Experimental investigation of pinching phenomena in cold rolling of thin steel sheets

During rolling of metal sheets defects may occur, such as local waviness, surface ruptures, and sometimes strip breaks. These phenomena, commonly referred to as 'pinching', have been observed in combination with snaking problems (strip sidewards movements) during tailing out, but even in continuous rolling processes. Severe pinches compromise the quality of the strip and damage to the work rolls can also be caused. This clearly affects the production, resulting in low product quality, process delays and, consequently, in extra costs. Even though pinching is a widely experienced issue, during both hot and cold rolling, it is not clear what mechanism is behind it. Pinches occur due to disruptions in the rolling process, therefore pinching sensitive operative regimes need to be identified such that mill operations can be performed in a way that keeps the process stable. Currently, pinching cannot be predicted by rolling simulation models due to the lack of knowledge about the circumstances leading to pinches. Therefore, rolling experiments were performed at the pilot mill in Tata Steel (IJmuiden), in order to understand under which process conditions pinches are more likely to occur. It is shown that pinching phenomena can be created in a single-stand mill, being triggered by perturbations introduced during the steady-state rolling process. Specifically, the lubrication level has been detected as an underlying factor, playing a role in the occurrence of these phenomena. An extensive characterization of pinching defects is provided within the field of metal rolling. Furthermore, part of a possible mechanism for pinches is discussed, based on the experimental evidence of the pinching tests. The aim of this work is to contribute to a more fundamental understanding of pinching, as a crucial step towards the prediction of these defects.