Stainless steel seamless pipe for oil country tubular goods and method for manufacturing the same

1 . A stainless steel seamless pipe for oil country tubular goods having:

a composition that consists of, in mass %, C: 0.10% or less, Si: 0.5% or less, Mn: 0.05 to 0.50%, P: 0.030% or less, S: 0.005% or less, O: 0.0040% or less, Ni: 3.0 to 8.0%, Cr: 10.0 to 11.9%, Mo: 0.5 to 0.94%, Al: 0.1% or less, V: 0.005 to 0.2%, N: 0.10% or less, Cu: 0.01 to 1.0%, Co: 0.01 to 1.0%, and Ca: 0.0005 to 0.0030%, optionally one or more selected from Nb: 0.50% or less, W: 1.0% or less, Ta: 0.1% or less, Zr: 0.20% or less, REM: 0.010% or less, Mg: 0.010% or less, B: 0.010% or less, Sb: 0.20% or less, and Sn: 0.20% or less, and in which the balance is Fe and incidental impurities;

a microstructure containing non-metallic inclusions of an oxide containing calcium oxide, CaO, and aluminum oxide, Al 2 O 3 , in the steel, wherein a number of the non-metallic inclusions in the steel having a composition ratio satisfying the formula (1) below and a major axis of 5 μm or more is at most 20 per 100 mm 2 ; and

a yield stress of 806 MPa or more,

(CaO)/(Al 2 O 3 )≥4.0  (1),

where (CaO) and (Al 2 O 3 ) in formula (1) represent the content of CaO and the content of Al 2 O 3 in the non-metallic inclusions in the steel, in mass %.

2 . The stainless steel seamless pipe for oil country tubular goods according to claim 1 , wherein the composition consists of, in mass %, C: 0.10% or less, Si: 0.5% or less, Mn: 0.05 to 0.50%, P: 0.030% or less, S: 0.005% or less, O: 0.0040% or less, Ni: 3.0 to 8.0%, Cr: 10.0 to 11.9%, Mo: 0.5 to 0.94%, Al: 0.1% or less, V: 0.005 to 0.2%, N: 0.10% or less, Cu: 0.01 to 1.0%, Co: 0.01 to 1.0%, and Ca: 0.0005 to 0.0030%, one or two or more selected from Nb: 0.50% or less, W: 1.0% or less, Ta: 0.1% or less, and Zr: 0.20% or less, and optionally one or more selected from REM: 0.010% or less, Mg: 0.010% or less, B: 0.010% or less, Sb: 0.20% or less, and Sn: 0.20% or less, and in which the balance is Fe and incidental impurities.

3 . The stainless steel seamless pipe for oil country tubular goods according to claim 1 , wherein the composition consists of, in mass %, C: 0.10% or less, Si: 0.5% or less, Mn: 0.05 to 0.50%, P: 0.030% or less, S: 0.005% or less, O: 0.0040% or less, Ni: 3.0 to 8.0%, Cr: 10.0 to 11.9%, Mo: 0.5 to 0.94%, Al: 0.1% or less, V: 0.005 to 0.2%, N: 0.10% or less, Cu: 0.01 to 1.0%, Co: 0.01 to 1.0%, and Ca: 0.0005 to 0.0030%, one or two or more selected from REM: 0.010% or less, Mg: 0.010% or less, B: 0.010% or less, Sb: 0.20% or less, and Sn: 0.20% or less, and optionally one or more selected from Nb: 0.50% or less, W: 1.0% or less, Ta: 0.1% or less, and Zr: 0.20% or less, and in which the balance is Fe and incidental impurities.

4 . The stainless steel seamless pipe for oil country tubular goods according to claim 2 , wherein the composition consists of, in mass %, C: 0.10% or less, Si: 0.5% or less, Mn: 0.05 to 0.50%, P: 0.030% or less, S: 0.005% or less, O: 0.0040% or less, Ni: 3.0 to 8.0%, Cr: 10.0 to 11.9%, Mo: 0.5 to 0.94%, Al: 0.1% or less, V: 0.005 to 0.2%, N: 0.10% or less, Cu: 0.01 to 1.0%, Co: 0.01 to 1.0%, and Ca: 0.0005 to 0.0030%, one or two or more selected from Nb: 0.50% or less, W: 1.0% or less, Ta: 0.1% or less, and Zr: 0.20% or less, one or two or more selected from REM: 0.010% or less, Mg: 0.010% or less, B: 0.010% or less, Sb: 0.20% or less, and Sn: 0.20% or less, and in which the balance is Fe and incidental impurities.

5 . A method for manufacturing the stainless steel seamless pipe for oil country tubular goods of claim 1 ,

the method comprising:

forming a steel pipe material of said composition into a steel pipe;

quenching in which the steel pipe is heated to a temperature equal to or greater than an A c3 transformation point, and is cooled to a cooling stop temperature of 100° C. or less; and

tempering in which the steel pipe is tempered at a temperature equal to or less than an Act transformation point.

6 . A method for manufacturing the stainless steel seamless pipe for oil country tubular goods of claim 2 ,

the method comprising:

forming a steel pipe material of said composition into a steel pipe;

quenching in which the steel pipe is heated to a temperature equal to or greater than an A c3 transformation point, and is cooled to a cooling stop temperature of 100° C. or less; and

tempering in which the steel pipe is tempered at a temperature equal to or less than an A c1 transformation point.

7 . A method for manufacturing the stainless steel seamless pipe for oil country tubular goods of claim 3 ,

the method comprising:

forming a steel pipe material of said composition into a steel pipe;

quenching in which the steel pipe is heated to a temperature equal to or greater than an A c3 transformation point, and is cooled to a cooling stop temperature of 100° C. or less; and

tempering in which the steel pipe is tempered at a temperature equal to or less than an A c1 transformation point.

8 . A method for manufacturing the stainless steel seamless pipe for oil country tubular goods of claim 4 ,

the method comprising:

forming a steel pipe material of said composition into a steel pipe;

quenching in which the steel pipe is heated to a temperature equal to or greater than an A c3 transformation point, and is cooled to a cooling stop temperature of 100° C. or less; and

tempering in which the steel pipe is tempered at a temperature equal to or less than an A c1 transformation point.