Steel pipe for fuel injection pipe and method for producing the same

1. A steel pipe for fuel injection pipe, the steel pipe having:

an outer diameter of 20 mm or less;

a tensile strength of 500 to 900 MPa and a yield ratio of 0.50 to 0.85; and

a critical internal pressure satisfying a following formula (i):

IP≥ 0.41× TS×α   (i)

α=[( D/d ) 2 −1]/[0.776×( D/d ) 2 ]  (ii)

where, in the above formula (i), IP denotes the critical internal pressure (MPa) of the steel pipe, TS denotes the tensile strength (MPa) of the steel pipe, and α is a value expressed by the above formula (ii), and where, in the above formula (ii), D denotes an outer diameter (mm) of the steel pipe, and d denotes an inner diameter (mm) of the steel pipe, and wherein

a circumferential-direction residual stress on an inner surface of the steel pipe is −20 MPa or lower after the steel pipe is split in half in a pipe axis direction.

2. The steel pipe for fuel injection pipe according to claim 1 , wherein the steel pipe has a chemical composition consisting, by mass percent, of

C: 0.12 to 0.27%,

Si: 0.05 to 0.50%,

Mn: 0.3 to 2.0%,

Al: 0.005 to 0.060%,

N: 0.0020 to 0.0080%,

Ti: 0.005 to 0.040%,

Nb: 0.015 to 0.045%,

Cr: 0 to 1.0%,

Mo: 0 to 1.0%,

Cu: 0 to 0.5%,

Ni: 0 to 0.5%,

V: 0 to 0.15%,

B: 0 to 0.005%, and

the balance: Fe and impurities, and

contents of Ca, P, S, and O in the impurities being

Ca: 0.001% or less,

P: 0.02% or less,

S: 0.01% or less, and

O: 0.0040% or less.

3. The steel pipe for fuel injection pipe according to claim 2 , wherein

the chemical composition consists of, by mass percent,

C: 0.12 to 0.27%,

Si: 0.05 to 0.50%,

Mn: 0.3 to 2.0%,

Al: 0.005 to 0.060%,

N: 0.0020 to 0.0080%,

Ti: 0.005 to 0.015%,

Nb: 0.015 to 0.045%,

Cr: 0 to 1.0%,

Mo: 0 to 1.0%,

Cu: 0 to 0.5%,

Ni: 0 to 0.5%,

V: 0 to 0.15%,

B: 0 to 0.005%, and the balance: Fe and impurities, and contents of Ca, P, S, and O in the impurities being

Ca: 0.001% or less,

P: 0.02% or less,

S: 0.01% or less, and

O: 0.0040% or less.

4. The steel pipe for fuel injection pipe according to claim 2 , wherein

the chemical composition contains, by mass percent,

one or more elements selected from

C: 0.12 to 0.27%,

Si: 0.05 to 0.50%,

Mn: 0.3 to 2.0%,

Al: 0.005 to 0.60%

N: 0.0020 to 0.0080%

Ti: 0.0050 to 0.040%,

Nb: 0.015 to 0.045%,

Cr: 0.2 to 1.0%,

Mo: 0.03 to 1.0%,

Cu: 0.03 to 0.5%,

Ni: 0.03 to 0.5%,

V: 0.02 to 0.15%, and

B: 0.0003 to 0.005%; and

the balance: Fe and impurities, and

contents of Ca, P, S, and O in the impurities being

Ca: 0.001% or less,

P: 0.02% or less,

S: 0.01% or less, and

O: 0.0040% or less.

5. The steel pipe for fuel injection pipe according to claim 1 , wherein

the outer diameter and the inner diameter of the steel pipe satisfy a following formula (iii):

D/d≥ 1.5  (iii)

where, in the above formula (iii), D denotes the outer diameter (mm) of the steel pipe, and d denotes the inner diameter (mm) of the steel pipe.

