Effect of gas absorption of liquid steel on steel cleanliness in tundish

초록

The evolution of non-metallic inclusions in various steel grades, along with reoxidation phenomena induced by gas absorption during casting, is examined. Gas absorption leads to a two-stage reoxidation process. In the first stage, a sharp drop in oxygen partial pressure occurs at the melt surface. This is followed by a second stage, characterized by a more gradual decrease in oxygen partial pressure due to the formation of an oxide layer that inhibits further oxygen uptake. Although alloying elements significantly influence gas absorption rates, the thermodynamic and kinetic aspects of their interaction with oxygen remain inadequately understood. The development of a novel CaO–Al2O3–SiO2-based single-layer flux has helped mitigate reoxidation caused by excess SiO2 content in rice husk ash. However, slag carryover from the ladle can increase the oxidizing potential of the flux in the tundish and enhance the permeability of atmospheric oxygen through the slag layer, raising the risk of reoxidation of liquid steel during a stable teeming process. Furthermore, reoxidation can also result from oxidizing gases produced by carbothermal reactions in refractory materials. Both theoretical and experimental studies confirm that, under high-temperature conditions in the tundish, carbon monoxide (CO) is the main oxidizing gas produced by these reactions. Dissolved CO increases localized oxygen content in the molten steel, leading to reoxidation and nozzle clogging. The composition of non-metallic inclusions formed during reoxidation is steel grade-specific. In Al-killed steels, they evolve into aggregates rich in Al2O3. For Ca-treated steels, however, their transformation is primarily governed by the concentrations of calcium and sulfur in the melt. At lower CaS content, reoxidation produces solid inclusions. Higher CaS content allows more dissolved oxygen to be absorbed, reducing the formation of solid inclusions. In ultra-low-carbon Ti-containing Al-killed steel, the reoxidation caused by gas absorption produces Al2O3 and Al2O3–TiOx composite inclusions. This steel type is also susceptible to oxidation by CO, forming liquid FetO–Al2O3–TiOx phases at the interior surface of the nozzle. These inclusions, along with Al2O3 and solidified steel, can contribute to nozzle clogging as Al in the steel reduces the FetO–Al2O3–TiOx phases. For Al-killed and Ce-treated steels, molten steel reoxidation transforms spherical Ce2O2S inclusions into clustered Al2O3–CeAlO3 inclusions. Large clusters of Al2O3–CeAlO3 can clog nozzles and create defects in continuous casting billets. In Si-killed stainless steel, reoxidation results in the simultaneous oxidation of Al, Ca, Mn, and Si by atmospheric oxygen, increasing the MnO and SiO2 contents in inclusions. As the casting stabilizes, MnO is progressively reduced by Al and Ca in the steel, causing MnO-rich inclusions to disappear over time.

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