The ladle slag line is the part where molten steel comes into direct contact with air. At present, magnesia carbon bricks are mostly used for the construction of ladle slag line. Due to the temperature difference and the existence of oxygen-rich environment, the erosion rate of this part is significantly faster than that of other parts. In addition, the tipping and slag discharge 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.
1. External environment
The ladle is a device for receiving molten steel and performing pouring operations. The temperature of molten steel is often around 1500℃. When the ladle slag line comes into contact with air at this temperature, a strong oxidation reaction will occur. In addition, the temperature difference of the contact surface between molten steel and air also has a very strong impact on the ladle slag line. The large temperature difference will severely test the thermal stability of the ladle slag line[20]. During frequent receiving and dumping operations, the refractory will produce a certain degree of cracking. 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 collapse of refractory materials, the ladle slag line is easily damaged, and then steel infiltration occurs.
LF refining slag is easy to cause oxidation and decarburization of magnesia carbon bricks. LF slag has a relatively low viscosity at high temperature, has a strong permeability in the decarburization layer, and has a high solubility in magnesium oxide. At the same time, the slag is easy to penetrate into the grain boundary of periclase to dissociate magnesia sand particles, as shown in Figure 2 (SA is slag in the figure; TA is the intersection of three pieces). Therefore, the service life of LF slag line magnesite carbon bricks is relatively low. Shen et al. systematically studied the damage mechanism of ladle magnesium carbon bricks in the LF refining process, indicating that smaller MgO grain aggregates are easily eroded by high-temperature slag. After erosion, the slag will continue to penetrate into the interior of the MgO aggregate along the periclase grain boundary, eventually causing the cleavage of the periclase aggregate.
2. Refractory quality
Currently, magnesite carbon bricks are mainly used for ladle slag lines. Both traditional magnesia carbon bricks and low-carbon magnesite carbon bricks, which are currently widely used, mainly use flake graphite as their carbon source. Flake graphite is generally selected from -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 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 is the main raw material for the production of magnesia carbon refractory bricks, mainly due to its excellent physical properties: ① non-wetting of slag, ② high thermal conductivity, and ③ low thermal expansion. In addition, graphite does not melt with refractory materials, and graphite has high refractoriness. It is precisely because of this characteristic that mag-c bricks are selected for slag lines with harsh operating environments [24]. For low carbon magnesia carbon bricks (mass fraction of carbon ≤8%) or ultra-low carbon magnesite carbon 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 magnesia-carbon bricks is relatively complex. On the contrary, the organizational structure design of high carbon mag-carbon bricks (mass fraction of carbon>10%) is relatively simple.
Due to the susceptibility of magnesite carbon bricks to moisture and the influence of formula selection, the performance of magnesia- carbon bricks will be affected to a certain extent. After magnesia carbon bricks are damp, the structure becomes loose, and water escapes at high temperature to produce multiple empty channels, which will have a negative impact on the thermal stability and corrosion resistance of this 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. Too little binder will cause the powder of magnesia carbon brick to be loosely bound and easily washed and peeled off; too much binder will cause the thermal shock stability and refractoriness of magnesia carbon brick to 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 refractory 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, at high temperature, Al reacts with C and N2 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.
3. 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 mag-c 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 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 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.
