Ladle air bricks are the most critical functional components in the secondary refining process. They are also the key functional materials in the bottom argon blowing process of secondary steel refining. Their main functions are as follows:
(1) The uniform distribution of molten steel temperature in the ladle can be adjusted to achieve the optimal casting temperature of the existing process.
(2) The alloy and deoxidizer in the ladle can be evenly distributed by blowing and stirring.
(3) Non-metallic inclusions in the molten steel can be brought into the slag to meet the cleanliness requirements of the molten steel.
To achieve the above functions, the inert gas used for refining needs to be blown into the ladle through the air bricks. At the contact surface between the air bricks and the molten steel, that is, the working surface of the air bricks, a large number of bubbles blown out under sufficient pressure form a gas jet beam, stirring the molten steel in the entire ladle, promoting the flow of the molten steel, and making the temperature and composition in the ladle uniform. At the same time, the continuously ejected bubbles bring the non-metallic inclusions in the molten steel into the slag under the action of the interface, thereby achieving the purpose of clean molten steel.
In order to meet the above metallurgical functions, breathable bricks must have the following main properties:
(1) Good permeability. Permeability is one of the important parameters for measuring the quality of air bricks. Studies have shown that the stirring energy of molten steel is proportional to the flow rate of the gas blown in; the stirring energy directly affects the stirring efficiency of the molten steel. Only sufficient stirring energy can achieve a good stirring effect on the molten steel. When the amount of argon blowing is constant, the more argon bubbles blown out, the more beneficial it is to the degassing and stirring of the molten steel.
(2) High-temperature corrosion resistance. The refining ladle has very strict requirements in terms of temperature and time. The highest temperature often reaches above 1750℃, and the refining time sometimes reaches tens of minutes. During the refining operation, the alkalinity of the slag has a great influence on the life of the air bricks. Therefore, the air bricks will be corroded by the alkaline slag with strong permeability at high temperature and will be damaged quickly.
(3) High-temperature wear resistance. When argon is blown from the bottom of the refining ladle, the flow speed of the molten steel in the ladle is very fast due to the bottom blowing of argon, and the erosion and wear of the molten steel on the furnace lining material and the bottom air bricks and seat bricks is significantly increased. During hot repair of the ladle, in order to remove the residual steel and slag on the surface of the air brick and restore the air permeability of the air brick, it is necessary to blow oxygen to clean the surface of the air brick to melt the steel slag adhering to the surface of the air brick; at the same time, blow gas into the air brick to blow away the slag. During the cleaning process, the air brick is scour by the high-speed air flow, so the air brick is required to have good high-temperature wear resistance.
(4) Good thermal shock resistance. Since the ladle is operated intermittently, when the ladle is filled with molten steel, the end of the air brick is affected by the high-temperature molten steel, and the temperature rises sharply. When argon is blown, it is cooled by the cold air flow, and a large thermal stress is generated inside the material. At the same time, when molten steel is poured into the empty ladle, a large temperature change will also occur. Therefore, the use conditions of the air brick are very harsh, and thermal peeling and structural peeling are very likely to occur.
(5) It is required to be easy to install, safe and reliable. Air bricks are installed inside the base bricks at the bottom of the ladle. Operating conditions are extremely harsh, and the lifespan of these bricks cannot keep pace with the entire ladle lifespan. Therefore, they need to be replaced regularly. Therefore, they must be easy to install and operate, safe and reliable to use, and avoid steel seepage and leakage.
Classification of ladle air bricks
Based on their structure, air bricks can be categorized as impermeable, narrow-slit, and straight-through-hole air bricks.
- Impermeable Air Bricks
Diffused air bricks are impermeable air bricks that require minimal or even no cleaning and offer customizable air permeability. They are manufactured by adding a specific amount of micropore-forming agent to the brick mix. After high-temperature firing, the agent decomposes or burns away, creating a uniform distribution of interconnected micropores throughout the brick, achieving the goal of refining and cleaning molten steel. This prevents molten steel from penetrating the air bricks when negative pressure develops within the argon blowing system, preventing the bricks from being eroded by infiltrating steel and slag, thus ensuring rapid and safe production.
