Patent ID: 7887649

Claim:
A high-tensile strength welded steel tube, having excellent low-temperature toughness, formability, and torsional fatigue endurance after being stress-relief annealed, for structural parts of automobiles, the tube having a composition which contains 0.03% to 0.24% C, 0.002% to 0.95% Si, 1.01% to 1.99% Mn, and 0.01% to 0.08% Al, which further contains 0.041% to 0.150% Ti and 0.017% to 0.150% Nb such that the sum of the content of Ti and that of Nb is 0.08% or more, and which further contains 0.019% or less P, 0.020% or less S, 0.01.0% or less N, and 0.005% or less O on a mass basis, the remainder being Fe and unavoidable impurities, P, S, N, and O being impurities; a microstructure containing a ferrite phase and a second phase other than the ferrite phase; and a yield strength of greater than 660 MPa, wherein the ferrite phase has an average grain size of 2 μm to 8 μm in circumferential cross section and a microstructure fraction of 60 volume percent or more and contains a precipitate of a (Nb, Ti) composite carbide having an average grain size of 2 to 40 nm.