Messi Biology states that in the continuous casting process of steel plants, magnesium oxide (MgO) is used as a separating agent and is a key functional component of the continuous casting mold flux. Its core value lies in regulating the physical and chemical properties (melting point, viscosity, stability, crystallization properties) of the mold flux, especially in inhibiting the reduction reaction of aluminum in molten steel with the mold flux, improving the slag’s ability to accommodate alumina, and maintaining the stability of the lubricating film. This ensures the smooth operation of the continuous casting process and obtaining cast billets with good surface quality. It can be said that without the reasonable application of magnesium oxide, modern, efficient, and high-quality continuous casting (especially casting aluminum-containing steel grades) would be difficult to achieve. Therefore, it is an indispensable strategic raw material in the mold flux formulation design.
Magnesium oxide (MgO) plays a crucial role as a separating agent (more accurately, as a core component of the continuous casting mold flux) in steel plants, particularly in continuous casting processes.
1. Core Application: Continuous Casting Mold Flux
- Position: Covers the surface of the molten steel flowing from the tundish to the mold.
- Main Functions: The core functions of the mold flux are to isolate air, provide thermal insulation, absorb inclusions, lubricate the billet, and regulate heat transfer. Magnesium oxide plays an irreplaceable role in these functions.
2. Key Roles of Magnesium Oxide in Mold Flux:
- Regulation of Melting Point and Melting Characteristics: Magnesium oxide is a high-melting-point alkaline oxide (melting point approximately 2852°C). Adding it to mold flux bases mainly composed of silicates (SiO₂) and calcium oxide (CaO) can effectively regulate the melting temperature and viscosity of the mold flux. A suitable melting point and viscosity range are crucial for the mold flux to form a stable three-layer structure (powder slag layer, sintered layer, liquid slag layer).
- Viscosity Control: Magnesium oxide can change the silicon-oxygen network structure of the slag, thereby affecting the fluidity (viscosity) of the slag. Adding an appropriate amount of magnesium oxide helps to obtain the specific viscosity required for the continuous casting process, ensuring that the liquid slag flows evenly between the billet and the copper wall of the mold to form a lubricating film.
- Improved Chemical Stability: During the continuous casting process, aluminum (Al) in the molten steel (especially Al-containing steel) reacts with SiO₂ in the mold flux: 4[Al] + 3(SiO₂) –> 2(Al₂O₃) + 3[Si]. This reaction consumes SiO₂ and increases the content of high-melting-point Al₂O₃ in the slag, leading to a sharp increase in the viscosity of the mold flux and disrupting its lubricating properties. Magnesium oxide is an alkaline oxide, and adding it to the mold flux can:
- (1) Inhibit the Al reduction of SiO₂: Magnesium oxide increases the basicity of the slag (usually expressed as (CaO+MgO)/SiO₂), reducing the oxidizing power of the slag, which reduces the thermodynamic driving force of the above reduction reaction, thereby inhibiting the reduction of SiO₂ by Al.
- (2) Accommodate Al₂O₃: Slag systems containing magnesium oxide have a higher ability to accommodate Al₂O₃. Even if some Al₂O₃ enters the slag, the presence of MgO helps form low-melting-point calcium-magnesium-aluminum-silicate phases, preventing the precipitation of high-melting-point spinel (MgAl₂O₄) or corundum (Al₂O₃) crystals in the slag, thus avoiding abnormal viscosity increases and maintaining the stability of the lubricating film.
- Optimization of Crystallization Properties: During the solidification of the mold flux, magnesium oxide can influence the type of precipitated crystal phases and the crystallization rate. By adjusting the MgO content, the heat transfer rate of the mold flux within the mold can be controlled, which is very important for preventing surface cracks in the billet (especially in medium-carbon steel). Suitable crystallization properties help form a uniform slag film and optimize heat flow distribution.
- Reduction of Fluoride Usage / Development of Low-Fluoride/Fluoride-Free Slag: Traditional mold fluxes often use fluorspar (CaF₂) to lower the melting point and viscosity. However, fluorides are corrosive, pollute the environment, and are harmful to health. MgO can be used as an effective fluxing agent to partially replace or reduce the amount of fluorspar, helping to develop more environmentally friendly low-fluoride or fluoride-free mold fluxes.
3. Requirements for Magnesium Oxide:
- Purity: High purity is required (usually MgO content above 95%, even 99%+) to reduce the interference of impurities (such as SiO₂, Al₂O₃, Fe₂O₃) on the performance of the mold flux.
- Particle Size: Specific fineness and particle size distribution are required to ensure uniform mixing in the mold flux batch and sufficient reaction during melting.
- Source: Mainly comes from calcined magnesite (MgCO₃) or magnesium hydroxide extracted from seawater/brine and then calcined.