| 摘要 |
Provided is a seamless steel pipe having high strength and toughness even with thick walls. This seamless steel pipe contains C in an amount of 0.03 to 0.08%, Si in an amount of 0.25% or less, Mn in an amount of 0.3 to 2.0%, P in an amount of 0.05% or less, S in an amount of 0.005% or less, Al in an amount of 0.001 to 0.10%, Cr in an amount of 0.02 to 1.0%, Ni in an amount of 0.02 to 1.0%, Mo in an amount of 0.02 to 0.8%, N in an amount of 0.002 to 0.008%, Ca in an amount of 0.0005 to 0.005%, and Nb in an amount of 0.01 to 0.1% (amounts given in percent by mass), the remainder comprising Fe and impurities, and the wall thickness being 50 mm or more. In a cross section orthogonal to the axial direction of the seamless steel pipe, the average crystal grain size of old austenite grains is less than 80 µm in a surface layer section, which is a 500×500 µm area centered around a position 2 mm below the surface, and there is a difference of less than 50 µm between the average crystal grain size of old austenite grains in the surface layer section, and the average crystal grain size of old austenite grains in a thick-wall center section, which is a 500×500 µm area centered around a center position of the thick wall in the cross section. |
