Seamless steel pipe and method of manufacture thereof

A seamless steel pipe having:

• a t/Dout of 0.05 to 0.40, where t is a wall thickness (mm) of the seamless steel pipe, and Dout is an outside diameter (mm) of the seamless steel pipe,
• a maximum depth dmax (mm)≤0.350 for defects in an inner surface of the seamless steel pipe in a cross section of the seamless steel pipe perpendicular to a pipe axis,
• an average defect depth dave (mm)≤0.200 for defects having a depth of 0.050 mm or more in the inner surface of the seamless steel pipe,
• per millimeter of an inner circumferential length of the seamless steel pipe, at most 30 defects having a depth of 0.050 mm or more in the inner surface of the seamless steel pipe,
• a chemical composition comprising, in mass %:
• C:
• 0.05 to 0.45%,
• Si:
• 0.05 to 0.45%,
• Mn:
• 0.05 to 1.2%, and
• Cr:
• less than 4.5%, and
• a microstructure comprising ferrite grains having an average grain size of 15 μm or less.