Low alloy oil well steel pipe and method for manufacturing same

1. A low-alloy oil-well steel pipe characterized by comprising:

a chemical composition consisting, in mass %, of

C: 0.40 to 0.65%,

Si: 0.05 to 0.50%,

Mn: 0.10 to 1.00%,

P: 0.020% or less,

S: 0.0020% or less,

Cu: 0.15% or less,

Cr: 0.40 to 1.50%,

Mo: 0.50 to 2.50%,

V: 0.05 to 0.25%,

Ti: 0 to less than 0.01%,

Nb: 0.01 to 0.2%,

sol. Al: 0.010 to 0.100%,

N: 0.006% or less,

B: 0 to 0.0015%, and

Ca: 0 to 0.003%,

the balance being Fe and impurities; and

a structure consisting of tempered martensite and 0 to less than 2% in volume ratio of retained austenite,

wherein the steel pipe has yield strength of 965 MPa or more;

wherein a grain size number of a prior-austenite grain in the structure is 9.0 or more; and

wherein in the tempered martensite, a equivalent circular diameter of a sub-structure surrounded by a boundary having a crystal orientation difference of 15° or more from among a packet boundary, a block boundary and a lath boundary is 3 μm or less.

2. A method for manufacturing a low-alloy oil-well steel pipe, which has a yield stress of 965 MPa or more, comprising:

a hot working step of hot-working a starting material having the chemical composition according to claim 1 to form a hollow shell;

a final quenching step of subjecting the hollow shell to quenching in which a cooling rate when a temperature of the hollow shell is between 500° C. and 100° C. is set to 1° C./s to less than 15° C./s and the temperature of the hollow shell at which cooling is stopped is set to 100° C. or less; and

a step of tempering the quenched hollow shell.

3. The method according to claim 2 , characterized in that

the method further comprises an intermediate heat treatment step of soaking the hollow shell at a temperature of an A 1 point or more after the hot working step and before the final quenching step.