Patent ID: 7530248

Claim:
A plug for use in a piercing mill, comprising a tip end portion, a taper portion, and a middle portion sequentially in the direction from the front end to the back end of said plug, wherein the surface of said tip end portion has a convex spherical surface in the axial direction of said plug, its radius of curvature is R 1 , the length of said tip end portion is L 1 that is smaller than said R 1 , the surface of said taper portion is formed continuously with the surface of said tip end portion, the outer diameter of said taper portion on the front end side is D 1 , the outer diameter of said taper portion on the back end side is D 2 that is larger than D 1 , the length of said taper portion is L 2 , the surface of said middle portion is formed continuously with the surface of said taper portion, the outer diameter of said middle portion gradually increases in the direction from the front end to the back end of said plug, and said plug satisfies Expression (1), Expression (2) if 0<L 1 /R 1 <0.5, and Expression (3) if 0.5≦L 1 /R 1 <1: 0.5 D 1 <L 1 +L 2≦2.5 D 1 (1) 1.0 <D 2 /D 1≦1.4 (2) 1.0 <D 2 /D 1<1.8−0.8 L 1 /R 1 (3).