STEEL PLATE FOR LINE PIPE, AND LINE PIPE

摘要

A steel plate for a line pipe, the steel plate having a plate thickness of 25 mm or more comprising in terms of mass%: 0.040 to 0.080% of C, 0.05 to 0.40% of Si, 1.60 to 2.00% of Mn, 0.020% or less of P, 0.0025% or less of S, 0.05 to 0.20% of Mo, 0.0011 to 0.0050% of Ca, 0.060% or less of Al, 0.010 to 0.030% of Nb, 0.008 to 0.020% of Ti, 0.0015 to 0.0060% of N, and 0.0040% or less of O, a remainder consisting of Fe and unavoidable impurities, wherein a content ratio of Ca to S [Ca/S] is from 0.90 to 2.70, and a content ratio of Ti to N [Ti/N] is 2.20 or higher" wherein Ceq, which is defined by the following Formula (1), is from 0.380 to 0.480: Ceq = C+Mn/6+(Ni+Cu)/15+(Cr+Mo+V)/5; and wherein: at a position of 1/4 of the plate thickness, a ferrite fraction (F1) is from 20 to 60% and a remainder is a structure of bainite; at a position of 1/2 of the plate thickness, a ferrite fraction (F2) is from 5 to 60% and a remainder is a structure of bainite or a structure of bainite and martensite; a ratio (F1/F2) of the ferrite fraction (F1) to the ferrite fraction (F2) is from 1.00 to 5.00; at a position of 1/4 of the plate thickness an average grain diameter of ferrite is from 2.0 to 15.0 µm, and at a position of 1/2 of the plate thickness the average grain diameter of ferrite is from 5.0 to 20.0 µm; and the hardness at a position of 1/2 of the plate thickness is 400 Hv or less, and the length of MnS at a position of 1/2 of the plate thickness is 1.00 mm or less.