Three-dimensional deformation of tubes in stretch reducing milll is simulated both by rigid-plastic finite-element method and by rolling experiments with a single three-roll stand. The mechanism responsible for circumferential wall thickness variation, so-called polygonization of the tube bore is clarified:
(1) The wall is thinned at the groove root and is thickened at the flange after a single-pass rolling.
(2) The degree of polygonization increases as the t/D ratio, the reduction in diameter and the ovality of the roll groove increase. It decreases as the front or/and back tension increases. The position of the neutral line which is closely related to the stress-strain distribution in the tube has a great influence on the degree of polygonization.
(3) The finite element analysis on the polygon formation appeared in a multi-pass rolling reveals that the circumferential position of the thickest wall lies between the groove root and the flange side. The degree of polygonization after each pass is also calculated.
The rolling conditions such as the reduction in diameter, the ovality of the roll groove and the magnitude of inter-stand tension are optimized to suppress the polygon formation. This leads to the expansion of the product range of thick-walled tubes of which the t/D ratio exceeds 30% in a small-diameter seamless tube mill.
