Reaction Mechanism of CA6, Al2O3 and CA6-Al2O3 Refractories with Refining Slag

In this study, to clarify the corrosion mechanism of CA6 based refractory by refining slag, the static crucible tests for CA6, CA6-Al2O3, and Al2O3 refractory, were carried out and the detail reaction processes were analyzed from the perspective of thermodynamic simulation and structural evolution. From the results, CaAl4O7 plays a vital role in the slag corrosion resistance of the three refractories. Regarding CA6 refractory, the double pyramid module in CA6 crystal structure was destroyed very quickly, leading to the rapid collapse of its structure to form the denser CaAl4O7 in high amounts. As a result, a reaction layer mainly composed of CaAl4O7 formed, which effectively inhibited the slag corrosion, so CA6 refractory exhibits the most excellent slag corrosion. Meanwhile, the formation of CaAl4O7 can also avoid CA6 particles entering the molten steel to introduce exogenous inclusions. For Al2O3 refractory, the generation of CaAl4O7 is much slower than that of CA6 and CA6-Al2O3 refractory, and the amount generated is also quite small, resulting in its worst slag corrosion among the three crucibles. Therefore, CA6 based refractory has excellent application potential in ladle refining and clean steel smelting.


Introduction
During the ladle refining process, the refractory is corroded severely due to the continuous reaction of the refining slag [1][2][3]. The slag corrosion will destroy the structure of the refractory and reduce the service life [4][5][6][7]. More importantly, under the continuous scouring effect of molten steel, the dislodged refractory and the corrosion products would enter the molten steel to form exogenous large size inclusions [8][9][10][11][12][13][14][15][16][17], which will have a fatally harmful effect on the purity of the molten steel and the quality of the final products [17][18][19][20]. Therefore, in order to eliminate the introduction of exogenous inclusion and to increase the service life of the refractories, it is absolutely essential to study corrosion behavior between the refractory and refining slag.
At present, the primary materials for the refining ladle are Al 2 O 3 system refractories, due to their high density and stable high temperature properties. Many researches have been conducted to investigate the corrosion process between Al 2 O 3 refractories and the refining slag [21][22][23], and many achievements have been obtained. It was found that the slag corrosion degree of Al 2 O 3 refractories is influenced by factors such as temperature and refining slag composition [24][25][26][27]. Some scholars pointed out that [28,29] a small amount of CaAl 12 O 19 generated on the surface of Al 2 O 3 refractories can prevent further corrosion effectively, which is a critical mechanism. After years of development, an understanding of the corrosion resistance of Al 2 O 3 refractories has been profound, but there are few reports on the introduction and control of inclusions. So far, as Al 2 O 3 refractories, the problem of introducing inclusions into molten steel is still unavoidable. With the development of cleanliness steel smelting, the effect of refractories on inclusions has attracted more and more attention. In order to prepare refractory materials with excellent corrosion resistance and less introduction of inclusions, in our previous work [30][31][32][33][34] we have synthesized the pure dense CA 6 (calcium hexaaluminate, CaAl 12 O 19 ) base raw material and studied the slag corrosion resistance. After that, some researchers focused on optimizing CA 6 structure and tried to dope N 3− or Zr 4+ into CA 6 [35,36], trying to improve performance further and achieve expected results. Besides that, from our recent experiments, the CA 6 -based refractories not only have excellent slag corrosion resistance but can also reduce the size of inclusions and absorb sulfur in steel. Researches on the preparation and structural improvement of CA 6 refractory are consummate. However, its slag corrosion mechanism is not clear, and the effect on the exogenous inclusion is not confirmed, which cannot provide strong support for promotion and application. Therefore, it is essential and meaningful to study the slag corrosion of CA 6 -based refractory, which has a great significance on refining ladle and even on the whole steelmaking process.
In this study, the reaction mechanism between two CA 6 -based refractory (pure CA 6 and CA 6 -Al 2 O 3 composite) and refining slag was investigated and analyzed, and the Al 2 O 3 refractory commonly used in the refining ladle was also used as a comparison. The microstructure and distribution of elements in the corrosion area were characterized, and the corrosion process was deduced and simulated by the thermodynamic software. The results of this work proved that CA 6 refractory has a great application prospect in the metallurgical industry due to excellent and distinctive slag corrosion resistance.

