A steel comprises, by mass percent, C: 0.10 to 0.15%, Si: not less than 0.02% and less than 0.10%, Mn: more than 0.90% and not more than 2.50%, P≦0.030%, S≦0.050%, Cr: 0.80 to 2.0%, V: 0.05 to 0.50%, Al: 0.01 to 0.07%, N≦0.0080%, O≦0.0030%, and one or more selected from Mo, Cu, Ni, Ti, Nb, Zr, Pb, Ca, Bi, Te, Se and Sb, the balance being Fe and impurities. The composition satisfies [35≦Mn/S≦200], [20≦(669.3×logeC−1959.6×logeN−6983.3)×(0.067×Mo+0.147×V)≦80], [140×Cr+125×Al+235×V≧160] and [150≦511×C+33×Mn+56×Cu+15×Ni+36×Cr+5×Mo+134×V≦200].