SEAMLESS STEEL PIPE FOR LINE PIPE AND METHOD FOR PRODUCING THE SAME

摘要

A seamless steel pipe for line pipe has a chemical composition consisting, by mass percent, of C: 0.03-0.15%, Si: ≦0.50%, Mn: 1.0-2.0%, P: ≦0.050%, S: ≦0.005%, Cr: 0.1-1.0%, Al: 0.001-0.10%, N: ≦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≦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.

主权项

1. A seamless steel pipe for line pipe having a chemical composition consisting, by mass percent, of
C: 0.03 to 0.15%, Si: 0.50% or less, Mn: 1.0 to 2.0%, P: 0.050% or less, S: 0.005% or less, Cr: 0.1 to 1.0%, Al: 0.001 to 0.10%, N: 0.01% or less, Ni: 0.05 to 2.0%, B: 0.0003 to 0.0015%, Ca: 0.0002 to 0.0050%, Mo: 0.10 to 0.50%, Ti: 0.001 to 0.05%, Cu: 0 to 2.0%, Nb: 0 to 0.05%, V: 0 to 0.10%, the balance: Fe and impurities, and satisfying the following formula (i), 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 to 5 μm 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 μm or longer:
2Nb+4V+Mo≦0.50  (i) where each symbol of element in formula (i) represents the content (mass %) of each element.