A piercing mill includes a plurality of inclined rolls, a plug, a pusher, and an undriven guide roll. The pusher pushes a round billet from the rear end at least until the round billet advances for a prescribed distance after contacting the inclined rolls. The guide roll is between the inclined rolls and the pusher and includes a roll shaft and a roll surface, with the roll shaft arranged obliquely with respect to a pass line PL. The sectional shape of the roll surface is a concave arch shape. The round billet is rotated by the guide roll and the friction force generated when the round billet contacts the inclined rolls is reduced. Consequently, the wear of the inclined rolls can be restrained if the round billet is pierced and rolled while it is pushed in between the inclined rolls using the pusher.
1. A piercing mill piercing and rolling a round billet into a hollow shell, comprising:
a plurality of inclined rolls provided around a pass line;
a pusher provided in front of said inclined rolls to push said round billet from a rear end of said round billet at least until said round billet advances for a prescribed distance after contacting said inclined rolls, said pusher including a billet push rod contacted to the rear end of said round billet; and
an undriven first guide roll provided between said inclined rolls and said pusher, including a first roll shaft arranged obliquely with respect to said pass line and a first roll surface having a concave arch sectional shape in the direction of said first roll shaft and rotating said round billet in advance in the same direction as the direction of rotation provided by said inclined rolls.
2. The piercing mill according to claim 1 , further comprising an undriven second guide roll provided opposed to said first guide roll with said pass line therebetween and including a second roll shaft crossing said first roll shaft and a second roll surface having a concave arch sectional shape in the direction of said second roll shaft.
3. The piercing mill according to claim 2 , comprising a plurality of said first guide rolls, wherein said plurality of first guide rolls and said second guide roll are arranged zigzag along said pass line.