Reasons For The Erosion Of Magnesium-Carbon Bricks By Vanadium-Containing Steel Slag

1. Test

Raw Materials and Specimens:

The specimens were made from large crystalline electrofused magnesium sand and large-scale graphite (with the main chemical composition shown in Table 1) as the primary raw materials. Thermosetting phenolic resin was used as the binding agent, and 4% Si powder and Al powder were added. Under a pressure of 180 MPa, the mixture was pressed into specimens with dimensions of 25 mm × 25 mm × 125 mm. These were heat-treated at 240°C for 9 hours before being prepared for use. The bulk density of the resulting specimen was 2.99 g/cm³, and the apparent porosity was 4.7%.

The particle size of the test slag was <0.5 mm, and the main chemical composition is shown in Table 2. Among these, slag A and B were vanadium- and titanium-containing slags from the Pangangang site, slag D was ordinary steel slag from other steel mills, and slag C was artificial slag synthesized from slag D and chemically pure V₂O₅. The numbering of the post-erosion specimens was consistent with that of the test slags used.

Test Method:

The test was conducted using a medium-frequency induction furnace, and the schematic diagram of the device is shown in Fig. 1. Eight kilograms of A3 steel and 200 grams of test slag were placed in a graphite crucible, heated electrically. Once the steel and slag had completely melted, the specimen was immersed in the crucible to a set height. After 10 minutes, approximately 50 grams of test slag were added. The erosion time was 20 minutes, and the temperature of the molten pool was maintained at (1650 ± 30)°C. The shape of the specimen was observed after erosion. After removing the hanging slag, the specimen was cut about 20 mm from its end, and its cross-sectional area was measured. The melt loss index was calculated using the following formula:

(1−cross-sectional area of the residual brick/cross-sectional area of the original brick)×100%,

which characterizes the erosion of the specimen. The microstructure of the residual bricks was analyzed using scanning electron microscopy.

02 Results.

Figure 2 illustrates the shape of the specimen after erosion by different slags. The end of the specimen after erosion is conical in shape.

Table 3 shows the melt loss index and the thickness of the penetration layer of the specimen after erosion by different slags. It can be seen that the vanadium-containing slag has a significantly higher erosion ability on the specimen than ordinary slag, and the melt loss index of the specimen generally increases with the increase in V₂O₅ content. The penetration depth of the different slags in the specimen, in order from largest to smallest, is as follows: A > C > B > D. In other words, the thickness of the penetration layer increases with the increase in V₂O₅ content in the slag.

From Table 2, we know that the alkalinity of A slag (CaO/SiO₂) is 2.2 times greater than that of D slag, and its Fe₂O₃ content is also lower than that of D slag. Therefore, according to common understanding, the aggressiveness of A slag on magnesium-carbon bricks should be smaller than that of D slag. However, as seen in Table 3, the actual erosive ability of A slag on MgO-C bricks is approximately 3.7 times greater than that of D slag. This suggests that the high erosiveness of A slag is likely due to its higher V₂O₅ content.

When comparing A slag and B slag, it is found that both have similar Fe₂O₃ contents, but the alkalinity of A slag is higher than that of B slag. Despite this, the erosive ability of A slag on MgO-C bricks is greater than that of B slag. The alkalinity and Fe₂O₃ content of C and D slags are similar. After adding 2% V₂O₅, the melt loss index of C slag is significantly higher than that of D slag.

All of these findings indicate that the V₂O₅ content in the slag has a greater impact on the erosion of MgO-C bricks. Specifically, the higher the V₂O₅ content in the slag, the greater the erosion of MgO-C bricks.

The microstructure of the reaction layer of specimen B is shown in Fig. 3. As can be seen from the figure, the residual MgO particles in the reaction layer are surrounded by the liquid phase, and the prisms have disappeared, indicating that the slag's erosion of MgO particles occurs via surface dissolution. The bright white phase in the figure represents Mn-Fe alloy particles formed after the reduction of FeₓO and MnO. EDS analysis shows that the main components in the slag are CaO, MgO, Al₂O₃, and SiO₂, primarily in the form of C₂AS and C₃MS₂, with a small amount of C₃S and C₃A detected in the matrix. In addition, TiCVC and Ti-V-Fe alloys were found at the edges of the metal particles, as well as V-Fe alloy.

Figure 4 shows the surface scanning results of the penetration layer of specimen B. It can be observed that V₂O₅ and TiO₂ have penetrated into the brick with the slag, primarily existing at the edges of the magnesia particles and within the matrix. When observed under high magnification, the morphology of the magnesium sand particles is similar to that of the reaction layer, and the graphite has completely disappeared in areas where a large amount of slag has infiltrated, leaving only brecciated cavities (>500 μm) and a small number of round pores (10-20 μm). In areas with less slag, graphite residues remain. Additionally, a large number of high-brightness metal particles (5-15 μm) are interspersed within the matrix. Energy spectrum analysis showed that the main components of the slag were CaO, MgO, Al₂O₃, and SiO₂, which were present in the forms of C₂AS and C₂MS₂. The metal phase consisted of a Mn-Fe alloy, with TiCVC present at the edges. The composition of the microzone was 46.84% C, 9.54% Ti, 39.25% V, and 4.37% Fe.

In summary, due to the fact that vanadium-containing slag has a lower melting point and viscosity than ordinary steel slag, the V₂O₅ in the slag significantly reduces the wetting angle between the slag and the refractory material while lowering the melting temperature. As a result, it not only penetrates more easily into the matrix of magnesium-carbon bricks, leading to the oxidation of carbon and the dissolution of the MgO particle surface, but also infiltrates the grain boundaries of the MgO particles. This reaction with the grain boundary bonding phases generates low-melting-point phases, causing the disintegration and spalling of magnesium sand particles. Additionally, the VC and TiC formed during the decarburization reaction can inhibit the further penetration of the slag to some extent.

03. Conclusion

Compared with ordinary steel slag, vanadium- and titanium-containing steel slag exhibits higher erosivity toward magnesium-carbon bricks, and this erosive capacity increases with the V₂O₅ content in the slag. On one hand, the V₂O₅ and TiO₂ in the slag penetrate into the magnesium-carbon brick matrix through the pores, reacting with carbon to form VC and TiC, which reduces the carbon content in the matrix and disrupts the matrix structure. On the other hand, the slag also penetrates the grain boundaries of magnesium sand, reacts with the grain boundary bonding phases, and disintegrates the magnesium sand particles, thereby accelerating the destruction of MgO-C bricks.