Installing a gas-permeable plug at the bottom of a steel ladle allows for the injection of inert gas to stir the molten steel; this process promotes the flotation of inclusions into the slag layer and homogenizes the steel’s composition and temperature. Consequently, the gas-permeable plug serves as a critical functional component in ladle refining. As requirements for steel quality and purity become increasingly stringent, the importance of these plugs has become more pronounced, as they directly influence steelmaking production rates and costs.

In recent years, research on slit-type gas-permeable plugs has focused on how additives—such as spinel micropowder, fused zirconia-corundum, TiO₂, and magnesia—affect performance; these additives modify the microstructure and enhance thermal shock resistance through mechanisms like micro-crack toughening and phase-transformation toughening. Regarding diffuse-type plugs, studies on the effects of additives and particle composition have established a theoretical foundation for performance improvement. Furthermore, finite element analysis and mathematical modeling have been employed to analyze the generation of internal thermal stresses during argon blowing, providing researchers with a theoretical basis for reducing these stresses and thereby extending the plug’s service life. Currently, the gas-permeable plug represents a bottleneck in further improving ladle service life and operational safety, necessitating deeper optimization. This paper reviews the current state of research on ladle gas-permeable plugs and outlines future development directions.

Performance Requirements for Ladle Purging Plugs

1.1 Thermal Shock Resistance

During ladle operations, gas-permeable bricks are subjected to frequent thermal shock; consequently, thermal shock resistance is one of their most critical properties. The working face of the brick comes into direct contact with molten steel at 1600°C, while argon gas at near-ambient temperature is blown in from the rear. This creates a steep temperature gradient within the brick, generating significant thermal stress. Such stress can easily lead to fracturing, spalling, or cracking of the working face, thereby impairing performance.

1.2 Slag Erosion Resistance

Gas-permeable bricks used in ladles must withstand erosion by steelmaking slag. Particularly during the initial stages of use, the working face contacts the molten slag. Components within the slag—such as CaO and SiO₂—react with the Al₂O₃ in the brick to form low-melting-point compounds (e.g., CaO·Al₂O₃, 3CaO·Al₂O₃, 12CaO·7Al₂O₃, CaO·Al₂O₃·2SiO₂, and 2CaO·Al₂O₃·SiO₂). These low-melting-point compounds can clog the gas-permeable channels on the working face. Furthermore, when these compounds come into contact with the high-temperature molten steel, they are washed away, causing erosion of the brick and reducing its service life.

1.3 Erosion/Scouring Resistance

During the refining process, argon gas enters the ladle through the brick to stir the molten steel. The high-speed flow of the steel combined with the gas stream creates a turbulent flow pattern that exerts strong scouring forces on the brick, accelerating its erosion. Therefore, the brick requires high spalling resistance, high-temperature strength, and overall high-temperature stability to ensure adequate resistance to this scouring action.

1.4 Oxygen Lancing Resistance

After casting is completed at the continuous casting platform, oxygen lancing is performed on the working face of the gas-permeable brick to remove residual steel and slag and maintain gas permeability. The heat generated by the oxidation of residual steel and the combustion of the oxygen lance melts the residue on the working face, effectively cleaning it. During the oxygen-burning process, the temperature at the tip of the oxygen lance is extremely high—potentially exceeding 2,000°C—which surpasses the refractoriness of the gas-permeable brick. This high heat can lead to the formation of low-melting-point phases on the brick’s working surface; therefore, the process must be strictly controlled to prevent damage caused by excessive burning. Consequently, high-purity raw materials are essential to enhance the brick’s refractoriness and, in turn, its resistance to oxygen-burning damage.

Types and Properties of Porous Plugs for Ladles

Regarding gas-permeable plugs for steel ladles—common types include slit-type and diffuse-type—issues such as clogging of gas channels, erosion and spalling, and low high-temperature strength persist. Consequently, it is essential to explore practical improvement measures to significantly enhance performance and better meet the rigorous demands of steel refining processes.

2.1 Slit-type gas-permeable plugs

Slit-type plugs are produced by embedding combustible polyester strips within the castable material; these strips oxidize at high temperatures to form gas-permeable channels. They remain the mainstream product for steel ladles both domestically and internationally. Through the rational design and layout of the slits, internal thermal stress during argon blowing can be reduced and thermal shock resistance improved, thereby extending the plug’s service life.

