Automatische Partikelanalyse zur Bestimmung der Verteilung nicht-metallischer Einschlüsse innerhalb von stranggegossenen Brammen eines ULC (Ultra Low Carbon)-Stahls

In two slabs of an ULC steel with different oxygen and nitrogen contents, the size and spatial distribution of non-metallic inclusions with a minimum size of 1 µm was investigated via automated particle analysis (SEM/EDS). The preparation of large sample areas enables an inclusion investigation over the whole slab thickness of 250 mm. The inclusions consist mostly of Al₂O₃ and TiN. The inclusion density is determined by the concentrations of oxygen and nitrogen, and the size distribution is primarily controlled by the solidification and cooling conditions. Due to the reduction of cooling rate during continuous casting from the surface towards the center of the slab, inclusions in the slab center are larger. This trend is especially pronounced for TiN and is attributed to diffusion-controlled growth of the particles. Since the highest cooling rates are present at the edges of the slab, the inclusions are smallest there. Al₂O₃ inclusions occasionally form clusters, especially close to the slab surfaces.