To obtain hardenable martensitic heat treated steel exhibiting increased heat resistance at 300 to 600°C compared with the case of steel X12 CrNiMo12 and having high ductility.

This steel has the following composition of, by mass, 9 to 13% Cr, 0.001 to 0.25% Mn, 2 to 7% Ni, 0.001 to 8% Co, at least one of W and Mo by 0.5 to 4% in total, 0.5 to 0.8% V, at least one selected from Nb, Ta, Zr and Hf by 0.001 to 0.1% in total, 0.001 to 0.05% Ti, 0.001 to 0.15% Si, 0.01 to 0.1% C, 0.12 to 0.18% N, ≤0.025% P, ≤0.015% S, ≤0.01% Al, ≤0.0012% Sb, ≤0.007% Sn and ≤0.012% As, and the balance iron with ordinary impurities.