Title: Key Factors Affecting the Life of Ladle Slag Brick Refractory
Keyword: ladle slag line，magnesia-carbon bricks
The ladle slag line is the part where the molten steel is in direct contact with the air, and it is also one of the parts with the most frequent maintenance. This article explains how to improve the life of ladle slag line from three aspects: external environment, refractory quality and masonry method.
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 ladle slag line masonry. Due to the temperature difference and the oxygen-enriched environment in this part, the erosion rate is significantly faster than other parts. In addition, the tipping and slag discharge operations of molten steel during operation will cause great damage to the slag line. Therefore, the ladle slag line is one of the most frequently repaired parts.
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, and the ladle slag line contacts with air at this temperature, which will produce a strong oxidation reaction. 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 larger temperature difference will severely test the thermal stability of the ladle slag line. In the frequent receiving and dumping operations, the refractory material will crack to a certain extent. Therefore, in the external environment, oxidation at high temperature 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.
Magnesia-carbon bricks have different damage and erosion mechanisms in different service temperature areas in the ladle and their different internal structures. In the high temperature area near the molten steel surface, the magnesia-carbon brick itself will react with MgO and carbon to form a decarburized layer. The wettability of slag and magnesia-carbon bricks at high temperature is better and the tendency of MgO to dissolve into slag is greater. 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 performed by the ladle will inevitably cause artificial damage to the ladle slag line. While the slag scraper and bale unpacker clean the cold steel and residues of the slag line, they will cause vibration and accidental injury to the ladle slag line, thereby causing a certain degree of damage to the ladle slag line. Although this damage has minimal impact on the overall quality of the slag line, it still increases the frequency of repairs to 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 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 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 with harsh 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 structure 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. At the same time, the scouring ability to deal with 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, 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 the 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, antioxidant 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 amount of additives are often added. Common additives are 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: one is from the point of view of thermodynamics. At the working temperature, the additive or the additive reacts with carbon to form other substances, which have a greater affinity with oxygen than carbon and oxygen, and are oxidized prior to carbon to protect 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 refractories, such as increasing the density and blocking pores.
It hinders the diffusion of oxygen and reaction products. At present, Al powder is mainly used in magnesia-carbon bricks to prevent the oxidation of carbon. Although Al has strong anti-oxidation ability, at high temperature, 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 this case. Some domestic refractory manufacturers have used powder, silicon powder and carbon powder as raw materials to prepare AI4SiC4 powder in vacuum sintering furnace, and apply it as an antioxidant in magnesia-carbon bricks. Research on 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.
(3) Masonry method
Ladle slag line magnesia-carbon bricks generally use dry laying (directly stacking bricks, no fire clay bonding) and wet laying (using fire clay combined with refractory bricks). The advantage of dry laying is that the influence of fire clay is minimized. At high temperature, due to the different materials of magnesia-carbon bricks 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% tight contact between magnesia-carbon bricks cannot be guaranteed. At the same time, when the magnesia-carbon brick is heated and expanded, there is no room for buffering between the bricks, causing the bricks to be squeezed and fractured; or due to the expansion of the magnesia-carbon brick. The whole ring of slag line is lifted as a whole, and the huge extrusion force deforms the package along 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.