SUPER HIGH STRENGTH UOE STEEL PIPE AND METHOD FOR PRODUCTION THEREOF

摘要

An UOE steel pipe for a line pipe, which has super high strength such as a strength in the circumferential direction of the pipe of 750 to 900 MPa, is excellent in the toughness of the base material and the heat affected zone thereof and in joint fracture characteristics, and exhibits good circumferential weldability, can be produced by using a hot-rolled steel plate, which has a chemical composition containing 0.03 to 0.08 % of C, 1.70 to 2.2 % of Mn, 0.0020 % or less of S, 0.005 to 0.025 % of Ti, and 0.0050 % or less of N, with the proviso that a carbon equivalent (Ceq) as defined below is 0.50 % or more and a weld cracking sensitivity index (Pcm) as defined below is 0.24% or less, and which has been produced by a process wherein the cooling with water after the hot-rolling is stopped at a temperature of 350°C or higher: Ceq = C + Mn/6 + (Cr + Mo + V)/5 + (Cu + Ni)/15; Pcm = C + Si/30 + Mn/20 + Cu/20 + Ni/60 + Cr/20 + Mo/15 + V/10 + B.