| 摘要 |
PURPOSE: To produce a high tensile strength steel improved in toughness at low temp. and hot dip galvanizing crack resistance by specifying respective contents of Cu, Mn, Si, Al, etc., and the relation among MnO, Al2 O3 , and Sin., respectively. CONSTITUTION: This high tensile strength steel has a composition consisting of, by weight, 0.03-0.09% C, 0.005-0.35% Si, 0.5-1.8% Mn, <=0.010% P, <=0.005% S, >0.7-1.2% Cu, 0.2-2.5% Ni, <=0.015% sol.Al, 0.0002-0.005% insol. Al, 0.001-0.005% O, 0.0020-0.0080% N, further one or two kinds among 0-0.1% V, 0-0.5% Cr, and 0-0.5% Mo, and the balance Fe. Further, the grains of <0.2μm diameter, which are the grains of a compound oxide composed essentially of ternary oxide having a composition within the region where respective ranges of, by weight, 23-56% MnO, 4-27% Al2 O3 , and 30-54% SiO2 overlap in the ternary diagram, are dispersed by >=6pieces/mm<2> . A cast slab of this steel is heated to 950-1280 deg.C and rolling is finished at >=750 deg.C. After cooling, heating is applied at 475-670 deg.C.
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