6. A method for producing a steel pipe for fuel injection pipe according to claim 1 , that has a critical internal pressure satisfying a following formula (i), the method comprising

subjecting a steel pipe starting material having an outer diameter of 20 mm or less, a tensile strength of 500 to 900 MPa, and a yield ratio of 0.50 to 0.85 to

auto-frettage treatment at an auto-frettage treatment internal pressure that is 450 MPa or lower and satisfies a following formula (iv):

IP≥ 0.41× TS ×α. . .   (i)

a =[( D/d ) 2 −]/[0.776×( D/d ) 2 ] . . .   (ii)

P AF <0.44× TS ×(1+ YR ) . . .   (iv)

where, in the above formula (i), IP denotes a critical internal pressure (MPa) of the steel pipe, TS denotes the tensile strength (MPa) of the steel pipe starting material, and a is a value expressed by the above formula (ii), and where, in the above formula (ii), D denotes an outer diameter (mm) of the steel pipe, and d denotes an inner diameter (mm) of the steel pipe, and where, in the above formula (iv), P AF denotes an auto-frettage treatment internal pressure (MPa), TS denotes the tensile strength (MPa) of the steel pipe starting material, and YR denotes the yield ratio of the steel pipe starting material.

7. The method for producing the steel pipe for fuel injection pipe according to claim 6 , wherein

the steel pipe has a chemical composition consisting by mass percent, of

C: 0.12 to 0.27%,

Si: 0.05 to 0.50%,

Mn: 0.3 to 2.0%,

Al: 0.005 to 0.060%,

N: 0.0020 to 0.0080%,

Ti: 0.0050 to 0.0040%,

Nb: 0.015 to 0.045%,

Cr: 0 to 1.0%,

Mo: 0 to 1.0%,

Cu: 0 to 0.5%,

Ni: 0 to 0.5%,

V: 0 to 0.15%,

B: 0 to 0.005%,

the balance: Fe and impurities,

contents of Ca, P, S, and O in the impurities being

Ca: 0.001% or less,

P: 0.02% or less,

S: 0.01% or less, and

O: 0.0040% or less.

8. The steel pipe for fuel injection pipe according to claim 7 , wherein the chemical composition consists of, by mass percent,

C: 0.12 to 0.27%,

Si: 0.05 to 0.50%,

Mn: 0.3 to 2.0%,

Al: 0.005 to 0.060%,

N: 0.0020 to 0.0080%,

Ti: 0.005 to 0.015%,

Nb: 0.015 to 0.045%,

Cr: 0 to 1.0%,

Mo: 0 to 1.0%,

Cu: 0 to 0.5%,

Ni: 0 to 0.5%,

V: 0 to 0.15%,

B: 0 to 0.005%, and

the balance: Fe and impurities, and

contents of Ca, P, S, and O in the impurities being

Ca: 0.001% or less,

P: 0.02% or less,

S: 0.01% or less, and

O: 0.0040% or less.

9. The steel pipe for the fuel injection pipe according to claim 7 , wherein the chemical composition consists of, by mass percent,

C: 0.12 to 0.27%,

Si: 0.05 to 0.50%,

Mn: 0.3 to 2.0%,

Al: 0.005 to 0.060%,

N: 0.0020 to 0.0080%,

Ti: 0.005 to 0.040%,

Nb: 0.015 to 0.045%,

one or more elements selected from:

Cr: 0.2 to 1.0%,

Mo: 0.03 to 1.0%,

Cu: 0.03 to 0.5%,

Ni: 0.03 to 0.5%,

V: 0.02 to 0.15%, and

B: 0.0003 to 0.005%; and

the balance: Fe and impurities, and

contents of Ca, P, S, and O in the impurities being

Ca: 0.001% or less,

P: 0.02% or less,

S: 0.01% or less, and

O: 0.0040% or less.

10. The method for producing the steel pipe for fuel injection pipe according to claim 6 , wherein

the outer diameter and the inner diameter of the steel pipe satisfy a following formula (iii):

D/d≥ 1.5 . . .   (iii)

where, in the above formula (iii), D denotes the outer diameter (mm) of the steel pip, and d denotes the inner diameter (mm) of the steel pipe.

