| 摘要 |
PROBLEM TO BE SOLVED: To provide high strength steel excellent in fracture toughness, in which yield strength of >=550 MPa and tensile strength of >=620 MPa are satisfied and suitable for shipbuilding and marine structures. SOLUTION: This steel has a composition containing, by mass, 0.02 to 0.10% C, 0.50 to 2.5% Mn, <=0.025% P,<=0.01% S, 0.50 to 2% Cu, O to 4% Ni, <=0.05% Nb, <=0.05% Ti, <=0.005% Se and <=0.1% Al, is heated at a heating temperature of 950 to 1,250 deg.C prior to hot rolling, is air-cooled or cooled to a temperature region of <=580 deg.C at a cooling rate of 1 to 50 deg.C/sec, is next reheated at 450 to 680 deg.C and is thereafter subjected to air cooling treatment.
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