Analysis Of Damage Causes Of Ladle Porous Plug

 In the modern steelmaking process, in order to uniform the temperature and composition of molten steel and promote the physical and chemical reactions of molten steel refining, the ladle refining technology is an important smelting link for modern steelmaking to improve quality. The ladle refining process adopts the bottom argon blowing technology.

 The argon blowing process at the bottom of the steel ladle is a necessary condition for realizing ladle refining. As a key material of this process, the performance of the porous plug is very important for the smooth implementation of the refining process, ensuring the quality of steel and increasing production. During the use of the porous plug, thermal stress will be generated due to large temperature differences and the erosion of molten steel, resulting in cracks and spalling of the porous plug.

 During on-site use, transverse cracks often occur on the heated surface of the slit-type corundum-spinel porous plug in contact with molten steel, causing the porous plug to spall from the transverse cracks, seriously shortening the service life of the porous plug. With the enlargement of steel ladles, the increase of smelting temperature and the extension of smelting time, its service environment is becoming more and more severe, requiring materials to have better performance and longer service life. Due to the special position and service environment of the porous plug in the steel ladle, its blowing success rate and service life have become key factors restricting the improvement of the service life of the steel ladle. High blowing rate, no need for oxygen blowing cleaning, and synchronization with the service life of the steel ladle lining or ladle bottom material are the common goals pursued by porous plug users, production enterprises and researchers.

 By analyzing the damage causes of the porous plug, this article expects to provide a certain basis for porous plug production enterprises and researchers, so as to improve the performance of refractory materials for porous plugs.

 In the steelmaking process, in order to promote the refining of molten steel and uniform the temperature and composition of molten steel, it is necessary to carry out the bottom argon blowing process through the porous plug at the bottom of the steel ladle. Porous plugs for steel ladles can be classified by gas channels or by installation methods.

1.1 There are mainly three types of porous plugs classified by the form of gas channels, namely the - type (diffusion - type), straight - through directional type, and slit directional type

1.1.1 Diffusion - type porous plug
The pores of the diffusion - type porous plug are fine and connected, with high porosity, low density and strength. However, the gas flow is unstable and the durability is poor.

1.1.2 Straight - through directional type porous plug
Compared with the diffusion - type porous plug, the gas flow and distribution in the straight - through directional type porous plug are reasonable, the blowing - through effect is better, and the service life is longer. But it is easy to be blocked in the later stage, which affects the blowing success rate.

1.1.3 Slit directional type porous plug
The slits of the slit directional type porous plug are integrally formed with the inlaid material during the manufacturing process. Due to the heating, melting and volatilization of the inlaid material during the high - temperature firing process, directional slits are formed. Its advantages are stable gas flow, smaller bubble size, good stirring effect on molten steel, small reverse impact of gas flow, dense material, high strength, and long service life, making up for the deficiencies of the first two types of porous plugs. However, its manufacturing process is complex and the manufacturing precision is high. An appropriate slit width can not only ensure the bottom argon blowing intensity, control and adjust the flow rate, but also avoid the danger of penetration and blockage. At present, slit directional type porous plugs are more commonly used in steel ladles.

1.2 Porous plugs are classified into two types by installation methods

1.2.1 Integral porous plug
The integral porous plug is convenient to install, has a high safety factor and a long service life, but it is difficult to replace the brick. Therefore, it is suitable for process conditions where the argon blowing time is short and the service life of the porous plug is synchronized with that of the steel ladle.

1.2.2 Externally - installed porous plug
The externally - installed porous plug is equipped with installation equipment, and its structure is relatively complex. There are stricter on - site installation requirements for the brick core. If the installation quality is not good, the phenomenon of steel leakage is likely to occur. However, the externally - installed porous plug can be replaced in a hot state without cooling the steel ladle, saving a lot of maintenance time. It is suitable for the process of long - time argon blowing and frequent replacement of porous plugs, especially for refining steel ladles.

 Porous plugs are key materials for the bottom argon blowing process in steel ladles.

The intermittent operation process of porous plugs for steel ladles is as follows: ladle receiving → argon blowing refining → steel casting → slag dumping → oxygen cleaning. The porous plug repeats the above process until its service life ends. Based on the working environment of the porous plug, it is affected by the following factors during operation, leading to damage.

2.1 Effect of Thermal Stress

Before use, the steel ladle is baked to about 1100°C, while the tapping temperature from the converter is as high as 1640–1700°C. After steel casting, the temperature inside the ladle drops to about 800°C. The refractory material on the working surface of the porous plug, especially around the air outlet, is in direct contact with high-temperature molten steel and affected by the continuous outflow of cold gas, creating a huge temperature gradient. This subjects the porous plug to rapid heating and cooling, generating significant thermal stress. After repeated use, the porous plug spalls.

2.2 Mechanical Wear Effect

During argon blowing, the interaction between high-speed gas flow and flowing molten steel forms a turbulent flow, which impacts the protruding part of the porous plug higher than the seat brick, causing erosion and shear stress. Additionally, the strong scouring of molten steel on the bottom of the ladle during tapping causes severe wear, accelerating the damage of the porous plug.

2.3 Chemical Erosion Effect

Throughout the service life, the working surface of the porous plug is in long-term contact with molten steel and slag, reacting with oxides in the molten steel or slag to form low-melting substances such as FeO·Al₂O₃, 2(MnO)·SiO₂·Al₂O₃, and 12CaO·7Al₂O₃. These substances are eroded by the gas flow, leading to the corrosion of the porous plug. The penetration and erosion of molten steel and slag are important causes of porous plug damage.

