Patent ID: 8544306

Claim:
A plug for use in a piercing-rolling mill for piercing-rolling a starting material and deforming the same into a hollow blank, the plug being coupled with a front edge of a mandrel and adapted to pierce the starting material while injecting a lubricant, the plug including: a front edge portion having a convex curvature; a cylindrical portion having a truly or nearly cylindrical shape; a trunk portion having an outer diameter gradually increasing toward a rear edge thereof, wherein a first end of the cylindrical portion is adjacent to the front edge portion and a second end of the cylindrical portion is adjacent to the trunk portion, wherein a maximum diameter of the front edge portion is equal to a diameter of the cylindrical portion at the first end of the cylindrical portion; a mandrel joint provided in a rear edge portion of the plug; a lubricant ejection hole running through the trunk portion from the mandrel joint that opens on the surface of the cylindrical portion; and a coating comprising oxides and Fe that is formed on the base metal surface of each of the front edge portion as well as the trunk portion by arc spraying using an iron wire, wherein a ratio of regions occupied by the oxides in the coating is not more that 40% in a portion adjacent to the base metal, and 55% to 80% in a surface layer portion.