Seamless steel pipe

A seamless steel pipe having an inner surface and an outer surface;

• having a chemical composition comprising, by mass %,
• C:
• 0.06% or less,
• Si:
• 0.55% or less,
• Mn:
• 0.70 to 1.40%,
• P:
• 0.020% or less,
• S:
• 0.0005 to 0.020%,
• N:
• 0.005% or less,
• O:
• 0.0005 to 0.0035%,
• Cu:
• 0.25 to 0.45%,
• Ni:
• 0.50% or less,
• Mo:
• 0.20% or less, and
• Sb:
• 0.05 to 0.15%,
• with the balance being Fe and impurities;
• and
• having a metallographic structure including, by area %,
• ferrite:
• 90% or more,
• wherein the ferrite has an average grain size of 3.0 to 20.0 μm in the inner surface layer of the pipe and has an average grain size of 3.0 to 25.0 μm in the outer surface layer of the pipe, and
• wherein:
• the inner surface layer of the pipe contains MnS and MnS-oxide, the number density of MnS having a maximum length of 2.0 μm or more is less than 100/mm2, and the ratio of the number density of MnS-oxide having a maximum length of 2.0 μm or more to the number density of MnS having a maximum length of 2.0 μm or more is 0.10 or more;
• and
• the outer surface layer of the pipe contains MnS and MnS-oxide, the number density of MnS having a maximum length of 2.0 μm or more is less than 100/mm2 and the ratio of the number density of MnS-oxide having a maximum length of 2.0 μm or more to the number density of MnS having a maximum length of 2.0 μm or more is 0.10 or more.