Void initiation close to a macro-inclusion during single pass reductions in the hot rolling of steel slabs: A numerical study

The work is focussed on the start of void formation close to a hypothetical single macro-inclusion in the hot rolling of steel slabs. The inclusion is chosen to be either three times harder or three times softer than the surrounding matrix. A commercial FE-code LS-Dyna3D has been utilized for the numerical analysis. Process data from the Swedish company “SSAB Tunnplåt AB” are used. The initial inclusion geometry is assumed to be cylindrical with an extension through the entire width of the slab. Different distances from the slab surface, on the vertical symmetry plane in rolling direction, are chosen as locations for the enclosure. Influence of different single pass reductions and roll radii are analysed. The only boundary force used on the matrix/inclusion interface is referred to friction. By treating a very big inclusion and neglecting bonding forces in radial direction, a worst-case scenario means to be studied. Two indices, one describing the deformation of the inclusion and one the surrounding matrix are used to estimate the likelihood for start of void formation. If the matrix index is bigger than that of the inclusion, voids are assumed to be initiated. This hypothesis gives trusty results. Near the hard particle, voids are likely to be formed. It became clear that the risk is smaller when large rolls are used. Furthermore, the threat for start of void formation turned out to be slighter for high reductions and in the vicinity of the slab surface. Soft particles were just elongated and no risk for void formation could be predicted.