The invention relates to a martensitic-hardenable heat-treated steel, having the following composition (data in % by weight): 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 in total between 0.5 and 4%, 0.5 to 0.8% V, at least one of Nb, Ta, Zr and Hf in total between 0.001 and 0.1%, 0.001 to 0.05% Ti, 0.001 to 0.15% Si, 0.01 to 0.1% C, 0.12 to 0.18% N, at most 0.025% P, at most 0.015% S, at most 0.01% Al, at most 0.0012% Sb, at most 0.007% Sn, at most 0.012% As, remainder iron and customary impurities, and the proviso that the ratio by weight of vanadium to nitrogen V/N lies in the range between 3.5 and 4.2. After solution annealing at 1050 to 1250° C., cooling to a temperature below 300° C., tempering treatment, partial or complete reaustenitization at 600 to 900° C., cooling to a temperature below 300° C. and annealing at a temperature from 550 to 650° C., it has a high resistance to heat combined, at the same time, with a high ductility.