Elucidation of inclusion distribution mechanism in continuously cast AISI 316L stainless steel blooms

A study was conducted to ascertain the mechanism of modification of the inclusion distribution along the cross-section of continuously cast AISI 316L grade stainless steel blooms. Specimens from various locations along the perpendicular and diagonal directions of 160 mm square AISI 316L grade stainless steel blooms were used. The inclusion distribution on the surface of the specimens was studied by the feature counting method using an optical microscope. Size distribution, mean size, mean distance of separation, and volume fraction of inclusions were investigated by measuring the sizes of inclusions using an image analyser and using the particle size distribution module of a computer program developed at Mukand Ltd which was based on the Schwartz-Saltykov method. The inclusion size was found to be finer near the surface of the bloom compared with that observed further away from the surface towards the Centre where the shell growth rate was lower than that at the surface. The number of inclusions per unit area was found to increase to a maximum value and then decrease from the surface to the centre of the bloom cross-section. Particle coarsening diffusional growth of larger silica inclusions at the expense of smaller ones is found to be operative during solidification. The coarsening rate constant matches fairly well that derived from the physical parameters of the melt and the inclusions. Inclusions were characterised by scanning electron microscopy and energy dispersive X-ray spectroscopy.