Screw joint for steel pipe

1. A screw joint for a steel pipe comprising:

a pin which includes a male screw portion formed by a two-step screw, and a seal surface including a tapered surface and a curved surface adjacent to the tapered surface, the seal surface of the pin being formed on an intermediate portion of the two-step screw; and

a box which includes a female screw portion formed by a two-step screw, and a seal surface including a tapered surface and a curved surface adjacent to the tapered surface, the seal surface of the box being formed on an intermediate portion of the two-step screw, wherein

a taper angle of the tapered surface of the pin is substantially the same as a taper angle of the tapered surface of the box,

the male screw portion and the female screw portion are made-up by screwing, such that the pin and the box interfere with each other in a radial direction while the seal surface of the pin and the seal surface of the box contact each other in a process of the makeup, whereby at least a part of the seal surface of the pin comes into close contact with at least a part of the seal surface of the box over an entire circumference, and a distribution of the contact pressure on the seal surface during the make-up includes a region where a value of the contact pressure is substantially constant along the axis direction of the seal surface,

a contact pressure amplification mechanism, which increases a contact pressure between the seal surface of the pin and the seal surface of the box at the completion of the make-up compared to a time point during the make-up, is further provided, and

the contact pressure amplification mechanism is configured to increase the contact pressure between the seal surface of the pin and the seal surface of the box by:

make-up between a dovetail-shaped screw which is formed in a place adjacent to the seal surface of the male screw portion and a dovetail-shaped screw which is formed in a place adjacent to the seal surface of the female screw portion, or

a hook shoulder which is adjacent to either a front side or a rear side of each of the seal surface of the pin and the seal surface of the box.

2. The screw joint for a steel pipe according to claim 1 ,

wherein the curved surface of the pin is formed on a tip side of the pin, and the curved surface of the box is formed on a tip side of the box.

3. The screw joint for a steel pipe according to claim 2 ,

wherein a peak contact pressure is generated in a contact portion between the curved surface and the tapered surface in a contact surface between the seal surface of the pin and the seal surface of the box at the completion of the make-up.

4. The screw joint for a steel pipe according to claim 2 ,

wherein the contact pressure amplification mechanism increases the contact pressure between the seal surface of the pin and the seal surface of the box by make-up between a dovetail-shaped screw which is formed in a place adjacent to the seal surface of the male screw portion and a dovetail-shaped screw which is formed in a place adjacent to the seal surface of the female screw portion.

5. The screw joint for a steel pipe according to claim 2 ,

wherein the contact pressure amplification mechanism increases a contact pressure between the seal surface of the pin and the seal surface of the box by a hook shoulder which is adjacent to either a front side or a rear side of each of the seal surface of the pin and the seal surface of the box.

6. The screw joint for a steel pipe according to claim 1 ,

wherein the curved surface of the pin is formed on a side opposite to a tip side of the pin, and the curved surface of the box is formed on a side opposite to a tip side of the box.

7. The screw joint for a steel pipe according to claim 6 ,

wherein a peak contact pressure is generated in a contact portion between the curved surface and the tapered surface in a contact surface between the seal surface of the pin and the seal surface of the box at the completion of the make-up.

8. The screw joint for a steel pipe according to claim 6 ,

wherein the contact pressure amplification mechanism increases the contact pressure between the seal surface of the pin and the seal surface of the box by make-up between a dovetail-shaped screw which is formed in a place adjacent to the seal surface of the male screw portion and a dovetail-shaped screw which is formed in a place adjacent to the seal surface of the female screw portion.

9. The screw joint for a steel pipe according to claim 1 ,

wherein the curved surface of the pin is formed to be adjacent to both sides of the tapered surface of the pin, and a length of the tapered surface of the box is longer than a length of the seal surface of the pin.

10. The screw joint for a steel pipe according to claim 9 ,

wherein a peak contact pressure is generated in a contact portion between the curved surface and the tapered surface in a contact surface between the seal surface of the pin and the seal surface of the box at the completion of the make-up.

11. The screw joint for a steel pipe according to claim 9 ,

wherein the contact pressure amplification mechanism increases the contact pressure between the seal surface of the pin and the seal surface of the box by make-up between a dovetail-shaped screw which is formed in a place adjacent to the seal surface of the male screw portion and a dovetail-shaped screw which is formed in a place adjacent to the seal surface of the female screw portion.

12. The screw joint for a steel pipe according to claim 1 ,

wherein the curved surface of the box is formed to be adjacent to both sides of the tapered surface of the box, and a length of the tapered surface of the pin is longer than a length of the seal surface of the box.

13. The screw joint for a steel pipe according to claim 12 ,

wherein a peak contact pressure is generated in a contact portion between the curved surface and the tapered surface in a contact surface between the seal surface of the pin and the seal surface of the box at the completion of the make-up.

14. The screw joint for a steel pipe according to claim 12 ,

wherein the contact pressure amplification mechanism increases the contact pressure between the seal surface of the pin and the seal surface of the box by make-up between a dovetail-shaped screw which is formed in a place adjacent to the seal surface of the male screw portion and a dovetail-shaped screw which is formed in a place adjacent to the seal surface of the female screw portion.

15. The screw joint for a steel pipe according to claim 1 ,

wherein a peak contact pressure is generated in a contact portion between the curved surface and the tapered surface in a contact surface between the seal surface of the pin and the seal surface of the box at the completion of the make-up.

16. The screw joint for a steel pipe according to claim 15 ,

wherein the contact pressure amplification mechanism increases the contact pressure between the seal surface of the pin and the seal surface of the box by make-up between a dovetail-shaped screw which is formed in a place adjacent to the seal surface of the male screw portion and a dovetail-shaped screw which is formed in a place adjacent to the seal surface of the female screw portion.

17. The screw joint for a steel pipe according to claim 1 ,

wherein the taper angle of the tapered surface of the pin and the taper angle of the tapered surface of the box are 2° to 10°.