PURPOSE: To increase the thermal resistance, wear resistance, and pitting resistance of steel, by carburizing low alloy steel of low- or medium-carbon contg. <0.5% C, <2.4% Cr to form pseudospheroidal or spheroidal carbide of a volume ratio not smaller than 30% in the range of 0.4mm from the surface.

CONSTITUTION: Low alloy steel of low- or medium-carbon contg., 0.05W0.50% C, 0.10W0.35% Si, 0.30W2.00% Mn, <5.5% Ni, <2.4% Cr, <0.7% Mo is preliminarily carburized at a carbon potential not higher than Acm point to a carbon quantity on the surface not lower than that of eutectoid, then it is air-cooled or hardened. The steel is heated from Ac1 point to 750W950°, the temp. at which carbide is formed, at a heating rate of steel surface not higher than 20°C/min. While maintaining the carbon potential exceeding the Acm point. Then the steel is carburized at the carbide-forming temp. while maintaining the carbon potential exceeding the Acm point to the depth 0.1W0.4mm from the surface, hereby pseudo- spheroidal or spheroidal carbide of volume ratio not smaller than 30% is formed.