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

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.

A manufacturing method of a thick welded steel pipe excellent in low temperature toughness having a thickness of 25 to 45 mm comprising:

• foaming a base material steel sheet obtained in a manner that a steel having 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 is cast, an obtained steel billet is heated to 950 to 1150° C., is subjected to hot rolling at an end temperature of Ar3 or higher, and is water-cooled down to 600° C. or lower, into a pipe shape;
• and
• when end portions of the base material steel sheet being butted to be seam welded by submerged arc welding from an inner surface and outer surface of the base material steel sheet, seam welding the end portions of the base material steel sheet such that heat input JI [kJ/cm] of the submerged arc welding from the inner surface, heat input JO [kJ/cm] of the submerged arc welding from the outer surface, and a sheet thickness t [mm] satisfy relationships of (Expression 3) and (Expression 4) below:
• 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)
• where [C], [Si], [Mn], [Cu], [Cr], [Ni], [Mo], and [V] denote the contents of C, Si, Mn, Cu, Cr, Ni, Mo, and V [mass %] respectively:
• −2.3≦JI−1.75 t≦27.8 (Expression 3)
• −9.6≦JO−2.42 t≦20.6 (Expression 4).

A steel sheet for a thick welded steel pipe used for manufacturing a thick welded steel pipe and having a thickness of 25 to 45 mm, the steel sheet for the thick welded steel pipe comprising:

• 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 consists of ferrite being 30 to 95% in an area ratio and a low temperature transformation structure,
• 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.