Abstract: A thick-wall oil-well steel pipe has a wall thickness of 40 mm or more, excellent SSC resistance and high strength. The thick-wall oil-well steel pipe has a composition containing, in mass %, C: 0.40 to 0.65%, Si: 0.05 to 0.50%, Mn: 0.10 to 1.0%, P: 0.020% or less, S: 0.0020% or less, sol. Al: 0.005 to 0.10%, Cr more than 0.40 to 2.0%, Mo: more than 1.15 to 5.0%, Cu: 0.50% or less, Ni: 0.50% or less, N: 0.007% or less, and O: 0.005% or less. The number of carbide which has a circle equivalent diameter of 100 nm or more and contains 20 mass % or more of Mo is 2 or less per 100 mm2. The thick-wall oil-well steel pipe has yield strength of 827 MPa or more. A difference between a maximum value and a minimum value of the yield strength in the wall-thickness direction is 45 MPa or less.

Abstract: A steel for oil country tubular goods includes, as a chemical composition, by mass %, C, Si, Mn, Al, Mo, P, S, O, N, and a balance containing Fe and impurities, wherein a full width at half maximum HW of a crystal plane corresponding to a (211) crystal plane of an ? phase and a carbon content expressed in mass % in the chemical composition satisfy HW×C1/2?0.38, the carbon content and a molybdenum content expressed in mass % in the chemical composition satisfy C×Mo?0.6, a number of M2C carbides having a hexagonal crystal structure and having an equivalent circle diameter of 1 nm or more is 5 pieces or more per one square micron, and an yield strength is 758 MPa or more.

Abstract: A thick-wall oil-well steel pipe has a wall thickness of 40 mm or more, excellent SSC resistance and high strength. The thick-wall oil-well steel pipe has a composition containing, in mass %, C: 0.40 to 0.65%, Si: 0.05 to 0.50%, Mn: 0.10 to 1.0%, P: 0.020% or less, S: 0.0020% or less, sol. Al: 0.005 to 0.10%, Cr more than 0.40 to 2.0%, Mo: more than 1.15 to 5.0%, Cu: 0.50% or less, Ni: 0.50% or less, N: 0.007% or less, and O: 0.005% or less. The number of carbide which has a circle equivalent diameter of 100 nm or more and contains 20 mass % or more of Mo is 2 or less per 100 mm2. The thick-wall oil-well steel pipe has yield strength of 827 MPa or more. A difference between a maximum value and a minimum value of the yield strength in the wall-thickness direction is 45 MPa or less.

Abstract: A steel for oil country tubular goods includes, as a chemical composition, by mass %, C, Si, Mn, Al, Mo, P, S, O, N, and a balance containing Fe and impurities, wherein a full width at half maximum HW of a crystal plane corresponding to a (211) crystal plane of an ? phase and a carbon content expressed in mass % in the chemical composition satisfy HW×C1/2?0.38, the carbon content and a molybdenum content expressed in mass % in the chemical composition satisfy C×Mo?0.6, a number of M2C carbides having a hexagonal crystal structure and having an equivalent circle diameter of 1 nm or more is 5 pieces or more per one square micron, and an yield strength is 758 MPa or more.