Experimental
The static crucible method was used to investigate the slag corrosion behavior of the three refractories. Put 70 g refining slag into the crucibles and then heat the crucibles to 1600 • C at a heating rate of 5 • C/min in the box furnace (KSL-1700X-M). After holding for 3h, the furnace stopped working and the crucible was taken out when it cooled to room temperature. The crucibles were cut along the center, and the corrosion area was made into a mosaic sample and then polished.

Characterization
The composition of the refining slag was analyzed by X-ray fluorescence spectrometry (XRF, Shimadzu, Japan). The micro-morphology of the slag-refractory interface was observed by scanning electron microscopy (SEM, FEI Nova nano 450, USA). The elemental distribution at the slag-resistant material interface was analyzed by SEM equipped with Energy Dispersive Spectrometer (EDS, EDAX Team, USA).

Thermodynamic Simulation
During the corrosion test, only the final result can be determined clearly and the intermediate process cannot be observed directly. Therefore, thermodynamic simulation for the slag corrosion process is necessary.
In this work, Factsage7.0 was used to simulate the corrosion process of three crucibles. The calculation mechanism is shown in Figure 1. The left side is refining slag and the right side is refractory, and the concentration of the refining slag and refractory at the interface area changes in the reverse cross between 0~1. During the calculations, the interface area is considered as a composition of multiple cross sections and the corrosion process of the refractory is simulated by predicting the generation of each section.

Experimental
The static crucible method was used to investigate the slag corrosion behavior of the three refractories. Put 70 g refining slag into the crucibles and then heat the crucibles to 1600 °C at a heating rate of 5 °C /min in the box furnace (KSL-1700X-M). After holding for 3h, the furnace stopped working and the crucible was taken out when it cooled to room temperature. The crucibles were cut along the center, and the corrosion area was made into a mosaic sample and then polished.

Characterization
The composition of the refining slag was analyzed by X-ray fluorescence spectrometry (XRF, Shimadzu, Japan). The micro-morphology of the slag-refractory interface was observed by scanning electron microscopy (SEM, FEI Nova nano 450, USA). The elemental distribution at the slag-resistant material interface was analyzed by SEM equipped with Energy Dispersive Spectrometer (EDS, EDAX Team, USA).

Thermodynamic Simulation
During the corrosion test, only the final result can be determined clearly and the intermediate process cannot be observed directly. Therefore, thermodynamic simulation for the slag corrosion process is necessary.
In this work, Factsage7.0 was used to simulate the corrosion process of three crucibles. The calculation mechanism is shown in Figure 1. The left side is refining slag and the right side is refractory, and the concentration of the refining slag and refractory at the interface area changes in the reverse cross between 0~1. During the calculations, the interface area is considered as a composition of multiple cross sections and the corrosion process of the refractory is simulated by predicting the generation of each section. The variable <X> in the Equilib module is used to calculate the inverse crossover interdiffusion, based on which to simulate the corrosion process of the refractory from thermodynamic aspect. When X = 0, the refining slag is all the composition of the system. When x = 1, the refractory is all the composition of the system. At the beginning of the reaction, X was defined as 1. With the decrease of X, more and more slag is involved in the reaction. The oxide data included in the calculation are available in the FToxid module. The calculated temperature was set at a constant 1873 k and the pressure was 1atm.

Composition Changes of Refining Slag
The composition of slag always changed due to the reaction with refractory. Figure  2 shows the variations of the refining slag composition after the corrosion test. As can be seen from Figure 2, the content of the CaO and MgO in the refining slag decreased, and The variable <X> in the Equilib module is used to calculate the inverse crossover interdiffusion, based on which to simulate the corrosion process of the refractory from thermodynamic aspect. When X = 0, the refining slag is all the composition of the system. When x = 1, the refractory is all the composition of the system. At the beginning of the reaction, X was defined as 1. With the decrease of X, more and more slag is involved in the reaction. The oxide data included in the calculation are available in the FToxid module. The calculated temperature was set at a constant 1873 k and the pressure was 1atm.

Composition Changes of Refining Slag
The composition of slag always changed due to the reaction with refractory. Figure 2 shows the variations of the refining slag composition after the corrosion test. As can be seen from Figure 2, the content of the CaO and MgO in the refining slag decreased, and the content of the Al 2 O 3 increased in all three crucibles. It indicates that the CaO and MgO in the slag reacted with or entered into the refractories, and Al 2 O 3 in the refractories diffused into the slag during the corrosion process. The content of the SiO 2 in the slag remained almost unchanged, which is due to the large ionic radius of the silicate ion. The composition of the slag variation shows that the three crucibles were corroded by the refining slag in varying degrees. the content of the Al2O3 increased in all three crucibles. It indicates that the CaO and MgO in the slag reacted with or entered into the refractories, and Al2O3 in the refractories diffused into the slag during the corrosion process. The content of the SiO2 in the slag remained almost unchanged, which is due to the large ionic radius of the silicate ion. The composition of the slag variation shows that the three crucibles were corroded by the refining slag in varying degrees.

