1.Overview of bottom-blowing gas-permeable bricks
Since the 1960s, ladle secondary refining processes have been adopted by steel producers. Industrial practice has demonstrated that effective secondary refining in a ladle is difficult to achieve without proper inert gas treatment. Initially, inert gas stirring was carried out using top-blown argon lances; however, this method proved limited, as it failed to adequately stir the molten steel near the bottom of the ladle. In contrast, bottom-blowing inert gas into the ladle enhances bath agitation, facilitating the removal of non-metallic inclusions, purifying the molten steel, and ensuring uniformity in both chemical composition and temperature. In China, the use of porous plugs for secondary refining began in the 1970s, and by the 1990s, the technique of bottom-blowing argon through porous plugs had become a vital method in steelmaking operations.
2.Metallurgical Functions of Porous Plugs
The porous plug in a steel ladle is a critical functional component in the secondary refining process, serving the following primary purposes:
(1) It helps regulate the temperature distribution of the molten steel within the ladle, ensuring the optimal casting temperature for the given process.
(2) It facilitates the uniform distribution of alloying elements and deoxidizers throughout the molten steel via gas-stirring.
(3) It enables the removal of non-metallic inclusions from the molten steel by transporting them into the slag, thereby achieving the required level of steel cleanliness.
To perform these functions, an inert gas—used for refining—is injected into the ladle through the porous plug. At the interface between the porous plug and the molten steel (the working face of the plug), the gas emerges under sufficient pressure, forming a stream of bubbles. This creates a stirring effect that promotes the circulation of the molten steel, ensuring uniformity in both temperature and chemical composition. Simultaneously, the continuous flow of bubbles carries non-metallic inclusions from the molten steel into the slag, effectively purifying the steel.
3.Properties required for porous plugs
To fulfill the aforementioned metallurgical functions, a gas-permeable plug (porous plug) must possess the following key properties:
(1) Good gas permeability. Gas permeability is a critical parameter for evaluating the quality of a gas-permeable plug. Research indicates that the stirring energy imparted to molten steel is directly proportional to the flow rate of the injected gas. Since stirring energy dictates the efficiency of the process, a sufficient level is essential to achieve effective agitation of the molten steel. For a given argon flow rate, a higher number of argon bubbles is more advantageous for both the degassing and the stirring of the molten steel.
(2) High-temperature corrosion resistance. Refining ladles operate under stringent temperature and duration requirements; temperatures can exceed 1750°C, and refining cycles may last for several tens of minutes. During the refining process, the basicity of the molten slag significantly impacts the service life of the gas-permeable plug. Consequently, the plug is susceptible to rapid degradation caused by highly penetrative basic slag at elevated temperatures.
(3) High-temperature wear resistance. During bottom-blown argon refining, the molten steel circulates rapidly within the ladle, leading to increased erosive wear on the furnace lining, the bottom-mounted gas-permeable plugs, and their surrounding seating blocks. During hot repairs of the ladle, oxygen is often blown onto the surface of the gas-permeable plug to melt away any residual steel or slag, thereby restoring its gas-permeability. Simultaneously, a gas stream is injected through the plug to clear away the molten slag. Given that the plug is subjected to high-velocity gas flow during this cleaning process, it must exhibit excellent high-temperature wear resistance.
(4) Good thermal shock resistance. Because ladles are used in an intermittent, cyclic manner, the tip of the gas-permeable plug experiences a sharp temperature increase upon contact with molten steel. Conversely, the introduction of argon gas—which is relatively cool—induces significant thermal stress within the material. Furthermore, when molten steel is poured into an empty ladle, it causes significant temperature fluctuations. Consequently, the operating conditions for porous plugs are extremely demanding, making them highly susceptible to both thermal spalling and structural spalling.
(5) Ease of installation, safety, and reliability are required. Porous plugs are installed within the well block at the bottom of the ladle, where they face exceptionally harsh operating conditions. Since the service life of a porous plug does not match that of the entire ladle, it must be replaced periodically. Therefore, the installation process must be simple, and the plug must be safe and reliable in use to prevent any incidents of steel penetration or leakage.
