ELECTRIC RESISTANCE WELDED TUBE HAVING EXCELLENT STRENGTH AND TOUGHNESS AND PRODUCTION METHOD THEREFOR

摘要

PROBLEM TO BE SOLVED: To provide an electric resistance welded tube for a line pipe which secures high strength of an API5L-X60 grade or higher, and has high toughness. SOLUTION: A hot rolled steel sheet having a composition containing 0.04 to 0.10% C, 0.8 to 1.6% Mn, 0.005 t 0.06% Nb and 0.005 to 0.05% Ti, and further containing one or more kinds selected from Cu, Ni, Cr, Mo and V, and having a metallic structure mainly consisting of bainitic ferrite is subjected to electric resistance welding, and is thereafter heat-treated, so that an electric resistance welded tube in which the ratio (H) of HAZ widths between the outside face and the inside face expressed by the formula (1) is controlled to 1.1 to 1.5, and the metallic structure of the electric resistance weld zone mainly consists of ferrite is produced: H=HOUT/HIN...(1). Alternatively, in the electric resistance welded tube produced by the same method, A value expressed by the formula (2) is <=15μm: A=0.43×(DL+0.90×DC)...(2); wherein, HOUT and HIN denote the widths of the heat affected zones in the inside face and the outside face of the tube, and DL and DC denote the average length (μm) of bainitic ferrite in the cross-sections parallel and vertical to the rolling direction.