Critical Factors For Extending Ladle Slag Line Refractory Brick Lifespan

The ladle slag line is the area where molten steel directly comes into contact with air. At present, most ladle slag lines are lined with magnesia-carbon bricks. Due to temperature fluctuations and an oxygen-rich environment, the corrosion rate in this area is significantly higher than in other parts. In addition, the tilting and slag-discharging operations during the transportation of molten steel cause substantial damage to the slag line. Therefore, the ladle slag line is one of the areas with the highest maintenance frequency. The service life of the ladle slag line is mainly influenced and restricted by three factors: the external environment, the quality of the refractory materials, and the lining construction method.

01 External Environment

A ladle is a piece of equipment used to hold molten steel and carry out pouring operations. The temperature of molten steel is typically around 1500 °C. Under such high temperatures, the ladle slag line, when exposed to air, undergoes intense oxidation reactions. Moreover, the temperature difference at the interface between molten steel and air exerts a severe impact on the ladle slag line. Large temperature fluctuations impose a rigorous test on the thermal stability of the slag line. During frequent charging and pouring operations, the refractory material may crack to a certain extent.

Therefore, in the external environment, high-temperature oxidation has a significant erosive effect on the slag line. Meanwhile, drastic temperature changes place high demands on the thermal stability of the refractory material. Under the combined effects of melting loss and cracking, the ladle slag line is easily damaged, which may further lead to steel penetration.

LF refining slag tends to cause oxidation and decarburization of magnesia-carbon bricks. LF slag has relatively low viscosity at high temperatures, exhibits strong penetration into the decarburized layer, and has high solubility for magnesium oxide. At the same time, molten slag easily penetrates the grain boundaries of periclase and decomposes magnesia grains. Consequently, the service life of magnesia-carbon bricks used in the LF slag line is relatively short.

Shen Ping et al. systematically investigated the degradation mechanism of ladle magnesia-carbon bricks during LF refining. Their research showed that MgO aggregates with small grain sizes are prone to erosion by high-temperature molten slag. After erosion, the molten slag continues to penetrate into the interior of the MgO aggregates along the periclase grain boundaries, eventually resulting in the cleavage of the periclase aggregates.

The degradation and erosion mechanisms of magnesia-carbon bricks vary due to the different service temperature zones inside the ladle and their internal microstructures. In the high-temperature zone near the steel surface, reactions between MgO and carbon occur within the magnesia-carbon bricks, forming a decarburized layer. At high temperatures, molten slag has better wettability with magnesia-carbon bricks, and MgO tends to dissolve into the molten slag more easily. Compared with the low-temperature zone near the air side, magnesia-carbon bricks suffer more severe erosion from molten slag.

In addition, slag skimming and ladle maintenance operations inevitably cause mechanical damage to the ladle slag line. While removing skull and residual slag from the slag line, slag skimming machines and ladle dismantling machines generate vibrations and accidental impacts on the slag line, resulting in certain damage to the ladle slag line. Although such damage has a negligible effect on the overall quality of the slag line, it still increases the maintenance frequency of the ladle slag line.

02 Refractory Quality

At present, ladle slag lines are mainly lined with magnesia-carbon bricks. Both conventional magnesia-carbon bricks and the widely used low-carbon magnesia-carbon bricks primarily use flake graphite as the carbon source. Flake graphite grades such as -197 and -196 are generally selected, with a particle size larger than 100 mesh and a purity higher than 97% or 96% (mass fraction). The binder is thermosetting phenolic resin. During the carbonization reaction, the cross-linked network structure formed by the polymer segments provides mechanical interlocking between magnesia grains and graphite.

As the main raw material for magnesia-carbon bricks, graphite benefits from its excellent physical properties:

① non-wettability by molten slag;

② high thermal conductivity;

③ low thermal expansion.

In addition, graphite does not form eutectic phases with refractories and has high refractoriness. Due to these characteristics, magnesia-carbon bricks are suitable for use in the harsh service environment of the slag line.

