01. Ladle slag line
The ladle slag line is the part where molten steel is in direct contact with air. At present, most of the ladle slag line masonry uses magnesia carbon bricks. Due to the temperature difference and the existence of oxygen-rich environment, the erosion rate of this part is significantly faster than other parts. In addition, the tipping and slag discharge operations of molten steel during operation cause great damage to the slag line. Therefore, the ladle slag line is one of the parts with the highest maintenance frequency.
The life of the ladle slag line is mainly affected and restricted by three aspects: external environment, refractory quality and masonry method.
02. External environment
The ladle is a device that receives molten steel and performs pouring operations. The temperature of molten steel is often around 1500℃. When the ladle slag line contacts air at this temperature, a strong oxidation reaction will occur. Not only that, the temperature difference of the contact surface between molten steel and air also has a very drastic effect on the ladle slag line. The large temperature difference will severely test the thermal stability of the ladle slag line. During frequent receiving and dumping operations, the refractory will have a certain degree of collapse. Therefore, in the external environment, oxidation at high temperature has a great impact on the erosion of the slag line. At the same time, the huge change in temperature puts forward high requirements on the thermal stability of refractory materials. Under the interaction of melting loss and cracking of refractory materials, the ladle slag line is easily damaged, and then the phenomenon of steel infiltration occurs.
03. Refractory quality
At present, the ladle slag line is mainly built with magnesia carbon bricks. Whether it is traditional magnesia carbon bricks or low-carbon magnesia carbon bricks currently used in large quantities, flake graphite is mainly used as its carbon source. Flake graphite is generally selected from -197, -196, etc., that is, the particle size is greater than 100 mesh, the purity is higher than 97% or 96% (mass fraction), and the binder is a thermosetting phenolic resin. During the carbonization reaction, the self-chain segments undergo cross-linking reactions to form a network structure that can form a mechanical interlocking force between magnesia sand particles and graphite. 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 form a co-melting state, and graphite has high refractoriness. It is precisely because of this characteristic that MgO-C bricks are selected for use in slag lines with harsh operating environments [24]. For low-carbon MgO-C bricks (mass fraction of carbon ≤8%) or ultra-low-carbon MgO-C bricks (mass fraction of carbon ≤3%), it is difficult to form a continuous network structure due to the low carbon content, so the organizational structure design of low-carbon MgO-C bricks is relatively complex. On the contrary, the organizational structure design of high-carbon MgO-C bricks (mass fraction of carbon>10%) is relatively simple.
Due to the susceptibility of MgO-C bricks to moisture and the influence of formula selection, the performance of MgO-C bricks will be affected to a certain extent. When MgO-C bricks are damp, the structure becomes loose, and water escapes at high temperatures to produce multiple porous channels, which will have a negative impact on the thermal stability and corrosion resistance of MgO-C bricks, and the ability to cope with molten steel will also be greatly weakened. MgO-C is very sensitive to thermomechanical abrasion because the thermal expansion coefficient of MgO has a high reversibility. The binder of magnesia carbon brick is also an important factor affecting the quality of magnesia carbon brick. Too much or too little binder will affect the performance of magnesia carbon brick. If the binder content is too little, the magnesia carbon brick powder is not tightly bound and is easily washed and peeled off; if the binder content is too much, the thermal shock stability and refractoriness of magnesia carbon brick will deteriorate, and too many harmful elements will be added to the molten steel.
When the ladle receives the molten steel from the converter, it will be accompanied by a large amount of slag. The low melting point 2CaO·SiO2 in the slag dissolves into the MgO grain boundary and reacts chemically with the trace impurity elements in the MgO layer, which plays a major role in the dissolution of magnesia refractory materials. From the perspective of converter slag, the research on the performance improvement of magnesia carbon bricks mainly focuses on magnesia sand, 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, 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, at the working temperature, additives or additives react with carbon to generate other substances. Their affinity with oxygen is greater than that of carbon with oxygen, and they are oxidized before carbon, thereby protecting carbon. On the other hand, from a kinetic point of view, 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, and hindering the diffusion of oxygen and reaction products [28]. At present, Al powder is mainly used in magnesia carbon bricks to prevent carbon oxidation. Although Al has strong anti-oxidation ability, Al reacts with C and N2 at high temperature to form Al carbon and nitrogen compounds. Among them, Al carbide is easy to hydrate in the process from high temperature to low temperature, resulting in the formation of voids inside the magnesia carbon brick, which causes the structure to loosen and cracks.
04. Masonry method
Magnesium carbon bricks in ladle slag line generally adopt dry masonry (directly stacking bricks without fire mud bonding) and wet masonry (using fire mud combined with refractory bricks). The advantage of dry masonry is that it minimizes the impact of fire mud. Under high temperature conditions, due to the different materials of magnesia carbon bricks and fire mud, the thermal expansion rate is different due to the temperature, which is easy to produce gaps on the contact surface. The disadvantage of this method is that the magnesia carbon bricks cannot be guaranteed to be 100% in close contact. At the same time, when the magnesia carbon bricks expand due to heat, there is no room for buffering between the bricks, which causes the bricks to be squeezed and broken; or due to the expansion of the magnesia carbon bricks, the whole ring of slag line is lifted as a whole, and the huge extrusion force causes the edge plate to deform, and the refractory material loses protection and is washed and peeled off, which poses a greater threat to the quality of the slag line.
The wet masonry method is similar to the masonry method in buildings, but it is more stringent in requirements. The advantage of this method is that it can well avoid the gaps that may occur in dry masonry. At the same time, the fire mud is weak at high temperatures. When the magnesia carbon bricks expand due to heat, they can flow to adapt to the changes in the gaps between bricks, dispersing the extrusion force between bricks, thereby avoiding the generation 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, there will still be gaps between bricks.
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