Thick-wall oil-well steel pipe and production method thereof
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 mm 2 . 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.
1. A thick-wall oil-well steel pipe having a wall thickness of 40 mm or more, and having a chemical composition consisting of, 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,
O: 0.005% or less,
V: 0 to 0.25%,
Nb: 0 to 0.10%,
Ti: 0 to 0.05%,
Zr: 0 to 0.10%,
W: 0 to 1.5%,
B: 0 to 0.005%,
Ca: 0 to 0.003%,
Mg: 0 to 0.003%, and
rare earth metal: 0 to 0.003%, with the balance being Fe and impurities, wherein
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 μm 2 , and wherein
the thick-wall oil-well steel pipe has yield strength of 827 MPa or more, and a difference between a maximum value and a minimum value of the yield strength in a wall-thickness direction is 45 MPa or less.
2. A method for producing a thick-wall oil-well steel pipe according to claim 1 , comprising the steps of:
producing a steel pipe having the chemical composition consisting of, 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,
O: 0.005% or less,
V: 0 to 0.25%,
Nb: 0 to 0.10%,
Ti: 0 to 0.05%,
Zr: 0 to 0.10%,
W: 0 to 1.5%,
B: 0 to 0.005%,
Ca: 0 to 0.003%,
Mg: 0 to 0.003%, and
rare earth metal: 0 to 0.003%, with the balance being Fe and impurities,
subjecting the steel pipe to quenching once or multiple times, wherein a quenching temperature in the quenching of at least once is 925 to 1100° C., and
subjecting the steel pipe to tempering after the quenching.