Threaded connection for steel pipe

A threaded connection for steel pipe, comprising:

• a tubular pin formed from a tip portion of the steel pipe;
• and
• a tubular box adapted to be made up on the pin as the pin is inserted therein,
• wherein the pin includes:
• a male thread provided on an outer periphery of the pin;
• a nose formed from a tip portion of the pin, the nose having an outer diameter smaller than an inner diameter of a portion of the box that faces the nose when the pin and the box have been made up;
• a pin shoulder surface provided on a tip of the nose;
• and
• a pin sealing surface provided on the outer periphery of the pin, the pin sealing surface located between the male thread and the nose,
• the pin sealing surface includes a first curvature surface, a second curvature surface and a first taper surface, the first curvature surface being located closer to the pin shoulder surface than the second curvature surface is, the second curvature surface being located more distant from the pin shoulder surface than the first curvature surface is, the first taper surface being located between the first curvature surface and the second curvature surface,
• the box includes:
• a female thread provided on an inner periphery of the box, the female thread corresponding to the male thread;
• a box shoulder surface facing the pin shoulder surface, the box shoulder surface being in contact with the pin shoulder surface when the pin and the box have been made up;
• and
• a box sealing surface facing the pin sealing surface, the box sealing surface being in contact with the pin sealing surface when the pin and the box have been made up,
• the box sealing surface includes a third curvature surface, a fourth curvature surface and a second taper surface, the third curvature surface being located closer to the box shoulder surface than the fourth curvature surface is, the fourth curvature surface being located more distant from the box shoulder surface than the third curvature surface is, the second taper surface being located between the third curvature surface and the fourth curvature surface,
• the box sealing surface has a seal point on the second taper surface located at a midpoint as determined along a direction of a pipe axis,
• a seal-point distance between the tip of the nose and the seal point as measured in the direction of the pipe axis is 13 mm or more,
• a shoulder angle between the pin shoulder surface or the box shoulder surface and a plane perpendicular to the pipe axis is 2 to 13 degrees, and
• a seal wall-thickness ratio of a wall thickness of the box to a wall thickness of the pin as measured at the seal point is 1.7 or higher.