| 摘要 |
PROBLEM TO BE SOLVED: To provide high tensile steel for superhigh heat input welding which has excellent heat affected zone(HAZ) toughness in superhigh heat input welding in which heat input is >=200 kJ/cm (even in the case of about 1,500 kJ/cm for example). SOLUTION: The steel contains (Mn, Mg, Cu) S whose grain size is 0.005 to 0.5μm by 1.0×105 to 1.0×107 pieces per 1 mm2. The steel contains, 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 and 0.0005 to 0.005% Mg. Further, as selective elements, a suitable amount of one or more kinds selected from Ni, Cr, Mo, Nb, V, Ti and B are incorporated therein. By the pinning action of the fine (Mn, Mg, Cu) S grains, the growth ofγgrains in superhigh heat input welding HAZ(heat affected zone) is suppressed, and its HAZ toughness is improved.
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