Sumitomo Metal Industries, Inc.

Tomoaki Ikeda

Nobuaki Takahashi

Akio Yamamoto

Masahiko Hamada

Tetsuya Fukuba

Shigemichi Yamauchi

A welding method includes performing welding to form a weld metal consisting essentially of, in mass %,  and a remainder of Fe and unavoidable impurities. The amounts of impurities are P: at most 0.03%, S: at most 0.03%, N: at most 0.01%, Ca or Mg: at most 0.005%. The value of Pcm expressed by the following Equation (3) is in the range of 0.23-0.35%, and the time T until the weld metal is cooled to 100&#xb0; C. after welding satisfies the following Equation (1):(seconds)&#x2267;exp(7.0&#xd7;+4.66)&#x2003;&#x2003;Equation (1)/60&#x2003;&#x2003;Equation (2)(mass %)=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B&#x2003;&#x2003;Equation (3)

High strength steel weld having improved resistance to cold cracking and a welding method

The high yield ratio problem intrinsic in high-strength steel pipes is solved and a method of producing a steel pipe, while securing the pipe roundness is provided. A steel pipe produced from a steel plate by forming and welding is subjected to 0.3 to 1.2% of expansion and then to 0.1 to 1.0% of reduction. The steel pipe produced by this method has a structure in which martensite and/or bainite accounts for more than 80% as expressed in terms of area percentage, and has a yield strength of not lower than 551 MPa and a yield ratio of not higher than 93%. The pipe reduction is desirably carried out at a lower work ratio as compared with the pipe expansion.

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Kashima, Japan

Tomoaki Ikeda