HIGH-STRENGTH STEEL TUBE HAVING EXCELLENT FORMABILITY AND BURST PROPERTY AND METHOD FOR MANUFACTURING IT

摘要

PROBLEM TO BE SOLVED: To provide a method for manufacturing a high-strength steel tube for line pipes by which crack and fracture in a seam-weld zone in a tube expanding stage are prevented and also burst properties along with excellent formability with which brittle fracture from the seam-weld zone is prevented from the load by internal pressure at the time of using the steel tube are imparted in the method for manufacturing the high-strength steel tube for line pipes the strength of the steel plate of which is >850 N/mm2 by the UOE system. SOLUTION: In the method for manufacturing the high-strength steel tube whose tensile strength is >850 N/mm2 by the UOE system, there is manufactured the high-strength steel tube having excellent formability and the burst property in which the ratio R/r of the average radius R of curvature within the range of 120 mm in the peripheral direction including the weld zone of the steel tube before expanding the tube to the radius r of the steel tube after expanding the tube is 0.65-2.0 in the tube expanding stage and, furthermore, the ratio R/r to the radius r of the steel tube is 0.90-2.0.