| 摘要 |
<p>A high-tension welded steel pipe for automotive structural members which has excellent formability and has excellent resistance to torsional fatigue after section forming and subsequent annealing for stress removal; and a process for producing the steel pipe. A raw steel material having a composition which contains C, Si, and Al in amounts in respective proper ranges and further contains 1.01-1.99% Mn, 0.041-0.150% Ti, and 0.017-0.150% Nb, provided that Ti+Nb is 0.08% or more, and in which the amounts of P, S, N, and O have been regulated so as to be not larger than given values is subjected to hot rolling in which the heating temperature and the finish-rolling completion temperature are in proper ranges. After completion of the hot rolling, the steel material is annealed in a temperature range of 750-650°C for 2 seconds or longer. The steel is then wound at a winding temperature of 660-510°C to obtain a hot-rolled steel strip comprising at least 60 vol.% ferrite phase having an average particle diameter of 2-8 mum and a structure comprising an (Nb/Ti) composite carbide having an average particle diameter of 2-40 nm and precipitated in the ferrite phase. This steel strip is subjected to the step of pipe formation by electric-resistance welding at a width reduction of 10% or lower to thereby form a welded steel pipe. Thus, a high-tension welded steel pipe is obtained which has a high yield strength exceeding 660 MPa and is excellent in low-temperature toughness, formability, and resistance to torsional fatigue after annealing for stress removal.</p> |
