PRESS ROLL PIERCING METHOD

摘要

A press roll piercing method for manufacturing a hollow shell is disclosed wherein a polygonal metal billet is pushed in its axial direction between a pair of rolling rolls mounted one above the other and having semicircular grooves so as to be rolled into circular or almost circular shape, and the piercing is effected in a core portion of the billet by means of a piercing plug supported between the rolling rolls. The improvement resides in selecting a pushing force of a predetermined value which corresponds to the stress above the yield stress of the billet at the start of the piercing although the actual stress occurring in the billet at the start of the piercing may or may not exceed the yield stress; selecting the pushing speed and/or the peripheral speed of the rolls to be of such values that the volume of the billet to be pushed in per unit time and the volume of the hollow shell to be discharged from the rolling rolls become substantially equal, and biting the billet between the rolling rolls at said set values.