Nippon Steel Corporation

Japan Atomic Energy Research Institute

Nippin Steel Corporation

Shigeru Ohkita

Takuya Hara

Hitoshi Asahi

Hiroshige Inoue

Hiroshi Tamehiro

Yoshio Terada

Hirofumi Yoshimura

Masayuki Yamamoto

Junichi Kobayashi

Tadashi Kasuya

Kunio Koyama

Hideo Nakajima

Naoomi Yamada

Masao Fuji

Naoki Saitou

Toshiaki Nishida

Takao Horiya

Toshihiko Chiba

Toshihiko Koseki

Toshinari Andou

To provide a super-high strength line pipe that is excellent in low temperature toughness, can be field welded easily, and has a tensile strength of at least 900 MPa (exceeding &times;100 of the API standard), and a production method thereof. The present invention relates to a super-high strength line pipe produced by shaping a steel plate into a pipe shape and arc welding seam portions, the strength of a base steel portion is 900 to 1,100 MPa and the strength of the weld metal is higher than the base steel strength &minus;100 MPa. In the steel pipe, the Ni content of the weld metal is higher by at least 1% than that of the base steel. The combination of the chemical components of the steel plate with those of the weld metal, for accomplishing these steel pipes by a U&O step is shown concretely. A production method of the steel plate and the welding method for achieving the steel pipe are also described. Furthermore, a method of reducing the strength of the inner surface of the weld metal to restrict cracking at the time of pipe expansion is also shown.

Super-high-strength line pipe excellent in low temperature toughness and production method thereof

In order to conduct welding so that the welded steel shows a decreased residual stress after welding without a post treatment such as a PWHT, the steel for welded structures and the welding wire of the present invention starts a transformation from austenite into martensite at a temperature (Ms temperature) from at least 200&deg; C. to up to 350&deg; C., and has a yield strength from at least 60 kg/mm to up to 120 kg/mm at the transformation starting temperature. The multipass welding process of the present invention comprises forming a weld metal having a Ms temperature of 150&deg; C. to 300&deg; C., and TIG remelt-run welding the surface of the final layer.

Steel for welded structures and welding wire

Wire for welding high-chromium steel

Titanium for conductor conduit in superconducting coil

Growing community of inventors

Futtsu, Japan

Shigeru Ohkita