Key factors affecting the service life of refractory bricks in steel ladle slag lines

Key factors affecting the service life of refractory bricks in steel ladle slag lines

The slag line in a ladle is the part where molten steel comes into direct contact with air. Currently, magnesia-carbon bricks are mostly used for slag line construction. Due to the temperature difference and oxygen-rich environment, the erosion rate in this area is significantly faster than in other parts. Furthermore, the tipping and slag removal operations during molten steel operation cause considerable damage to the slag line. Therefore, the slag line is one of the parts with the highest maintenance frequency.

The lifespan of the ladle slag line is mainly affected and constrained by three factors: external environment, refractory material quality, and construction method.

External Environment

A ladle is a device used to receive molten steel for pouring operations. The temperature of the molten steel is often around 1500℃. At this temperature, the slag line of the ladle will undergo a strong oxidation reaction upon contact with air. Furthermore, the temperature difference between the molten steel and the air contact surface has a significant impact on the slag line. A large temperature difference severely tests the thermal stability of the slag line. During frequent receiving and pouring operations, the refractory material will experience a certain degree of cracking. Therefore, in the external environment, high-temperature oxidation has a significant impact on the erosion of the slag line. At the same time, drastic temperature changes place high demands on the thermal stability of the refractory material. Under the interaction of melting and cracking of the refractory material, the slag line of the ladle is easily damaged, leading to steel seepage.

LF refining slag easily causes oxidation and decarburization of magnesia-carbon bricks. LF slag has relatively low viscosity at high temperatures, resulting in strong penetration into the decarburized layer and high solubility for magnesium oxide. Simultaneously, the slag easily penetrates into the periclase grain boundaries, dissociating magnesia particles. Therefore, the service life of magnesia-carbon bricks in LF slag lines is relatively short. Shen et al. systematically studied the damage mechanism of ladle magnesia-carbon bricks during the LF refining process, showing that smaller MgO grains are easily eroded by high-temperature slag. After erosion, the slag continues to penetrate into the MgO aggregate along the periclase grain boundaries, ultimately causing cleavage of the periclase aggregate.

The different service temperatures and internal microstructures of magnesia-carbon bricks within the ladle lead to varying damage and erosion mechanisms. In the high-temperature region near the molten steel surface, magnesia-carbon bricks undergo a reaction between MgO and carbon, forming a decarburized layer. At high temperatures, the wettability between the molten slag and the magnesia-carbon bricks is better, and MgO tends to dissolve into the slag. Compared to the low-temperature region near the air side, magnesia-carbon bricks are more severely eroded by the molten slag. Furthermore, slag removal and repair operations on the ladle inevitably cause human-induced damage to the slag line. Slag removal and unloading machines, while cleaning cold steel and residue from the slag line, will vibrate and accidentally damage it, thus causing some degree of harm. Although this damage has a negligible impact on the overall quality of the slag line, it still increases the frequency of maintenance.

Refractory Material Quality

Currently, magnesia-carbon bricks are mainly used for slag lines in steel ladles. Whether traditional magnesia-carbon bricks or the widely used low-carbon magnesia-carbon bricks, they primarily utilize flake graphite as their carbon source. The flake graphite is generally selected from types such as -197 and -196, meaning a particle size greater than 100 mesh and a purity higher than 97% or 96% (mass fraction). The binder is thermosetting phenolic resin. During the carbonization reaction, the network structure formed by the cross-linking reaction of its own chain segments creates a mechanical interlocking force between the magnesia particles and graphite. Graphite, as the main raw material for producing magnesia-carbon bricks, benefits primarily from its excellent physical properties: ① non-wetting property to slag, ② high thermal conductivity, and ③ low thermal expansion. Furthermore, graphite does not eutectic with refractory materials, and graphite has high refractoriness. It is precisely because of these characteristics that magnesia-carbon bricks are selected for slag lines operating in harsh environments. For low-carbon magnesia-carbon bricks (carbon mass fraction ≤ 8%) or ultra-low-carbon magnesia-carbon bricks (carbon mass fraction ≤ 3%), the low carbon content makes it difficult to form a continuous network structure, thus the microstructure design of low-carbon magnesia-carbon bricks is more complex. Conversely, the microstructure design of high-carbon magnesia-carbon bricks (carbon mass fraction > 10%) is relatively simple.