4.Structural Development of Porous Plugs
Through years of development, three primary structural types of porous plugs (gas-permeable bricks) have emerged: the diffuse-porosity type, the slit type, and the straight-channel type.
The diffuse-porosity type is the earliest form of porous plug. Due to the high inherent porosity of the material, the numerous pores serve as channels for inert gas. However, this type suffers from low mechanical strength and poor resistance to erosion; it is susceptible to penetration by molten steel and slag, which can lead to spalling. Furthermore, it provides relatively ineffective stirring of the molten steel. Consequently, this type is rarely used in ladle porous plugs in China.
Slit-type porous plugs come in two varieties. In one version—often called the “assembled-slit” type—the central section of the plug is constructed by arranging several thin, pre-formed plates to create slits, with the surrounding area filled using castable refractory material. A drawback of this design is the limited control over the gas flow. The second version features dozens of straight, pre-cast slits within the brick body; this is the standard “slit-type” design. Compared to the assembled-slit version, this design offers a longer service life, a higher gas-permeability rate, greater gas flow capacity, and superior stirring efficiency.
Straight-channel porous plugs are manufactured by embedding a number of thin steel tubes into the brick, creating a network of straight, micro-fine gas channels; these are typically formed using a casting process. Compared to the diffuse-porosity type, straight-channel plugs provide better stirring and a service life that is two to three times longer. Their primary limitation, however, is a restricted gas flow capacity; toward the end of their service life, the gas flow often diminishes or becomes completely blocked, which can lead to the failure of the refining process.
5.Selection for the Installation of Porous Plugs
The configuration of porous plugs is determined by factors such as the volume of molten steel in the ladle, the stirring requirements for the specific steel grade being produced, and the overall metallurgical process route. The placement of bottom-blowing gas components in the ladle should be decided based on the specific objectives of the ladle treatment. Installing a porous plug at the center of the ladle bottom versus at an off-center position (typically at a radius of 1/2 to 1/3 from the center) results in different stirring dynamics. Central bottom-argon blowing promotes reactions between the slag and the molten steel—such as desulfurization by the top slag—whereas off-center blowing enhances mixing, ensures uniform temperature distribution, and facilitates the flotation of inclusions. For instance, in the CAS/CAS-OB process, the porous plug must be positioned at the center of the ladle bottom; this allows the argon flow to clear the slag from the surface of the molten steel beneath the immersion bell, facilitating alloying or oxygen injection. In the LF (Ladle Furnace) process, the porous plug is typically positioned near the furnace door to accommodate operations such as recarburization, alloying, and deoxidation. Increasing the number of porous plugs generates a greater volume of fine gas bubbles, which improves the removal of inclusions and enhances the quality of the molten steel; however, a higher number of plugs also increases the risk of steel breakouts. Generally, for the production of plain carbon steel, a single porous plug is sufficient for ladles with a capacity of up to 70 tons, while ladles with a capacity exceeding 70 tons require two porous plugs.
Porous plugs can be installed using either an internal or an external mounting method. The internal method involves pre-assembling the porous plug with its seating block outside the ladle. During the lining of the ladle, the designated area at the bottom is cleared, and the pre-assembled unit (porous plug and seating block) is hoisted into position before the surrounding bottom bricks and ladle lining are installed. The external mounting method consists of a seating block, a sleeve brick, and the porous plug itself. When lining a steel ladle, the bottom and sidewalls are constructed after the well block has been installed in the ladle bottom. Subsequently, a layer of refractory mortar is applied evenly to the outer surfaces of the sleeve brick and the porous plug; these are then firmly inserted into the well block. Following this, a base brick is placed beneath the sleeve brick and porous plug, the flange is secured, and the ladle undergoes a preheating (drying) process. “Internal-mounting” porous plugs are intended for scenarios where the service life of the ladle lining matches that of the porous plug, whereas “external-mounting” (bottom-loading) designs are suitable for situations requiring frequent replacement of the porous plug. Due to the lower safety, poorer reliability, and cumbersome replacement procedures associated with internal-mounting porous plugs, virtually all steel ladles now utilize the external-mounting design.
