Identifying Heterogeneous Friction Coefficients on the Hot Forming Tools in Mannesmann Cross-Roll Piercing

Abstract

As in all metal rolling processes, Mannesmann cross-roll piercing relies on entrainment by friction between the billet and the rolls. But contrary to other rolling processes, a strong back-push is imposed by the piercing force (Fig. 1). Entrainment of the billet through the mill is therefore a critical problem which can be solved by optimizing the surface state of all tools (rolls, guide rolls, piercing plug). This is why the effect of friction with all the tools on the tube entrainment speed and on the state of stress has been investigated using the 3D Finite Element Method (FEM, ForgeNxT). It has been found that friction between billet and cross rolls is a driving force on the first (upstream) half of the rolls but may become resistant on the downstream part for certain process settings. Friction between piercer plug and hot metal is always resistant. Friction with rotating Diescher guide disks is a driving force in the piercing direction, but works against pierced shell rotation, causing shell torsion (“twist”). If static lateral guide shoes are preferred, their pure sliding friction is resistant in both directions. Friction on the upstream part of rolls must therefore be as high as possible for correct entrainment and process stability, which explains the practice of giving it very high roughness. The surface of the piercer plug must be smoother to minimize its frictional resistance. The surface states of lateral guides and of the downstream half of rolls can be used as process optimization variables. Results suggest how to obtain estimates of friction coefficients from mill measurements.