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).