LOW ALLOY HIGH STRENGTH THICK-WALLED SEAMLESS STEEL PIPE FOR OIL COUNTRY TUBULAR GOODS

摘要

A low alloy high strength thick-walled seamless steel pipe for oil country tubular goods is provided having a wall thickness of 40 mm or more and a yield strength of 758 MPa or more, the steel pipe including a composition containing, in terms of mass %, C: 0.25 to 0.31%, Si: 0.01 to 0.35%, Mn: 0.55 to 0.70%, P: 0.010% or less, S: 0.001% or less, O: 0.0015% or less, Al: 0.015 to 0.040%, Cu: 0.02 to 0.09%, Cr: 0.8 to 1.5%, Mo: 0.9 to 1.6%, V: 0.04 to 0.10%, Nb: 0.005 to 0.05%, B: 0.0015 to 0.0030%, Ti: 0.005 to 0.020%, and N: 0.005% or less, and having Ti/N of 3.0 to 4.0, with the balance being Fe and inevitable impurities, wherein a cumulative frequency rate at a measurement point at which a Mo segregation degree by a predetermined expression is 1.5 or more is 1% or less.