Investigation for improving process reliability and strip quality during cold rolling of slit strip

For the production of cold rolled narrow precision strips the manufactures use starting material from the wide strip mills. After hot rolling, and sometimes also cold rolling, in full width the wide coil is slit into two or more narrow strands by slitting before they are delivered to the precision strip mill. Asymmetrical thickness profiles in slit strip, e.g. wedge shape, often cause problems during the rolling process. Irregular strip geometry over the strip width on the coiler causes uncontrolled changes in tension over the strip width, which may seriously affect flatness measurement and control. Strip asymmetry also can result in undesirable and unpredictable effects during coiling. Coiling slit strip showing wedge shaped thickness profile leads a lateral displacement of the layers leading to undesired telescoping of the coiled strip accompanied by the effect of a stress related warping (longitudinal ripples) in the strip between coiler and roll gap. The stress warping, above all, causes falsifications of the measurement of the tensile stress distribution across the strip width, and thus rendering it impossible to achieve reliable process control and automatic stand pre-setting. The investigations were focussed on the theoretical analysis of these effects by FE-calculations and on experimental studies at a pilot plant as well as at an industrial rolling mill where slitted hot strip material is processed. Whereas the theoretical studies were mainly concentrated on the telescoping effect, the major concern of the experimental studies consisted in developing and creating applicable solutions to avoid falsifications in flatness measuring and flatness control. By multivariate analysis of collected process parameters combined with designed experiments the underlying parameters promoting the strip lateral movement were to be identified. FEM simulation of the cold rolling of slit strip in general and with different rolling parameters and external control possibilities in particular were to be done in order to analyse cases which could not be performed in practical tests.