METHOD OF PRODUCING STEEL PIPES, AND WELDED PIPES

摘要

The high yield ratio problem intrinsic in high-strength steel pipes is solv ed and a method of producing a steel pipe, while securing the pipe roundness is provided. A steel pipe produced from a steel plate by forming and welding is subjected to 0.3 to 1.2% of expansion and then to 0.1 to 1.0% of reduction. The steel pipe produced by this method has a structure in which martensite and/o r bainite accounts for more than 80% as expressed in terms of area percentage, and has a yield strength of not lower than 551 MPa and a yield ratio of not high er than 93%. The pipe reduction is desirably carried out at a lower work ratio as compared with the pipe expansion.