Analysis of profile and flatness in flat hot rolling based on non-linearly coupled models for elastic roll-stack deflection and pseudo-steady-state elasto-viscoplastic strip

An enhanced iterative concept for the effective numerical simulation of flat hot rolling processes is presented. The underlying physical process is the forming of metal within a flat rolling stand, i.e. between a lower and an upper roll set, each of them consisting of one or more rolls. The strip material is described elasto-viscoplastically, whereas the roll stack is deformed elastically. The accurate coupling of the strip model with the routines for the elastic roll stack deflection is a precondition to get reliable results concerning profile transfer and incompatible residual strains inside the strip, which allows the prediction of flatness defects, such as buckling. Especially for thin, wide strips and heavy plates, where the aspect ratio width over thickness is extremely unfavourable, the determination of profile transfer and flatness obviously leads to extremely high calculation times with commercial FEM-programs. Therefore, a tailor-made FEM-code for the efficient simulation of the elasto-viscoplastic material flow inside the roll gap was developed and programmed in C++. It is based on pseudo-steady-state, fully implicit stress-update approaches, where the incremental material objectivity is satisfied exactly. The developed model is well suited for systematic parameter studies to investigate flatness defects in more detail and to develop enhanced flatness criteria for thin hot and cold strips and plates.