The ladle slag line is the part where molten steel is in direct contact with the air. At present, magnesia carbon bricks are mostly used in ladle slag line masonry. Due to the temperature difference and the existence of an oxygen-rich environment, the erosion rate of the slag line is significantly faster than that of other parts. The tipping and slag discharge operations of molten steel during operation also cause great damage to the slag line. Therefore, the ladle slag line is one of the most frequently repaired parts.
The service life of a ladle slag line is mainly affected and restricted by three aspects: external environment, refractory quality and masonry methods.
The ladle is a device for receiving molten steel and performing pouring operations. The temperature of molten steel is often around 1500°C. When the ladle slag line comes into contact with air at this temperature, a strong oxidation reaction will occur. The temperature difference at the contact surface between molten steel and air also has a severe impact on the ladle slag line. During frequent receiving and dumping operations, the refractory material will crack to a certain extent. Therefore, in the external environment, oxidation at high temperatures has a great impact on the erosion of slag lines. At the same time, huge changes in temperature place high requirements on the thermal stability of the refractory material. Under the interaction of melting loss and cracking of the refractory material, the ladle slag line is easily damaged, resulting in steel penetration.
The slag removal and repair operations performed on the ladle will inevitably cause man-made damage to the ladle slag line. While cleaning the cold steel and residue of the slag line, the slag scraper and unpacking machine will cause vibration and accidental damage to the ladle slag line, thereby causing a certain degree of damage to the ladle slag line. Although this damage has a minimal impact on the overall quality of the slag line, it will still increase the frequency of maintenance of the ladle slag line.
At present, ladle slag lines are mainly built with magnesia carbon bricks. Whether it is traditional magnesia carbon bricks or low carbon magnesia carbon bricks that are currently widely used, flake graphite is mainly used as its carbon source. Flake graphite generally uses -197, -196, etc., that is, the particle size is greater than 100 mesh and the purity is higher than 97% or 96% (mass fraction). The binding agent is thermosetting phenolic resin. Graphite, as the main raw material for the production of magnesia-carbon bricks, mainly benefits from its excellent physical properties:
① Non-wetting of slag
② High thermal conductivity
③ Low thermal expansion. In addition, graphite and refractory materials do not fuse together, and graphite has high refractoriness. It is precisely because of this characteristic that magnesia-carbon bricks are selected for use in slag lines with harsh operating environments. For low carbon magnesia carbon bricks (mass fraction of carbon ≤ 8%) or ultra-low carbon magnesia carbon bricks (mass fraction of carbon ≤ 3%), it is difficult to form a continuous network structure due to the low carbon content, so low carbon magnesia carbon bricks The design of the organizational structure of high carbon magnesia carbon bricks (mass fraction of carbon >10%) is relatively simple.
Since magnesia carbon bricks are susceptible to moisture and are affected by formula selection, the performance of magnesia carbon bricks will be affected to a certain extent. After the magnesia-carbon bricks become damp, the structure becomes loose, and moisture escapes at high temperatures to create porous channels, which will have a negative impact on the thermal stability and corrosion resistance of the magnesia-carbon bricks. At the same time, the ability to cope with the erosion of molten steel will also be greatly weakened. MgO-C is sensitive to thermomechanical abrasion because of the high reversibility of MgO’s thermal expansion coefficient. The binder of magnesia carbon bricks is also an important factor affecting the quality of magnesia carbon bricks. Too much or too little binder content will affect the performance of magnesia carbon bricks. If the binder content is too little, the magnesia carbon brick powder will not be tightly bonded and will be easily washed and peeled off; if the binder content is too much, the thermal shock stability and refractory resistance of the magnesia carbon brick will become worse. At the same time, too many harmful elements will be added to the molten steel.
In addition, the addition of antioxidants to magnesia carbon bricks also affects their quality. In order to improve the oxidation resistance of magnesia carbon bricks, a small amount of additives are often added. At present, Al powder is mainly used in magnesia carbon bricks to prevent the oxidation of carbon. Although Al has strong ability to resist oxidation, at high temperatures, Al reacts with C and N2 to form carbon and nitrogen compounds of Al. Among them, Al carbide is prone to hydration in the process from high temperature to low temperature, causing the formation of voids inside the magnesia carbon brick, resulting in loose structure and cracks. In view of this situation, AI4SiC4 powder prepared in a vacuum sintering furnace using silicon powder and carbon powder as raw materials has been applied to magnesia-carbon bricks as an antioxidant. Studying its effect on the antioxidant properties of magnesia-carbon bricks found that AI4SiC4 not only has strong antioxidant properties but also can avoid the hydration cracking problem of traditional antioxidants.
Ladle slag line magnesia carbon bricks are generally made of two types: dry laying (direct stacking of bricks without fire mud bonding) and wet laying (using fire mud combined with refractory bricks). The advantage of dry-laying is that it minimizes the impact of fireclay. Under high temperature conditions, due to the different materials of magnesia carbon bricks and fire clay, the thermal expansion rates are different due to temperature, which easily creates gaps on the contact surface. The disadvantage of this method is that there is no guarantee of 100% close contact between magnesia carbon bricks. At the same time, when the magnesia-carbon bricks expand when heated, there is no room for buffering between the bricks, causing the bricks to be squeezed and broken; or due to the expansion of the magnesia-carbon bricks, the entire ring slag line rises as a whole, and the huge extrusion force causes the bricks to break along the board. Deformation, refractory material loses protection and is washed away and peeled off, posing a greater threat to the quality of the slag line.
The advantage of wet masonry is that it avoids the gaps that may occur during dry masonry. At the same time, the strength of the fire clay is weak at high temperatures. When the magnesia carbon bricks are heated and expand, they can flow to adapt to the changes in the gaps between the bricks, dispersing the extrusion force between the bricks, thereby well avoiding the occurrence of gaps. The disadvantage of this method is that the use of fire mud makes the structure of the slag line unstable and increases the difficulty of masonry. If the fire mud is uneven, gaps will still occur between bricks.
Maybe you will be interested in refractory materials for steel making.
Specializing in refractory materials for over 20 years, we provide professional refractory solutions for the global high temperature industry.