Abstract: In the current study, inclusions in the molten steel and the continuous casting slab of a ultra-low carbon Al-killed steel were analyzed using automatic scanning electron microscopy-energy dispersive spectrometer. The evolution of inclusions in the steel during continuous casting were analyzed and discussed by combining with thermodynamic calculation. The composition of each inclusion in the steel during continuous casting was mainly compounds of titanium and aluminum, with an average content of 98 %. Inclusions of the molten steel in the continuous casting tundish were mainly Al₂O₃ and Al₂O₃-TiO_x, and those in the continuous casting slab were mainly Al₂O₃, Al₂O₃-TiN and TiN. Inclusions of Al₂O₃ and Al₂O₃-TiO_x in the molten steel were significantly removed by attaching to the submerged entry nozzle and by removing to mold slag during continuous casting. As the temperature decreased, many TiN particles precipitated in the solid and mushy zone of the steel and became the dominant ones in the slab. A thermodynamic software was used to calculate the transformation of inclusions during cooling and solidification of the steel, showing a result agreeing with the measurement. According to the calculation results of Scheil equation, when the solid fraction was greater than 0.58, TiN can precipitate in the solid steel around the solidification front; when the solid fraction was greater than 0.83, TiN can precipitate in the molten steel around the solidification front. In the continuous casting slab, TiN inclusions accounted for 86 % of the total inclusions, and the TiN inclusions with Al₂O₃ cores accounted for 8 %.

Key words: ultra-low carbon Al-killed steel, non-metallic inclusions, continuous casting, evolution