Refractory Materials For Slag Line And Low Erosion Areas Of Ladle Furnace: Configuration Guide

Ladle linings can be divided into monolithic cladding and brick cladding. In the former case, mainly Al₂O₃ raw material (natural sintered bauxite) and synthetic Al₂O₃ (white corundum, sub-alumina corundum, and slab Al₂O₃, etc.) are used to produce so-called MgO-containing Al₂O₃-based low-cement (LCC) or ultra-low-cement (ULCC) refractory castables. These are also equipped with materials like Sp, MgO, etc. to enhance performance.

In recent years, alkaline refractory castables have also been investigated to further improve erosion resistance. However, they are still in the experimental stage, except for some special applications. Although the issues related to MgO hydration, bonding systems, and residual shrinkage have been largely controlled, problems such as slag penetration, slag sticking, and structural spalling still limit the application of alkaline refractory castables in ladles.

Brick ladle linings are primarily constructed using neutral bricks (such as high-alumina bricks, Al₂O₃-MgO-C bricks, etc.) and MgO-C bricks.

Due to differences in refining processes, operating conditions, and refractory design concepts, the actual application of ladle linings varies significantly from one steel mill to another and across regions. The design of ladle linings needs to consider a balance between refining conditions and economic efficiency. It must also account for the wear life of the slag line, ladle wall, bottom of the ladle, and the impact zone. To achieve optimal design, zoning of the ladle areas is essential.

In the desulfurization process (using saturated CaO slag and CaF₂ additives), the slag line requires lining with MgO-C bricks. For environmental protection reasons, chromium treatment is required for used MgO-Cr₂O₃ bricks, limiting their use worldwide to specific conditions, such as in VOD ladles. MgO-Cr₂O₃ bricks show the best results in such applications due to the wide fluctuation of alkalinity in the VOD ladle slag (ranging from 0.6 to 4.0 or higher).

Part 01: Refractories for Ladle Slag Line Parts

In the past, MgO-Cr₂O₃ bricks and high-alumina bricks were mainly used as refractory materials for lining the slag line and low-erosion areas in the refining ladle. As a result of environmental protection requirements, chromium-free solutions have been achieved, and MgO-C bricks or MgO-CaO-C bricks have replaced MgO-Cr₂O₃ bricks as refractory materials for the slag line of the ladle. In the case of steel vacuum refining (including the refining of low-carbon and ultra-low-carbon steel), the slag line of the ladle is typically lined with low-carbon MgO-C bricks.

The choice between MgO-C bricks and MgO-CaO-C bricks for refining ladle slag lines is mainly determined by the type of steel being refined, the characteristics of the slag, and the operating conditions.

Research has shown that in CaO-SiO₂ slag (low-alkalinity slag), MgO-CaO-C bricks offer higher erosion resistance. This is because when MgO-CaO particles come into contact with CaO-SiO₂ slag, the fCaO dissolved from the MgO-CaO particles reacts immediately with the slag components. This reaction generates high-melting-point compounds such as 2CaO-SiO₂ and 3CaO-SiO₂, which solidify and densely deposit on the surface of the bricks. This forms a strong protective layer, making the slag highly viscous and improving the erosion resistance of the bricks. The formation of this protective layer enhances the durability of the MgO-CaO-C brick lining.

In contrast, in CaO-Al₂O₃-based slag, the loss of MgO-CaO-C bricks is more severe and increases with the amount of CaO in the bricks. This is because the fCaO dissolved from the bricks reacts with the CaO-Al₂O₃ slag to generate low-melting-point substances like 12CaO-7Al₂O₃. This accelerates the dissolution of the bricks by causing them to dissolve into the slag in molten form. On the other hand, MgO-C bricks are more easily eroded by CaO-SiO₂-based slag but exhibit higher resistance to erosion when exposed to CaO-Al₂O₃-based slag.

Based on this, the selection of refractory materials for the refining ladle slag line should be made according to the specific refining conditions and slag characteristics.

The primary damage mechanism for MgO-C lining in the refining ladle slag line is the oxidation of carbon. In addition to the direct oxidation of carbon, this issue is mainly caused by the operating temperature (which exceeds 1650°C) and high-temperature decompression operations, particularly under high-temperature decompression conditions, where oxidation is more pronounced.

Part.02 Refractories for low erosion zone of ladle

When refractory castables are used in low-erosion zones, such as the sidewalls and bottom of the refining ladle, synthetic raw materials like fused corundum, slab Al₂O₃, and Al₂O₃-rich Sp (76A or 90A) are typically used as the main components of the castables. These are designed according to the LCC and ULCC schemes. This approach results in refractory materials with high refractoriness, excellent resistance to slagging, and good thermodynamic stability. Al₂O₃-rich Sp releases excess Al₂O₃ at low temperatures, which then reacts with CaO in the slag to form CaO·6Al₂O₃, accompanied by a volume increase, as shown below:

Moreover, MnO and FeO in the solid solution slag lead to an increase in viscosity, which improves slag resistance. The appropriate amount of Sp added is between 15% and 30% (by mass), with the corresponding MgO content ranging from 4% to 10% (by mass).

