| 摘要 |
A plug 1 includes a first columnar portion 20, a tapered portion 30, and a second columnar portion 40. The first columnar portion 20 has an outside diameter D1. The second columnar portion 40 has an outside diameter D2 which is larger than the outside diameter D1. The tapered portion 30 is formed between the first columnar portion 20 and the second columnar portion 40. The tapered portion 30 has a tapered surface 31 provided with an outside diameter which is gradually increased from the first columnar portion 20 to the second columnar portion 40, and an axial direction length L. The outside diameters D1 and D2, and the axial direction length L meet the following expressions (1) to (4): 0.25≦̸&rgr;≦̸2.00  (1), 0.06≦̸L/D2≦̸0.8  (2), L/D2≦̸0.3×&rgr;+0.575  (3), and L/D2≧0.1×&rgr;  (4) where &rgr;=(D2−D1)/D1×100. Therefore, the cold drawing plug according to the present invention can reduce the tensile residual stress on the outer surface of a metal pipe after the cold drawing.
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