Research and application of low-carbon and carbon-free refractory materials for ladle

Magnesia carbon bricks remain the most widely used refractory material in steelmaking today due to their high cost-effectiveness. While they offer significant advantages in corrosion resistance and spalling resistance, they also suffer from poor thermal insulation and the tendency to carburize ultra-low carbon molten steel. With the increasing specialization and segmentation of steel grades, higher demands are being placed on refractory materials. Low-carbon and carbon-free refractories can effectively reduce carbon addition to molten steel, lower ladle shell temperatures, and reduce superheat. Improving quality and reducing costs will be key priorities for future development in the metallurgical industry. To meet customer requirements, refractory companies have introduced low-carbon and carbon-free materials in recent years, including low-carbon magnesia carbon bricks, alumina-magnesia spinel castables, and precast blocks. To reduce costs, ladle casting and gunning processes have also been adopted. This article focuses on the research and application of low-carbon and carbon-free materials for ladle casting.

1.Low carbon magnesia carbon bricks

To reduce carbon migration from low-carbon MgO-C bricks into molten steel, the goal is to minimize both carbon content and erosion rate, rather than simply minimizing carbon content. Low-carbon MgO-C bricks containing 4% to 5% carbon (excluding resin carbon) (see Table 1 for key physical and chemical indicators) have been used in several domestic steel mills. In actual use, their erosion rate is similar to that of MT-14A MgO-C bricks, with no noticeable spalling, resulting in relatively stable performance. Low-carbon MgO-C bricks are primarily made from magnesia, fine graphite, and resin. Given that low-carbon MgO-C bricks are often used in the smelting of ultra-low-carbon steel, where molten steel contains high oxygen content and slag often has low alkalinity, the focus is on increasing their density to enhance their oxidation and erosion resistance. The developed low-carbon MgO-C bricks have an apparent porosity of approximately 2.5% and a bulk density of approximately 3.17 g/cm³, making them comparable in density to conventional MgO-C bricks. In some applications, their erosion rate is lower than that of conventional MgO-C bricks with a carbon content of 10% or more. Improving spalling resistance is key to developing low-carbon MgO-C bricks. During the development process, the thermal expansion rate and elastic modulus at high temperatures were controlled. After sintering at 1600°C, the developed product exhibited a linear change rate of ≤0.6%, and an elastic modulus of ≤4000 MPa after 30 minutes of carbon burial at 1600°C. After 50 uses (at the slag line) in a well-known steel mill’s 150t refining ladle, the low-carbon magnesia-carbon bricks remained smooth and flat, with no signs of breakage or flaking.

2.Carbon-free machine-pressed bricks

Based on the characteristics of existing magnesia-carbon brick production lines, we developed carbon-free machine-pressed bricks (alumina-magnesia unfired bricks) for refined steel ladles. The production of carbon-free machine-pressed bricks fully utilizes existing magnesia-carbon brick production equipment and processes, resulting in high production efficiency. Carbon-free machine-pressed bricks are primarily made of alumina, corundum, magnesia, alumina-magnesia spinel, and alumina powder, bonded with resin or inorganic binders. Because they are formed using large-tonnage presses, carbon-free machine-pressed bricks are relatively dense. They are primarily used in the ladle bath and bottom, and are identical to magnesia-carbon brick ladles in terms of brick shape, masonry, and baking. In the early stages of development, carbon-free machine-pressed bricks exhibited flaking. After several adjustments, they have achieved generally stable performance. Ladle linings constructed with machine-pressed bricks have now been used for 80 cycles in a single operation on a 180t refined steel ladle at a certain plant without repair, with minimal flaking and no visible cracking in the brick joints.

3.Aluminum-magnesium top-grade stone castables, prefabricated blocks

Aluminum-magnesium spinel castables have been extensively researched and applied in China. While the corundum-magnesium oxide system is considered cost-effective, the corundum-spinel system offers more stable overall performance. Products using pre-synthesized spinel exhibit relatively gentle thermal expansion throughout the heating process, with slight expansion after cooling and minimal expansion after re-firing. Their bulk density and apparent porosity are also relatively good. Samples using pre-synthesized spinel exhibit superior high-temperature flexural strength, indicating better overall performance at actual operating temperatures. Samples using pre-synthesized spinel also demonstrate excellent resistance to converter slag erosion, demonstrating high slag resistance. Castables made primarily of white corundum, tabular corundum, pure calcium aluminate cement, and spinel offer ease of construction and a long service life (comparable to magnesia-carbon bricks). The key lies in controlling volume stability and re-firing rate, with a design that balances spalling resistance with erosion resistance. Based on experience with a 120t refining ladle at a certain steel mill, the erosion rate of this castable in the most severely corroded areas is less than 1mm per heat. In some steel mills, due to suboptimal construction conditions, ladle shell deformation, or the inability to bake the ladle for extended periods, the ladle lining must be constructed using aluminum-magnesium precast blocks. These blocks are produced by casting, each weighing approximately 10kg, and can be laid and baked like magnesia-carbon bricks. It is generally believed that as cement content decreases, the permeability resistance of corundum-spinel castables decreases while their corrosion resistance increases. Aluminum-magnesium precast blocks, made from cement-free corundum-magnesia, exhibit a better erosion rate than low-cement aluminum-magnesium castables, but their production efficiency is relatively low.

4.Ladle gunning materials

Many companies have conducted trials on gunning the magnesia-carbon brick lining of ladles, but few have applied it in practice, and the results are not ideal. Gunning the lining of aluminum-magnesium castables is feasible. Gunning technology for ladles is a good supplement and improvement to aluminum-magnesium castables. It can repair potential and existing weak spots in the ladle, ensuring the stability and safety of the ladle during operation. It can directly cover the aluminum-magnesium gunning material on the surface of the original castable to form a new working layer, avoiding the waste of resources and environmental pollution caused by the castable during removal. At present, cold gunning on a 120t refined steel ladle at a steel plant can achieve the effect of 40 uses of gunning once. This allows a variety of methods such as pouring a new ladle, casting over, and gunning to ensure the use effect and control the overall cost.