The corrosion resistance of alternative materials to different types of slag, their thermal shock resistance and flexural strength at 1400°C were studied. The relevant properties and usage conditions of such materials for sliding nozzle slides were discussed.
Sliding nozzles are widely used in continuous casting to control the flow of steel. Since the slide plate of the sliding nozzle is subject to strong erosion, it is required to have high corrosion resistance and thermal shock resistance.
As the refractory material of skateboards, Al₂O₃-C material is usually used. However, when casting special grades of steel, Al₂O₃-C materials are strongly corroded. We believe that due to the influence of some special additives added to the molten steel during smelting, the slide gate plate material can be corroded. CaO, MnO and FeO are particularly likely to react with Al₂O₃, resulting in the formation of fusible compounds, which cause Al₂O₃-C materials to be quickly corroded.
Therefore, when casting special grades of steel containing the above components, the Al₂O₃-C sliding plate is severely corroded. Figure 1 (omitted) shows the condition of the Al₂O₃-C sliding plate after casting a special grade of steel. The area around the casting hole that regulates the flow of steel was severely eroded.
If the damage of Al₂O₃-C material is so serious, it is unreasonable to use it, and slide gate plates of other materials should be used (for example: MgO-spinel, ZrO₂ material, MgO-C material, etc.). But in terms of physical properties, they are different from each other. Therefore, attention must be paid to the chemical and physical properties of alternative materials.
We prepared 4 types of samples: MgO-spinel, ZrO₂, MgO-C and Al₂O₃-C samples for test comparison. Their properties are shown in Table 1. The above performance values are those before the material is impregnated with coal tar pitch.
Evaluation of corrosion resistance
A corrosion test was conducted in a rotary crucible at 1650°C for 3 hours, and the corrosion resistance was evaluated. A schematic diagram of the test setup is shown in Figure 2. Four specimens (80 mm × 60 mm × 30 mm) were arranged so as to form a rectangular container, and slag was poured into the container. Use gas oxygen burner for heating.
Table 1 Performance of specimens
|Chemical composition / %
Figure 2 Schematic diagram of slag resistance test device
The composition of the slag is listed in Table 2. The slag is pre-synthesized and fed into the crucible at a rate of 200g per hour. In order to study the effect of slag composition on corrosion resistance, three different compositions were used: slag A with high CaO content; slag B with high MnO-SiO₂ content; slag C with high FeO content. The oxides chosen for the study are all aggressive components of slide gate plate materials.
Table 2 Chemical composition of slag
|Chemical composition / %
Thermal shock resistance evaluation
Thermal shock resistance is evaluated by heating with an acetylene lamp to approximately 1800°C for firing. A schematic diagram of the thermal shock resistance test is shown in Figure 3 . The upper surface of the specimen was heated and the number and width of cracks were evaluated in cross-section of the specimen. The flexural strength was measured at 1400°C.
Figure 3 Schematic diagram of thermal shock resistance test
The high-temperature flexural strength was measured using the three-point bending method at 1400°C in an inert gas (N₂) atmosphere using a sample with dimensions of 150mm×25mm×25mm. The distance between the fulcrums was 125mm.
Results and discussion
Evaluation of corrosion resistance
Results of slag A (CaO-Al₂O₃-FeO system)
The corrosiveness test results of slag A with high CaO content are shown in Figure 4 . The test results show that the slag resistance of MgO-spinel, ZrO₂ and MgO-C materials to the slag is greater than that of Al₂O₃-C materials. It is speculated that the reason is that no low melting point phase is produced when MgO or ZrO₂ reacts with CaO. Due to the reaction with CaO, Al₂O₃-C materials are severely damaged. In order to improve corrosion resistance, MgO-spinel, ZrO₂ and MgO materials should be used.
Results of slag B (MnO-SiO₂-FeO series)
Figure 5 shows the results of the slag resistance test for slag B with high MnO and SiO₂ contents. Obviously, the slag resistance of Al₂O₃-C and MgO-spinel materials is lower than that of ZrO₂ and MgO-C materials.
Although the MgO content in MgO-C materials is similar to that in MgO-spinel materials, the corrosion resistance of MgO-C materials is significantly higher than that of MgO-spinel materials. The reasons are as follows: The carbon in the MgO-C material can prevent the slag from intruding into the refractory matrix. Therefore, the reaction between MgO-C materials and slag is limited to the surface.
Figure 4 Result of slag resistance test of material against slag A
Figure 5 Result of slag resistance test of material against slag B
MgO- spinel material does not contain carbon, but contains 10% Al₂O₃. Al₂O₃ easily reacts with the MgO-SiO₂ component. Therefore, the slag easily invades the matrix of the MgO-spinel slide. The ceramic bond of the sliding plate was damaged due to the intrusion of slag, and undamaged particles were separated from the refractory structure. Therefore, the intrusion of slag causes severe damage to the product structure.
Impregnation with coal tar pitch is effective in improving the performance of MgO-spinel materials. This process can limit the intrusion of slag.