- Slit Air Bricks
Slit air bricks are the most commonly used air brick structure in the market. By controlling the number and length of slits and rationally designing them based on actual field conditions, the performance of this type of air brick can be maximized during use.
- Straight Hole Breathing Bricks
Currently, straight hole breather bricks are primarily used in converters. Because straight hole breather bricks used in ladles are complex to manufacture and have low pore permeability, straight slit breather bricks have replaced straight hole breather bricks.
4 common materials of ladle air bricks
The materials of breathable bricks mainly include sintered magnesia, magnesia-chromium, high alumina and corundum.
- Sintered Magnesia Breathable Bricks
Sintered magnesia refers to refractory materials with a MgO content of over 80%. These materials are alkaline and possess advantages such as high refractoriness, strong resistance to alkaline slag corrosion, resistance to corrosion by calcium and CaO, and no contamination of molten steel. However, their high coefficient of thermal expansion and poor thermal shock resistance make them susceptible to spalling, significantly reducing their service life.
- Magnesia-Chromium Breathable Bricks
Magnesia-chromium refractory materials are primarily composed of MgO and Cr₂O₃, with periclase and sphene as the primary mineral components. Because chromium ore is relatively inert to steelmaking slag and is compatible with other refractories, the emergence of magnesia-chromium refractory materials has significantly improved the thermal shock resistance of magnesia.
- High-Alumina Breathable Bricks
High-alumina refractory materials refer to refractory materials with an Al₂O₃ content greater than 48%. They feature high hot and cold strength, excellent wear resistance, thermal shock resistance, spalling resistance, and excellent volume stability at high temperatures. However, its resistance to slag erosion and penetration is poor, insufficient to withstand the infiltration and penetration of molten slag into the bricks during the entire service life.
- Corundum Breathable Bricks
Corundum material refers to a refractory material with an Al2O3 content greater than 90%. Corundum is produced by sintering or fusion of industrial alumina or bauxite. When fused with industrial alumina, white corundum is obtained with an Al2O3 content greater than 98.5%. When bauxite is used as the raw material, ordinary corundum is obtained. Adding iron filings produces brown corundum. Adding germanium quartz or germanium oxide produces germanium corundum, and adding Cr2O3 produces chromium corundum. Fused tabular corundum is formed directly from industrial alumina in a semi-molten state in an electric arc furnace at 1900-2000°C. The ultra-high temperature within the formation zone of fused tabular corundum facilitates the volatilization of volatile impurities such as Na2O, resulting in a self-purification process. The characteristics of plate-like alumina materials are high thermal conductivity, good thermal stability, high high-temperature strength, and strong corrosion resistance, but the production cost is relatively high.
Under high temperature and vacuum, the wetting angle of molten steel with respect to several refractory oxides follows the order: Cr2O3 > Al2O3 > MgO. The stability of the oxides follows the order: Al2O3 > CaO > MgO > Cr2O3. Considering these two factors, corundum is the preferred primary crystalline phase for breathable bricks. Furthermore, the melting point of Cr2O3 is 2275°C, higher than that of Al2O3 (2050°C). Aluminum oxide and chromium oxide can form a continuous solid solution. This solid solution of Al2O3-Cr2O3 significantly enhances its resistance to corrosion by iron oxide or slag. Adding a small amount of Cr2O3 can inhibit the excessive growth of alumina crystals, thereby reducing internal stress and improving the material’s physical properties. However, if excessive Cr2O3 is added, the growth rate of the corundum grains is severely affected, thereby reducing the material’s physical properties. Therefore, the appropriate addition of Cr2O3 can improve the material’s thermal shock stability, erosion resistance, and corrosion resistance. However, from an environmental perspective, the addition of Cr2O3 can cause environmental pollution, so its addition should be carefully considered.
To implement the bottom argon blowing process for ladle breather bricks, the ladle breather bricks must possess excellent high-temperature resistance, corrosion resistance, thermal shock resistance, high-temperature volume stability, high strength, good permeability, stable operation, accurate dimensions, and minimal molten steel penetration.
