Continuous casting of Al-killed Ti-stabilized ULC steels is still linked to the problem of nozzle clogging. Until today the reason behind this phenomenon is not entirely clarified. One possible cause is the attachment of agglomerated deoxidation products (e.g., Al2O3) to the nozzle wall. Therefore, different tracing techniques are applied to track alumina inclusions and their possible modification over the production route. Besides the direct addition of rare earth elements (e.g., La, Ce) to the melt, a second method, the rare earth element (REE) fingerprint, is also discussed.
The present study compares tracing on a laboratory scale with trials in the industry. The experiments in the laboratory were carried out in a resistance-heated Tammann-type furnace since an inert atmosphere can be adjusted, and the production route can be depicted through consecutive alloying additions and continuous sampling. In both cases, Lanthanum or Cerium was added to the melt after the deoxidation with Aluminium. Furthermore, samples were taken during the process to detect the change in morphology of non-metallic inclusions.
Differences between the industrial and the laboratory scale appear mainly concerning the cooling conditions, the inclusion size and their amount. Moreover, the possibility of investigating the clogged material in the submerged entry nozzle leads to additional output from the industrial trials. Ti-modified REE-traced alumina inclusions were found in all experiments. Together with the investigation of the clogged material from the industrial trial, it can be suggested that preexisting deoxidation products agglomerate and attach to the nozzle wall. The traced inclusions form heterogeneous microscopic multiphase inclusions in all cases.