass, at most 0.02% of C, from 0.01 to 0.30% of Si, at most 1% of Mn, at most 0.04% of P, at most 0.03% of S, from more than 21 to 26% of Cr, at most 2% of Mo, from 0.05 to 0.6% of Nb, from 0.05 to 0.4% of Ti, at most 0.025% of N, and from 0.02 to 0.3% of Al, and optionally containing at least one of at most 2%, preferably from 0.1 to 2% of Ni and at most 1%, preferably from 0.1 to 1% of Cu, with a balance of Fe and inevitable impurities.

Abstract: Stainless steel sheets are plated with zinc and further phosphated so as to have chromaticness indices of L=45-53, a=0.0-0.4 and b=1.3-4.4. Thus treated stainless steel sheets have sufficient strength, excellent corrosion resistance and a color well harmonizing with other building materials and environment and do not suffer from color change for a prolonged period of time.