Analysis of secondary oxide-scale failure at entry into the roll gap

Both numerical analysis based on finite-element (FE) modeling and experimen tal evidence concerning the secondary oxide-scale failure at entry into the roll gap are presented and reviewed for a better understanding of events a t the roll-workpiece interface, in turn, leading to better definition of th e boundary conditions for process models. Attention is paid to the two limi t modes leading to oxide-scale failure, which were observed earlier during tensile testing under rolling conditions. These are considered in relation to the temperature, the oxide-scale thickness, and other hot-rolling parame ters. The mathematical model used for the analysis is composed of macro and micro parts, which allow for simulation of metal/scale flow, heat transfer , cracking of the oxide scale, as well as sliding along the oxide/metal int erface and spallation of the scale from the metal surface. The different mo des of oxide-scale failure were predicted, taking into account stress-direc ted diffusion, fracture and adhesion of the oxide scale, strain, strain rat e, and temperature. Stalled hot-rolling tests under controlled conditions h ave been used to Verify the types of oxide-scale failure and have shown goo d predictive capabilities of the model. The stock temperature and the oxide -scale thickness are important parameters, which, depending on other rollin g conditions, may cause either through-thickness cracking of the scale at t he entry or lead to entry of a nonfractured scale when the scale/metal inte rface is not strong enough to transmit the metal deformation.