Corrosion of CA6 Crucible
The microstructure results of the CA6 crucible after slag corrosion are shown in Figure 3. The left side is the slag layer and the right side belongs to the original brick layer, and the reaction layer is in the middle, as shown in the red dotted area. It can be seen that many pores exist in the original brick layer. The average diameter of the pores ranged from 12 to 180 μm. In general, the higher porosity, the more severe corrosion of the crucible. However, the CA6 crucible shows excellent slag resistance. At the slag-crucible interface, the width of the reaction layer is 50~60 μm, which is the thinnest of the three crucibles ( Figures 5 and 7).

Corrosion of CA 6 Crucible
The microstructure results of the CA 6 crucible after slag corrosion are shown in Figure 3. The left side is the slag layer and the right side belongs to the original brick layer, and the reaction layer is in the middle, as shown in the red dotted area. It can be seen that many pores exist in the original brick layer. The average diameter of the pores ranged from 12 to 180 µm. In general, the higher porosity, the more severe corrosion of the crucible. However, the CA 6 crucible shows excellent slag resistance. At the slag-crucible interface, the width of the reaction layer is 50~60 µm, which is the thinnest of the three crucibles ( Figures 5 and 7). the content of the Al2O3 increased in all three crucibles. It indicates that the CaO and MgO in the slag reacted with or entered into the refractories, and Al2O3 in the refractories diffused into the slag during the corrosion process. The content of the SiO2 in the slag remained almost unchanged, which is due to the large ionic radius of the silicate ion. The composition of the slag variation shows that the three crucibles were corroded by the refining slag in varying degrees.