2.2 Diffuse-type gas-permeable plugs

Diffuse-type plugs are manufactured by creating interconnected pores through the loose packing of particles, which serve as gas-permeable channels. During steel refining, these plugs generate smaller gas bubbles, increasing the probability of capturing inclusion particles and facilitating the production of clean steel; however, they suffer from poor resistance to oxygen lancing and low strength.

2.3 Other types of gas-permeable plugs

Assembled-seam plugs are constructed by joining grooved ceramic elements to form gas-permeable channels. The minute gaps resulting from this assembly buffer thermal stress, leading to fewer structural failures during use and ensuring consistent gas permeability. The ceramic elements may be composed of materials such as corundum-mullite, corundum-spinel, or chrome-corundum-spinel. Currently, assembled-seam plugs from certain manufacturers are being utilized in steel plants. Favored by these plants for their ability to reduce or even eliminate the need for oxygen lancing, they prevent the environmental pollution caused by smoke during on-site thermal maintenance (oxygen lancing) and reduce the physical workload for operators. To ensure the service life and safety of steel ladles, the “twin” porous plug design was developed by combining the advantages of both monolithic and split-type porous plugs. This design incorporates two porous plug elements within a single well block; once the usable portion of the first element is exhausted, the second element can be switched into service while the position of the first is sealed with repair material, thereby guaranteeing safe operation. The use of “twin” porous plugs eliminates the need for the cold-installation or hot-replacement procedures associated with split-type plugs, allowing for an extended ladle service life without compromising operational safety. Building on this concept, a steel plant abroad is currently utilizing “triplet” porous plugs, and “multi-element” designs may be applied to ladles in the future. However, the use of such plugs can easily lead to a shift in the argon-blowing position; therefore, numerical simulations or water-modeling experiments are required to evaluate the setup and select an optimal configuration, ensuring no adverse effects on the ladle’s refining performance or the refractory materials in other areas.

Development Trends of Ladle Purging Plugs

3.1 Influence of additives on the performance of gas-permeable bricks

Incorporating nanopowders or their precursors into refractory materials serves to fill voids, promote sintering, and modify internal structures, thereby further enhancing material performance. Future research could focus on the effects of nanoscale additives—such as nano-ZrO₂ and nano-TiO₂—on the properties of gas-permeable bricks.

3.2 Development of environmentally friendly gas-permeable bricks

Environmentally friendly gas-permeable bricks should meet the following criteria: 1) Avoid or minimize the use of environmentally harmful raw materials, such as those containing chromium oxide; 2) Vigorously develop low-temperature firing processes to reduce high-temperature firing requirements, thereby cutting energy waste and production costs; and 3) Minimize or eliminate oxygen-burning operations to reduce environmental pollution.

3.3 Influence of equipment on the performance of gas-permeable bricks

Utilizing intelligent, automated production equipment minimizes human-induced variables. The use of dust collection systems and sealed, high-speed mixing equipment prevents dust pollution and ensures material uniformity. Computerized automatic control systems for curing, drying, and high-temperature firing kilns ensure consistent conditions throughout the kiln, thereby guaranteeing stable product quality.

Currently, some steel plants install on-site infrared monitoring equipment to measure ladle shell temperatures. This allows for the automatic calculation of residual refractory lining thickness and triggers alarms for anomalies, enabling timely removal of ladles from service for maintenance and ensuring safe operation.

Audio-visual alarm devices are installed at the oxygen-burning stations used for ladle hot repairs. These devices alert operators to stop the oxygen-burning process once the required gas permeability level is reached. Achieving optimal performance and a long service life for gas-permeable bricks requires not only sound production formulas and processes but also close cooperation from the customer on-site—including proper masonry installation, curing, preheating, and argon pressure stabilization—to ensure safe and durable operation.

This paper reviews the current research status of gas-permeable bricks, proposes measures to enhance their performance, and outlines future development directions: 1) investigating the effects of additives—such as nanomaterials and non-oxides—on brick performance; 2) developing environmentally friendly gas-permeable bricks; and 3) optimizing production processes and utilizing advanced manufacturing equipment to achieve both extended service life and safe operation.

By integrating factors such as new technologies, production processes, and on-site operational practices, high-quality gas-permeable bricks can be produced to improve steel quality, reduce smelting costs, and drive high-quality development within the steel industry.