| 摘要 |
PROBLEM TO BE SOLVED: To provide a high-tensile welded steel pipe for automobile structural member, having excellent formability and excellent torsion fatigue-resistant characteristic after cross-sectional forming-stress removal annealing. SOLUTION: To a steel blank having the composition adjusted as the followings, in which C, Si, Al are made into suitable ranges and 0.8-1.99% Mn, 0.001-0.07% Ti, 0.001-0.05% Nb are contained and P, S, N, O are made to prescribed values or less, respectively; A hot-rolling is applied in the suitable ranges of the heating temperature and the finish-rolling temperature, and after completing the hot-rolling, a slow-cooling is performed at≥2 s in the temperature range of 750-650°C, and the steel strip is coiled at 660-410°C coiling temperature, and to the hot-rolled steel strip having≥60 vol% ferrite-phase of 2-8μm average grain diameter and the structure precipitating Nb-carbide of 2-40 nm average grain diameter in the ferrite phase, an electric resistance welded steel pipe-making process having≤10% width drawing rate, is applied to make the welded steel pipe. In this way, the high-tensile welded steel pipe excellent in the formability, and the torsion fatigue-resistant characteristic after applying the cross-sectional forming-stress removal annealing treatment is obtained. COPYRIGHT: (C)2007,JPO&INPIT
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