Kinematic and Thermal Analysis of Hot Piercing Rolling of Seamless Tubes

Based on the formulation of dragging axial velocity of the rolls, a new kinematic model gives the sliding velocity distribution at the roll billet interface and the effective axial velocity of the billet during the piercing operation. Starting from this preliminary result, an original flow approach has been developed to calculate plastic flow patterns and strain rates in the deformed zone of the billet. It is shown that strain rate and adiabatic temperature increment strongly increase in the vicinity of the neck plug. The occurence and dimensions of the central cavity which appears ahead the neck plug also modify the strain rate gradients. Then a thermo-viscoplastic finite element simulation has been performed to confirm the above results. The new strain rate predictions are very close to those calculated from the flow model. Roll-piercing produces strong non-homogeneity of the plastic flow and of adiabatic temperature increment. According to the above models, it is now possible to have accurate analysis of the influence of process parameters first, interface and bulk thermo-mechanical properties of the billet second, and to define new efficient operating conditions.