Thick welded steel pipe excellent in low temperature toughness, manufacturing method of thick welded steel pipe excellent in low temperature toughness, and steel plate for manufacturing thick welded steel pipe

摘要

The present invention provides a thick welded steel pipe excellent in low temperature toughness in which contents of Mn and Mo satisfy (Expression 1) below, Pcm obtained by (Expression 2) below is 0.16 to 0.19, and a metal structure of a base material steel plate consists of ferrite being 30 to 95% in an area ratio and a low temperature transformation structure, and in a metal structure of a 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)andPcm=[C]+[Si]/30+([Mn]+[Cu]+[Cr])/20+[Ni]/60+[Mo]/15+[V]/10  (Expression 2).

主权项

1. 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.