Patent ID: 8333092

Claim:
A piercer for piercing and rolling a round billet in the axial direction to form the round billet into a hollow shell, comprising: a plug having an injection hole for injecting a lubricant; a mandrel having a through hole in the axial direction for providing the lubricant therethrough and having a tip end coupled with the rear end of said plug; and an injection device injecting the lubricant from said injection hole through said through hole, said plug comprising: a tip end portion having a raised curvature in the axial direction; a cylindrical portion adjacent to said tip end portion and having a substantially cylindrical surface; a barrel portion adjacent to said cylindrical portion and having an outer diameter gradually increased from a front end to a rear end thereof; and a mandrel coupling portion provided at the rear end of said plug to couple with said mandrel, said injection hole penetrating from the surface of said cylindrical portion to a surface of said mandrel coupling portion and being communicated with said through hole, and a length L 1 (mm) of said cylindrical portion satisfying the following Expression (1): 0.05 ×BD≦L 1≦0.30 ×BD (1) where BD represents the diameter (mm) of the round billet.