Nippon Steel Corporation

Nippon Steel Sumitomo Metal Corporation

Taishi Fujishiro

Takuya Hara

Hitoshi Asahi

Yoshio Terada

Yasuhiro Shinohara

Shinya Sakamoto

Taro Muraki

Naoki Doi

Eiichi Yamashita

Izuru Minato

Go Suzuki

Naoshi Ayukawa

A high-strength welded steel pipe is obtained by welding a seam weld portion of a steel plate that are formed in a pipe shape. In the high-strength welded steel pipe, 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 the balance of Fe and inevitable impurities, Ceq is 0.30 to 0.53, Pcm is 0.10 to 0.20, [N]&#x2212;[Ti]/3.4 is less than 0.003, the average grain size of the prior &#x3b3; grains in heat affected zones in the steel plate is 250 &#x3bc;m or less, and the prior &#x3b3; grains include bainite and intragranular bainite.

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

The present invention provides high strength steel plate with excellent low temperature toughness, high strength steel pipe using this as a base metal, and methods of production of the same. The steel plate of the present invention contains Mo: 0.05 to 1.00% and B: 0.0003 to 0.0100%, has a Ceq of 0.30 to 0.53, has a Pcm of 0.10 to 0.20, and has a metal structure which has an area percentage of polygonal ferrite of 20 to 90% and has a balance of a hard phase comprised of one or both of bainite and martensite. To obtain this steel plate, strain-introducing rolling is performed with a start temperature of not more than Ar+60&#xb0; C., an end temperature of Aror more, and a reduction ratio of 1.5 or more, then the plate is air-cooled and then acceleratedly cooled from Ar&#x2212;100&#xb0; C. to Ar&#x2212;10&#xb0; C. in temperature by 10&#xb0; C./s or more.

High strength steel plate, steel pipe with excellent low temperature toughness, and method of production of same

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Tokyo, Japan

Taishi Fujishiro