Improvement of steel cleanness requires detailed knowledge about the formation of non-metallic inclusions. Tracing inclusions
and potential sources is an effective tool for studying inclusion evolution. In the paper, two tracing approaches are evaluated.
First, classical tracer experiments are performed, where rare earth elements (REE) like La or Ce are deliberately added to the
melt on an industrial scale to mark the inclusions directly. Second, the natural REE multi-element fingerprint of the inclusions
is compared to the REE multi-element pattern of potential sources such as alloying and auxiliary materials. These REE
distributions profiles are interpreted by normalizing the REE fractions to reference values. REE contents are analyzed by
inductively coupled plasma mass spectrometry in all cases. Both approaches are applied to track the source of clogged material
in the submerged entry nozzle from Ti-ULC steel production. Results show a clear connection between inclusions resulting
from deoxidation and the clogged layer in the SEN.