STEEL PLATE EXCELLENT IN DEFORMABILITY FOR ULTRAHIGH STRENGTH WELDED STEEL PIPE, WELDED STEEL PIPE, AND METHOD FOR MANUFACTURING THE SAME

摘要

PROBLEM TO BE SOLVED: To provide a steel plate for an ultrahigh strength welded steel pipe which is high in bend buckling occurrence limit strain, has fully safe performance, and has a high strength equivalent to the X150 grade, and a welded steel pipe and a method for manufacturing the same.;SOLUTION: The steel plate for an ultrahigh strength welded steel pipe, excellent in deformability, contains, by mass%, C: 0.11-0.15%, Si: 0.20-0.50%, Mn: 1.5-2.5%, Al: 0.01-0.08%, Cu: 2.0-4.0%, Nb: 0.005-0.05%, Ti: 0.005-0.015%, one or more selected from Cr: 0.05-1.0%, Mo: 0.05-1.0%, V: 0.01-0.1%, and B: 0.0005-0.003%, and the balance being Fe and inevitable impurities. The metal structure includes two phases of bainite and island shaped martensite. The area fraction of the island shaped martensite is 10-25%. In addition, the product of the tensile strength (MPa) in the rolling direction and the uniform elongation (%) in one direction is 8000 or more and the ratio of the 1.5% proof stress to the 0.5% proof stress is 1.1 or more.;COPYRIGHT: (C)2013,JPO&INPIT