Patent ID: 8765269

Claim:
A high strength steel pipe for low-temperature usage comprising: a base material portion in which a base material has a composition containing, in mass percentage, C: more than 0.03% and 0.08% or less, Si: 0.01% to 0.5%, Mn: 1.5% to 3.0%, P: 0.015% or less, S: 0.003% or less, Al: 0.01% to 0.08%, Nb: 0.005% to 0.025%, Ti: 0.005% to 0.025%, N: 0.001% to 0.010%, O: 0.005% or less, and B: 0.0003% to 0.0020% and one or more of Cu: 0.01% to 1%, Ni: 0.01% to 1%, Cr: 0.01% to 1%, Mo: 0.01% to 1%, and V: 0.01% to 0.1%, wherein a P CM value (in %) calculated by a formula (1) below satisfies 0.19≦P CM ≦0.25, the balance is Fe and unavoidable impurities, the base material has a tensile strength of 760 MPa or more and 930 MPa or less, a uniform elongation of 5% or more, a yield ratio of 85% or less, a Charpy absorbed energy at a test temperature of −40° C. is 210 J or more when the plate thickness is less than 25 mm and is 150 J or more when the plate thickness is 25 mm or more, and a microstructure mainly constituted of bainite as a soft phase containing island martensite as a hard phase having an area fraction of 4% or more and 12% or less, the island martensite contained has a long axis size of 2 μm or less, and bainite surrounded by boundaries having a misorientation of 15° or more in the microstructure of the base material portion has a long axis size of 20 μm or less; and a weld metal portion in which weld metal for seam welding has a composition containing, in mass percentage, C: 0.03% to 0.10%, Si: 0.5% or less, Mn: 1.5% to 3.0%, P: 0.015% or less, S: 0.005% or less, Al: 0.05% or less, Nb: 0.005% to 0.05%, Ti: 0.005% to 0.03%, N: 0.010% or less, O: 0.015% to 0.045%, and B: 0.0003% to 0.0050% and one or more of Cu: 0.01% to 1%, Ni: 0.01% to 2.5%, Cr: 0.01% to 1%, Mo: 0.01% to 1.5%, and V: 0.1% or less, and the balance is Fe and unavoidable impurities, wherein a microstructure of a welded heat affected zone in which a prior austenite grain size is 50 μm or more near a fusion line in a seam weld zone of the steel pipe is a lower bainite structure or a multi-phase structure containing lower bainite having an area fraction of 50% or more and upper bainite and/or martensite, wherein P CM (%)=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5×B (1) where each element represents content (mass %).