| 摘要 |
PROBLEM TO BE SOLVED: To control diameter reduction of the rear end portion of a hollow material pipe by moving a plug in the direction opposite to the piercing rolling direction when a predetermined portion on the rear end side in the rolling direction is rolled. SOLUTION: When a predetermined portion of a billet reaches a gorge portion 9, a plug 5 is moved in the direction opposite to the advancing direction of the billet 2, thus making thinner the wall thickness of the rear end portion of a hollow material pipe 8 in comparison with the center portion. The plug can be moved continuously till the completion of piercing rolling with a drive unit 10 provided at the rear end of a mandrel, but may be stopped after the wall thickness is reduced to prescribed thickness. When the plug 5 moves, the wall thickness of the rear end portion is made thinner and the outside diameter and the inside diameter are increased, thus preventing diameter reduction of the rear end portion. If the thin portion in wall thickness of the rear end portion is too long, a trouble tends to occur during rolling in the following process, and therefore, the timing for starting to move the plug 5 should be within the range where the distance from the rear end face 2b of the billet 2 to the gorge portion 9 is 0.5 times or more but 5 times or less of the billet diameter.
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