Formation of residual stresses owing to tension levelling of cold rolled strip

Unacceptable shape defects in light gauge strip products may be introduced as a result of differential reduction being applied across the width of the material at rolling mills. Consumer specifications and quality procedures demand that the resulting defects are removed by subsequent processing oper ations. Such an operation is tension levelling, which attempts to remove th e resulting defects such as edge waves by ensuring that all longitudinal ma terial 'fibres' possess the same length. Shape removal is achieved by subje cting the material to a series of alternate bends under superimposed tensio n. Consequently, imbalanced residual stress fields may be present in the as levelled strip, resulting in bowed material in a further downstream slitti ng operation. Much work has been published on the mechanics of the process, and many advances have been made, especially in the design of levelling sy stems to improve shape removal and reduce internal stress levels. However, residual stresses and specifically the imbalance of these stresses post-lev elling is an issue that has failed to be addressed in its entirety. To inve stigate the generation and effects of residual stresses owing to tension le velling, a finite element (FE) model has been developed with validation car ried out on an experimental laboratory leveller. The analysis is performed in real time using the commercial FE code Abaqus. The present paper gives d etails of investigations carried out to examine the effectiveness of materi al work hardening models and the associated implications for residual stres s fields; validation of the geometric modelling and boundary conditions are also discussed. Results for the analyses reported here show that a charact eristic (longitudinal) residual stress field is produced across the width o f the material as a fundamental consequence of the levelling process. The c urrent model produces predictions comparable to experimental results from t he laboratory leveller.