| 摘要 |
PROBLEM TO BE SOLVED: To improve toughness in HAZ in extra-large heat input welding by specifying a chemical composition in a steel, precipitating sulfides and nitrides around Mg oxides having a specific grain size, and specifying the diameter and proportion of dispersion of the resultant composite grains, respectively. SOLUTION: This steel has a composition consisting of, by weight, 0.02-0.20% C, 0.02-0.50% Si, 0.3-2.0% Mn, <=0.03% P, 0.0001-0.03% S, 0.005-0.01% Al, 0.003-0.05% Ti, 0.0001 0.015% Mg, 0.0005-0.008% O, one or more kinds among 0.0001-0.05% Zr, 0.0001-0.05% Ta, and 0.0001-0.05% Nb, and the balance Fe. Composite grains consisting of sulfides and nitrides precipitated around Mg oxides of 0.2 to 5.0μm grain size are incorporated by 10 to 10000 pieces per mm<2> . Further, composite grains of 0.01 to 2.0μm consisting of nitrides precipitated around Mg oxides of 0. 005 to 0.1μm grain size, are incorporated by (1.0×10<4> to 1.0×10<8> ) pieces per mm<2> .
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