Introduction to the Diverse Applications of Magnesia Carbon Bricks in Steelmaking

Magnesia-carbon bricks are an ideal choice for steel smelting due to their excellent high-temperature resistance, slag erosion resistance, and good thermal shock stability. The non-wetting properties of carbon materials, combined with the high refractoriness, slag resistance, and solvent resistance of magnesia, make magnesia-carbon bricks particularly suitable for critical areas such as severely eroded slag lines and tapping holes. With the widespread application of magnesia-carbon bricks in the steelmaking process, combined with improvements in steel smelting technology, significant economic benefits have been brought to enterprises.

1. Application Differences of Magnesia-Carbon Bricks in Converter Linings

Different parts of the converter working lining face different service conditions, so the effectiveness of magnesia-carbon bricks will vary. The furnace mouth of the lining is continuously impacted by hot and cold molten steel, which requires the refractory material used here to be able to resist the erosion of high-temperature slag and high-temperature exhaust gas, and at the same time be easy to clean hanging steel. The furnace cap area not only suffers from severe slag erosion but also has to cope with rapid temperature changes, as well as the combined effects of high-temperature airflow and dust exhaust gas generated by carbon oxidation. Therefore, it is necessary to select magnesia-carbon bricks with strong slag erosion resistance and spalling resistance. The charging side requires magnesia-carbon bricks to have not only excellent slag erosion resistance but also excellent high-temperature strength and spalling resistance. High-strength versions with added metal antioxidants are usually selected. Studies have shown that magnesia-carbon bricks containing metallic aluminum have lower high-temperature strength at low temperatures than samples with composite additions of metallic aluminum and metallic silicon, but their strength is higher in high-temperature environments. As a key point where furnace lining refractory materials, high-temperature molten slag, and furnace gas converge, the slag line is the area with the most severe slag erosion. Therefore, it is necessary to select magnesia-carbon bricks with excellent slag erosion resistance and a high carbon content.
2. Application of Magnesia-Carbon Bricks in Electric Furnaces

Currently, almost all electric furnace walls are built with magnesia-carbon bricks. Therefore, the durability of magnesia-carbon bricks directly determines the service life of electric furnaces. The key factors affecting the quality of magnesia-carbon bricks for electric furnaces include: the purity and types of impurities of the MgO source magnesia, as well as the bonding state and grain size of the periclase crystals; at the same time, the purity, degree of crystallization, and flake size of the flake graphite, which is the source of carbon introduction, are also crucial. In addition, the amount and residual carbon content of thermosetting phenolic resin as a binder are also factors that cannot be ignored.

It is worth noting that although adding antioxidants to magnesia-carbon bricks can improve their matrix structure, antioxidants are not necessary under normal operating conditions of electric furnaces. However, for electric arc furnaces using high FeOn slag, such as direct reduced iron or areas with irregular oxidation, as well as electric furnace hot spots, the addition of metal antioxidants is particularly important.

In the slag line area, the erosion behavior of magnesia-carbon bricks is manifested as the formation of obvious reaction dense layers and decarburized loose layers. The reaction dense zone, also known as the slag erosion zone, is an erosion area formed by the formation of a large number of pores after the decarburization of magnesia-carbon bricks, and the penetration of high-temperature liquid phase slag into the interior of the brick. In this area, FeOn in the slag is reduced to metallic iron, and even the exsolved phase and intergranular Fe2O3 dissolved in MgO are reduced. The depth of slag penetration into the brick mainly depends on the thickness of the decarburized loose layer, usually ending at the remaining graphite. Under normal circumstances, due to the presence of graphite, the decarburized layer of magnesia-carbon bricks remains thin.

There are two types of tapping methods for electric furnace tapping holes: tapping with a tapping spout and tapping at the bottom of the furnace. When tapping with a tapping spout, magnesia-carbon bricks are usually not selected, but Al2O3 or ZrO2 materials are selected, and non-oxides such as C, SiC, and Si3N4 are added. When tapping at the bottom of the furnace, the tapping hole is composed of an outer sleeve brick and an inner tube brick, in which the tube brick is made of magnesia-carbon brick, and its pore size is determined according to factors such as furnace capacity and tapping time. The inner diameter is generally 140~260mm.

An electric furnace in a steel plant used medium and low-grade magnesia-carbon bricks for the tapping hole and both sides of the copper tapping hole, replacing the original sintered magnesia bricks, and achieved significant results. The furnace age has been greatly increased from about 60 furnaces to more than doubled. The magnesia-carbon bricks in the slag line area after use remain intact and have no slag sticking phenomenon, thereby reducing labor intensity and improving steel purity and production efficiency.
3. Application of Alumina-Magnesia-Carbon Bricks on Ladles

In refining ladle furnaces and ladles, MgO-C bricks are mainly used in key areas such as the freeboard and slag line. These parts require refractory materials to resist high temperatures, thermal shock, and mechanical erosion such as slag erosion. In the past, magnesia-chrome refractory materials were used in these parts, but in view of the potential environmental impact of chromium, magnesia-carbon bricks are now more often used.

However, the magnesia-carbon bricks in the new ladle will suffer severe damage during the preheating process, and the loose decarburized layer can be 30~60mm thick. This layer will be washed away when molten steel is injected, causing magnesia particles to enter the slag. Therefore, preventing the carbon in the magnesia-carbon bricks from being oxidized during preheating is the key to extending the service life of the magnesia-carbon bricks in the freeboard and slag line areas of the ladle. In addition to adding composite antioxidants to the magnesia-carbon bricks, covering the surface of the magnesia-carbon bricks with an alkali-containing low-melting glass phase liquid after lining can effectively protect the carbon in the bricks from being burned during preheating.