Seamless Steel Tube

Abstract: A seamless steel tube having an inner surface and an outer surface, and having a chemical composition of, by mass, 0.06% or less of C, 0.55% or less of Si, 0.70-1.40% of Mn, 0.020% or less of P, 0.0005-0.020% of S, 0.005% or less of N, 0.0005-0.0035% of O, 0.25-0.45% of Cu, 0.50% or less of Ni, 0.20% or less of Mo, and 0.05-0.15% of Sb, with the remainder made up by Fe and impurities, the metal structure containing 90.0% or more of ferrite by area, the average crystal grain diameter of ferrite in the tube inner surface layer being 3.0-20.0 µm, the average crystal grain diameter of ferrite in the tube outer surface layer being 3.0-25.0 µm, the tube inner surface layer and the tube outer surface layer containing MnS and MnS oxide, the number density of MnS being 100/mm2, and the ratio of the number density of MnS oxide relative to the number density of MnS being 0.10 or more.

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