PURPOSE: To produce a high-toughness sub-hot forged non-temperature steel having excellent characteristics by heating a low-carbon steel having a specific compsn. to the specific temp. above the Ac3 transformation point according to the structure thereof, then subjecting the steel to hot rolling followed by direct water cooling thereby converting the structure to martensite and bainite or the mixed structure composed thereof.
CONSTITUTION: The steel contg., by weight %, 0.04W0.20% C, 0.05W1.0% Si, 0.8W2.0% Mn, 0.01W0.15% Nb, 0.01W0.05% Ti, 0.0003W0.005% B, and 0.005W0.10% Al, further contg. 1 or ≥2 kinds among ≤0.15% S, ≤0.30% Pb, ≤0.01% Ca, ≤0.30% Se, ≤0.30% Te, and ≤0.30% Bi at need, and has the structure of ≥#6 grain size number in the case of the structure essentially consisting of proeutectoid ferrite after hot rolling or ≥#6 old austenite grain size number in the case of the structure essentially consisting of bainite or martensite is subjected to hot forging at the Ac3 temp. W1,100°C then to the direct water cooling. The non-tempered steel products adequate as the stock for various machine parts are obtd.
KANATSUKI YUTAKA
SATO MOTOO
MATSUMOTO OSAMU
