Towards hot levelling strategies for steel heavy plates: analysis of flatness evolution in accelerated cooling, hot levelling, and final air cooling via thermo-mechanical FE modelling

Abstract

AbstractIn the production of heavy plates, the flatness of the products is an essential feature that must be set within narrow tolerances. Thus, the understanding of the flatness evolution throughout all relevant production steps in the rolling mill is paramount for high-quality products. It is known that most flatness defects arise either due to roll gap deviations in rolling or because of changing thermal conditions in the subsequent cooling processes. Severe defects induced during rolling are treated by a pre-leveller while the final heavy plate flatness after cooling is set in a dedicated hot leveller. Consequently, thermally induced defects need to be removed in this dedicated hot leveller as well. However, it is not yet feasible to model the flatness evolution throughout the relevant processing sequence consisting of accelerated cooling after hot rolling, hot levelling, and final air cooling. In order to close this gap and to analyse the process sequence with regard to flatness evolution, within this study, two cooling and a levelling model were developed and coupled with respect to thermal history, deformation, and residual stresses. These coupled thermo-mechanical finite element (FE) models were successfully applied to predict the flatness evolution in heavy plate production. Firstly, the formation of centre waves caused by inhomogeneous cooling conditions was captured. Secondly, the change in flatness during hot levelling was predicted. Thirdly, a transition from centre to edge waves was observed during final air cooling after levelling. This phenomenon (sometimes observed in industrial heavy plate production) was successfully reproduced in a sequence of coupled simulations for the first time. Finally, the hot levelling process was analysed via a parameter study to understand the root cause of this transition and to propose alternative hot levelling strategies that avoid it.