Nippon Steel Sumitomo Metal Corporation

Nippon Steel Corporation

Naoki Doi

Takuya Hara

Yasuhiro Shinohara

Taishi Fujishiro

Eiichi Yamashita

Naoshi Ayukawa

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&#x2266;(0.85&#xd7;[Mn]&#x2212;[Mo])&#x2266;1.5215&#x2003;&#x2003;(Expression 1)andPcm=[C]+[Si]/30+([Mn]+[Cu]+[Cr])/20+[Ni]/60+[Mo]/15+[V]/10&#x2003;&#x2003;(Expression 2).

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

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Tokyo, Japan

Naoki Doi