Abstract: Manufacturing equipment of electric resistance welding pipes having excellent characterization of welded seams, includes an apparatus that supplies a strip, an apparatus that reforms a shape, an apparatus that performs roll forming, an induction heater, and a contacting apparatus of strip edges by pressing, and having a finpass forming apparatus incorporating a fin having at least two stages of tapering in a middle portion of the roll forming, wherein tapering of the strip immediately before electric resistance welding is conducted such that an angle from a surface of a strip edge toward a vertical direction is in a range of 25 degrees to 50 degrees, and length of a perpendicular angle from a starting position to an end position of the tapering at one side is 20% to 45% of strip thickness, and by shaping the tapering with a fin, having at least two stages of tapering, incorporated in a finpass forming end stand.

Abstract: A high-tensile strength welded steel tube has excellent formability and torsional fatigue endurance after being formed into cross-sectional shape and then stress-relief annealed. A steel material used has a composition which contains C, Si, Al, 1.01% to 1.99% Mn, 0.041% to 0.150% Ti, 0.017% to 0.150% Nb, P, S, N, and O such that the sum of the content of Ti and that of Nb is 0.08% or more, the content of each of C, Si, and Al being within an appropriate range, the content of each of P, S, N, and O being adjusted to a predetermined value or less.

Abstract: A high-tensile strength welded steel tube has excellent formability and torsional fatigue endurance after being formed into cross-sectional shape and then stress-relief annealed. A steel material used has a composition which contains C, Si, Al, 1.01% to 1.99% Mn, 0.041% to 0.150% Ti, 0.017% to 0.150% Nb, P, S, N, and O such that the sum of the content of Ti and that of Nb is 0.08% or more, the content of each of C, Si, and Al being within an appropriate range, the content of each of P, S, N, and O being adjusted to a predetermined value or less.

Abstract: Manufacturing equipment of electric resistance welding pipes is provided in which lateral edges of a strip for an electric resistance welding pipe is shaped with appropriate tapering immediately before electric resistance welding so that welding quality is kept excellent. Cutting or shaving means is provided after a leveler. At least one finpass forming stand is provided which includes a fin having tapering having at least 2 stages.