Electric resistance welded steel pipe for oil well

1. An electric resistance welded steel pipe for an oil well, comprising, in terms of mass %:

0.02 to 0.14% of C,

0.05 to 0.50% of Si,

1.0 to 2.1% of Mn,

0.020% or less of P,

0.010% or less of S,

0.010 to 0.100% of Nb,

0.010 to 0.050% of Ti,

0.010 to 0.100% of Al, and

0.0100% or less of N,

wherein contents of Cu, Ni, Cr, Mo, V, B, Ca and Ce are:

0 to 0.50% of Cu,

0 to 1.00% of Ni,

0 to 0.50% of Cr,

0 to 0.30% of Mo,

0 to 0.10% of V,

0 to 0.0030% of B,

0 to 0.0050% of Ca, and

0 to 0.0050% of Ce respectively,

a remainder consisting of Fe and unavoidable impurities,

and wherein, in a pipe axis direction tensile test with a full thickness specimen, a tensile strength is 780 MPa or more, a ratio of a 0.2% proof stress to the tensile strength [0.2% proof stress/tensile strength] is 0.80 or more, and a ratio of a 2% flow stress to the tensile strength [2% flow stress/tensile strength] is from 0.85 to 0.98.

2. The electric resistance welded steel pipe for an oil well according to claim 1 , comprising, in terms of mass %, one or more of:

more than 0 but equal to or less than 0.0050% of Ca,

more than 0 but equal to or less than 0.30% of Mo,

more than 0 but equal to or less than 0.10% of V,

more than 0 but equal to or less than 0.50% of Cr,

more than 0 but equal to or less than 1.00% of Ni,

more than 0 but equal to or less than 0.50% of Cu,

more than 0 but equal to or less than 0.0030% of B, and

more than 0 but equal to or less than 0.0050% of Ce.

3. The electric resistance welded steel pipe for an oil well according to claim 1 , wherein a residual stress measured by a Crampton method is 300 MPa or less.

4. The electric resistance welded steel pipe for an oil well according to claim 1 , wherein a weld crack sensitivity composition Pcm defined by the following Formula (1) is 0.1800 or more:

Pcm =C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B  Formula (1):

wherein, in Formula (1), each of C, Si, Mn, Cu, Ni, Cr, Mo, V, and B shows a content (mass %) of each element.

5. The electric resistance welded steel pipe for an oil well according to claim 1 , wherein an average positron lifetime measured by a positron annihilation method is from 120 ps to 140 ps.

6. The electric resistance welded steel pipe for an oil well according to claim 1 , wherein 50% or more of prior austenite grains observed in a cross section parallel to a pipe axis direction and a wall thickness direction are prior austenite grains having an aspect ratio of 1.5 or more.

7. The electric resistance welded steel pipe for an oil well according to claim 1 , wherein a base metal toughness in a circumferential direction of the pipe, determined by subjecting a full size specimen with a V-notch to a Charpy impact test, is 30 J or more at 0° C.

8. The electric resistance welded steel pipe for an oil well according to claim 2 , wherein a residual stress measured by a Crampton method is 300 MPa or less.

9. The electric resistance welded steel pipe for an oil well according to claim 2 , wherein a weld crack sensitivity composition Pcm defined by the following Formula (1) is 0.1800 or more:

Pcm =C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B  Formula (1):

wherein, in Formula (1), each of C, Si, Mn, Cu, Ni, Cr, Mo, V, and B shows a content (mass %) of each element.

10. The electric resistance welded steel pipe for an oil well according to claim 2 , wherein an average positron lifetime measured by a positron annihilation method is from 120 ps to 140 ps.

11. The electric resistance welded steel pipe for an oil well according to claim 2 , wherein 50% or more of prior austenite grains observed in a cross section parallel to a pipe axis direction and a wall thickness direction are prior austenite grains having an aspect ratio of 1.5 or more.

12. The electric resistance welded steel pipe for an oil well according to claim 2 , wherein a base metal toughness in a circumferential direction of the pipe, determined by subjecting a full size specimen with a V-notch to a Charpy impact test, is 30 J or more at 0° C.

13. The electric resistance welded steel pipe for an oil well according to claim 1 , wherein the tensile strength is more than 780 MPa.

14. The electric resistance welded steel pipe for an oil well according to claim 1 , wherein the tensile strength is 790 MPa or more.

15. The electric resistance welded steel pipe for an oil well according to claim 1 , having a wall thickness of from 5 to 17 mm, wherein a ratio [D/t] of an outer diameter (D) to a wall thickness (t) is from 10.0 to 25.0.

16. An electric resistance welded steel pipe for an oil well, comprising, in terms of mass %:

0.02 to 0.14% of C,

0.05 to 0.50% of Si,

1.0 to 2.1% of Mn,

0.020% or less of P,

0.010% or less of S,

0.010 to 0.100% of Nb,

0.010 to 0.050% of Ti,

0.010 to 0.100% of Al, and

0.0100% or less of N,

wherein contents of Cu, Ni, Cr, Mo, V, B, Ca, and Ce are:

0 to 0.50% of Cu,

0 to 1.00% of Ni,

0 to 0.50% of Cr,

0 to 0.30% of Mo,

0 to 0.10% of V,

0 to 0.0030% of B,

0 to 0.0050% of Ca, and

0 to 0.0050% of Ce respectively,

a remainder comprising Fe and unavoidable impurities,

and wherein, in a pipe axis direction tensile test with a full thickness specimen, a tensile strength is 780 MPa or more, a ratio of a 0.2% proof stress to the tensile strength [0.2% proof stress/tensile strength] is 0.80 or more, and a ratio of a 2% flow stress to the tensile strength [2% flow stress/tensile strength] is from 0.85 to 0.98.

17. The electric resistance welded steel pipe for an oil well according to claim 16 , wherein the tensile strength is more than 780 MPa.

18. The electric resistance welded steel pipe for an oil well according to claim 16 , wherein the tensile strength is 790 MPa or more.

19. The electric resistance welded steel pipe for an oil well according to claim 16 , having a wall thickness of from 5 to 17 mm, wherein a ratio [D/t] of an outer diameter (D) to a wall thickness (t) is from 10.0 to 25.0.