The formation mechanism behind nozzle clogging during continuous casting of Ti-stabilized ultra-low carbon (ULC) steels is not entirely clarified today. One of the main reasons for the clogging layer formation is the deposition of pre-existing deoxidation products and the possibility of re-oxidation of the steel at the steel/refractory interface. By applying tracing techniques, the source of interfering inclusions and the formation of the clogging layer during continuous casting can be studied in detail.
In this work, two different approaches to identify the source behind the alumina inclusions observed in the clogged nozzle are applied. First, direct tracing by means of rare earth elements (REEs) was performed. For this technique, REEs are added to the liquid steel after deoxidation. Hence, pre-existing alumina inclusions are modified. The advantage of this technique is that REE-containing inclusions appear brighter than the steel matrix in backscattered electron images of scanning electron microscopy with energy-dispersive spectroscopy (SEM/EDS) analysis. It is the state-of-the-art method to track the formation of deoxidation products over the process.The second concept that was examined in this work is REE fingerprint analysis. Up to now, this method has been mainly applied in the research field of food chemistry and geology. For REE fingerprint analysis, the pre-existing concentration of REEs for all essential auxiliaries in the production process – such as Al-granules or casting powders – are measured by inductively coupled plasma-mass spectrometry (ICP-MS) and normalized to a reference data set in order to make REE patterns easier to recognize. The resulting pattern is then compared to the detected pattern of the clogging layer and existing mesoscopic inclusions. Similarities in the REE patterns indicate materials that may have contributed to the formation of the clogging layer or inclusions.