PURPOSE: To obtain the titled steel plate in which security of high strength under low carbon equivalent, security of superior toughness at low temp. in the steel including the center of plate thickness, and improvement in weldability are simultaneously enabled, by specifying steel components and also by limiting the heating, rolling, and cooling conditions of a slab.
CONSTITUTION: A steel consisting of, by weight, 0.03W0.20% C, 0.05W0.60% Si, 0.50W2.50% Mn, 0.001W0.10% Nb, 0.005W0.1% Al, and the balance Fe with inevitable impurities is heated to 900W1,150°C. After heating, the steel is cooled after rolled to a thickness of intermediate stage or is cooled in a state of a slab without rolling, and the steel then is reheated before the temp. in the center of plate thickness drops below the Ar3 point, so that surface temp. of ≤Ar3 point is subjected to temp. raise to Ar3 point W 1,150°C. After that, rolling is resumed and rolling is applied to the whole plate thickness at a temp. between Ar3 point and (Ar3+100)°C at 50W70% draft. After rolling, the plate is cooled at 1W10°C/sec cooling rate down to 250W600°C and successively air-cooled, so that above-mentioned steel plate can be manufactured.
TOMITA YUKIO
YAMABA RYOTA
