High strength stainless steel seamless steel pipe for oil well excellent in low temperature toughness and method of manufacturing the same

摘要

PROBLEM TO BE SOLVED: To provide a high strength seamless steel pipe excellent in low temperature toughness and a manufacturing method therefor.SOLUTION: A high strength seamless steel pipe has a structure containing 50% or more of M phase, 3 to 15% of γ phase and the balance F phase in volume ratio in a thickness center, having a ratio between Ni concentration in the γ phase and Ni concentration in the M phase, (C)/(C)of 1.15 or more and further consisting of 3 or more particle martensite having particle diameter of 10 μm or less distributed per ferrite particle of 100 μmor less by conducting a first treatment of heating a steel raw material having a composition containing, by mass%, C:0.050% or less, Si:0.50% or less, Mn:0.20 to 1.80%, Cr:15.5 to 18.0%, Ni:1.5 to 5.0%, Mo:1.0 to 3.5%, V:0.02 to 0.20%, Al:0.05% or less, N:0.01 to 0.15% to a ferrite single phase zone, accelerate cooling to a predetermined cooling stop temperature, obtaining a phase distribution of non-equilibrium state and conducting hot processing, cooling and then further heating to two phase temperature zone and quick cooling and a second treatment of heating to 680°C or less. Thereby the structure of the seamless steel pipe can be refined to a thickness center with relatively small processed amount and low temperature toughness is achieved even in especially thick seamless steel pipe of which processed amount in the thickness center cannot be large.SELECTED DRAWING: None