Steel for oil well pipe having excellent sulfide stress cracking resistance and method for manufacturing seamless steel pipe for oil well

摘要

A steel for an oil well pipe, having high strength and excellent SSC resistance, and a method for producing a seamless steel pipe for an oil well having these characteristics. The steel consists of, by mass %, C: 0.30 to 0.60%, Si: 0.05 to 0.5%, Mn: 0.05 to 1.0%, Al: 0.005 to 0.10%, Cr+Mo: 1.5 to 3.0%, wherein Mo is 0.5% or more, V: 0.05 to 0.3%, Nb: 0 to 0.1 %, Ti: 0 to 0.1%, Zr: 0 to 0.1%, N (nitrogen): 0 to 0.03%, Ca: 0 to 0.01%, and the balance Fe and impurities. Among impurities P is 0.025% or less, S is 0.01% or less, B is 0.0010% or less and O (oxygen) is 0.01% or less. The method is characterized by heating a steel ingot having the above chemical composition at 1150°C or more; producing a seamless steel pipe by hot working; water-cooling the pipe to a temperature in a range of 400 to 600°C immediately after finishing the working; and subjecting the pipe to a heat treatment for bainite isothermal transformation in a range of 400 to 600°C. A complementary heating treatment may be performed in a range of 900 to 950°C before water-cooling.