High-tensile strength welded steel tube for structural parts of automobiles and method of producing the same

A method of producing a high-tensile strength welded steel tube having a yield strength of greater than 660 MPa, excellent low-temperature toughness, excellent formability, and excellent torsional fatigue endurance after being stress-relief annealed, for structural parts of automobiles, the method comprising an electrically welded tube-making step of forming a steel tube material into a welded steel tube, wherein the steel tube material is a hot-rolled steel strip that is obtained in such a manner that a steel material is subjected to a hot-rolling step including a hot-rolling sub-step of heating the steel material to a temperature 1160° C. to 1320° C. and then finish-rolling the steel material at a temperature of 760° C. to 980° C., a slow cooling sub-step of slow cooling the rolled steel material at a temperature of 650° C. to 750° C. for 2 s or more, and a coiling sub-step of coiling the annealed steel material at a temperature of 510° C. to 660° C.;

• the steel material has 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, 0.010% 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;
• the electrically welded tube-making step includes a tube-making step of continuously roll-forming the steel tube material at a width reduction of 10% or less and then electrically welding the steel tube material into the welded steel tube;
• and the width reduction of the steel tube material is defined by the following equation:
• width reduction (%)=[(width of steel tube material)−π{(outer diameter of product)−(thickness of product)}]/π{(outer diameter of product)−(thickness of product))}×(100%) (1).