ULTRAHIGH STRENGTH STEEL PIPE HAVING SEAM WELD ZONE WITH EXCELLENT COLD-CRACK RESISTANCE, AND ITS MANUFACTURING METHOD

摘要

PROBLEM TO BE SOLVED: To provide an ultrahigh strength steel pipe free from cold-crack of weld metal in performing two-side one-pass seam welding with a large amount of a heat input in a method for manufacturing an ultrahigh strength line pipe of >=900 MPa (>X100 by API standard) tensile strength, and also to provide a method for manufacturing the ultrahigh strength steel pipe by which cold- crack of weld metal can be prevented. SOLUTION: In the ultrahigh strength steel pipe having seam weld zone with excellent cold-crack resistance, >=1% retained austenite phase is contained in the structure of at least the inside permanent weld metal between the inside permanent weld metal and the outside permanent weld metal each formed by permanent seam welding, as to the weld metal of the seam weld zone.