| 摘要 |
PROBLEM TO BE SOLVED: To provide a high tensile strength steel excellent in the toughness of a heat-affected zone in the case of superhigh heat input welding of >=200 kJ/cm heat input (even in the case of about 1500 kJ/cm). SOLUTION: This steel contains MgO-TiN composite precipitates of 0.05 to 0.5μm dimensions in which MgO of 0.005 to 0.1μm grain size is used as the nuclei and having TiN at the vicinities thereof by 1.0×10<5> to 1.0×10<7> pieces per mm<2> . It has a compsn. contg., by weight, 0.04 to 0.2% C, 0.02 to 0.5% Si, 0.6 to 2.0% Mn, <=0.02% P, <=0.02% S, <0.003% Al, 0.005 to 0.025% Ti, 0.002 to 0.008% N, 0.0002 to 0.005% Mg and 0.0005 to 0.008% O, contg., as selective elements, one or >= two kinds among Cu, Ni, Cr, Mo, Nb, V and B, and the balance Fe with inevitable impurities. By the suppression of the growth ofγgrains in the superhigh heat input weld HAZ and the promotion of ferritic transformation in the grains, the toughness of the HAZ can be improved.
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