Prediction of the fracture due to Mannesmann effect in tube piercing

Mannesmann piercing process is a well-known hot rolling process used in industry for seamless tube production. Its special feature is the so-called Mannesmann effect, that is the cavity formation in the center of the cylindrical billet and its propagation along the axis due to stress state caused by the rolls. The cavity is then expanded and sized in its internal diameter by an incoming plug. The industrial requirement is to know quite precisely the characteristics of the cavity especially in terms of its location along the billet axis in order to minimize the plug wear and the oxidation of the pierced bar. However, the scientific knowledge about the fracture mechanism leading to the Mannesmann effect is limited, even if several theories have been proposed; this lack makes the design and optimization of the process through numerical simulations still challenging [1]. The aim of this work is then to develop a suitably calibrated FE model of the piercing process in its first stage before the plug arrival, in order to investigate the Mannesmann effect using both energy-based criteria and continuous damage mechanics [2]. Different workability tests, capable to reproduce the industrial conditions in terms of temperature, strain rate, and stress states, are proposed and carried out to determine the parameters of the damage models on specimens machined from continuous-casting steel billets. The calculated parameters are implemented in the model of the process and a sensitivity analysis to the different criteria is carried out. Numerical results are then compared with non-plug piercing tests conducted in the industrial plant, allowing the choice of the most suitable test and damage criterion.