| 摘要 |
PROBLEM TO BE SOLVED: To provide high tensile strength steel for large heat input welding, which has excellent HAZ(heat affected zone) toughness even if the heat input is >=200 kJ/cm (e.g. about 1,500 kJ/cm). SOLUTION: The high tensile strength steel contains one or more kinds of Cu-Ca sulfides with a particle size of 0.005 to 0.5μm having a composition consisting of (Cu, Ca) S or (Cu, Mn, Ca) S, or (Cu, Mn, Mg, Ca) S by 1.0×10<5> to 1.0×10<7> pieces per square mm in total. Further, in the high tensile strength steel, by mass, 0.04 to 0.25% C, 0.02 to 0.5% Si, 0.1 to 2.0% Mn, <=0.02% P, 0.002 to 0.02% S, 0.03 to 1.5% Cu, 0.015 to 0.5% Al, 0.0005 to 0.005 Mg and 0.0005 to 0.005% Ca are contained. As the selective elements, a suitable amount of one or more kinds selected from Ni, Cr, Mo, Nb, V, Ti and B contained, and the balance Fe with inevitable impurities.
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