Influence of Reoxidation in Tundish on Inclusion for Ca‐Treated Al‐Killed Steel

Reoxidation usually happens during steelmaking, especially in tundish. Severe reoxidation contaminates the steel cleanliness, and detrimentally affects the steel quality and performance. The purpose of this study is to highlight the adverse effect of reoxidation in tundish on non-metallic inclusion in liquid steel and the end product for Ca-treated Al-killed steel. Inclusions of RH refining, tundish, slab, and plate were investigated on API-X70 pipeline steel. To get a more reliable result, automatic scanning electron microscopy ASPEX PSEM EXPLORER was employed to analyze the inclusion with a much larger scan area. It was found that reoxidation in tundish converted the well-modified liquid calcium aluminates into solid Al2O3-rich inclusions, which are more likely to cause nozzle clogging than pure Al2O3, and the inclusion number dramatically increased form 2.8 µm−2 after RH treatment to 13.5 µm−2 in tundish, further decreased to about 6.0 µm−2 in slab and plate. It indicated that reoxidation in tundish put the calcium treatment and strong capacity of inclusion removal during RH treatment in vain. The ratio of liquid inclusions was proportional to average CaO/Al2O3 in inclusions. Big aggregates and stringer shaped inclusions with CaO/Al2O3 locating between 0.08 and 0.13 were found at the 1/2 and 1/4 slab and plate thickness. The stringer shaped inclusions in the plate were the product of big aggregates in the slab after rolling, and made the plate fail to pass ultrasonic testing.