Abstract: A heat resisting steel whose metal structure is entirely martensite phase produced by tempering after quenching. The steel comprises, by weight, 0.05 to 0.20% C, not more than 0.15% Si, not more than 1.5% Mn, not more than 1.0% Ni, 8.5 to 13.0% Cr, not more than 3.50% Mo, not more than 3.5% W, 0.05 to 0.30% V, 0.01 to 0.20% Nb, not more than 5.0% Co, 0.001 to 0.020% boron, 0.005 to 0.040% nitrogen, 0.0005 to 0.0050% oxygen and 0.00001 to 0.0002% hydrogen. The steel has preferably not more than 10 of the Cr equivalent. The steel has 10 kgf/mm.sup.2 or more of 100,000 hours creep rupture strength at 650.degree. C.

Abstract: A method of producing forged article by casting a molten alloy steel containing 0.02 to 0.15 wt % of niobium and 9 to 12 wt % of chromium in a metal mold to form an ingot and subjecting the ingot to a forging. The ingot is formed to have a diameter greater than the height thereby to prevent generation of eutectic NbC and sedimental crystals.