Effect of slag-steel reaction on the structure and viscosity of CaO-SiO2-based mold flux during high-Al steel casting

Highlight
• IVAl species mainly formed AlO4 and AlO3F in the mold flux with Al2O3 and CaF2.
• AlO4 and AlO3F attributed to the formation of 3D-network structure of slag.
• The slag-steel reaction affected the structural stability of mold flux.
• Improving the structural stability is important to design the mold flux.

In this study, to establish the structure–viscosity relationship of mold flux before and after the slag-steel reaction, the structure of CaO-SiO2-based and CaO-SiO2-Al2O3-based mold fluxes was investigated by Raman spectroscopy and magic angle spinning nuclear magnetic resonance. The results show that Si atoms are replaced by Al atoms and act as the network formers for the slag-steel reaction. Al has three types of coordination number (IVAl, VAl, and VIAl species) in the mold flux, but only the IVAl species mainly forming AlO4 and AlO3F can take part in the formation of the three-dimensional (3D) network. During the slag-steel reaction process, the degree of polymerization decreases first and then increases. The 3D-network of the original CaO-SiO2-based slag is connected by the Si–O–Si linkages; however, it is converted to the CaO-SiO2-Al2O3-based slag mainly connected by Si–O–Si, Al–O–Al, and Al–O–Si linkages during this conversion. The change in the degree of polymerization and chemical bond properties will affect the structural stability and deteriorate rheological characteristic during this conversion. Hence, the effects of slag-steel reaction on the structural stability and rheological characteristic must be controlled in the design process of CaO-SiO2-based mold flux for high-Al steel casting.