Some fundamental considerations for the control of residual flatness in tension levelling

Tension levelling is a process used in the steel industry in order to remove any shape defects present in temper rolled strip. It is generally the final process before the cold rolled product is despatched to the customer, and therefore plays an important rôle in delivering the desired material properties and the product standards required. In this paper a designed factorial analysis has been employed in order to study the effect that tension levelling process parameters have on the shape characteristics of the levelled product. These characteristics include residual flatness (longitudinal curvature or longbow), centreline elongation and residual (or internal) stress imbalances; criteria which determine whether the customer accepts the material or not. It has been identified that in a basic five-roll leveller the final, adjustable roll wrap angle, has the most significant influence on these characteristics in over 70% of cases. It has also been determined that final flatness is dictated by many second-order process interactions.