Surface defect evolution analysis in high silicon steels during hot rolling processes: experimental and numerical simulation

Experimental and numerical approaches were used in the current research to investigate the evolution of superficial defects that develop on slabs of High Silicon content during the first stand of hot rolling processes. Understanding the deformation mechanisms of the initial surface features is essential for the prevention of critical defect formation on final products.
Hot rolling experiments were carried out to determine the evolution of superficial cavities when the slabs are subjected to hot rolling. In addition, descaling experiments and hot ring compression tests were carried out to identify the friction coefficient present during the process. A model reproducing the hot rolling experiment was produced and a multilevel modelling approach was used to refine the mesh in the defects zone. The predictions are found in good agreement with the experiments.
The results obtained from hot rolling experiments indicated the buckling of the lateral sides of the cavity to be the main cause of oxide entrapment and consequent defect formation. This is because the oxide inclusions beneath flaps of metal are most likely to resurface during successive slab thickness reductions appearing as surface defects. Experiments demonstrated the importance of depth to width aspect ratio of the initial surface features on the severity of the final defect. With regards to friction, isothermal ring tests demonstrate that friction coefficient increases with temperature and decreases with the thickness of the oxide present on the surface of the slab.
The material models, the contact conditions and the modelling approaches developed are used to reproduce a full scale single rolling stand of a hot rolling process. The full-scale model was used to test the effect of different process parameters on the defects evolution and of different geometries on the deformation mechanisms.
The model developed revealed that the friction is the main process parameter affecting the defect evolution during rolling. Higher friction coefficients facilitate the closure of the defect, increasing the severity of the final features formed. The geometry of the initial indentation before rolling was identified to be the main parameter which determines its deformation mechanisms. In particular, spherical cavities are easily eliminated during rolling. Conversely, pyramidal and cubical features tend to deform in a more severe way. For a given initial geometry, the initial aspect ratio of the defect was confirmed to play an important role in the final severity. The major limitation of the model was the excessive simplicity of the contact conditions between roll and slab in the defect zone.
Nevertheless, the approaches used to produce the models can be extended to predict the deformation of different initial features presents on the surface of slab subjected to hot rolling processes and to reproduce the behaviour of defects for multistand rolling procedures.