Investigation of correlation between material properties, process parameters and residual stresses in roller levelling

When processing metal strips, roller levelling is the first process the sheet metal undergoes after decoiling. The process of roller levelling mainly aims on the flatness of the material. Additionally, the resulting residual stress distribution after levelling is of great importance since a strip might be flat but have a disadvantageous residual stress distribution for further processing. In this paper the correlations between the incoming strip condition, the roll intermeshes and the residual stresses are evaluated in order to achieve both, a constant flatness as well as a distinct residual stress distribution after levelling. The investigations are focusing on a seven-roll levelling machine with three individual load triangles. The first load triangle ensures a constant plastification of the incoming strip, the load triangle in the middle aims on the residual stress distribution and the last load triangle sets the flatness of the sheet material. Within a numerical model of this setup, a variation of the incoming material properties by means of yield stress and kinematic hardening is done. From the numerical model both global values such as the forces acting on the rolls as well as the local residual stress distribution are obtained. Accompanying experiments are conducted on a laboratory-scale roller levelling machine. Furthermore, the residual stresses across the sheet thickness after levelling are measured using diffraction analysis. The results indicate a distinct relationship between process parameters and the target values of flatness and residual stress distribution. Finally, a strategy for an online process control acting on both flatness and residual stress distribution is introduced based on these results. (C) 2018 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the 17th International Conference on Metal Forming.