PURPOSE: To improve the high-temp. tensile strength, creep rupture strength and initial toughness of a thick Cr-Mo steel plate during quenching, while reducing its sensitivity to embrittleness, by previously heat treating it under a specified condition in advance of the quenching.

CONSTITUTION: When the thick Cr-Mo steel plate of 300W500mm in thickness containing 0.08W0.17% C, Si<0.50%, 0.10W1.0% Mn, 1.5W8.5% Cr, 0.7W1.8% Mo, 0.003W0.080% acid-soluble Al, 0.0015W0.015% N, and optionally Cu<0.20%, Ni<0.30% and V<0.20% is to be quenched, it is preheated at 900W1,050°C. Thereafter, it is slowly cooled in the temp. range of 800W600°C at an average cooling speed of 80°C/hr and then quenched. By properly controlling the amounts of Al and N in the composition and making austenitic grains in the structure 6.5W4.0 by ASTM No. in size, various quenching properties are improved.