To provide a method for piercing a round billet by which hot only the generation of inside surface flaws is suppressed but productivity is raised than that by conventinal methods.
In a method for piercing the round billet in which the round billet is fed between a pair of rolls, which are inclined to each other, of a piercer and a stock tube for seamless steel tubes is manufactured by piercing the round billet using a shell-shaped plug arranged between these rolls, the inclined angle of the roll is taken as 12-14° and also, when the tip of the plug bites into the round billet, a piercer piercing mill is operated at 0.4-0.6 in the X/Db value which is determined by the equation: X/Db=(1/2 tanβ).(1-E/Db). Where, Db is the outside diameter of the billet before piercing, E is the distance between positions of the max. diameters of the roll, β is the inclined angle of the inlet-side surface of the roll, X is the length where the round billet is not drilled with the roll, L is the length where the billet is drilled with the roll and plug.
ITAYA SUSUMU
KIMURA TOMOMITSU
OKA HIROSHI
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