Steel pipe for line pipe and method of manufacturing the same

1. A steel pipe for line pipe, having a chemical composition of, in mass %:

C: 0.02 to 0.11%;

Si: 0.05 to 1.0%;

Mn: 0.30 to 2.5%;

P: up to 0.030%;

S: up to 0.006%;

Cr: 0.05 to 0.36%;

Mo: 0.02 to 0.33%;

V: 0.02 to 0.20%;

Ti: 0.001 to 0.010%;

Al: 0.001 to 0.100%;

N: up to 0.008%;

Ca: 0.0005 to 0.0040%;

Cu: 0 to 1.5%;

Ni: 0 to 1.5%;

Nb: 0 to 0.05%; and

balance Fe and impurities,

the chemical composition satisfying Equation (1) below,

the steel pipe having a microstructure containing tempered martensite and/or tempered bainite and

further containing recrystallized ferrite in at least one of a portion between an outer surface of the steel pipe and a depth of 1 mm from the outer surface, and a portion between an inner surface of the steel pipe and a depth of 1 mm from the inner surface,

Cr+Mo+V≤0.40  (1),

wherein each of the chemical symbols in equation (1) is substituted for by the content of the corresponding element in mass %.

2. The steel pipe for line pipe according to claim 1 , wherein a Vickers hardness of a portion between a depth of 1 mm from the inner surface and a depth of 1 mm from the outer surface as measured in a wall thickness direction is not higher than 230 Hv.

3. The steel pipe for line pipe according to claim 1 , wherein, in a cross section perpendicular to an axial direction of the steel pipe, an area ratio of recrystallized ferrite at at least one of a depth of 1 mm from the outer surface of the steel pipe and a depth of 1 mm from the inner surface of the steel pipe is not lower than 5%.

4. The steel pipe for line pipe according to claim 1 , having a yield strength not lower than 415 MPa.

5. The steel pipe for line pipe according to claim 1 , wherein the chemical composition includes one or more elements selected from the group consisting of, in mass %:

Cu: 0.05 to 1.5%;

Ni: 0.05 to 1.5%; and

Nb: 0.001 to 0.05%.

6. The steel pipe for line pipe according to claim 2 , wherein, in a cross section perpendicular to an axial direction of the steel pipe, an area ratio of recrystallized ferrite at at least one of a depth of 1 mm from the outer surface of the steel pipe and a depth of 1 mm from the inner surface of the steel pipe is not lower than 5%.

7. The steel pipe for line pipe according to claim 2 , having a yield strength not lower than 415 MPa.

8. The steel pipe for line pipe according to claim 2 , wherein the chemical composition includes one or more elements selected from the group consisting of, in mass %:

Cu: 0.05 to 1.5%;

Ni: 0.05 to 1.5%; and

Nb: 0.001 to 0.05%.

9. The steel pipe for line pipe according to claim 3 , having a yield strength not lower than 415 MPa.

10. The steel pipe for line pipe according to claim 3 , wherein the chemical composition includes one or more elements selected from the group consisting of, in mass %:

Cu: 0.05 to 1.5%;

Ni: 0.05 to 1.5%; and

Nb: 0.001 to 0.05%.

11. The steel pipe for line pipe according to claim 4 , wherein the chemical composition includes one or more elements selected from the group consisting of, in mass %:

Cu: 0.05 to 1.5%;

Ni: 0.05 to 1.5%; and

Nb: 0.001 to 0.05%.

12. The steel pipe for line pipe according to claim 6 , having a yield strength not lower than 415 MPa.

13. The steel pipe for line pipe according to claim 6 , wherein the chemical composition includes one or more elements selected from the group consisting of, in mass %:

Cu: 0.05 to 1.5%;

Ni: 0.05 to 1.5%; and

Nb: 0.001 to 0.05%.

14. The steel pipe for line pipe according to claim 7 , wherein the chemical composition includes one or more elements selected from the group consisting of, in mass %:

Cu: 0.05 to 1.5%;

Ni: 0.05 to 1.5%; and

Nb: 0.001 to 0.05%.

15. The steel pipe for line pipe according to claim 9 , wherein the chemical composition includes one or more elements selected from the group consisting of, in mass %:

Cu: 0.05 to 1.5%;

Ni: 0.05 to 1.5%; and

Nb: 0.001 to 0.05%.

16. The steel pipe for line pipe according to claim 12 , wherein the chemical composition includes one or more elements selected from the group consisting of, in mass %:

Cu: 0.05 to 1.5%;

Ni: 0.05 to 1.5%; and

Nb: 0.001 to 0.05%.

17. A method of manufacturing a steel pipe for line pipe, comprising:

preparing a hollow shell having a chemical composition of, in mass %: C: 0.02 to 0.11%; Si: 0.05 to 1.0%; Mn: 0.30 to 2.5%; P: up to 0.030%; S: up to 0.006%; Cr: 0.05 to 0.36%; Mo: 0.02 to 0.33%; V: 0.02 to 0.20%; Ti: 0.001 to 0.010%; Al: 0.001 to 0.100%; N: up to 0.008%; Ca: 0.0005 to 0.0040%; Cu: 0 to 1.5%; Ni: 0 to 1.5%; Nb: 0 to 0.05%; and balance Fe and impurities;

quenching the hollow shell such that at least one of a maximum Vickers hardness of a portion between an outer surface of the hollow shell and a depth of 1 mm from the outer surface, and a maximum Vickers hardness of a portion between an inner surface of the hollow shell and a depth of 1 mm from the inner surface is not lower than 310 Hv; and

tempering the quenched hollow shell,

the chemical composition satisfying Equation (1) below,

Cr+Mo+V≤0.40  (1),

wherein each of the chemical symbols in equation (1) is substituted for by the content of the corresponding element in mass %.

18. The method of manufacturing a steel pipe for line pipe according to claim 17 , wherein a tempering parameter TP defined by Equation (2) below is not higher than 18800,

TP=( T+ 273)×(20+log( t ))  (2),

where T is the tempering temperature and t is the holding time for that temperature, and the unit of T is ° C. and the unit oft is the hour.

19. The method of manufacturing a steel pipe for line pipe according to claim 17 , wherein the chemical composition includes one or more elements selected from the group consisting of, in mass %:

Cu: 0.05 to 1.5%;

Ni: 0.05 to 1.5%; and

Nb: 0.001 to 0.05%.

20. The method of manufacturing a steel pipe for line pipe according to claim 18 , wherein the chemical composition includes one or more elements selected from the group consisting of, in mass %:

Cu: 0.05 to 1.5%;

Ni: 0.05 to 1.5%; and

Nb: 0.001 to 0.05%.