The permeable brick of a ladle is the most critical functional component in the secondary refining process and a key functional material for the bottom-blown argon process in steel secondary refining. Its main functions are as follows:
(1) It can regulate the uniform distribution of molten steel temperature in the ladle to achieve the optimal casting temperature for the existing process.
(2) Through gas stirring, it can evenly distribute alloys and deoxidizers in the ladle.
(3) It can carry non-metallic inclusions in the molten steel into the slag to meet the cleanliness requirements of the molten steel.
To achieve the above functions, inert gas used in refining needs to be blown into the ladle through the permeable brick. On the working surface of the permeable brick (i.e., the contact surface between the permeable brick and the molten steel), a large number of bubbles blown out under sufficient pressure form a gas injection jet to stir the molten steel in the entire ladle, promote the flow of molten steel, and homogenize the temperature and composition in the ladle. At the same time, the continuously ejected bubbles carry non-metallic inclusions in the molten steel into the slag under the action of the interface, achieving the purpose of cleaning the molten steel.
To fulfill the above metallurgical functions, permeable bricks must possess the following key properties:
(1) Good gas permeability. Permeability is one of the important parameters for measuring the quality of permeable bricks. Studies have shown that the stirring energy of molten steel is directly proportional to the flow rate of the blown gas; stirring energy directly affects the stirring efficiency of molten steel, and only sufficient stirring energy can achieve good stirring effects. When the argon blowing volume is constant, the more argon bubbles blown out, the more favorable it is for degassing and stirring of the molten steel.
(2) High-temperature corrosion resistance. Refining ladles have extremely strict requirements in terms of temperature and time, with the highest temperature often exceeding 1750°C and the refining time sometimes reaching dozens of minutes. During refining operations, the basicity of the slag has a great impact on the service life of permeable bricks. Therefore, permeable bricks are eroded by highly permeable alkaline slag at high temperatures and deteriorate rapidly.
(3) High-temperature wear resistance. When bottom-blowing argon in a refining ladle, due to the argon blowing, the flow rate of molten steel in the ladle is very high, significantly increasing the scouring and wear of the lining materials, bottom permeable bricks, and seat bricks by the molten steel. During ladle hot repair, in order to remove the residual steel and slag on the surface of the permeable brick and restore its gas permeability, oxygen blowing cleaning is required to melt the steel slag adhered to the permeable brick surface; at the same time, blowing gas into the permeable brick to remove the molten slag. During the cleaning process, the permeable brick is scoured by high-velocity gas flows, so it is required to have good high-temperature wear resistance.
(4) Good thermal shock resistance. Due to the intermittent operation of the ladle, when molten steel is poured into the ladle, the end of the permeable brick is subjected to high-temperature molten steel, causing a sudden temperature rise. When blowing argon, the material is cooled by cold gas flows, generating large thermal stresses inside. Meanwhile, significant temperature changes also occur when molten steel is poured into an empty ladle. Therefore, the service conditions of permeable bricks are extremely harsh, making them prone to thermal spalling and structural spalling.
(5) Simple installation and safe and reliable performance. Permeable bricks are installed inside the seat bricks at the bottom of the ladle, and their service conditions are extremely harsh. The service life of ladle permeable bricks cannot be synchronized with the entire ladle's life, so they need to be replaced. Therefore, they are required to be simple to install, safe and reliable in use, and avoid incidents of steel penetration or leakage.
| Corresponding Steel Grade | Service Life Requirement | Blow - through Rate Requirement | Molten Steel Cleanliness Requirement | Flow Rate Requirement | Applicable Type |
| Plain Carbon Steel | High | Medium | None | High | Slit - type |
| Plain Carbon and Low - Medium Carbon Steel | High | High | Medium | High | Slit - type, Ceramic Tube - type |
| Stainless Steel | Medium | High | High | Medium | Core Plate - type |
| Special Steel | Medium | High | High | Medium | Diffusion - type, Core Plate - type |
Common Classification of Ladle Permeable Bricks
After years of development, the common structural types of permeable bricks mainly include three types: diffusion type, slit type (including integral casting type and core plate joint type), and straight-through microporous type.
The diffusion-type permeable brick is the earliest form of permeable bricks. Due to the high porosity of the material itself, the large number of pores provide channels for inert gases. The disadvantages of this surface-porous diffusion-type permeable brick include low strength, poor erosion resistance, easy spalling caused by penetration of molten steel and slag, and relatively poor stirring effect on molten steel. At present, it is rarely used in domestic ladle permeable bricks.
Slit-type permeable bricks include two forms. One is that the central part of the permeable brick is formed by assembling several molded thin plates to create slits, with the exterior cast using castable material, namely the so-called "joint-type". The disadvantage of this permeable brick is poor controllability of the blown gas. The other is pre-casting dozens of straight-through slits in the brick body, commonly known as the "slit-type". Compared with the former, slit-type permeable bricks have advantages such as longer service life, higher gas blowing efficiency, larger gas flow rate, and better stirring effect.
The normal erosion process of slit-type permeable bricks commonly used in steel plants is shown in the following figure:
Compared with diffusion - type permeable bricks, slit - type permeable bricks have higher density, better erosion resistance and better resistance to oxygen cleaning.
Straight - through hole - type permeable bricks are made by embedding fine steel pipes of varying numbers into the bricks. The gas channels are composed of many straight, tiny pipes, and they are formed by the casting method. Compared with diffusion - type permeable bricks, straight - through hole - type permeable bricks have a better stirring effect and their service life is 2 - 3 times longer. However, their drawback is that the gas flow rate they can provide is limited. In the later stage of use, refining often fails due to a decrease in gas permeability or the inability to blow through.
The gas permeability of permeable bricks is commonly evaluated by the blow-through rate, which primarily focuses on whether the permeable brick has the basic function of gas permeability - a performance that steel enterprises first examine. Its convenience and quickness during operation have made it widely accepted by steel enterprises and permeable brick manufacturers. However, with the improvement of steel quality, using the blow-through rate as the sole functional evaluation index has become somewhat one-sided. Gas permeability can be subdivided into the following indicators: blow-through rate, maximum flow rate, flow rate controllability, initial blowing performance, and bubble characteristics.
High - turnover ladles require greater stirring power in the shortest possible time. Therefore, the indicator of maximum flow rate has been proposed. The maximum flow rate depends on the structural type of the permeable brick and the designed flow rate. The designed flow rate of slit - type permeable bricks is often higher than that of other types. So, for large - volume ladles of 300 t or more, slit - type permeable bricks are mostly used. Flow rate controllability means that the flow rate can be adjusted by pressure. As the pressure increases, the flow rate should increase accordingly. Good initial blowing performance reflects that the gas channels of the permeable brick can be quickly opened under a relatively small shell - breaking pressure.
Numerous studies have shown that the bubbles blown out by diffusion-type permeable bricks have smaller diameters; compared with slit-type permeable bricks, diffusion-type permeable bricks generate a larger number of bubbles at the same flow rate. A large number of smaller-diameter bubbles can adsorb more fine-sized non-metallic inclusions. The bubbles carry the non-metallic inclusions to float up and are eventually captured by the slag layer. This gas permeability characteristic is conducive to the removal of non-metallic inclusions in molten steel during soft blowing. However, diffusion-type permeable bricks are difficult to meet the flow requirements of strong blowing in large-volume ladles, and during strong blowing, the permeable bricks are eroded and worn more quickly, resulting in a lower service life.
Compared with diffusion-type permeable bricks, slit-type permeable bricks show greater advantages in meeting flow requirements. However, they have poor comprehensive metallurgical efficiency for special steel with high requirements for molten steel purity.