To further improve the slag penetration resistance of the refractory castables, a MgO-based refractory castable solution was adopted. This solution involves the in-situ generation of Sp through the reaction MgO + Al₂O₃ → Sp at high temperature, accompanied by a volume increase of about 15%. This volume increase helps compensate for the volumetric shrinkage during sintering and simultaneously enhances the structural densification of the material. The addition of a small amount of ufSiO₂ promotes the formation of Sp while controlling the expansion of the refractory castables to meet the required specifications. Additionally, CA helps control the residual linear expansion (PLC) of the Al₂O₃-MgO castables, as shown in Table 1.

The construction of high-alumina bricks in the low-erosion zone of the refining ladle is another solution. Modern refining ladles use natural beryl and bauxite to produce high-alumina bricks, which have higher purity and better thermo-mechanical stability (e.g., thermal strength) than bauxite. However, because high-alumina bricks contain more SiO₂, they have poor corrosion resistance, especially under alkaline slag conditions. The use of bauxite clinker as the main raw material for high-alumina brick masonry in refining ladle linings has the following disadvantages, which are being phased out:

①Volume contraction leads to severe penetration and erosion by steel and slag, especially at the brick joints, where a thick layer of slag forms.

②The inherent brittleness of the brick and its organizational structure causes the ladle lining to develop a thicker spalling zone and slag layer.

③The wettability of the lining to steel and slag accelerates erosion and penetration, leading to flake spalling.

To overcome the above shortcomings of high-alumina bricks in refining ladle linings, the development of Al₂O₃-MgO-C bricks and Al₂O₃-Sp-C bricks has been proposed as replacements. The advantages of these new materials compared to high-alumina bricks include:

(1)Good resistance to high temperatures and spalling of the structure.

(2)Good resistance to steel and slag erosion.

(3)Good residual expansion properties, ensuring that even at higher temperatures, the brick joints will not crack.

This is because, at temperatures above 1650°C, high-alumina bricks exhibit significant shrinkage, which can lead to the penetration of steel and slag into the brick joints. In contrast to high-alumina bricks, Al₂O₃-MgO-C bricks do not exhibit shrinkage within the temperature range encountered during continuous steel casting (1650°C to 1670°C).

The results of comparative slag resistance tests of high-alumina bricks, Al₂O₃-MgO-C bricks, and MgO-C bricks using the rotary impregnation method showed the following: high-alumina bricks experienced more severe erosion and penetration, while MgO-C bricks had the least erosion and penetration, with Al₂O₃-MgO-C bricks falling in between. Field trials have also shown that Al₂O₃-MgO-C bricks have a longer service life compared to high-alumina bricks, which is why they are widely used as refractory materials for lining the low-erosion areas of refining ladles.

The alkaline bricks used to line the low-erosion zone of the refining ladle are primarily MgO-CaO bricks and MgO-C bricks. To prevent hydration of fired dolomite bricks, they can be impregnated with organic substances to reduce their apparent porosity to less than 10%. These organic substances can be removed by heating before use in the ladle. MgO-rich dolomite bricks are used between the slag and steel lines to reduce wear in this area. One advantage of dolomite bricks in ladle use is their reactivity with steel; the dolomite brick lining serves as a reaction zone in the refining ladle, providing a large contact surface with the steel. However, the wear rate of dolomite brick ladle liners is generally higher than that of MgO-C bricks or Al₂O₃-MgO-C bricks.

When MgO-C bricks are used for refining ladle sidewall linings, their carbon content is generally less than 15%. Carbon is essential in magnesium refractories to improve their thermal spalling resistance and to counteract the high thermal expansion of MgO. The addition of carbon to MgO bricks improves their slag resistance, but it reduces their oxidation resistance. Typically, the oxidation susceptibility of MgO-C bricks is addressed by adding antioxidants.

A problem with adding metal antioxidants to MgO-C bricks is the increase in thermal conductivity, which can rise to 10 to 15 W/(m·K), leading to excessive heat loss from the steel. Therefore, it may be necessary to install a heat insulation layer in refining ladles that have a full sidewall MgO-C lining, including both slag and metal lines.

Additionally, as trial results show, MgO-C bricks have performed well in the steel impact zone. However, when using carbon-containing refractories across the entire ladle, MgO-C bricks with less than 10% carbon (by mass) are considered optimal in terms of temperature and inclusion control. However, it is important to note that low carbon content can lead to accelerated deterioration due to spalling.