A steel material contains, in mass %, C: 0.005 to 0.050, N: 0.05 to 0.30, Si: 0.1 to 1.5, Mn: 0.1 to 7.0, P: 0.005 to 0.100, S: 0.0001 to 0.0200, Cr: 18.0 to 28.0, Cu: 0.1 to 3.0, Ni: 0.1 to 8.0, Mo: 0.1 to 5.0, Al: 0.001 to 0.050, B: 0.0001 to 0.0200, and Ca: 0.0001 to 0.0100. An area ratio of an austenitic phase ranges from 30% to 70% and formulae (I) and (II) below are satisfied. 1.03≤[% Cr*F]/[% Cr]≤1.40  Formula (I) 1.05≤[% Mn*A]/[% Mn]≤1.80  Formula (II) [% symbol of an element]: a content of the element in the steel [% symbol of an element*F]: a content of the element in a ferrite phase [% symbol of element*A]: a content of the element in the austenitic phase