| 摘要 |
PROBLEM TO BE SOLVED: To provide a high tensile strength steel (a steel sheet, and a steel pipe including a welded pipe) having unstable fracture resisting characteristics, in which 85% ductile fracture transition temperature SATT 85% in a DWTT test in API standards is <=-30 deg.C, and absorbed energy EDWTT at -30 deg.C is >=5,000 J, and to provide a production method therefor. SOLUTION: The high strengh steel has a composition containing 0.01 to 0.10% C, <=0.30% Si, 1.00 to 2.50% Mn, <=0.010% P, <=0.0008% S, 0.005 to 0.06% Nb, 0.004 to 0.025% Ti, <=0.05% sol. Al, <=0.0040% N and <=0.003% O, and the balance Fe with impurities, and also satisfying the inequality of 20×S+P+5×(N+O)}<=0.045, and has TS(tensile strength) of <=750 MPa.
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