Non-metallic inclusions in aluminium killed steels

Heats of medium carbon aluminium killed, low carbon silicon-aluminium killed, and low carbon aluminium killed steels were sampled with short time intervals during ladle metallurgy. Non-metallic inclusions were extracted from the steel matrix and investigated using scanning electron microscopy and energy dispersive spectrometry. Six inclusion morphologies were recognised, i.e. spherical, faceted, platelike, and dendritic shapes, as well as clusters and aggregates. For each sample also the total oxygen content was measured. Spherical inclusions were most abundant, but clusters, aggregates, and large faceted inclusions made up most of the oxide volume fraction. The present research shows that clusters, which are formed during the deoxidation operation, are removed within ~15 min after aluminium addition. Aggregates and large polyhedra appear after 5-10 min and their sizes increase with holding time. Small, mainly spherical and polyhedral, and to some extent also platelike, inclusions do not show evolution in size or in composition.