Threaded connection for steel pipe

A threaded connection for steel pipe, comprising:

• a tubular pin provided at a tip of a steel pipe;
• and a tubular box adapted to be made up on the pin as the pin is screwed therein, wherein the pin includes a male threadform formed on an outer periphery of the pin, the box includes a female threadform formed on an inner periphery of the box and adapted to engage the male threadform when the connection is made up, each of the male threadform and the female threadform includes a load flank, a stab flank, a thread-crest surface and a thread-groove/root surface, where a stab-flank pitch of each of the male threadform and the female threadform is smaller than a load-flank pitch of each of the male threadform and the female threadform and, when the connection is made up, the load flank of the male threadform is in contact with the load flank of the female threadform and the stab flank of the male threadform is in contact with the stab flank of the female threadform, and where each of the load flanks and the stab flanks of the male threadform and the female threadform has a negative flank angle,
• wherein the load flank and the thread-groove/root surface in a portion of the male threadform for a predetermined range in a thread-helix direction starting at an end of the male threadform associated with a pin tip are connected by a first curved surface having a radius of curvature r1 as seen in a longitudinal cross section satisfying the following expression, (1):
• r1≥Th×0.14 (1),
• where Th is the thread height, measured at the load flank, in the male threadform as measured in the predetermined range in the thread-helix direction starting at the end of the male threadform associated with the pin tip, and satisfies 1.8 mm≤Th≤3.0 mm.