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

摘要

The present invention provides a thick welded steel pipe having excellent low-temperature toughness. The Mn and Mo content of this thick welded steel pipe having excellent low-temperature toughness satisfies formula 1 below, and Pcm, derived according to formula 2 below, is 0.16 to 0.19. The metal structure of the steel sheet base material comprises, in area ratio, 30 to 95% ferrite and a low-temperature transformation structure, in the metal structure of a coarse-grained HAZ, the area ratio of a ferrite grain boundary is at least 1.5%, the total area ratio of the ferrite grain boundary and intragranular ferrite is 25% to 90%, the area ratio of MA is not more than 10%, and the remainder comprises bainite. 1.2325 ≤ (0.85 × [Mn]-[Mo]) ≤ 1.5215 (formula 1) Pcm = [C]+[Si]/30+([Mn]+[Cu]+[Cr])/20+[Ni]/60+[Mo]/15+[V]/10 (formula 2)