Thick-walled high-strength seamless steel pipe with excellent sour resistance for pipe for pipeline, and process for producing same

1. A seamless steel pipe having a yield strength exceeding 450 MPa and prepared by performing a quenching and tempering treatment, wherein a Vickers hardness HV5 that can be measured at an outermost side or an innermost side of the pipe under a 5 kgf load (test load: 49 N) is 250 HV5 or less.

2. The seamless steel pipe according to claim 1 , wherein a hardness distribution across the entire region of the seamless steel pipe in a wall thickness direction has an M-shape profile.

3. The seamless steel pipe according to claim 1 , wherein a hardness distribution across the entire region of the seamless steel pipe in a wall thickness direction has a U-shape profile.

4. The seamless steel pipe according to claim 2 , wherein the hardness distribution across the entire region of the seamless steel pipe in a wall thickness direction has the M-shape profile and a maximum hardness in terms of a Vickers hardness HV5 measured under a 5 kgf load (test load: 49 N) is 250 HV5 or less.

5. The seamless steel pipe according to claim 1 , wherein the seamless steel pipe has a composition that contains, in terms of mass %,

C: 0.03 to 0.15%, Si: 0.02 to 0.5%,

Mn: 0.7 to 2.5%, P: 0.020% or less,

S: 0.003% or less, Al: 0.01 to 0.08%,

Ti: 0.005 to 0.05%, N: 0.005% or less,

and the balance being Fe and unavoidable impurities, where Ti and N are contained to satisfy expression (1) below:

N≤Ti×14/48≤N+10  (1)

where Ti and N represent contents of respective elements (mass ppm).

6. The seamless steel pipe according to claim 5 , wherein the composition further contains, in terms of mass %, at least one selected from Cr: 0.5% or less, Mo: 0.3% or less, Ni: 0.3% or less, Cu: 0.3% or less, V: 0.05% or less, and Nb: 0.05% or less.

7. The seamless steel pipe according to claim 5 , wherein the composition further contains, in terms of mass %, Ca: 0.002% or less.

8. A method for producing a seamless steel pipe, the method comprising performing a quenching treatment and a tempering treatment on a crude steel pipe so as to form a product steel pipe having a yield strength exceeding 450 MPa,

wherein the crude steel pipe is a seamless steel pipe that has a composition that contains, in terms of mass %,

C: 0.03 to 0.15%, Si: 0.02 to 0.5%,

Mn: 0.7 to 2.5%, P: 0.020% or less,

S: 0.003% or less, Al: 0.01 to 0.08%,

Ti: 0.005 to 0.05%, N: 0.005% or less, and

the balance being Fe and unavoidable impurities, where Ti and N are contained to satisfy expression (1) below:

N≤Ti×14/48≤N+10  (1)

where Ti and N represent contents of respective elements (mass ppm);

the quenching treatment includes performing heating at a temperature equal to or higher than the Ac3 transformation point and then performing rapid cooling; and

after the quenching treatment, surface layers are ground from surfaces by a depth of 0.3 mm or more in a wall thickness direction and then the tempering treatment is performed.

9. A method for producing a seamless steel pipe, the method comprising performing a quenching treatment and a tempering treatment on a crude steel pipe so as to form a product steel pipe having a yield strength exceeding 450 MPa,

wherein the crude steel pipe is a seamless steel pipe that has a composition that contains, in terms of mass %,

C: 0.03 to 0.15%, Si: 0.02 to 0.5%,

Mn: 0.7 to 2.5%, P: 0.020% or less,

S: 0.003% or less, Al: 0.01 to 0.08%,

Ti: 0.005 to 0.05%, N: 0.005% or less, and

the balance being Fe and unavoidable impurities, where Ti and N are contained to satisfy expression (1) below:

N≤Ti×14/48≤N+10  (1)

where Ti and N represent contents of respective elements (mass ppm); and

the quenching treatment includes performing heating and then performing rapid cooling, the heating includes holding a heating temperature equal to or higher than the Ac3 transformation point for 120 seconds or more in an air atmosphere, and the rapid cooling includes performing a water-cooling treatment in a nucleate boiling region.

10. The method for producing a seamless steel pipe according to claim 9 wherein the rapid cooling includes, instead of performing the water-cooling treatment in a nucleate boiling region, performing water-cooling in a film boiling region and then performing water-cooling in a nucleate boiling region.

11. The method for producing a seamless steel pipe according to claim 8 , wherein the heating is performed through a heating furnace injection system, an electric resistance heating system, or an induction heating system.

12. The method for producing a seamless steel pipe according to claim 8 , wherein the composition further contains, in terms of mass %, at least one selected from Cr: 0.5% or less, Mo: 0.3% or less, Ni: 0.3% or less, Cu: 0.3% or less, V: 0.05% or less, and Nb: 0.05% or less.

13. The method for producing a seamless steel pipe according to claim 8 , wherein the composition further contains, in terms of mass %, Ca: 0.002% or less.

14. The seamless steel pipe according to claim 6 , wherein the composition further contains, in terms of mass %, Ca: 0.002% or less.

15. The method for producing a seamless steel pipe according to claim 9 , wherein the composition further contains, in terms of mass %, at least one selected from Cr: 0.5% or less, Mo: 0.3% or less, Ni: 0.3% or less, Cu: 0.3% or less, V: 0.05% or less, and Nb: 0.05% or less.

16. The method for producing a seamless steel pipe according to claim 9 , wherein the composition further contains, in terms of mass %, Ca: 0.002% or less.

17. The method for producing a seamless steel pipe according to claim 12 , wherein the composition further contains, in terms of mass %, Ca: 0.002% or less.

18. The method for producing a seamless steel pipe according to claim 15 , wherein the composition further contains, in terms of mass %, Ca: 0.002% or less.