PURPOSE: To prevent the formation of oxide scale on the inside surface of a hollow blank material in the stage of piercing a blank billet by means of inclined rolls and a plug by supplying an inert gas to the pierced hollow part.
CONSTITUTION: Cooling water is fed through a supply pipe 28 to the inside surface of a plug 13 to cool the plug 13 and to prevent the decrease in the abrasion resistance and strength thereof by the effect of high temp. in the stage of piercing a blank billet 10 with inclined rolls 11 and the plug 13 at the forward end of a plug bar 12 and producing a hollow blank pipe for a seamless steel pipe. At the same time, gaseous nitrogen is ejected from a gas ejection hole 37 through a supply pipe 29 and supply path 36 for the gaseous nitrogen to the fresh inside surface of the hollow part 10A formed in the billet 10 in the stage of piercing to prevent the oxidation of the inside surface in the part 10A by air. The sufficient gas is ejected over the entire part of the hollow part 10A when the plug 13 is pulled to prevent oxidation by the intrusion of air and to prevent the formation of oxide scale on the inside surface in the part 10A.
HATAKEYAMA SHIROU
AIYAMA SHIGEKI