Nippon Steel Corporation

Nippon Steel Sumitomo Metal Corporation

Yukimasa Ueda

Yuji Arai

Yasuyoshi Hidaka

Kei Murata

A seamless steel pipe for line pipe has a chemical composition consisting, by mass percent, of C: 0.03-0.15%, Si: &#x2264;0.50%, Mn: 1.0-2.0%, P: &#x2264;0.050%, S: &#x2264;0.005%, Cr: 0.1-1.0%, Al: 0.001-0.10%, N: &#x2264;0.01%, Ni: 0.05-2.0%, B: 0.0003-0.0015%, Ca: 0.0002-0.0050%, Mo: 0.10-0.50%, Ti: 0.001-0.05%, Cu: 0-2.0%, Nb: 0-0.05%, V: 0-0.10%, the balance: Fe and impurities, and satisfying the conditions of 2Nb+4V+Mo&#x2264;0.50, wherein a metal micro-structure of the steel pipe contains 50% or more of bainite, in an area fraction, a wall thickness of the steel pipe is 25 mm or larger, and in a scale formed on the surface of the steel pipe, metal particles consisting mainly of Ni or Cu having an average circle-equivalent diameter of 0.1-5 mm exist. A distance from a boundary between the base metal of the steel pipe and the scale to a region in which the metal particles do not exist is 20 mm or longer.

Seamless steel pipe for line pipe and method for producing the same

There is provided a seamless steel pipe having high strength and high toughness even if having a thick wall. A seamless steel pipe according to the present embodiment consists of: in mass %, C: 0.03 to 0.08%, Si: not more than 0.25%, Mn: 0.3 to 2.0%, P: not more than 0.05%, S: not more than 0.005%, Al: 0.001 to 0.10%, Cr: 0.02 to 1.0%, Ni: 0.02 to 1.0%, Mo: 0.02 to 0.8%, N: 0.002 to 0.008%, Ca: 0.0005 to 0.005%, and Nb: 0.01 to 0.1%, the balance being Fe and impurities, and has a wall thickness of not less than 50 mm. In a cross section perpendicular to an axial direction of the seamless steel pipe, an average crystal grain size of prior austenite grains in a near surface portion is less than 80 &#x3bc;m, the near surface portion being a 500 &#x3bc;m&#xd7;500 &#x3bc;m area centered at a position of a depth of 2 mm from a surface, and a difference between the average crystal grain size of the prior austenite grains in the near surface portion and an average crystal grain size of prior austenite grains in a central portion of a wall thickness of the cross section is less than 50 &#x3bc;m, the central portion being a 500 &#x3bc;m&#xd7;500 &#x3bc;m area centered at a center position of the wall thickness.

Seamless steel pipe and method for producing the same

There is provided a seamless steel pipe for line pipe, wherein a chemical composition consists, by mass percent, of C: 0.03-0.10%, Si: &#x2266;0.50%, Mn: 1.0-2.0%, P: &#x2266;0.050%, S: &#x2266;0.005%, Cr: 0.05-1.0%, Mo: 0.01-0.30%, Al: 0.001-0.10%, N: &#x2266;0.01%, Ni: 0.04-2.0%, Ca: 0.0005-0.0050%, Cu: 0-2.0%, Ti: 0-0.05%, Nb: 0-0.05%, V: 0-0.10%, the balance: Fe and impurities, and satisfies the conditions of Cu+Ni: &#x2267;0.10%, and Mo+V: &#x2266;0.30%, wherein in a scale formed on the surface of the steel pipe, metal particles consisting mainly of Ni or Cu having an average circle-equivalent diameter of 0.1-5 &#x3bc;m exist, and a distance from a boundary between the base metal of the steel pipe and the scale to a region in which the metal particles do not exist is 20 &#x3bc;m or longer.

Growing community of inventors

Tokyo, Japan

Yukimasa Ueda