New Refractory Castable for Improving Thermal Insulation Performance During Ladle Service Life
Ladle
The ladle is an indispensable vessel in the steelmaking process. With the continuous development of steelmaking technology-especially the development of external refining processes such as LF (Ladle Furnace) and RH (Ruhrstahl–Heraeus)-its function has evolved from simply holding molten steel to serving as a melting furnace that undertakes certain smelting functions. The long-term retention of molten steel in the ladle will inevitably lead to temperature loss. However, a ladle lining with high thermal conductivity intensifies heat dissipation from the molten steel, causes more serious deformation of the ladle shell, increases the temperature drop rate of the molten steel, and results in severe slag adhesion, nodulation, and cold steel deposition on the ladle wall. Excessively low temperatures may even force the tundish to stop pouring due to flow interruption. This not only affects the quality of cast billets and reduces molten steel yield but also increases production costs. Therefore, insulated ladles have increasingly become a necessity for steel plants.
The refractory structural design of an insulated ladle typically consists of a lightweight insulation board or nano-board, a permanent layer castable, and a working layer refractory. The insulation board, which provides the primary thermal insulation, requires a layer of refractory material for protection. This protective material must first prevent excessive pressure on the insulation material and second ensure that the insulation material does not exceed its service temperature (e.g., must remain below 1000 ℃). Thus, the ladle permanent layer castable is a crucial material. An ideal permanent layer refractory for insulated ladles should have excellent thermal stability, thermal insulation, and erosion resistance, along with safe and reliable application, structural integrity, and long service life.
To maintain good thermal insulation performance of the ladle and reduce the molten steel temperature drop, this work evaluated the performance of a new type of CA6 castable and a lightweight mullite castable. Their application in the ladle achieved a comparatively good overall thermal insulation effect.
01 Experiment
At present, the permanent layer material used in ladles is mostly ordinary high-alumina castable, with bauxite as the main raw material. This type of refractory castable has the following shortcomings during use:
First, it has high thermal conductivity, which leads to energy loss during service.
Second, it has low refractoriness. If abnormalities occur in the working layer and molten steel directly contacts the permanent layer, the probability of ladle penetration and steel leakage is high, resulting in a low safety factor.
Third, it has a high bulk density, making the empty ladle heavy.
Therefore, it cannot meet the special requirements for ladle thermal insulation, and it is necessary to develop a new type of permanent layer castable with excellent comprehensive performance. In this study, sample preparation and testing experiments were carried out on the new CA6 castable and lightweight mullite castable.
1. Raw Materials and Experimental Scheme
The main raw materials used in this experiment and their chemical compositions are shown in Table 1.
The CA6 raw material (CaAl₁₂O₁₉, abbreviated as CA6) is the calcium aluminate phase with the highest Al₂O₃ content in the CaO–Al₂O₃ system. It has a melting point of 1875 ℃, a thermal expansion coefficient of 8.0×10⁻⁶ ℃⁻¹, a particle bulk density of 2.70 g·cm⁻³, and an apparent porosity of 26.8%. This material exhibits refractory performance similar to that of tabular corundum, while its thermal conductivity is only one-third that of corundum, making it a new type of high-quality thermal insulation raw material that has emerged in recent years.
The CA6 castable is prepared using CA6 as the aggregate, and its matrix is composed of tabular corundum fine powder, alumina powder, and calcium aluminate cement as the binder. The spherical lightweight mullite particles have a bulk density of 1.59 g·cm⁻³ and an apparent porosity of 38.9%. The lightweight mullite castable uses microporous M70 spherical lightweight mullite spheres as the aggregate, and its matrix consists of tabular corundum fine powder, alumina powder, and calcium aluminate cement as the binder, ensuring good slag erosion resistance and improving the safety of the permanent layer.
2. Experimental Process and Performance Testing
After casting and forming the two types of castables, physical property tests and erosion resistance tests were carried out, respectively. All physical test methods were conducted in accordance with national standards or industry standard methods.