2.4 Mechanical Stress Effect

Due to direct contact with high-temperature molten steel, the working surface of the porous plug has a high temperature, while the non-working surface is relatively cooler. During the cycle of ladle receiving → refining → casting → hot repair → ladle receiving, the temperature of the ladle continuously changes, creating a temperature gradient between the working and non-working surfaces of the porous plug. The temperature gradient, combined with different expansion coefficients between the material 变质层 (metamorphic layer) and the original layer, causes different volume changes in each layer, generating shear force on the porous plug. This leads to transverse cracks and fracture, causing damage.

2.5 Oxygen Blowing Cleaning Effect

During the cycle of ladle receiving → argon blowing refining → steel casting → slag dumping → oxygen cleaning, molten steel or slag may penetrate into the slits of the porous plug or cracks caused by thermal stress, blocking the slits. Therefore, after steel casting and slag dumping, oxygen blowing cleaning is required for the porous plug. The high-temperature flame directly burning the surface of the porous plug causes steel slag to react with the plug material, forming low-melting phases and resulting in melting and depressions. Steel clamping in slits and failure to blow through during use are also main reasons for frequent replacement of porous plugs.

Zhang Gang et al. studied that poor thermal shock resistance is the main cause of problems such as steel clamping in slits and spalling of porous plugs in Ansteel's steel ladles. Li Bingqiang et al. used ANSYS software to analyze that temperature gradient and thermal stress cause layered spalling of the working surface of porous plugs, leading to their damage.

Cheng Ying et al. pointed out that the thermal shock resistance of materials is an important factor affecting the erosion and penetration of molten steel and slag. Cracks generated by thermal shock provide channels for the penetration of molten steel or slag, and the reaction between molten steel/slag and the porous plug material during cyclic use leads to structural spalling.

In summary, the thermal stress caused by huge temperature differences during the cyclic use of the steel ladle subjects the porous plug to thermal shock. The scouring and thermal shock damage from high-speed gas flow and molten steel during refining are the main causes of porous plug damage. With the enlargement of steel ladles, the increase in smelting temperature, and the extension of smelting time, the service environment becomes harsher, requiring materials with better performance and longer life. The thermal shock resistance of materials is a key factor affecting their life. Cracks from thermal shock provide channels for the penetration and erosion of molten steel and slag, and the residual molten steel/slag in cracks accelerate structural spalling and damage during cyclic use, reducing service life. Thermal stress causes transverse cracks and fractures in porous plugs, which are also main damage mechanisms. Due to the special position and service environment of porous plugs in steel ladles, their blowing success rate and life have become key factors limiting the improvement of ladle life.

Therefore, improving the thermal shock resistance of materials to promote the life of porous plugs is worthy of in-depth and systematic research.

The function of the bottom argon blowing process in a steel ladle is to stir the molten steel, enabling heating, composition adjustment, inclusion removal, and purification of the molten steel. These processes are completed through continuous argon blowing and stirring to make the molten steel circulate. In terms of thermal stress, the materials used for the ladle must have good thermal shock resistance. In terms of anti - penetration, materials that are not easily wetted by molten steel at working temperatures should be selected. The order of wetting angles between oxide refractories and molten steel is ZrO₂ > Al₂O₃ > MgO.

Corundum castables, characterized by high strength and excellent high - temperature properties, are widely used in ladle linings and other fields. However, due to their large elastic modulus, they are prone to cause material spalling. When the castable contains both corundum and spinel phases, the thermal shock resistance and high - temperature properties of the sample are significantly improved, and the performance of the castable is enhanced.

Corundum - spinel refractories are refractories with corundum and magnesium aluminate spinel as the main phases, usually with a high corundum content. The performance of corundum - spinel materials is closely related to the composition, relative content, and particle size of corundum and spinel. Corundum can react with CaO in the slag to form high - melting - point compounds such as CA₆ and CA₂, accompanied by volume expansion, leading to densification of the material structure. Spinel can absorb FeO and MnO in the slag to form solid solutions, all of which will increase the SiO₂ content of the slag, increase its viscosity, and enhance the slag penetration resistance of the material.

The spinel phase in corundum - spinel refractories is usually introduced in the following two ways: ① adding pre - synthesized spinel; ② adding MgO to react with Al₂O₃ to form in - situ spinel. Since the in - situ reaction of MgO and Al₂O₃ is accompanied by a large volume expansion effect, forming spinel according to the theoretical composition will produce about 8% volume expansion. To obtain volume - stable magnesium aluminate spinel refractory products, pre - synthesized magnesium aluminate spinel raw materials can be added.

Due to their good high - temperature properties, corundum - spinel materials have been used in the harsh service conditions of steel ladles. At present, the materials of ladle porous plugs are mainly corundum - spinel and chrome corundum, which are usually produced by casting and high - temperature firing processes. Since chrome corundum porous plugs will generate hexavalent chromium to pollute the environment during high - temperature firing, in recent years, corundum - spinel castable porous plugs have gradually replaced sintered magnesia, magnesia - chrome, high - alumina, and chrome corundum porous plugs due to their excellent high - temperature properties.

Although there are literature reports on the improvement of properties such as thermal shock performance of corundum - spinel castable porous plugs, the optimization of corundum - spinel material properties, with the focus on improving the thermal shock stability of the material, needs further research.

(1) Due to the special position and service environment of the porous plug in the steel ladle, its blowing success rate and service life are key factors limiting the improvement of the steel ladle's service life.

(2) The scouring and thermal shock damage caused by high-speed gas flow and molten steel during the use of the porous plug are the main causes of damage to the ladle porous plug. Steel clamping in the slits of the porous plug and failure to blow through are the main reasons for frequent replacement of the porous plug. The performance of materials used for porous plugs needs to be further optimized and improved.