Steel for oil country tubular goods and method of producing the same

1. A steel for oil country tubular goods, the steel consisting of, as a chemical composition, by mass %,

C: more than 0.35% to 1.00%,

Si: 0.05% to 0.5%,

Mn: 0.05% to 1.0%,

Al: 0.005% to 0.10%,

Mo: more than 1.0% to 10%,

P: 0.025% or less,

S: 0.010% or less,

O: 0.01% or less,

N: 0.03% or less,

Cr: 0% to 2.0%,

V: 0.07% to 0.30%,

Nb: 0% to 0.1%,

Ti: 0% to 0.1%,

Zr: 0% to 0.1%,

Ca: 0% to 0.01%,

B: 0% to 0.003%, and

a balance consisting of Fe and impurities,

wherein, when a full width at half maximum of a crystal plane corresponding to a (211) crystal plane of an a phase, which is obtained by an X-ray diffraction using CoKα1, is an HW in unit of °, the full width at half maximum HW and a carbon content expressed in mass % in the chemical composition satisfy a following Expression 1,

the carbon content and a molybdenum content expressed in mass % in the chemical composition satisfy a following Expression 2,

a number of M 2 C 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, wherein M of the M 2 C represents Mo, or Mo and V, and

a yield strength is 875 MPa or more,

HW ×C 1/2 ≤0.38  (Expression 1),

C×Mo≥0.60  (Expression 2).

2. The steel for the oil country tubular goods according to claim 1 , wherein the steel includes, as the chemical composition, by mass %, Cr :0.1% to 2.0%.

3. The steel for the oil country tubular goods according to claim 1 , wherein the steel includes, as the chemical composition, by mass %, at least one of

Nb: 0.002% to 0.1%,

Ti: 0.002% to 0.1%, and

Zr: 0.002% to 0.1%.

4. The steel for the oil country tubular goods according to claim 1 , wherein the steel includes, as the chemical composition, by mass %,

Ca: 0.0003% to 0.01%.

5. The steel for the oil country tubular goods according to claim 1 , wherein the steel includes, as the chemical composition, by mass %,

B: 0.0003% to 0.003%.