11. The steel pipe for fuel injection pipe according to claim 3 , wherein

the chemical composition contains, by mass percent,

one or more elements selected from

Cr: 0.2 to 1.0%,

Mo: 0.03 to 1.0%,

Cu: 0.03 to 0.5%,

Ni: 0.03 to 0.5%,

V: 0.02 to 0.15%, and

B: 0.0003 to 0.005%.

12. The steel pipe for fuel injection pipe according to claim 2 , wherein

the outer diameter and the inner diameter of the steel pipe satisfy a following formula (iii):

D/d≥ 1.5  (iii)

where, in the above formula (iii), D denotes the outer diameter (mm) of the steel pipe, and d denotes the inner diameter (mm) of the steel pipe.

13. The steel pipe for fuel injection pipe according to claim 3 , wherein

the outer diameter and the inner diameter of the steel pipe satisfy a following formula (iii):

D/d≥ 1.5  (iii)

where, in the above formula (iii), D denotes the outer diameter (mm) of the steel pipe, and d denotes the inner diameter (mm) of the steel pipe.

14. The steel pipe for fuel injection pipe according to claim 4 , wherein

the outer diameter and the inner diameter of the steel pipe satisfy a following formula (iii):

D/d≥ 1.5  (iii)

where, in the above formula (iii), D denotes the outer diameter (mm) of the steel pipe, and d denotes the inner diameter (mm) of the steel pipe.

15. The steel pipe for fuel injection pipe according to claim 11 , wherein

the outer diameter and the inner diameter of the steel pipe satisfy a following formula (iii):

D/d≥ 1.5  (iii)

where, in the above formula (iii), D denotes the outer diameter (mm) of the steel pipe, and d denotes the inner diameter (mm) of the steel pipe.

16. The steel pipe for the fuel injection pipe according to claim 8 , wherein the chemical composition contains consists of, by mass percent,

C: 0.12 to 0.27%,

Si: 0.05 to 0.50%,

Mn: 0.3 to 2.0%,

Al: 0.005 to 0.060%,

N: 0.0020 to 0.0080%,

Ti: 0.005 to 0.015%,

Nb: 0.015 to 0.045%,

one or more elements selected from:

Cr: 0.2 to 1.0%,

Mo: 0.03 to 1.0%,

Cu: 0.03 to 0.5%,

Ni: 0.03 to 0.5%,

V: 0.02 to 0.15%, and

B: 0.0003 to 0.005%, and

the balance: Fe and impurities, and

contents of Ca, P, S, and O in the impurities being

Ca: 0.001% or less,

P: 0.02% or less,

S: 0.01% or less, and

O: 0.0040% or less.

17. The method for producing the steel pipe for fuel injection pipe according to claim 7 , wherein

the outer diameter and the inner diameter of the steel pipe satisfy a following formula (iii):

D/d≥ 1.5 . . .   (iii)

where, in the above formula (iii), D denotes the outer diameter (mm) of the steel pipe, and d denotes the inner diameter (mm) of the steel pipe.

18. The method for producing the steel pipe for fuel injection pipe according to claim 8 , wherein

the outer diameter and the inner diameter of the steel pipe satisfy a following formula (iii):

D/d≥ 1.5 . . .   (iii)

where, in the above formula (iii), D denotes the outer diameter (mm) of the steel pipe, and d denotes the inner diameter (mm) of the steel pipe.

19. The method for producing the steel pipe for fuel injection pipe according to claim 9 , wherein the outer diameter and the inner diameter of the steel pipe satisfy a following formula (iii):

D/d ≥ 1.5 . .   (iii)

where, in the above formula (iii), D denotes the outer diameter (mm) of the steel pipe, and d denotes the inner diameter (mm) of the steel pipe.

20. The method for producing the steel pipe for fuel injection pipe according to claim 16 , wherein

the outer diameter and the inner diameter of the steel pipe satisfy a following formula (iii):

D/d≥ 1.5 . . .   (iii)

where, in the above formula (iii), D denotes the outer diameter (mm) of the steel pipe, and d denotes the inner diameter (mm) of the steel pipe.