The aim of this study is to determine the optimal chemical composition of the tundish flux, which can efficiently absorb macro and micro inclusions from 16MnCrS5 case hardening steel. Computation of phase diagram is used as an approach to understand properties and behavior of tundish slags in casting conditions. Thermodynamic computation of steel − slag reactions is performed to find out the capability of different tundish slags to absorb inclusions. Industrial trials are, then, carried out with the slags to verify their functionality, when casting several heats in sequence. Results show that by tailoring tundish slag composition the steel cleanliness can be improved. Basic Ca–Mg–aluminate type slags have potential for wide use, as they can be easily adjusted chemically stable against the steel and have ability to absorb inclusions. The slag optimization is, however, a complex issue depending for example on the steel grade.