High-strength welded steel pipe and method for producing the same

A high-strength welded steel pipe obtained by welding a seam weld portion of a steel plate into forming a pipe shape,

• wherein a base metal of the steel plate includes, by mass %,
• C:
• 0.010% to 0.080%,
• Si:
• 0.01% to 0.50%,
• Mn:
• 0.50% to 2.00%,
• S:
• 0.0001% to 0.0050%,
• Ti:
• 0.003% to 0.030%,
• Mo:
• 0.05% to 1.00%,
• B:
• 0.0003% to 0.0100%,
• O:
• 0.0001% to 0.0080%,
• N:
• 0.006% to 0.0118%,
• P:
• limited to 0.050% or less,
• Al:
• limited to 0.008% or less, and
• a balance of iron and inevitable impurities,
• wherein a Ceq that is obtained from a following equation <7> is 0.30 to 0.53, a Pcm that is obtained from a following equation <8> is 0.10 to 0.20, the [N] and the [Ti] satisfy a following equation <9>, wherein an average grain size of prior γ grains in a heat affected zone in the steel plate is 250 μm or less, and the prior γ grains include bainite and intragranular bainite,
• Ceq=[C]+[Mn]/6+([Ni]+[Cu])/15+([Cr]+[Mo]+[V])/5 <7>
• Pcm=[C]+[Si]/30+([Mn]+[Cu]+[Cr])/20+[Ni]/60+[Mo]/15+[V]/10+5[B] <8>
• [N]−[Ti]/3.4<0.003% <9>
• wherein [C], [Si], [Mn], [Ni], [Cu], [Cr], [Mo], [V], [B], [N], and [Ti] are defined as the amounts of C, Si, Mn, Ni, Cu, Cr, Mo, V, B, N, and Ti in mass %, respectively.