Magnesia-Carbon bricks are highly suitable for the requirements of steel metallurgy due to their superior high-temperature resistance, resistance to slag erosion, and excellent thermal shock stability. Leveraging the high refractory properties, excellent slag resistance, and solubility resistance of carbon materials and the low creep at high temperatures of magnesia, these bricks find applications in severely eroded slag lines and steel teeming nozzles. Thus far, the extensive use of magnesia-carbon bricks in the steelmaking process, coupled with process improvements in iron and steel metallurgy, has led to significant economic benefits.
1.Usage of Magnesia-Carbon Bricks in Converter Furnace Lining
The application effectiveness of magnesia-carbon bricks varies across different parts of the converter furnace lining due to varying working conditions.
The furnace lining and mouth area are continuously subjected to impacts from both cold and hot steel liquids. Therefore, refractory materials used for the furnace mouth must withstand high-temperature slag and the scouring of high-temperature exhaust gas, while also being resistant to steel hanging and easy to clean promptly. The furnace cap area not only suffers from severe slag erosion but also experiences rapid temperature changes from hot to cold. It is also subjected to a combination of high-temperature gas flow due to carbon oxidation and the scouring of hot dust-laden exhaust gas. Hence, magnesia carbon bricks with strong resistance to slag erosion and spalling are used.
For the charging side, magnesia carbon bricks are required not only to have high resistance to slag erosion but also high-temperature strength and excellent resistance to spalling. Therefore, high-strength magnesia carbon bricks with added metal anti-oxidants are commonly used. Research indicates that the addition of metallic aluminum to magnesia carbon bricks reduces the high-temperature strength at lower temperatures compared to samples with both metallic aluminum and metallic silicon. However, at higher temperatures, the high-temperature strength of such bricks increases instead.
The slag line is the interface between the furnace lining refractory, high-temperature slag, and furnace gas, and it is the area most severely affected by slag erosion. Hence, it requires the construction of magnesia carbon bricks with excellent resistance to slag erosion. The slag line area demands magnesia carbon bricks with higher carbon content.
2.The use of magnesia carbon bricks on electric furnaces
Currently, almost all electric furnace walls are constructed using magnesia carbon bricks. Therefore, the lifespan of these bricks determines the overall lifespan of the electric furnace. The main factors influencing the quality of magnesia carbon bricks used in electric furnaces include the purity of magnesia sand as the source of MgO, the types of impurities present, the bonding state, and grain size of periclase (MgO) crystals. Additionally, the purity, degree of crystallization, and size of graphite flakes, which serve as a source of carbon, are important considerations. Typically, thermosetting phenolic resins are chosen as binders, and the amount and residual carbon content of the binder are significant factors.
It has been proven that the addition of antioxidants to magnesia carbon bricks can alter and improve their matrix structure. However, under normal operating conditions of electric furnaces, antioxidants are not necessarily required as essential components of magnesia carbon bricks. They become crucial only when dealing with high FeO-containing slags in electric arc furnaces used for direct reduced iron or irregularly oxidized regions and hotspots of the furnace. In such cases, various metallic antioxidants are added to the magnesia carbon bricks to enhance their performance.
The slag line area of magnesia carbon bricks exhibits evident erosion behavior characterized by the formation of a distinct reaction dense layer and a decarburization loose layer. The reaction dense zone, also known as the slag invasion zone, is the region where high-temperature liquid phase slag infiltrates into the interior of the magnesia carbon bricks after the formation of numerous gas pores due to decarburization. In this zone, FeO in the slag is reduced to metallic iron, and even the intergranular Fe2O3 and decarbonized phases dissolved in MgO are reduced to metallic iron. The depth of slag penetration into the bricks is mainly determined by the thickness of the decarburization loose layer and typically stops at the location of remaining graphite. Under normal conditions, the decarburization layer in magnesia carbon bricks is relatively thin due to the presence of graphite.
For the steel tapping process in electric furnaces, there are two methods: tilting tapping and bottom tapping. When tilting tapping is employed, magnesia carbon bricks are generally not used, and instead, materials like Al2O3 or ZrO2 are chosen, with the addition of non-oxide materials such as C, SiC, and Si3N4. On the other hand, for bottom tapping, the tapping port consists of external brick lining and internal pipe bricks. Magnesia carbon bricks are used for the pipe bricks, and the dimensions of the pipe holes are determined based on factors such as furnace capacity and tapping time, typically ranging from 140 to 260 mm.
In a certain steel plant, magnesia carbon bricks were utilized in the middle and lower parts of the tapping port, replacing the previously used sintered magnesia bricks, resulting in significant improvements. The furnace lifespan increased from around 60 heats to more than double. The magnesia carbon bricks in the slag line area remained intact without slag sticking, eliminating the need for relining, which not only reduced labor intensity but also improved steel purity and productivity.
3.The Use of Aluminum-Magnesia-Carbon Bricks in Steel Ladles
MgO-C bricks are used in refining ladles and during ladle operations, primarily in areas such as the freeboard and slag line. Depending on the operating conditions, these areas require refractory materials with high-temperature resistance, thermal shock resistance, and resistance to mechanical erosion caused by slag attack. In the past, magnesia-chrome refractory materials were used in these areas. However, due to environmental concerns regarding chromium pollution, their usage has decreased, and now aluminum-magnesia-carbon bricks are preferred.
During the preheating process, newly installed aluminum-magnesia-carbon bricks in the ladle can experience severe damage, resulting in a loose decarburization layer of about 30 to 60 mm thick. This layer is washed away during the steel pouring process, carrying magnesium sand particles into the slag. Obviously, preventing the carbon in the aluminum-magnesia-carbon bricks from being burned off during preheating becomes one of the crucial steps in extending the lifespan of these bricks in the freeboard and slag line areas. The technical measure to achieve this is to incorporate composite antioxidants into the aluminum-magnesia-carbon bricks and, crucially, to cover the surface of the bricks with a low-melting alkali-containing glass phase liquid after installation. This protective measure ensures that the carbon in the aluminum-magnesia-carbon bricks is not burned off during the ladle’s preheating process.