The performance of magnesia-carbon bricks is affected by moisture and the choice of formulation. When magnesia-carbon bricks are damp, their structure becomes loose, and at high temperatures, moisture escapes, creating porous channels. This negatively impacts the thermal stability and corrosion resistance of the magnesia-carbon bricks, and significantly weakens their ability to withstand the erosion of molten steel. MgO-C is very sensitive to thermomechanical abrasion because the coefficient of thermal expansion of MgO has high reversibility. The binder in magnesia-carbon bricks is also a crucial factor affecting their quality. Too much or too little binder will negatively impact their performance. Insufficient binder results in loosely bonded magnesia-carbon brick powder, making them prone to erosion and peeling. Excessive binder leads to poor thermal shock stability and refractoriness, and also introduces excessive harmful elements into the molten steel.

When a ladle receives molten steel from a converter, it is accompanied by a large amount of steel slag. The low-melting-point 2CaO·SiO2 in the slag dissolves at the MgO grain boundaries and reacts chemically with trace impurities in the MgO layer, playing a major role in the erosion of magnesia refractories. From the perspective of converter slag, research on improving the performance of magnesia-carbon bricks mainly focuses on magnesia sand, antioxidants, and microstructure.

Furthermore, the addition of antioxidants to magnesia-carbon bricks also affects their quality. To improve the oxidation resistance of magnesia-carbon bricks, small amounts 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 is mainly twofold: Firstly, from a thermodynamic point of view, at the operating temperature, the additives or additives react with carbon to form other substances. These substances have a greater affinity for oxygen than carbon has for oxygen, and are preferentially oxidized before carbon, thus protecting the carbon. Secondly, from a kinetic point of view, the compounds formed by the reaction of additives with O2, CO, or carbon alter the microstructure of carbon composite refractory materials, such as increasing density, blocking pores, and hindering the diffusion of oxygen and reaction products. Currently, Al powder is mainly used in magnesia-carbon bricks to prevent carbon oxidation. Although Al has strong antioxidant properties, at high temperatures, it reacts with C and N2 to form Al carbon and nitrogen compounds. These Al carbides are prone to hydration during the transition from high to low temperatures, leading to voids within the magnesia-carbon brick, resulting in a loose structure and cracks. In light of this, some domestic refractory manufacturers have begun using Al₄SiC₄ powder, made from powder, silicon powder, and carbon powder, to prepare Al₄SiC₄ powder in a vacuum sintering furnace. This powder is then applied as an antioxidant to magnesia-carbon bricks. Studies on its impact on the antioxidant properties of magnesia-carbon bricks have revealed that Al₄SiC₄ not only possesses strong antioxidant properties but also avoids the hydration cracking problems associated with traditional antioxidants.

Construction Methods

Masonry methods for magnesia-carbon brickwork in steel ladle slag lines are generally divided into two types: dry laying (bricks are directly stacked without fire mortar bonding) and wet laying (fire mortar is used to bond refractory bricks). The advantage of dry laying is that it minimizes the impact of fire mortar. However, at high temperatures, due to the different materials of magnesia-carbon bricks and fire mortar, their thermal expansion rates differ, easily leading to gaps at the contact surface. The disadvantages of this method are that 100% tight contact cannot be guaranteed between the magnesia-carbon bricks. Furthermore, when the magnesia-carbon bricks expand due to heat, there is no buffer between the bricks, causing them to break under pressure; or, due to the expansion of the magnesia-carbon bricks, the entire slag line ring rises, and the enormous compressive force deforms the rim plate, leaving the refractory material unprotected and causing it to be eroded and peeled off, posing a significant threat to the quality of the slag line.

Wet-laying is similar to traditional bricklaying, but with stricter requirements. Its advantages include effectively avoiding gaps that can occur in dry-laying. Furthermore, the refractory clay, being weaker at high temperatures, allows it to flow when the magnesia-carbon bricks expand, adapting to changes in the gaps between bricks and dispersing the compressive stress, thus effectively preventing gaps. The disadvantages are that the use of refractory clay makes the slag line structure unstable and increases the difficulty of laying the bricks. If the refractory clay is not evenly distributed, gaps will still appear between the bricks.