For low-carbon magnesia-carbon bricks (carbon mass fraction ≤ 8%) or ultra-low-carbon magnesia-carbon bricks (carbon mass fraction ≤ 3%), it is difficult to form a continuous carbon network due to the low carbon content, resulting in a relatively complex microstructure design. In contrast, high-carbon magnesia-carbon bricks (carbon mass fraction > 10%) have a simpler structural design.

The properties of magnesia-carbon bricks can be affected by moisture absorption and formulation design. After moisture absorption, the brick structure becomes loose. Water evaporation at high temperatures creates porous channels, which negatively affect the thermal stability and corrosion resistance of the bricks and greatly weaken their resistance to molten steel erosion. MgO-C bricks are sensitive to thermomechanical abrasion because MgO has a highly reversible thermal expansion coefficient.

The binder is also a critical factor affecting the quality of magnesia-carbon bricks. Insufficient binder content leads to poor bonding of the matrix, making the bricks prone to erosion and spalling. Excessive binder content degrades thermal shock resistance and refractoriness and introduces excessive harmful elements into molten steel.

When a ladle receives molten steel from a converter, a large amount of slag is introduced simultaneously. The low-melting-point 2CaO·SiO₂ in the slag dissolves along MgO grain boundaries and reacts with trace impurities in the MgO layer, which is the main cause of corrosion of magnesia refractories. From the perspective of converter slag, studies on improving the performance of magnesia-carbon bricks mainly focus on magnesia, antioxidants, and microstructure.

In addition, the addition of antioxidants affects the quality of magnesia-carbon bricks. To improve oxidation resistance, small amounts of additives are commonly introduced, such as Si, Al, Mg, Al-Si, Al-Mg, Al-Mg-Ca, Si-Mg-Ca, SiC, B₄C, BN, Al-B-C, and Al-SiC-C systems.

The functions of additives are twofold.

Thermodynamically, at service temperatures, additives or their reaction products with carbon have a higher affinity for oxygen than carbon does and are oxidized preferentially to protect carbon.

Kinetically, compounds formed by reactions between additives, O₂, CO, or carbon modify the microstructure of carbon-bonded refractories, for example by increasing density, blocking pores, and hindering the diffusion of oxygen and reaction products.

Currently, Al powder is widely used as an antioxidant in magnesia-carbon bricks. Although Al provides strong oxidation resistance, it reacts with C and N₂ at high temperatures to form aluminum carbides and nitrides. Aluminum carbide easily hydrates during cooling, causing voids, a loose structure, and cracking inside the bricks.

In view of this, some domestic refractory manufacturers have prepared Al₄SiC₄ powder in a vacuum sintering furnace using aluminum powder, silicon powder, and carbon powder, and applied it as an antioxidant in magnesia-carbon bricks. Studies have shown that Al₄SiC₄ not only exhibits excellent oxidation resistance but also avoids the hydration and cracking problems associated with conventional antioxidants.

03 Laying Method

Two laying methods are commonly used for magnesia-carbon bricks in the ladle slag line: dry laying (direct stacking without mortar bonding) and wet laying (bonding refractory bricks with mortar).

The advantage of dry laying is that it minimizes the influence of mortar. At high temperatures, gaps tend to form at the contact interface due to the different materials and different thermal expansion coefficients of magnesia-carbon bricks and mortar. The disadvantage of this method is that complete tight contact between magnesia-carbon bricks cannot be guaranteed. Meanwhile, when the bricks expand under heat, there is no buffer space between them, which can cause the bricks to squeeze against each other and fracture. Alternatively, the expansion of the magnesia-carbon bricks may lift the entire circumferential slag line, and the huge squeezing force may deform the ladle flange plate. As a result, the refractory loses protection and is scoured and spalled, which poses a serious threat to the quality of the slag line.

Wet laying is similar to construction masonry but with stricter requirements. The advantage of this method is that it effectively avoids gaps that may occur in dry laying. Meanwhile, mortar has relatively low strength at high temperatures. When magnesia-carbon bricks expand under heat, the mortar can flow to adapt to changes in the gaps between bricks, dispersing the squeezing force and thus effectively preventing gap formation.

The disadvantage of this method is that the use of mortar makes the slag line structure less stable and increases the difficulty of laying. If the mortar is uneven, empty gaps will still form between the bricks.