Modelling of oxide failure in hot metal forming operations
Oxide scale can affect many aspects of hot metal forming operations, such as heat transfer, friction and the final surface finish of the rolled product. Surface oxide scale is always present on the steel slab and sometimes on cold rolls. Therefore, the study of the thermo-mechanical behaviour of oxide scale, particularly under conditions as close as possible to the steel manufacturing process is very important. In order to undertake a detailed study of oxide scale behaviour, several hightemperature testing techniques were applied. First, high-temperature tensile tests were carried out to investigate the brittle fracture and cracking of the surface oxide scales. Second, high-temperature compression tests were developed and the results obtained revealed many different failure mechanisms that are present in the compressed oxide scale. Finally, a tensile-compressive test was developed and performed under thermal conditions which were as close as possible to hot rolling. The best results in the understanding of oxide scale failure were achieved by closely linked combination of laboratory testing and measurements coupled with detailed finite element analysis. A close microstructural examination of the morphology of oxides was carried out after each experiment and finite element modelling was performed. The three-dimensional finite element simulations helped to improve the interpretation of the thermo-mechanical testing and to obtain more accurate heat transfer and stress-strain relationships. In this work the following thermo-mechanical failure modes of the oxide scales were observed and investigated: brittle fracture (through-thickness cracking, blister failure), indications of plastic behaviour (arrested cracks, unbroken top layers of the oxide scale) and a sticking effect (equivalent to the mill pick-up).