PURPOSE: To improve the inside surface shape of a metallic pipe and to improve the durability of a plug in the stage of producing the metallic pipe by a Mannesmann double piercing system by injecting cooling water from the forward end of the plug of the 2nd piercer.
CONSTITUTION: The thick walled hollow blank pipe which is formed with the 1st piercer is expanded and rolled to a final metallic pipe product with the 2nd piercer in a pipe making process by Mannesmann plug mill by a double piercing system. The plug 4 to be used in this stage is connected to a mandrel 1 by means of an end fitting 3 to provide a cavity 4a in the inside. An ejection hole 4b for cooling water is provided obliquely forward in the forward end part thereof. In the stage of expanding and rolling the metallic blank pipe with the 2nd piercer, the cooling water supplied from the forward end of the plug 4 through the mandrel 1 is injected to cool the inside surface of the blank pipe and to increase the deformation resistance thereof. The generation of pit flaws on the inside surface by the plug and the seizure between the inside surface of the metallic blank pipe and the plug is eliminated and the durability of the plug is improved.