Threaded joint for tube

A threaded joint for tubes forming a tube body comprising:

• a pin including an externally-threaded member, a nose extending toward a tube end side from the externally-threaded member, and a shoulder formed at an end of the nose;
• and
• a box including an internally-threaded member threadedly joined with the externally-threaded member, a box seal surface facing a nose outer peripheral surface of the pin in an opposed manner, and a shoulder brought into contact with the shoulder of the pin;
• wherein
• the threaded joint is a radial seal that the nose outer peripheral surface and the box seal surface are brought into metal-to-metal contact in a tube radial direction as a result of joining of the pin and the box by being threadedly joined, and the contact portion forms a seal portion, and
• tensile efficiency TE (%) is defined by a ratio Sc/Sp of a cross-sectional area Sc including a portion where tensile stress and strain become highest in a tube circumferential cross section of the box to a cross-sectional area Sp of the tube body,
• the tensile efficiency TE (%) satisfies formula (1) having a ratio t/ρ of an internally-threaded height t of the box to a radius of curvature ρ of a corner portion on an internally-threaded bottom load surface side:
• TE(%)≥2.25×t/ρ+99.9 (1).