The vent brick is a key functional component used in metallurgical engineering for external refining in steel ladles. It facilitates steel stirring by bottom-blowing argon gas, offering high thermal stability and excellent erosion resistance. As a long-lasting, energy-saving, and consumption-reducing product, the vent brick is primarily manufactured from materials such as corundum, corundum spinel, and chromite-corundum. Among these, corundum spinel permeable bricks are increasingly replacing traditional chromite-corundum products due to their superior corrosion resistance. For instance, a vent brick product developed by Luoyang Jinkai Company achieves a service life of 30 cycles per unit in a 40-ton LF-VD furnace, resulting in annual argon cost savings of approximately 5 million yuan.
The vent brick structures are categorized into three types: dispersed, straight-through oriented, and slit-oriented. The slit-oriented type has become the mainstream due to its high gas flow rate and long service life. The primary causes of damage include thermal stress shock, mechanical erosion, and steel slag infiltration.
While the addition of Cr₂O₃ can enhance impermeability, the vent brick carries the risk of Cr⁶⁺ pollution. Current research and development efforts focus on the application of nanomaterials, the selection of high-temperature solid-solution phase additives, and the optimization of slit structures. A new type of vent brick incorporating nano calcium carbonate has achieved chromium-free production, combining a micro-pore structure with higher toughness. Furthermore, the adoption of a composite structure design for the seat brick, along with separate casting technology, has reduced its erosion rate by more than 0.5 mm per heat.
Performance in Use: Slag Resistance
To enhance the slag resistance and metal penetration resistance of alumina spinel vent brick, Cr₂O₃ or chromium-containing compounds are typically added. Cr₂O₃ shares the same crystal structure as α-Al₂O₃. Its addition not only improves the slag resistance of the material but also increases the wetting angle between the brick and the molten steel. This significantly reduces pore blockage caused by steel infiltration.
The aluminum-chromium solid solution, formed at high temperatures from Cr₂O₃ fine powder and Al₂O₃, along with the independently existing chromium-containing glass phase, creates a viscous liquid phase upon contact with slag during the steelmaking process. This phase hinders the slag's corrosion of the vent bricks. Simultaneously, it absorbs iron oxide (FeO) and magnesium oxide (MgO) from the slag, forming dense spinel phases in the working layer of the brick, thereby enhancing its slag resistance.
However, adding Cr₂O₃ to the material poses an environmental risk, as Cr³⁺ can be oxidized to toxic Cr⁶⁺ during high-temperature firing or service. Consequently, to save energy and protect the environment, the use of Cr₂O₃ should be minimized. The research goal is to achieve comparable high-temperature performance without Cr₂O₃-by using alternative raw materials-to match the level of performance attained when Cr₂O₃ is added.
Thermal Shock Resistance
The primary failure mode of vent brick is thermal shock damage. As steelmaking temperatures continuously increase, the significant temperature gradient between the hot face of the vent brick during operation and its cooler regions during idle periods demands extremely high thermal shock resistance from the material. Introducing a spinel phase into the castable composition can enhance the thermal shock resistance of permeable bricks.
Additionally, oxides or non-oxides added to the vent brick form solid solution phases with the aggregate at high temperatures. This increases the brick's high-temperature strength, improves its structural stability (or permeability, if accurate), and enhances resistance to slag erosion in the ladle. Following high-temperature heat treatment, the overall performance of the bricks is significantly improved, meeting all required service conditions.
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