Control of residual stresses by roller leveling with regard to process stability and one-sided surface removal

Abstract

Roller leveling is a widely used production process to eliminate the coil set from metal strips. Therefore, this process aims mainly on removing the sheet curvature, while the created residual stresses are not systematically adjusted. Nonetheless, a straight strip can possess a disadvantageous residual stress distribution leading to a curvature when cutting the strip into parts or when applying a one-sided surface removal. Thus, it is desirable to address both curvature removal and residual stress distribution by roller leveling. Therefore, a seven-roll levelling machine is investigated to check for possible degrees of freedom to address those two target values. A respective control concept is introduced employing the different load triangles of a leveling machine. In order to investigate the possibility to influence curvature as well as residual stresses, first a numerical parameter study is conducted. In particular, the influence of the three individual load triangles of such a machine on the resulting residual stresses is investigated. The first load triangle is used to set a defined overstretch to the outer sheet fiber and the center load triangle is used as an actuator on the residual stress distribution. In all cases, the last load triangle is able to provide a straight sheet after leveling. To check for the influence of the achieved residual stress distributions on subsequent production processes, investigations on process robustness as well as a one-sided surface removal are done. For both cases it is possible to define an optimized set of roll positions.