| 摘要 |
PROBLEM TO BE SOLVED: To provide a method for producing a high strength steel having≥500 MPa yield stress and≥570 MPa tensile strength and excellent in toughness of welding heat-affected part. SOLUTION: A steel slab containing, by mass, 0.005 to <0.040% C, <0.05% Si, 1.0 to <1.5% Mn, 0.10-0.50% Mo, 0.02-0.08% Nb, 0.005-0.020% Ti, 0.0005-0.0030% B, 0.001-0.010% Al, 0.0010-0.0070% N and satisfies the relation of [B]/10.8≥[N]/14.0-[Ti]/47.9, is heated at 1,020-1,300°C, and the rolling is performed to <60% accumulated rolling-reduction ratio at 920-1,020°C and to 50-90% accumulated rolling-reduction ratio at >Ar<SB>3</SB>point to 920°C, and after completing the rolling, an accelerated cooling is started at≥1°C/sec within 60 sec, and the cooling is stopped at 300 to <550°C, and thereafter, the air-cooling is performed. COPYRIGHT: (C)2009,JPO&INPIT
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