02 Results and Analysis of Material Properties
1. Physical Properties
It can be seen from Table 2 that the properties of different ladle permanent layer castables vary.
The bulk density of the lightweight mullite castable is 2.17 g·cm⁻³, with its unit weight reduced by 24% compared with the high-alumina castable currently in use, and its thermal conductivity decreased by 16% at the same time. This enables the goals of ladle lightweighting and low thermal conductivity to be achieved. The CA6 castable is also 5.6% lighter than ordinary high-alumina castable, and its thermal conductivity is reduced by 26%.
2. Erosion Resistance Test
Crucibles were cast using the original ladle permanent layer castable B, the new CA6 castable C, and the lightweight mullite castable 3#. Converter final slag was added to conduct the crucible slag erosion resistance test under the condition of heat preservation at 1500 ℃ for 3 h. The melting loss and penetration resistance of the different materials were observed. After the test, the crucibles were cut open, and the erosion conditions are shown in Figure 1 below.
The CA6 castable exhibits the best resistance to slag erosion and penetration, with a large amount of residual slag remaining in the crucible cavity. The lightweight mullite castable ranks second in erosion and penetration resistance. For the ordinary high-alumina permanent layer castable, the boundary between the slag and the refractory material is unclear, and the crucible refractory material melts together with the slag, showing slightly better penetration resistance than erosion resistance. This indicates that the high-alumina castable generates a significant amount of liquid phase at 1500 ℃, so the material needs to be improved to enhance its high-temperature resistance, which is crucial for ladle safety. The microporosity of spherical lightweight mullite is beneficial for improving erosion and penetration resistance, meaning this lightweight material can also achieve good performance in both thermal insulation and erosion resistance.
03 Application Status
The three types of castables described above were applied in the construction of 300 t ladles. The lightweight mullite castable and CA6 castable used for the permanent layer of insulated ladles are designed to provide both good molten steel erosion resistance and thermal insulation performance, while maintaining structural integrity and a stable thermal insulation effect throughout the service life of the ladle. After three service cycles, the surface conditions of the permanent layers of different ladles are shown in the photos below.
All of the above ladle permanent layers showed cracks of varying degrees in the later stage of the service cycle. This is related to the thermal shock resistance and strength of the materials, as well as damage during ladle dismantling. Further in-depth research is still needed on comprehensive techniques for reducing cracking in the permanent layer materials of insulated ladles.
From the perspective of the average temperature at the slag line of each ladle during each service cycle, the temperature of the slag line steel shell in non-insulated ladles was consistently above 320 ℃, whereas the average temperature of the slag line in the four test ladles with thermal insulation was below 280 ℃. This corresponds to a general temperature drop of 50–100 ℃ for the slag line steel shell. The temperature drop of the steel shell at the ladle wall ranged from 20–50 ℃, depending slightly on the configuration of the thermal insulation and permanent layer materials.
The thermal insulation materials of the insulated test ladles maintained a good condition throughout the service cycle, and the average temperatures of the steel shells at both the slag line and the ladle wall were lower than those of ordinary ladles. This indicates that the permanent layer provided effective protection. For safety assurance, the current issue-where cracks in the new permanent layer materials become obvious after two service cycles-still needs to be optimized to ensure they can withstand four service cycles.
04 Conclusions
Insulated ladles have become an important technical measure for iron and steel enterprises to save energy, reduce emissions, and improve the quality of steel products, placing higher demands on ladle permanent layer materials. The developed CA6 castable and lightweight mullite castable possess physical properties, including strength, that meet the application requirements for ladle permanent layers. At the same time, they provide better thermal insulation and erosion resistance than conventional high-alumina castables. These materials can protect thermal insulation layers and help ladles maintain good thermal insulation performance throughout the service cycle. Further research is still needed on comprehensive technologies to reduce cracking in the permanent layer materials of insulated ladles.