The corrosion resistance of ZrO₂-based products is higher than that of Al₂O₃-C materials. However, MgO-C materials have the best corrosion resistance against slag with high MnO-SiO₂ content.
Results of slag C (FeO-MnO-SiO₂ system)
The test results of slag C with high FeO content are shown in Fig. 6 . MgO-C materials have higher corrosion resistance than other materials.
ZrO₂ material has good corrosion resistance to slags A and B, but it has poor corrosion resistance to slag C. It seems that the melting point of the phase produced when ZrO₂ material reacts with slag with high FeO content drops from 2000°C to 1400°C.
Thermal shock resistance
A cross-sectional view of the specimen after the thermal shock resistance test is shown in Figure 7 . A few cracks were found in the Al₂O₃-C material, and other samples also had cracks. However, the Al₂O₃-C sample suffered less damage.
Figure 6 Result of slag resistance test of material against slag C
Figure 7 Photo of the sample cross-section after the thermal shock resistance test
MgO-C materials have wider cracks. Figure 8 shows the thermal expansion of the sample. It seems that the thermal expansion rate of MgO-C material is large due to its low thermal shock resistance.
Figure 8 Thermal expansion rate of the sample
The cracks of the ZrO₂ sample are narrower than those of the MgO-C material. Therefore, we speculate that the probability of damage is smaller than that of MgO-C materials.
MgO-spinel materials have fewer cracks and good thermal shock resistance. Since the MgO-spinel material is composed of periclase sand and spinel (MgO·Al₂O₃), the thermal expansion rates between periclase sand and spinel are different, causing microscopic cracks to form during the production stage. These microscopic cracks cause a decrease in the elastic modulus and prevent cracking during application. For this reason, MgO-spinel materials have good thermal shock resistance.
Flexural strength at 1400℃
The test results of the flexural strength at 1400°C are shown in Figure 9 . This parameter of the MgO-spinel material is smaller than that of the other three samples. It appears that the microscopic cracks described above lead to a reduction in the high-temperature flexural strength of these specimens. The values of this parameter for other samples are quite high and the differences are not significant.
Figure 9 Flexural strength at 1400℃
Discussion about testing
The test results of slag resistance are shown in Figure 10 .
Figure 10 Result of slag resistance test
MgO-spinel material has good corrosion resistance to slag containing CaO, but has poor corrosion resistance to slag with high MnO-SiO₂ content and high FeO content. ZrO₂-based materials are severely damaged by slag with high FeO content.
Therefore, MgO-spinel and ZrO₂ materials are not omnipotent in use, especially when casting special grades of steel. Therefore, when selecting materials for slide gate plates it is necessary to find out which components have the strongest corrosive effect.
MgO-C materials have good corrosion resistance to all slags (including slags with high CaO content, high MnO-SiO₂ content and high FeO content).
Since the thermal shock resistance of MgO-C samples is not high, they may be damaged when subjected to thermal shock. Therefore, when selecting materials for sliding gate slides, attention should be paid to the balance between slag resistance and thermal shock resistance.
In the case where the thermal shock resistance of MgO-spinel materials or ZrO₂ materials is higher than that of MgO-C materials, such materials can be used to improve corrosion resistance. Although their corrosion resistance is not high enough, these materials can still be used when thermal conditions are severe.
When solving the problem of which material to use to make a slide gate plate, service conditions, corrosion resistance and thermal shock resistance (that is, aggressive components, casting hole diameter, ladle turnover rate, etc.) play an important role.
In order to improve materials for slide gate plates, MgO-C materials need to improve their thermal shock resistance, and MgO-spinel and ZrO₂ materials need to improve their corrosion resistance.
In addition, since the flexural strength at 1400°C affects erosion resistance and wear resistance, MgO-spinel materials need to improve this index.
We evaluated the corrosion resistance, thermal shock resistance and 1400°C flexural strength of certain types of materials used in slide gate plates:
●MgO-spinel material has good thermal shock resistance, but it only has good corrosion resistance against slag with high CaO content.
●The low flexural strength at 1400°C is due to the formation of microscopic cracks.
●ZrO₂ material has good corrosion resistance to slag with high CaO content and high MnO-SiO₂ content, but its thermal shock resistance is lower than MgO-spinel material. The high-temperature flexural strength of ZrO₂-based materials is higher than that of MgO-spinel materials, and its resistance to mechanical effects is also higher than that of MgO-spinel materials.
MgO-C materials have good corrosion resistance to certain aggressive components, but their thermal shock resistance is low. Therefore, MgO-C materials are only used where the amount of thermal shock is minimal.
When selecting suitable materials for specific usage conditions, it is extremely important to evaluate the many properties of slide gate plate materials.
Therefore, in order to improve the performance of slide gate plate materials, MgO-spinel materials need to improve their corrosion resistance and high-temperature flexural strength. ZrO₂ quality materials should improve their corrosion resistance and thermal shock resistance. MgO-C materials should improve their thermal shock resistance. To solve the above problems, extending the service life of the sliding gate slide is very important.