The ladle slag line is the part where the molten steel is in direct contact with the air. At present, magnesia-carbon bricks are mostly used in the masonry of the ladle slag line. Due to the existence of temperature difference and oxygen-enriched environment, the erosion rate of this part is significantly faster than that of other parts. The tipping and slag discharge operations during operation cause great damage to the slag line, so the ladle slag line is one of the most frequently repaired parts.
Figure 1: Schematic diagram of the ladle lining refractory structure
The service life of the ladle slag line is mainly affected and restricted by the external environment, the quality of the refractory materials, and the masonry method.
(1) External environment
The ladle is a kind of equipment for receiving molten steel and performing pouring operations. The temperature of the molten steel is often around 1500°C. When the ladle slag line contacts with air at this temperature, a strong oxidation reaction will occur. Not only that, the temperature difference between the contact surface of molten steel and air has a very severe impact on the ladle slag line, and a large temperature difference will severely test the thermal stability of the ladle slag line [20]. In the middle, the refractory will have a certain degree of cracking. Therefore, in the external environment, oxidation at high temperatures has a great influence on the erosion of the slag line, and at the same time, the huge change in temperature puts forward high requirements on the thermal stability of the refractory material. Under the interaction with cracking, the ladle slag line is easily damaged, resulting in the phenomenon of steel infiltration.
LF refining slag is easy to causes oxidation and decarburization of magnesia-carbon bricks. LF slag has low viscosity at high temperatures, strong penetration ability in the decarburized layer, and high solubility for magnesia. The magnesia particles are dissociated at the grain boundary of periclase, as shown in Figure 2 (SA is slag; TA is the intersection of three blocks). Therefore, the service life of LF slag line magnesia-carbon bricks is relatively low. Shen et al. systematically studied the damage mechanism of ladle magnesia-carbon bricks in the LF refining process, indicating that the smaller MgO grain aggregates are easily eroded by high-temperature slag, and the slag will continue to infiltrate along the periclase grain boundaries after erosion. Inside the MgO aggregate, eventually causing the cleavage of the periclase aggregate.
Fig. 2 Slag infiltration along the periclase grain boundary
Magnesia-carbon bricks have different damage and erosion mechanisms in different service temperature areas in the ladle and their own internal structures, resulting in different damage and erosion mechanisms. In the decarburized layer, the wettability of the slag and the magnesia-carbon brick at high temperature is better and the tendency of MgO to dissolve into the slag is greater. Compared with the low-temperature area near the airside, the magnesia-carbon brick is more seriously eroded by the slag. In addition, the slag removal and repair operations performed by the ladle will inevitably cause artificial damage to the ladle slag line. Vibration and accidental injury will cause a certain degree of damage to the ladle slag line. Although this damage has little impact on the overall quality of the slag line, it will still increase the maintenance frequency of the ladle slag line.
(2) Refractory quality
At present, the ladle slag line is mainly built with magnesia-carbon bricks. Whether it is a traditional magnesia-carbon brick or a low-carbon magnesia-carbon brick that is widely used at present, it mainly uses flake graphite 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), and the binder is a 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. The mechanical interlocking force with graphite etc. As the main raw material for producing magnesia-carbon bricks, graphite mainly benefits from its excellent physical properties: ① non-wetting to slag, ② high thermal conductivity, ③ low thermal expansion. In addition, graphite and refractory materials do not eutectic, and graphite has high refractoriness. It is because of this characteristic that magnesia-carbon bricks are selected for slag lines in harsh environments [24]. 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 structure is more complicated, on the contrary, the structure design of the high-carbon magnesia-carbon brick (the mass fraction of carbon> 10%) is relatively simple.
Because magnesia-carbon bricks are easily affected by moisture and the selection of formula, the performance of magnesia-carbon bricks will be affected to a certain extent. After the magnesia-carbon brick is damp, the structure is loose, and the water escapes at high temperature to generate multiple empty channels, which will have a negative impact on the thermal stability and corrosion resistance of the magnesia-carbon brick, and the scour ability of the molten steel will also be greatly weakened. MgO-C is sensitive to thermomechanical abrasion due to the high reversibility of the thermal expansion coefficient of MgO. 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. Tight and easy to be washed and peeled off; too much binder content will deteriorate the thermal shock stability and refractoriness of magnesia-carbon bricks, and at the same time, 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 in the MgO grain boundary and chemically reacts with the trace impurity elements in the MgO layer, which plays a major role in the dissolution of magnesia refractories. From the perspective of converter slag, the research on the performance improvement of magnesia-carbon bricks mainly focuses on magnesia, antioxidants, and microstructure.
In addition, the addition of antioxidants in magnesia-carbon bricks also affects its quality. In order to improve the oxidation resistance of magnesia-carbon bricks, a small number of additives are often added. Common additives are 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: one is from the point of view of thermodynamics, in the At the working temperature, additives or additives react with carbon to form other substances. Their affinity with oxygen is greater than that of carbon and oxygen, and they are oxidized in preference to carbon to protect carbon. On the other hand, from a kinetic point of view consider the compounds generated by the reaction of additives with O2, CO, or carbon to change the microstructure of carbon composite refractories, such as increasing the density, blocking pores
It hinders the diffusion of oxygen and reaction products, etc. [28]. At present, Al powder is mainly used in magnesia-carbon bricks to prevent the oxidation of carbon. Although Al has a strong anti-oxidation ability, at high temperatures, Al reacts with C and N2 to form Al carbon and nitrogen compounds. Al carbide is prone to hydration 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 a vacuum sintering furnace and used it as an antioxidant in magnesia-carbon bricks to study its resistance to magnesia-carbon bricks. The effect of oxidation performance found that AI4SiC4 not only has strong antioxidant performance but also can avoid the problem of hydration cracking existing in traditional antioxidants.
(3) Masonry method
Ladle slag line magnesia-carbon bricks generally use dry laying (directly stacking bricks, without fire clay bonding) and wet laying (using fire clay combined with refractory bricks). Influence, at high temperatures, due to the different materials of magnesia carbon brick and fire clay, the thermal expansion rate is different due to the influence of temperature, and it is easy to create gaps in the contact surface. The disadvantage of this method is that 100% close contact between magnesia-carbon bricks cannot be guaranteed. At the same time, when magnesia-carbon bricks are heated and expanded, there is no room for buffering between bricks, which will cause bricks to be squeezed and broken; or due to magnesia-carbon bricks. The carbon brick expands, the whole ring of slag line is lifted as a whole, the huge extrusion force deforms the package along with the plate, and the refractory material loses its protection and is washed and peeled off, which poses a great threat to the quality of the slag line.
The wet laying method is similar to the building method, except that the requirements are stricter. The advantage of this method is that it can avoid the possible gaps in dry laying. When magnesia-carbon bricks are heated and expanded, flow can be generated to adapt to the change of the gap between the bricks, dispersing the extrusion force between the bricks, thus avoiding the generation of gaps. The disadvantage of this method is that the use of fire mud makes the structure of the slag line in an unstable state, and at the same time increases the difficulty of masonry. If the fire mud is not uniform, there will still be gaps between the bricks.
