Tension levelling is used to improve the flatness of buckled and curved sheet.
Buckles are caused by some portions of the sheet across the sheet width being shorter in length in the rolling direction than other parts. The buckles can be removed by extending the sheet sufficiently in the rolling direction such that it is permanently deformed and the sheet length differences are small.
Curvature is caused by a through thickness strain distribution in the sheet. It can be a curvature in the rolling direction and across the sheet width. In both cases the strain distribution can be altered by passing the sheet through the roller leveller.
Tension levellers are used after a rolling mill to correct some of the off-flatness caused by the rolling process. Mills may have flatness control systems to help mill operators minimise sheet defects from the rolling process, but some levelling is usually required to produce extremely flat sheet that some customers now demand for their processes.
A typical sheet may have a thickness of the order of a millimetre and is passed round the rolls in a tension leveller. It is bent backwards and forwards whilst under tension. The sheet surface undergoes tension and compression cycles that exceed the yield stress of the material and thus causes plastic deformation which is permanent. The roll diameters need to be small enough such that the sheet surface can exceed the yield strain. Tension in the sheet will also help the bending mechanism to the reach the yield strains. The yield strain or stress will depend on the alloy and temper of the sheet.
Our validated, fast running tension leveller model can predict the through thickness stresses and strains in a sheet as it goes through a tension leveller.
As the strip passes over each roll, the stresses and strain in the strip are calculated to give the curvature after each roll.
A selection of stress, yield stress and strain for any roll can be plotted as a function of thickness. Results, shown in either graphical or tabular format, include the strip tension after each roll, the average yield stress, total strain, plastic strain, curvature and friction.