Ladle Porous Plugs: Performance Requirements And Future Development Trends

Porous plugs are installed at the bottom of ladles to inject inert gas into molten steel, stirring the liquid to promote the flotation of inclusions to the slag layer and to homogenize the composition and temperature of the molten steel. Therefore, porous plugs serve as key functional components in ladle refining. With increasingly stringent requirements for steel quality, cleanliness, and related performance indicators, porous plugs have become even more critical, while also influencing the production rhythm and cost of steelmaking.

In recent years, extensive studies on slit-type porous plugs have investigated the effects of additives such as spinel fines, fused zirconia–corundum, TiO₂, and magnesia on their properties. These additives modify the microstructure and improve thermal shock resistance through mechanisms such as microcrack toughening and phase transformation toughening. For diffuse-type porous plugs, research has examined the influence of additives and particle size distribution on performance, providing a theoretical basis for performance improvement. Finite element method simulations have been applied to analyze the generation of thermal stress within porous plugs in detail. Mathematical modeling of thermal stress during argon blowing provides theoretical guidance for reducing internal thermal stress and extending the service life of porous plugs.

At present, porous plugs have become a bottleneck restricting further improvements in ladle service life and safe operation, requiring more in-depth research and development. Accordingly, this paper reviews the current research status of ladle porous plugs and outlines their future development trends.

Performance Requirements of Ladle Porous Plugs

1.1 Thermal Shock Resistance

During ladle service, porous plugs are subjected to frequent thermal shocks, making thermal shock resistance one of their most critical properties. The working face of a porous plug is in direct contact with molten steel at 1600 °C, while argon gas at near room temperature is injected from the back end. This creates a large temperature gradient within the porous plug, resulting in high thermal stress. Under such conditions, the working face is prone to fracture, spalling, cracking, and other forms of damage, which impair its service performance.

1.2 Slag Corrosion Resistance

Ladle porous plugs must withstand corrosion from molten slag. Especially in the early stage of service, the working face comes into contact with molten slag, where components such as CaO and SiO₂ react with Al₂O₃ in the porous plug to form low-melting phases, including CaO·Al₂O₃, 3CaO·Al₂O₃, 12CaO·7Al₂O₃, CaO·Al₂O₃·2SiO₂, and 2CaO·Al₂O₃·SiO₂. These low-melting compounds may block the gas-permeable channels on the working face. Furthermore, they can be eroded upon contact with high-temperature molten steel, leading to melting loss of the porous plug and a reduction in its service life.

1.3 Erosion Resistance

During refining, argon gas is injected into the ladle through the porous plug to stir the molten steel. The high-speed flow of molten steel, together with the gas stream, forms a "plume flow," which exerts strong erosion on the porous plug and accelerates its melting loss. Therefore, porous plugs are required to have excellent spalling resistance, high-temperature strength, and refractoriness to ensure adequate erosion resistance.

1.4 Oxygen Lance Burning Resistance

After casting on the continuous casting platform, oxygen lancing is required to remove residual steel and slag from the working face of the porous plug to maintain its gas permeability. The oxidation of residual steel by oxygen, along with the heat released from oxygen lance combustion, melts the residual steel and slag, thereby cleaning the working face. The tip temperature of the oxygen lance during lancing can exceed 2000 °C, which is higher than the refractoriness of the porous plug. Low-melting phases tend to form on the working face during oxygen lancing; therefore, strict control is required to avoid over-burning and damage. Accordingly, porous plugs should be made from high-purity raw materials to enhance refractoriness and improve resistance to oxygen lance burning.

Types and Properties of Ladle Porous Plugs

Common types of ladle porous plugs include slit-type and diffuse-type, both of which suffer from issues such as clogging of gas-permeable channels, erosion and spalling, and insufficient high-temperature strength. Practical and effective improvement measures still need to be explored to achieve significant performance enhancement of porous plugs and to better meet the stringent requirements of steel refining processes.

2.1 Slit-Type Porous Plugs

Slit-type porous plugs are produced by embedding combustible polyester filaments in the castable, which are oxidized at high temperatures to form gas-permeable slits. They remain the mainstream type of ladle porous plug used both domestically and internationally. Rational design and arrangement of the slits can reduce internal thermal stress during argon blowing to a certain extent, improve thermal shock resistance, and thereby prolong service life.

