| 摘要 |
PROBLEM TO BE SOLVED: To provide a high tensile strength welded steel pipe for an automobile structural member having excellent low temperature toughness, excellent formability and excellent twisting fatigue resistance after cross-sectional forming-stress relief annealing, and to provide a method for producing the same. SOLUTION: A steel stock having a composition comprising C, Si and Al in proper ranges, comprising 1.01 to 1.70% Mn, 0.017 to 0.15% Nb and 0.045 to 0.45% Mo, and comprising P, S, N and O regulated to prescribed values is subjected to hot rolling in which heating temperature and finish rolling completing temperature are controlled to proper ranges, is annealed in the temperature range of 750 to 650°C for≥2s after the completion of the hot rolling, and is coiled at a coiling temperature of 660 to 510°C, so as to be a hot rolled steel strip having a structure where the ratio of a ferrite phase with the average grain size of 2 to 14μm is≥60 vol.%, and Nb carbides with the average grain size of 1.5 to 30 nm are precipitated into the ferrite phase. The hot rolled steel strip is subjected to an electric resistance welding process where a width contraction rate is≤10%. In this way, the high tensile strength welded steel pipe having high strength satisfying the tensile strength of≥660 MPa, and having excellent low temperature toughness, formability and twisting fatigue resistance after cross-sectional forming-stress relief annealing treatment is obtained. COPYRIGHT: (C)2008,JPO&INPIT |
