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Improve the service life of the ladle slag line

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 for ladle slag line masonry. Due to the temperature difference and the presence of an oxygen-enriched environment, the erosion rate of this part is significantly faster than that of other parts. The tipping and slag discharge operations in operation can cause a lot of damage to the slag line, so the ladle slag line is one of the most frequently repaired components.

 

Schematic diagram of the ladle lining refractory structure
(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

 

A ladle is a device that receives molten steel and performs pouring operations. The temperature of 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 molten steel and the air contact surface has a very serious impact on the ladle slag line, and a large temperature difference will seriously test the thermal stability of the ladle slag line [20]. In the middle, the refractory material will have a certain degree of cracking. Therefore, in the external environment, high temperature oxidation has a great influence on the erosion of the slag line, and at the same time, the huge temperature change 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 can easily cause oxidative decarburization of magnesia-carbon bricks. LF slag has low viscosity at high temperature, strong penetration ability to decarburized layer, and high solubility to magnesium oxide. The magnesium oxide particles dissociate at the periclase grain boundaries, as shown in Figure 2 (SA is slag; TA is the intersection of three pieces). Therefore, the service life of LF slag line magnesia-carbon brick is relatively low. The researchers systematically studied the damage mechanism of ladle magnesia-carbon bricks in the LF refining process, showing that the small aggregates of MgO grains are easily eroded by high-temperature slag, and the slag will continue to infiltrate along the periclase grain boundaries after erosion. Inside the MgO aggregates, cleavage of the periclase aggregates eventually occurs.

The slag infiltrates along the periclase grain boundary)
(2 The slag infiltrates along the periclase grain boundary)

 

Magnesia-carbon bricks have different damage and erosion mechanisms in different service temperature regions in the ladle and their different internal structures. In the high temperature area near the surface of the molten steel, the magnesia-carbon brick itself will react with MgO and carbon to form a decarburized layer. At high temperature, the wettability of the slag and magnesia-carbon bricks is good, and the tendency of MgO to dissolve into the slag is large. Compared with the low temperature zone near the air side, magnesia-carbon bricks are more seriously eroded by slag. In addition, the slag removal and repair work carried out by the ladle will inevitably cause man-made damage to the ladle slag line. Vibration and accidental injury can cause some damage to the ladle slag line. Although this kind of damage has little effect 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 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. 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 with 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 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. If the binder content is too small, the magnesia-carbon brick powder is not tightly combined, and it is easy to be washed and peeled off. If the binder content is too much, the thermal shock stability and refractoriness of magnesia-carbon bricks will be deteriorated, 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 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, 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 To 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 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 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, no fire clay bonding) and wet laying (using fire clay combined with refractory bricks). The advantage of dry laying is to minimize the damage caused by fire mud Influence, at high temperature, 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, resulting in bricks being squeezed and fractured; or due to magnesia-carbon bricks. The carbon brick expands, and the whole ring of slag line is lifted as a whole. 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.

 

magnesia carbon brick for Ladle1 (3)

 

The wet laying method is similar to the building method, except that the requirements are stricter. The advantage of this method is that the gaps that may be generated in the dry laying are well avoided. 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, so as to avoid 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.

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