HIGH-STRENGTH STEEL SHEET AND HIGH-STRENGTH WELDED STEEL PIPE EXCELLING IN DUCTILE FRACTURE PERFORMANCE AND PROCESS FOR PRODUCING THEM

摘要

A high-strength steel sheet and high-strength welded steel pipe excelling in ductile fracture performance that have a tensile strength corresponding to X100 class of API standards; and a process for producing them. There is provided a high-strength steel sheet excelling in ductile fracture performance characterized by being composed of, in terms of mass%, 0.01 to 0.5% C, 0.01 to 3% Si, 0.1 to 5% Mn, =0.03% P, =0.03% S and the balance of Fe and unavoidable impurities, and by having a microstructure wherein 1 to 60%, in terms of area ratio, of ferrite is contained with the balance composed of bainite and martensite, and characterized in that the maximum of degree of {100} accumulation in section resulting from 20° to 50° rotation of through-thickness section on the rolling direction as an axis is =3, and that the through-thickness parallel crack measured by ultrasonic inspection method is < 1 mm. Further, there is provided a high-strength welded steel pipe produced from the high-strength steel sheet as a parent material.