Patent ID: 8871039

Claim:
A thick welded steel pipe excellent in low temperature toughness having a thickness of 25 to 45 mm that is formed of a base material steel sheet formed into a pipe shape being seam welded and has a coarse-grained HAZ, wherein the base material steel sheet has a component composition containing, in mass %, C: 0.03% to 0.085%, Mn: 1.45% to 1.85%, Ti: 0.005 to 0.020%, Nb: 0.005 to 0.050%, and O: 0.0005 to 0.005%, and Si: limited to 0.15% or less, Al: limited to 0.015% or less, P: limited to 0.02% or less, S: limited to 0.005% or less, and Mo: limited to 0.20% or less, and having the contents of Mn and Mo satisfying (Expression 1) below, a Pcm obtained by (Expression 2) below being 0.16 to 0.19, and a balance being composed of Fe and inevitable impurities, a metal structure of the base material steel sheet consists of ferrite being 30 to 95% in an area ratio and a low temperature transformation structure, and in a metal structure of the coarse-grained HAZ, an area ratio of grain boundary ferrite is 1.5% or more, the total area ratio of the grain boundary ferrite and intragranular ferrite is not less than 11% nor more than 90%, an area ratio of MA is 10% or less, and its balance is composed of bainite: 1.2325≦(0.85×[Mn]−[Mo])≦1.5215 (Expression 1) Pcm=[C]+[Si]/30+([Mn]+[Cu]+[Cr])/20+[Ni]/60+[Mo]/15+[V]/10 (Expression 2) In (Expression 2), [C], [Si], [Mn], [Cu], [Cr], [Ni], [Mo], and [V] denote the contents of C, Si, Mn, Cu, Cr, Ni, Mo, and V [mass %] respectively.