6.Materials and Use of Porous Plugs
Currently, the materials used for porous plugs include corundum, chrome-corundum, high-alumina, and magnesia-chrome. The table below presents the physical and chemical specifications for porous plugs and well blocks used in steel-refining ladles.
A. Corundum-Spinel System Porous Plugs
Single-phase corundum castables exhibit suboptimal slag resistance and thermal shock resistance, whereas spinel-based materials offer superior resistance to slag erosion. Based on the principle of multi-phase modification to enhance the performance of refractory materials, high-purity fused spinel is incorporated into corundum castables. The raw materials typically consist of tabular corundum (as coarse aggregates), along with fused white corundum, spinel, and reactive α-Al₂O₃ micropowder (as fine components), with calcium aluminate cement serving as the binder. While this approach significantly improves thermal shock resistance and slag resistance, it presents a drawback: the volume changes occurring in the spinel during high-temperature processing compromise the volume stability of the porous plugs, making the manufacturing process difficult to control.
B. Corundum-Chromium Oxide System Porous Plugs
To further enhance the resistance of porous plugs to steel slag erosion, a specific quantity of chromium oxide (Cr₂O₃) micropowder is added to the composition. The primary raw materials include tabular corundum (as coarse aggregates), with fine tabular corundum powder and chromium oxide micropowder serving as the fine components, and calcium aluminate cement as the binder. At high temperatures, chromium oxide and aluminum oxide form a solid solution; additionally, they form a partial solid solution with a small amount of magnesium oxide (MgO·Cr₂O₃–MgO·Al₂O₃). This solid solution significantly improves resistance to erosion by Fe₂O₃ or furnace slag; furthermore, it exhibits high viscosity, which effectively inhibits the penetration and erosion of steel slag at high temperatures. Moreover, a small amount of Cr₂O₃ helps suppress excessive grain growth of Al₂O₃, reduces internal crystalline stress, and enhances the physical properties of the material. However, an excessive addition of Cr₂O₃ can overly inhibit the growth of corundum grains, leading to the development of internal stress that degrades the material’s physical properties. Additionally, Cr₂O₃ is relatively expensive—increasing costs significantly—and it poses a serious environmental pollution risk. C. Corundum-Spinel System Porous Plugs
Corundum-spinel system porous plugs are the most widely used type. Their primary raw materials include tabular corundum, α-Al₂O₃ micropowder, and spinel, bonded with pure calcium aluminate cement. Their advantages stem from the fact that spinel is a high-melting-point compound with strong resistance to both acids and bases. Magnesium-aluminate spinel, in particular, exhibits high resistance to alkaline slag and remains relatively stable when exposed to iron oxides; at high temperatures, it reacts with magnetite to form a solid solution, which enhances the plug’s resistance to high-temperature corrosion. Furthermore, the formation of a solid solution—incorporating excess MgO or Al₂O₃ into the spinel—improves thermal shock resistance due to differences in the thermal expansion coefficients of the constituent minerals. A drawback is that the formation of spinel from stoichiometric proportions of MgO and Al₂O₃ results in a volume expansion of approximately 8%, making it difficult to achieve high density during firing and complicating the control of volume changes in the porous plugs.
D. Corundum-Chromium Oxide System Porous Plugs
Corundum-chromium oxide (Cr₂O₃) system porous plugs were developed based on the corundum-spinel system to improve high-temperature spalling resistance. The primary raw materials consist of tabular corundum, α-Al₂O₃ micropowder, industrial-grade chromium oxide, and spinel, bonded with pure calcium aluminate cement. Beyond the performance benefits provided by spinel, the formation of an Al₂O₃-Cr₂O₃ solid solution significantly enhances resistance to corrosion by iron oxide-rich slag. Additionally, the inclusion of a small amount of Cr₂O₃ inhibits excessive growth of aluminum oxide crystals, thereby reducing internal crystal stress and improving the plug’s thermal shock resistance, erosion resistance, and corrosion resistance. A disadvantage is that an excessive addition of Cr₂O₃ severely disrupts the growth rate of corundum grains, which degrades the material’s physical properties. Moreover, Cr₂O₃ poses a significant environmental pollution risk, which runs counter to national requirements for sustainable development.