2.2 Diffuse-Type Porous Plugs

Diffuse-type porous plugs are manufactured by loosely packing particles to form interconnected open pores that act as gas-permeable channels. During molten steel refining, they generate fine bubbles that improve the capture efficiency of inclusions, thereby benefiting the production of clean steel. However, they have disadvantages such as poor resistance to oxygen lancing and low mechanical strength.

2.3 Other Types of Porous Plugs

Joint-type porous plugs form gas-permeable channels by assembling grooved ceramic segments. The micro-gaps between the ceramic segments help buffer thermal stress, resulting in reduced fracture during service and stable air permeability. The ceramic segments can be composed of materials such as corundum–mullite, corundum–spinel, and chrome corundum–spinel. Joint-type porous plugs produced by some manufacturers have been applied in several steel plants. Due to their ability to reduce or even eliminate the need for oxygen lancing, they help avoid air pollution from fumes generated during hot repair and reduce labor intensity, making them popular among steel mills.

To ensure ladle service life and operational safety, "twin" porous plugs have been developed by integrating the advantages of monolithic and split-type structures. In this design, two porous elements are embedded in the surrounding seat brick. After one element is consumed, the other can be activated while the first is sealed with repair material, ensuring safe and continuous operation. The application of twin porous plugs eliminates the need for cold installation and hot replacement required for conventional split-type plugs, thereby improving ladle campaign life under safe operating conditions. Following this concept, "triple" porous plugs have been adopted in some overseas steel plants, and "multi-cell" porous plugs may be further developed for ladle applications. However, such configurations tend to shift the argon blowing center; therefore, numerical simulation using computer software or water modeling is required to evaluate the results and determine a rational layout. This ensures that there is no negative impact on refining efficiency or on other refractory linings of the ladle.

Development Trends of Ladle Porous Plugs

3.1 Influence of Additives on the Properties of Porous Plugs

Introducing nanopowders or their precursors into refractories can not only exert a filling effect but also promote sintering and modify the internal microstructure, thereby further improving the properties of the refractories. Further research can be conducted on the effects of nanoscale additives such as nano-ZrO₂ and nano-TiO₂ on the performance of porous plugs.

3.2 Development of Environment-Friendly Porous Plugs

Environment-friendly porous plugs should meet the following requirements:

Avoid or reduce the use of environmentally harmful raw materials, such as chromium oxide–containing materials;

Actively develop low-temperature-fired porous plugs to reduce high-temperature firing during production, thereby lowering energy consumption and production costs;

Minimize or eliminate oxygen lancing of porous plugs to reduce environmental pollution.

3.3 Influence of Equipment on the Properties of Porous Plugs

Intelligent and automated equipment should be adopted in the production of porous plugs to minimize human-induced uncertainties. Dedusting systems and sealed high-speed mixing equipment should be used to prevent dust pollution and ensure material uniformity. Curing kilns, drying kilns, and high-temperature firing kilns should be operated under computerized automatic control to guarantee uniform conditions throughout the kiln and ensure stable product quality.

At present, some steelmaking plants have installed infrared monitoring equipment on site. By measuring the temperature of the ladle shell, this equipment can automatically calculate the residual lining thickness of the refractories and issue automatic alarms under abnormal conditions, allowing the ladle to be taken offline in time to ensure safe operation.

An acousto-optic alarm device is installed at the ladle hot-repair oxygen lancing station. When the required gas permeability is achieved during oxygen lancing, the device promptly alerts operators to stop the operation.

The satisfactory performance of porous plugs depends not only on rational formulation and production technology but also on close on-site cooperation from users, including proper installation, curing, baking, and stable argon pressure control. Only in this way can a long service life be achieved while ensuring the safe application of porous plugs.

Conclusion

This paper reviews the current research status of porous plugs, proposes measures for improving their performance, and outlines their development trends as follows:

Investigate the effects of additives such as nanomaterials and non-oxide materials on the properties of porous plugs;

Develop environment-friendly porous plugs;

Optimize the production design of porous plugs in combination with advanced manufacturing equipment to achieve long service life while ensuring safe operation.

By integrating new technologies, production processes, and on-site application and operation, high-quality porous plugs can be manufactured to improve steel quality, reduce smelting costs, and promote the high-quality development of the iron and steel industry.