Modelling the boundary conditions for thermo-mechanical processing - oxidescale behaviour and composition effects

It is known that better understanding of events at the tool-workpiece inter face will lead to better definition of the boundary conditions for the proc ess models. The deformation of the oxide scale on the surface of steel bein g hot worked contributes to the interface complexity. This scale has a comp licated microstructure, and can deform or fracture. In hot rolling, the def ormation of the scale begins before the steel makes contact with the roll, under the influence of a tensile stress as the roll draws the metal into th e roll gap. Tension tests under controlled conditions have been used to sim ulate this initial deformation and have revealed ductile and brittle behavi our, according to the operating conditions. This behaviour is also very sen sitive to small changes in the chemical composition of the underlying steel . This is illustrated for two low-carbon steels. The laboratory observation s art: complemented by finite element modelling of the tensile deformation, which is capable of reproducing both ductile and brittle behaviour of the oxide scale with the same model. The model is adjusted to mimic the effect of changing the chemical composition of the steel.