Determination of mean flow stress and friction coefficient by the modified two-specimen method in cold rolling

The aim of the study is to investigate the possibility of determining the on-line accurate mean value of both friction coefficient and flow stress for anisotropic materials during cold rolling. For this purpose, the rolling process was carried out by FE-simulations instead of by experiments. In the FEM, elastic flattening of work-rolls and material anisotropy have been taken into account. Based on the rolling data obtained from the FE-simulations, the influence of elastic flattening on estimation of the friction coefficient has been verified by forward slip method. Also the best combination of rolling force models together with equations describing the radius of flattened work-rolls has been selected by means of the modified two-specimen method (MTSM). The work material is low carbon steel provided by SSAB. The strain rate sensitivity of the material was tested in plane strain compression tests at three different strain rates. And its anisotropy was tested in different orientations in cylinder upsetting and plane strain compression tests. Results show that the flow stress determined by off-line methods under low strain rate conditions cannot adequately describe its plastic behavior in rolling process if the rolling speed is extremely fast. Results also show that if the elastic flattening of work-rolls is not taken into account, the friction coefficient can be overestimated by up to 35% based on the forward slip method, meaning that the influence of elastic flattening on determination of the friction coefficient do exist. Comparing different combinations of the rolling force models and the equations describing the radius of flattened work-rolls, results identify that the Ekelund’s model combined with the Hitchcock equation modified by Roberts gives the best result. Meanwhile, results demonstrate that the influence of anisotropy on determination of the friction coefficient can be negligible when MTSM is utilized. Finally, it is concluded that MTSM can be an efficient method to determine the on-line mean value of both friction coefficient and flow stress in the rolling process, even if materials possess anisotropy.