Research Progress on Refractories for LF Ladle Refining
Ladle refining refers to the metallurgical process in which molten steel, initially smelted in a converter or electric arc furnace, is transferred to another vessel for further refining. It is also known as "secondary refining" or "secondary metallurgy." The development of ladle refining is closely associated with the rapid progress of continuous casting, as well as with the specialization of product mix adjustment and the optimization of production in the steel industry.
With rapid social development, the iron and steel industry has advanced swiftly, placing increasingly stringent requirements on steel quality. Consequently, ladles have evolved from simple containers for molten steel into multifunctional vessels with refining capabilities. With simple equipment configuration, diverse metallurgical functions, and favorable economic benefits, LF refining has become an indispensable part of the metallurgical process.
Refractories are important basic materials and high-temperature structural materials for the steelmaking industry, playing a role throughout the entire steelmaking process. As vessels in direct contact with molten steel, they must minimize contamination of the molten steel to avoid impairing its quality, while also maintaining a long service life to reduce negative impacts on production efficiency and costs. Molten steel, slag, and refractories interact with one another throughout the entire smelting process.
Since LF refining is a near-final stage in the metallurgical process, impurities generated during LF refining are difficult to remove in subsequent procedures. Therefore, refractories for LF ladles have a significant influence on molten steel quality. Accordingly, research on refractories for LF ladles is of great importance for improving the quality of finished steel produced through LF ladle refining.
LF Refining Process
The study of LF (Ladle Furnace) ladle refining technology began in 1968. It uses a dedicated ladle to refine molten steel initially smelted in a primary furnace, with three electrodes serving as the heat source. Effective reduction refining is achieved through the pre-preparation of reducing slag in the electric arc furnace, the combined tapping of molten steel and slag, and argon blowing treatment in the ladle. The schematic diagram of the LF refining ladle unit is shown in Figure 1.
After electric arc furnace steelmaking, LF refining can shorten the reduction time of the electric arc furnace and improve production efficiency, thereby forming a short-flow steelmaking process of "EAF + LF refining + continuous casting." After converter steelmaking, LF refining is used for the production of special steels, forming a process of "converter + LF refining + continuous casting" for manufacturing various high-quality steels.
Refractories for LF Ladles
During the smelting process, refractories, slag, and molten steel interact with one another. Molten steel and slag erode the refractories, while the refractories dissolve into the slag and molten steel, thereby altering their compositions. Changes in slag composition affect its refining effect on molten steel, and the incorporation of refractory materials into molten steel leads to the formation of non-metallic inclusions, which impair the quality of finished steel products. Refractories are required in various parts of the LF ladle, including the ladle cover, slag line, ladle wall, and permeable bricks.
Due to erosion by molten steel and slag, as well as thermal shock during intermittent operation, refractories used for LF ladle covers must exhibit excellent high-temperature performance, thermal shock resistance, and spalling resistance. At present, high-alumina castables and corundum castables are mainly used.
Zinc-aluminum spinel was synthesized via a solid-phase sintering method. The influence of raw material ratios on the structural evolution of the product was investigated, and the effect of zinc-aluminum spinel addition on the properties of high-alumina castables was discussed. With increasing Al₂O₃ content, the surface of the formed zinc-aluminum spinel particles transformed from a microporous structure to a smoother one. Excessive Al₂O₃ resulted in a rough particle surface. When the molar contents of ZnO and Al₂O₃ were equal, the prepared corundum castables exhibited a denser microstructure, higher strength, and optimal slag resistance.
The influence of calcium dialuminate addition on the properties of corundum castables was also studied. It was found that with increasing calcium dialuminate content, the bulk density of the corundum castables decreased, accompanied by reductions in the average thermal expansion coefficient and thermal conductivity. The permeability of the castables was effectively improved without compromising their resistance to hydrogen reduction.
Under the combined effects of molten steel, slag, and atmospheric conditions, the slag line of LF ladles suffers severe erosion. Moreover, repeated operation and shutdown cycles lead to frequent thermal shock. Therefore, improving the service life of refractories used in the slag line of LF ladles has long been a key research focus in the metallurgical and refractory industries. The most commonly used refractories for LF ladle slag lines are magnesia-carbon bricks, along with magnesia-chrome bricks, spinel-carbon bricks, and magnesia-lime-carbon bricks. Among them, magnesia-carbon bricks (graphite > 14 wt.%) exhibit the best thermal shock resistance and erosion resistance. MgO–C refractories are multiphase materials composed mainly of magnesia clinker and graphite, supplemented by antioxidant additives and resin binders.
The preparation process of MgO–C refractories is shown in Figure 2. First, magnesia clinker is crushed and screened into coarse aggregates and fine powders of different particle sizes. The magnesia fine powders are premixed with additive powders such as silicon–aluminum alloys for later use. The magnesia coarse aggregates are mixed with a binder to ensure uniform coating, followed by the addition of graphite for further mixing. Finally, the premixed magnesia fine powders and additive powders are introduced, and thorough mixing yields the MgO–C mixture.
The ideal mixing model of the MgO–C mixture is shown in Figure 3. The inner part consists of magnesia aggregates coated with a binder, the middle layer is graphite, and the outermost layer comprises fine magnesia powder and additives. During service, the fine magnesia powder and antioxidants in the outermost layer protect the graphite and reduce its oxidation. Graphite lowers the overall thermal expansion coefficient of the multiphase refractory and reduces the wettability between molten slag and the refractory, thereby improving its slag resistance and thermal shock resistance. The fused magnesia aggregates in the innermost layer act as a skeleton to support the refractory. Effective mixing of the MgO–C mixture enhances the oxidation resistance, slag resistance, and thermal shock resistance of the refractory during use.
