Transient Evolution of Al–Ti Oxide Inclusions During Reoxidation in Ti-Added Ultra-Low Carbon Steel: Thermodynamic–Diffusion Simulation with Experimental Consistency

Abstract Ti-added Ultra-Low Carbon (Ti-ULC) steel is an interstitial-free grade with excellent formability and elongation, widely used for automotive outer panels. However, the low carbon content, achieved by Ti addition during refining, leads to surface defects and nozzle clogging associated with non-metallic inclusions (NMIs). Understanding the phase stability and transient evolution of these inclusions is therefore essential. In the present study, the inclusion system was investigated using thermodynamic modeling within the CALPHAD framework, coupled with diffusion analysis. Particular attention was given to the Fe–Al–Ti–O system under reoxidation conditions. The results revealed that the presence of Ti promotes the formation of NMIs beyond conventional alumina, such as pseudobrookite ((Ti,Fe,Al) $$_3$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mn>3</mml:mn> <mml:mrow/> </mml:mmultiscripts> </mml:math> O $$_5$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mn>5</mml:mn> <mml:mrow/> </mml:mmultiscripts> </mml:math> ) and molten oxide (Fe $$_t$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mi>t</mml:mi> <mml:mrow/> </mml:mmultiscripts> </mml:math> O–Al $$_2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mn>2</mml:mn> <mml:mrow/> </mml:mmultiscripts> </mml:math> O $$_3$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mn>3</mml:mn> <mml:mrow/> </mml:mmultiscripts> </mml:math> –TiO $$_x$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mi>x</mml:mi> <mml:mrow/> </mml:mmultiscripts> </mml:math> ). These inclusions incorporate significant amounts of Fe oxide, which can act as an “oxygen carrier” in molten steel, thereby deteriorating steel cleanliness. The transient evolution of NMIs predicted by the thermodynamic–diffusion analysis was examined through a high-temperature experiment, which demonstrated qualitative consistency with the predicted inclusion evolution. Microstructural characterization confirmed the presence of pseudobrookite, corundum, and metallic Fe droplets formed by the reduction of the pseudobrookite. The findings highlight that the combined effect of Ti and high-oxygen potential facilitates oxidation reactions in Ti-ULC steel. A comprehensive reaction mechanism is proposed based on integrated thermodynamic–diffusion simulations with experimental consistency, providing new insights into the control of inclusion evolution and steel cleanliness in Ti-ULC steels. Graphical Abstract