Ti-added ultra-low carbon (ULC) steel is produced by adding Al and Ti during the secondary refining
process to deoxidise molten steel and to bind C and N which are detrimental to the deep drawing
quality of the steel product. However, the production of Ti-added ULC steel faces challenges,
including submerged entry nozzle clogging and surface defects on cold rolled coils. The troubles
were often attributed to ‘Al-Ti complex inclusions’ in the molten steel. However, the evolution of oxide
inclusions including their stability has been still unclear. In the present study, the evolution of these
inclusions in Ti-added ULC steel was investigated by phase diagram measurement, CALPHAD
thermodynamic modelling, high-temperature stability test for the oxides, and observation of transient
behaviour of oxide inclusions in the steel. The phase diagram of the Al-Ti-O system was elucidated
with an emphasis on oxygen potential, which significantly controls the stability of the oxides.
CALPHAD thermodynamic models and a self-consistent database were developed. The transient
behaviour of Al-Ti complex inclusions in Ti-added ULC steel was observed in situ using a confocal
scanning laser microscope. This was additionally validated by the new thermodynamic model. The
equilibrium phases of the oxide inclusions were investigated using a high-temperature resistance
furnace, and were analysed using a secondary electron microscope, energy dispersive
spectrometer, and electron back-scattered diffraction. By considering the morphology and crystal
structure of these inclusions, the evolution of Al-Ti oxide inclusions was elucidated. Various
behaviour of Al-Ti inclusions and their influence on steel cleanliness are discussed.