Ladle slag line
The ladle slag line is the part where molten steel is in direct contact with the air. Currently, magnesia carbon bricks are mostly used in the construction of the ladle slag line. Due to the temperature difference and the existence of oxygen-rich environment, the corrosion rate of this part is significantly faster than that of other parts. In addition, the molten steel The tipping and slag discharge operations during operation will cause great damage to the slag line, so 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.
01 External environment
The ladle is a kind of equipment that accepts molten steel and performs pouring operations. The temperature of the molten steel is often around 1500°C. When the ladle slag line comes into contact with the air at this temperature, a strong oxidation reaction will occur. Not only that, the temperature difference at the contact surface between molten steel and air also has a severe impact on the ladle slag line. A large temperature difference will severely test the thermal stability of the ladle slag line. In frequent receiving and dumping operations, the resistance to The 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. When the refractory material is subject to melting damage Under the interaction with cracking, the ladle slag line is easily damaged, resulting in steel penetration.
LF refining slag can easily cause oxidation and decarburization of magnesia carbon bricks. LF slag has relatively low viscosity at high temperatures, has strong penetration ability in the decarburization layer, and has high solubility for magnesium oxide. At the same time, molten slag can easily penetrate into Magnesia particles are dissociated at the grain boundaries of periclase, as shown in Figure 1 (SA in the picture is the slag; TA is the intersection of the three pieces). Therefore, the service life of the LF slag line magnesia carbon bricks is relatively low. The damage mechanism of ladle magnesia carbon bricks during the LF refining process was systematically studied, which showed that the smaller MgO grain aggregates are easily eroded by high-temperature slag, and after erosion, the slag will continue to penetrate into the MgO bone along the periclase grain boundaries. inside the material, eventually causing cleavage of periclase aggregate.
Different service temperature areas of magnesia carbon bricks in the ladle and different internal organizational structures result in different damage and erosion mechanisms. In the high temperature area near the steel liquid level, magnesia carbon bricks themselves will react with MgO and carbon to form In the decarburization layer, the wettability of slag and magnesia-carbon bricks is better at high temperatures and MgO has a greater tendency to dissolve into the slag. Compared with the low-temperature area near the air side, magnesia-carbon bricks are more seriously eroded by slag. In addition, the slag removal and repair operations of the ladle will inevitably cause artificial damage to the ladle slag line. While the slag removal machine and unpacking machine clean the cold steel and residues of the slag line, they will also cause damage to the ladle slag line. Vibration and accidental damage will cause a certain degree of damage to the ladle slag line. Although this damage has minimal impact on the overall quality of the ladle slag line, it will still increase the frequency of maintenance of the ladle slag line.
02 Refractory quality
At present, ladle slag lines are mainly built with magnesia carbon bricks. Whether they are traditional magnesia carbon bricks or low carbon magnesia carbon bricks that are widely used at present, flake graphite is mainly used as its carbon source. The flake graphite is generally -197, -196 etc., that is, the particle size is greater than 100 mesh, the purity is higher than 97% or 96% (mass fraction), the binding agent is thermosetting phenolic resin, during the carbonization reaction, the network structure formed by the cross-linking reaction of its own chain segments can form magnesia particles. Mechanical interlocking force with graphite, etc. 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 the water escapes at high temperatures to create multiple air 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 combined. Tight and easy to be washed away and peeled off; if the binder content is too much, the thermal shock stability and refractory resistance of magnesia carbon bricks will become worse, and too many harmful elements will be added to the molten steel.
When the ladle receives molten steel from the converter, it will be accompanied by a large amount of steel slag. The low melting point 2CaO·SiO2 in the steel slag dissolves into the MgO grain boundaries and chemically reacts with trace impurity elements in the MgO layer, playing a major role in the dissolution of magnesia refractory materials. From the perspective of converter slag, research on improving the performance of magnesia carbon bricks mainly focuses on magnesia, antioxidants and microstructure.
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. Common additives include Si, Al, Mg, Al-S, and Al-Mg. , Al-Mg-Ca, Si-Mg-Ca, SiC, B4C, BN and Al-B-C and Al-SiC-C series additives. The role of additives mainly has two aspects: on the one hand, from a thermodynamic point of view, in At the working temperature, additives or additives react with carbon to form other substances. Their affinity with oxygen is greater than the affinity between carbon and oxygen. They are oxidized in priority to carbon and thus play a role in protecting carbon. On the other hand, from a kinetic perspective Let’s consider that the compounds generated by the reaction of additives with O2, CO or carbon change the microstructure of carbon composite refractory materials, such as increasing density, blocking pores, hindering the diffusion of oxygen and reaction products, etc.
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 Al carbon and nitrogen compounds, among which Al carbide is prone to hydration in the process from high temperature to low temperature, resulting in the formation of voids inside the magnesia carbon brick, resulting in loose structure and cracks. In view of this situation, some domestic refractory manufacturers have used powder, silicon powder and carbon powder as raw materials to prepare AI4SiC4 powder in vacuum sintering furnaces, and applied it as an antioxidant to magnesia carbon bricks to study its resistance to magnesia carbon bricks. Effect of oxidation properties It was found that AI4SiC4 not only has strong antioxidant properties but also can avoid the hydration cracking problem of traditional antioxidants.
03 Masonry method
Ladle slag line magnesia carbon bricks are generally made of two types: dry laying (direct stacking of bricks without fire clay bonding) and wet laying (using fire clay combined with refractory bricks). The advantage of dry laying is that it minimizes the damage caused by fire clay. Under high temperature conditions, due to the different materials of magnesia carbon bricks and fire clay, the thermal expansion rates are different due to the influence of temperature, which easily creates gaps on the contact surface. The disadvantage of this method is that the magnesia-carbon bricks cannot ensure 100% close contact. At the same time, when the magnesia-carbon bricks expand when heated, there is no buffer room between the bricks, causing the bricks to be squeezed and broken; or due to the magnesia-carbon bricks The carbon bricks expand, and the entire slag line rises as a whole. The huge extrusion force causes the bale edge plate to deform, and the refractory material loses protection and is washed away and peeled off, posing a greater threat to the quality of the slag line.
The wet masonry method is similar to the masonry method in buildings, but the requirements are more stringent. The advantage of this method is that it can well avoid the gaps that may occur in dry masonry. At the same time, the strength of fire clay is weak at high temperatures. When magnesia carbon bricks expand when heated, 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 clay makes the structure of the slag line unstable and increases the difficulty of masonry. If the fire clay is uneven, gaps will still appear between bricks.
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