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

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.