Modelling Friction in Cold Rolling

The cold rolling process is an essential step in the production chain of high quality steel strip that is used in applications such as automotive, packaging or construction. Cold rolling models are of crucial importance for setup of the cold rolling mill, defect analysis, development of new steel grades or for design of new rolling mills. Unlike conventional cold rolling models, mixed lubrication models explicitly take the influence of the lubricant into account. This thesis aims to further advance mixed lubrication models for cold rolling, with particular attention to:
- A physically based description of the friction mechanisms that play a role in the roll bite.
- The validity of the main assumptions in cold rolling models.
- An extensive experimental validation of the complete cold rolling model.
The proposed mixed lubrication model takes into account three friction mechanisms that play a role during cold rolling: boundary layer shear stress (or ‘adhesive friction’), ploughing friction and viscous shear stress. The contribution of these mechanisms to the total friction in the roll bite is determined by ploughing simulations with the Material Point Method and an experimental characterisation of the rheological properties of the rolling lubricant. The isotropic von Mises yield criterion that is commonly used in cold rolling models is replaced in the mixed lubrication model by the Hill48 yield criterion; this criterion better describes the experimentally determined plastic yielding behaviour of cold rolled strip.

The resulting mixed lubrication model is experimentally validated resulting in the following conclusions:
- Strip can enter the mill asymmetrically resulting in uneven lubricant layers on top/bottom strip surface. An additional experimental procedure guarantees a perfectly symmetrical material entrance.
- Taking anisotropic plastic yielding behaviour into account leads to a significant improvement in model performance.
- Viscous shear heating significantly influences viscous shear stress in the work zone. When this effect is accounted for (by a simple relation), the model outcome corresponds better to the experimental results.
- The complete mixed lubrication model predicts significantly better the rolling force and forward slip than a conventional rolling model, also the lubricant film thickness is accurately predicted.