| 摘要 |
PROBLEM TO BE SOLVED: To provide steel with the maximum plate thickness of 100 mm having tensile strength in a class of 600 MPa, and in which HAZ toughness in the vicinity of melting is >=100 J/cm2 expressed in terms of the average value of Charpy absorbed energy at 0 deg.C in case that the quantity of heat for welding is 100 kJ/mm. SOLUTION: This thick 600 MPa-class steel excellent in heat affected zone toughness has chemical components containing, by mass, 0.08 to 0.15% C, <=0.4% Si, 1.0 to 1.8% Mn, <=0.015% P, <=0.006% S, 0.005 to 0.05% Nb, 0.001 to 0.05% Al, 0.007 to 0.025% Ti, 0.0003 to 0.005% Mg, 0.0003 to 0.003% B, 0.001 to 0.004% O and 0.001 to 0.005% N, in which Ti/N, i.e., the ratio in mass% between Ti and N further satisfies 3 to 6, and the balance iron with inevitable impurities, and in which TiN of 0.01 to 0.05μm including oxides composed of Mg and Al is present by >=10,000 pieces/mm2.
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