Influence of roll speed mismatch on strip curvature during the roughing stages of a hot rolling mill

Abstract

Asymmetrical conditions occurring during the rolling of hot strip can cause strip curvature and can lead to poor product shape, profile, quality and cause reduced productivity. In the most severe of cases, strip with a head-end turning towards the upper work roll may fail to enter the roll gap on its next pass and result in a cobble. In these investigations, finite element techniques have been used to determine the effect of roll speed mismatch on the direction and severity of strip curvature in the roughing stages of a hot mill. A plane strain finite element model has been constructed to simulate the rolling of low carbon steel strip at high temperatures. Five ingoing strip thicknesses were considered, each one representing a particular pass of the reversing rougher at Corus Port Talbot hot mill. Alterations were made to the finite element model's boundary conditions, i.e. roll gap height, to determine how the reduction affects the magnitude of strip curvature. Each model has a 5% roll speed mismatch applied. Results have identified, on selected passes, an optimum reduction range where a straight slab is the predicted outcome. In addition, the results also show the quantified reduction range, where the sensitivity of strip curvature to roll speed mismatch is at a maximum. This is of particular interest, since in many industrial cases the rougher operator utilises roll speed mismatch to counteract the effect of turn down/up caused by other asymmetrical factors, for example temperature differentials or work roll frictional differences.