Improvement of cleanliness in continuous casting

The present work describes an innovative method and its application. Strand specimens are rolled transverse to the direction of casting and tested using ultra-sonics. In the case of sequence casting, particular attention must be devoted to the non-steady-state phases of the casting process. Reoxidation and slag carry-over must be prevented. Castability and 'nozzle clogging' can be controlled by means of Ca treatment. In steels with specifications of S > 0.025% and Al content from 0.020 to 0.040%, 'black' nozzle depositions of CaS, CaO&middot2Al2O3, CaO&middotAl2O3, are formed and infiltrated by liquid steel, which subsequently oxidizes superficially to Fe3O4. The emulsification of carried-over desulphurization slag from the ladle creates an inclusion collective in the strand, with diameters ranging from 25 to 160 μm. Techniques of flow control, argon rinsing and increase of residence time in the tundish contribute to an improvement in the cleanliness of the strands. An important factor appears to be a reduction in casting rate, in order to allow the inclusions adequate time to separate out in the ladle, tundish and mould.