After mixing, the mixture is pressed into shape and then subjected to heat treatment to cure the phenolic resin binder. The curing process is usually carried out at about 200 ℃ for more than 24 hours, and the cured MgO–C refractory exhibits excellent mechanical, thermal, and chemical properties.
High elemental carbon content gives MgO–C refractories good thermal shock resistance and slag resistance. However, carburization of molten steel occurs during the smelting process. In line with carbon peaking and carbon neutrality goals, reducing carbon content has become an important development direction for MgO–C refractories. Domestic and international scholars have focused on low-carbon MgO–C refractories by nanosizing carbon sources, adding various composite high-efficiency antioxidants, and catalyzing the formation of ceramic phases, among other approaches.
The effects of graphite, carbon black, and pitch on the properties of low-carbon magnesia–carbon bricks were investigated. The results show that specimens containing pitch exhibit the best mechanical properties, specimens with graphite have the best oxidation resistance but slightly inferior mechanical properties compared to pitch-containing specimens, while specimens with carbon black display the poorest mechanical properties and intermediate oxidation resistance between those of pitch and graphite specimens.
The role of Al₄SiC₄ in low-carbon MgO–C refractories was also studied. Al₄SiC₄ possesses excellent oxidation resistance, erosion resistance, and stable physicochemical properties. Its addition improves the mechanical properties of refractories, reduces the apparent porosity of specimens, enhances oxidation and erosion resistance, mitigates degradation of thermal and chemical properties, and effectively prolongs service life, demonstrating significant application value.
Refractories for the ladle wall require good erosion resistance and high-temperature shrinkage resistance and are evolving from shaped to unshaped forms. Currently used materials mainly include high-alumina bricks, magnesia–alumina–carbon castables, alumina–magnesia–carbon castables, and unfired magnesia–lime castables. The volumetric erosion wear rates of the original formed surface and the internal cut surface of high-alumina refractories were tested under different erosion angles, abrasive speeds, and durations. It was found that the internal cut surface exhibits superior erosion resistance compared to the original formed surface under all test conditions. Moreover, two erosion stages-acceleration and steady state-are observed on the original formed surface during erosion. Water-bonded lime-containing castables prepared by hydration and carbonation treatment of magnesia–lime clinker exhibit physical properties equivalent to or better than conventional magnesia castables. The thickness of the modified layer of magnesia–lime clinker is approximately 8–9 μm, with no adverse effects on castable performance.
High-alumina–spinel large bricks are used in the impact zone of the ladle bottom, ultra-low cement high-alumina castables containing chromium and steel fibers are used in the base seat, and high-lime magnesia dry ramming mixes are applied elsewhere. Straight-through slit-type corundum or chrome–corundum permeable bricks are mainly adopted as purge plugs. Slit-type permeable bricks are widely used in domestic ladles due to their stable gas flow, small reverse impact force, dense structure, and high strength. They are produced by pre-embedding combustible organic strips during casting; the number and thickness of slits after high-temperature firing are controlled to ensure gas permeability and smooth purging. Common slit configurations include linear and circular slits, with distributions such as star, spiral, tubular, and eight-trigram patterns.
The influence of nano-CaCO₃ addition on castable flowability, permeable brick properties, and microstructure was investigated. Increasing nano-CaCO₃ content decreases castable flowability and bulk density while increasing apparent porosity and room-temperature mechanical properties, with no significant change in gas permeability. Under a holding time of 4 h, specimens heat-treated at 1600℃ show better physical properties than those treated at 1200℃. Optimal comprehensive performance is achieved with a nano-CaCO₃ addition of 1.5 wt.%.
Repair of LF ladles can extend the service life of the lining. According to application methods, repair materials are classified into flame gunning mixes, semi-dry gunning mixes, and castable repair mixes. Flame gunning mixes are mainly alumina–chrome, alumina–spinel, and alumina–zirconia based; semi-dry gunning mixes are mainly magnesia and alumina–silica based; and castable repair mixes are mainly magnesia–carbon castables. Extensive research has been conducted on novel magnesia–carbon castables, in which conventional binders are replaced by a pure carbon-based binder system composed of special condensed resins and other graphitizable carbon materials. Standard dosages of dispersants and deflocculants are also incorporated to maintain low moisture content. Combined with an optimized drying schedule and forming method, the hydration of magnesia can be nearly suppressed. The prepared novel magnesia–carbon castables are expected to replace conventional magnesia–carbon bricks.
Conclusion
With the increasing role of LF ladle refining in the steelmaking process, the importance of refractories for LF ladles has grown significantly. Based on a summary and analysis of the current status of LF ladle refractories, the following directions are proposed for the design of new materials:
1.Under carbon reduction and carbon neutrality goals, refractories should be developed with lower carbon content to reduce costs while meeting industrial carbon emission requirements.
2.Since LF ladle refining is a near-final stage of steelmaking, refractories must exhibit excellent spalling resistance to prevent large inclusions from entering the molten steel, which are difficult to remove in subsequent processes.
3.The thermal shock resistance and slag resistance of LF ladle refractories should be further improved to extend service life, reduce frequent ladle repairs or refractory replacements, and enhance production efficiency.