High-strength steel material for oil well and oil country tubular goods

1. A high-strength steel material for oil well having a chemical composition consisting, by mass percent, of

C: 0.70 to 1.8%,

Si: 0.05 to 1.00%,

Mn: 12.0 to 25.0%,

Al: 0.003 to 0.06%,

P: 0.03% or less,

S: 0.03% or less,

N: 0.10% or less,

V: more than 0.5% and 2.0% or less,

Cr: 0 to 2.0%,

Mo: 0 to 3.0%,

Cu: 0 to 1.5%,

Ni: 0 to 1.5%,

Nb: 0 to 0.5%,

Ta: 0 to 0.5%,

Ti: 0 to 0.5%,

Zr: 0 to 0.5%,

Ca: 0 to 0.005%,

Mg: 0 to 0.005%,

B: 0 to 0.015%,

the balance: Fe and impurities,

satisfying the following formula (i),

wherein a metal microstructure consists of an austenite single phase with a total volume fraction of α′ martensite and ferrite being less than 0.1%, and a volume fraction of ε martensite being 10% or less,

V carbides having circle equivalent diameters of 5 to 100 nm exist at a number density of 20 pieces/μm 2 or higher, and

a yield strength is 654 MPa or higher;

0.6≤C-0.18V-0.06Cr<1.44  (i)

where, the symbol of an element in the formula represents a content in mass % of the element contained in the high-strength steel material, and is made zero in a case where the element is not contained.

2. The high-strength steel material for oil well according to claim 1 ,

wherein the yield strength is 758 MPa or higher.

3. Oil country tubular goods, which are comprised of the high-strength steel material for oil well according to claim 1 .

4. Oil country tubular goods, which are comprised of the high-strength steel material for oil well according to claim 2 .