High-strength seamless steel pipe for oil country tubular goods and method of producing the same

A high-strength seamless steel pipe for oil country tubular goods having the composition which contains, by mass %, 0.20 to 0.50% C, 0.05 to 0.40% Si, 0.3 to 0.9% Mn, 0.015% or less P, 0.005% or less S, 0.005 to 0.1% Al, 0.008% or less N, 0.6 to 1.7% Cr, 0.4 to 1.0% Mo, 0.01 to 0.30% V, 0.01 to 0.06% Nb, 0.0003 to 0.0030% B, 0.0030% or less O (oxygen), and Fe and unavoidable impurities as a balance, wherein

• the steel pipe has the microstructure where a volume fraction of a tempered martensitic phase is 95% or more, and prior austenitic grains have a grain size number of 8.5 or more, and a segregation degree index Ps which is defined by a following formula (1) using XM which is a ratio between a segregated portion content obtained by performing an area analysis of respective elements by an electron probe micro analyzer (EPMA) in a region having the center thereof positioned at ¼ t (t:
• wall thickness) from an inner surface of the steel pipe and an average content is set to less than 65, and a yield strength YS is 758 MPa or more,
• wherein
• PS=8.1(XSi+XMn+XMo)+1.2XP (1)
• where XM:
• (segregated portion content (mass %) of element M)/(average content (mass %) of element M).