Low-alloy steel for oil well pipe and method of manufacturing low-alloy steel oil well pipe

1. A low-alloy steel for oil well pipe, having a chemical composition consisting of, by mass percent,

C: more than 0.45 and up to 0.65%;

Si: 0.05 to 0.50%;

Mn: 0.10 to 1.00%;

P: up to 0.020%;

S: up to 0.0020%;

Cu: up to 0.1%;

Cr: 0.40 to 1.50%;

Ni: up to 0.1%;

Mo: 0.50 to 2.50%;

Ti: up to 0.01%;

V: 0.05 to 0.25%;

Nb: 0.005 to 0.20%;

Al: 0.010 to 0.100%;

B: up to 0.0005%;

Ca: 0 to 0.003%;

O: up to 0.01%;

N: up to 0.007%; and

the balance: Fe and impurities,

the steel having a microstructure consisting of tempered martensite and retained austenite in less than 2% in volume fraction,

a crystal grain size number of prior austenite grains of the microstructure being 9.0 or larger,

a number density of carbonitride-based inclusions with a grain diameter of 50 μm or larger being 10 inclusions/100 mm 2 or smaller, and

a yield strength being 965 MPa or higher.

2. The low-alloy steel for oil well pipe according to claim 1 , wherein a number density of carbonitride-based inclusions with a grain diameter of 5 μm or larger is 600 inclusions/100 mm 2 or smaller.

3. The low-alloy steel for oil well pipe according to claim 1 , wherein an equivalent circle diameter of sub-structures defined by those boundaries between packets, blocks and laths in the tempered martensite that have a crystal misorientation of 15° or larger is 3 μm or smaller.

4. A method of manufacturing a low-alloy steel oil well pipe, the low-alloy steel oil well pipe, comprising:

preparing a raw material having a chemical composition consisting of, by mass percent, C: more than 0.45 and up to 0.65%; Si: 0.05 to 0.50%; Mn: 0.10 to 1.00%; P: up to 0.020%; S: up to 0.0020%; Cu: up to 0.1%; Cr: 0.40 to 1.50%; Ni: up to 0.1%; Mo: 0.50 to 2.50%; Ti: up to 0.01%; V: 0.05 to 0.25%; Nb: 0.005 to 0.20%; Al: 0.010 to 0.100%; B: up to 0.0005%; Ca: 0 to 0.003%; 0: up to 0.01%; N: up to 0.007%; and the balance: Fe and impurities;

casting the raw material to produce a cast material;

hot working the cast material to produce a hollow shell;

quenching the hollow shell; and

tempering the quenched hollow shell to manufacture the low-alloy steel for oil well pipe according to claim 1 ,

wherein, in the casting, a cooling rate for a temperature range of 1500 to 1000° C. at a position of ¼ of a wall thickness of the cast material is 10° C./min or higher.

5. The method of manufacturing a low-alloy steel oil well pipe according to claim 4 , wherein, in the casting, the cooling rate for the temperature range of 1500 to 1000° C. at the position of ¼ of the wall thickness of the cast material is 30° C./min or lower.

6. The method of manufacturing a low-alloy steel oil well pipe according to claim 4 , wherein the quenching includes:

heating the hollow shell to a temperature equal to or higher than Ac 3 point; and

cooling the heated hollow shell to a temperature equal to or lower than 100° C.,

in the cooling, a cooling rate for a temperature range from 500° C. to 100° C. is equal to or higher than 1° C./sec and lower than 15° C./sec.

7. The low-alloy steel for oil well pipe according to claim 2 , wherein an equivalent circle diameter of sub-structures defined by those boundaries between packets, blocks and laths in the tempered martensite that have a crystal misorientation of 15° or larger is 3 μm or smaller.

8. The method of manufacturing a low-alloy steel oil well pipe according to claim 5 , wherein the quenching includes:

heating the hollow shell to a temperature equal to or higher than Ac 3 point; and

cooling the heated hollow shell to a temperature equal to or lower than 100° C.,

in the cooling, a cooling rate for a temperature range from 500° C. to 100° C. is equal to or higher than 1° C./sec and lower than 15° C./sec.

9. The low-alloy steel for oil well pipe according to claim 1 , wherein the C content of the chemical composition is 0.51 to 0.65.