PURPOSE: To obtain a high tensile steel plate having excellent toughness and weldability by heating a steel slab of specific compsns. to form a solid solution of Nb in the compsn. and rolling the same under control of temp. and draft at specific values.
CONSTITUTION: The steel slab consisting basically of 0.04W0.10% C, 0.05W0.30% Si, 1.25W2.50% Mn, 0.010W0.100% Nb, 0.0005W0.0030% B, ≤0.0080% N, 0.005W 0.040% Ti, 0.005W0.050% Al, ≥0.10 Cu, ≥0.10% Ni (0.20W1.00% Cu+Ni) is rolled at ≥(Ar3 point+150°C) and ≥50% cumulative draft. In succession, the slab is rolled at ≤(Ar3 point+150°C), and in an unrecrystallization austenite region higher than the Ar3 point at ≥50%. Further the slab is rolled in a temp. region of two phases according to need and is then air-cooled. Thus the structure consisting essentially of ferrite and fine bainite as well as island-like martensite is formed in the hot-rolled steel plate.
HATOMURA TANEO
AMANO KENICHI