Corrosion of CA6 Crucible
The microstructure results of the CA6 crucible after slag corrosion are shown in Figure 3. The left side is the slag layer and the right side belongs to the original brick layer, and the reaction layer is in the middle, as shown in the red dotted area. It can be seen that many pores exist in the original brick layer. The average diameter of the pores ranged from 12 to 180 μm. In general, the higher porosity, the more severe corrosion of the crucible. However, the CA6 crucible shows excellent slag resistance. At the slag-crucible interface, the width of the reaction layer is 50~60 μm, which is the thinnest of the three crucibles ( Figures 5 and 7).  main reaction during the corrosion test. The corrosion of the refractory by the liquid slag was effectively inhibited due to the high viscosity of the CaAl 4 O 7 . As the time increased, CaAl 4 O 7 continued to react with CaO in the liquid slag to form CaAl 2 O 4 in the reaction layer. In addition, a very small amount of slag phase of Al 2 O 3 -CaO-SiO 2 -MgO was found in the reaction layer. It can be clearly found that the microstructure of the reaction layer was denser than that of the original bricklayer. One of the reasons is the gaps and pores in the reaction layer were filled by CaAl 4 O 7 and CaAl 2 O 4 , which is also an important reason for preventing further corrosion of the refining slag. To reveal the penetration degree of the refining slag, area 2 was selected randomly for EDS analysis. The very limited Mg and Si were detected, indicating that the CaAl 4 O 7 layer effectively prevents the penetration of the refining slag. Figure 4 gives the element distribution result from the slag layer to the original brick layer. It can be seen that the amount of Al in the original brick layer is significantly more than that in the refining slag, while the amount of Ca was the opposite. Thus, it can be deduced that the Ca in the slag diffused into the refractory during the reaction, while Al diffused from the refractory to the refining slag. The amount of Mg and Si in the refractory was minimal, which can also be proved by the EDS result of area 2, indicating that only a very limited Mg and Si in the refining slag penetrate into the crucible through the pores or the gaps between grains. The EDS results show that CaAl4O7 generated at the slag-refractory interface (reaction layer), and most of the components in the reaction layer are CaAl4O7 (such as the EDS results of area 1). At the experimental temperature, liquid slag penetrated into the refractory through the pores and reacted with refractory to form CaAl4O7, which is the main reaction during the corrosion test. The corrosion of the refractory by the liquid slag was effectively inhibited due to the high viscosity of the CaAl4O7. As the time increased, CaAl4O7 continued to react with CaO in the liquid slag to form CaAl2O4 in the reaction layer. In addition, a very small amount of slag phase of Al2O3-CaO-SiO2-MgO was found in the reaction layer. It can be clearly found that the microstructure of the reaction layer was denser than that of the original bricklayer. One of the reasons is the gaps and pores in the reaction layer were filled by CaAl4O7 and CaAl2O4, which is also an important reason for preventing further corrosion of the refining slag. To reveal the penetration degree of the refining slag, area 2 was selected randomly for EDS analysis. The very limited Mg and Si were detected, indicating that the CaAl4O7 layer effectively prevents the penetration of the refining slag. Figure 4 gives the element distribution result from the slag layer to the original brick layer. It can be seen that the amount of Al in the original brick layer is significantly more than that in the refining slag, while the amount of Ca was the opposite. Thus, it can be deduced that the Ca in the slag diffused into the refractory during the reaction, while Al diffused from the refractory to the refining slag. The amount of Mg and Si in the refractory was minimal, which can also be proved by the EDS result of area 2, indicating that only a very limited Mg and Si in the refining slag penetrate into the crucible through the pores or the gaps between grains.  Figure 5 exhibits the microstructure of the Al2O3 crucible after the static crucible test. As can be seen from Figure 5, from left to right are the slag layer, reaction layer, penetration layer, and original brick layer, respectively. The width of the reaction layer was 300 μm, and the maximum width of the penetration layer was more than 880 μm that was the widest among the three crucibles. As can be seen from Figure 5, from left to right are the slag layer, reaction layer, penetration layer, and original brick layer, respectively. The width of the reaction layer was 300 µm, and the maximum width of the penetration layer was more than 880 µm that was the widest among the three crucibles. The EDS results show MgO·Al2O3 generated in the reaction layer and it was formed by the reaction between MgO in the refining slag and Al2O3 in the refractory, which is the main component in the reaction layer. Some researchers [4,27] pointed out that the presence of MgO·Al2O3 at the interface has some hindrance to the penetration of slag. Meanwhile, trace amounts of CaAl4O7 were also detected in the reaction layer, but its generation mechanism is different from the CA6 crucible. The Al2O3 in the refractory and CaO in the slag reacted to form CaAl12O19 first, and then the CaAl12O19 continued to react with CaO in the slag to form CaAl4O7 in the reaction layer. Compared with the CA6 crucible, EDS The EDS results show MgO·Al 2 O 3 generated in the reaction layer and it was formed by the reaction between MgO in the refining slag and Al 2 O 3 in the refractory, which is the main component in the reaction layer. Some researchers [4,27] pointed out that the presence of MgO·Al 2 O 3 at the interface has some hindrance to the penetration of slag. Meanwhile, trace amounts of CaAl 4 O 7 were also detected in the reaction layer, but its generation mechanism is different from the CA 6 crucible. The Al 2 O 3 in the refractory and CaO in the slag reacted to form CaAl 12 O 19 first, and then the CaAl 12 O 19 continued to react with CaO in the slag to form CaAl 4 O 7 in the reaction layer. Compared with the CA 6 crucible, EDS results (area 3) show that the content of Mg in the reaction layer increased obviously and the amount of Ca decreased. It is confirmed again that the main phase was the MgO·Al 2 O 3 and the content of CaAl 4 O 7 was limited in the reaction layer. Meanwhile, it can be noticed that the liquid slag phase was found in the reaction layer, and its content is more than that in the CA 6 crucible. In the penetration layer, except for the Al 2 O 3 particles, the CaAl 12 O 19 is also found in this area, proving that the Ca in the refining slag gradually penetrated into the refractory through the pores or cracks and reacted with the refractory. In addition, some of slag phase of Al 2 O 3 -CaO-SiO 2 -MgO was found in the penetration layer, proving that the reaction layer does not have an advantage in preventing slag penetration. The EDS results of area 4 show that the content of Mg and Si in the penetration layer is more than that of CA 6 and CA 6 -Al 2 O 3 crucibles, indicating the Al 2 O 3 crucible has the worst slag corrosion resistance of the three crucibles. Figure 6 is the element distribution result of the slag-crucible interface. It can be seen that the content of Al in the crucible was more than that in slag and Ca was detected in the slag at higher amounts. The element of Ca in the refractory mainly came from the penetration of the refining slag because of the absence of CaO in the raw material. A small amount of Mg and Si were found in the refractory, combined with the EDS result of area 4 in the penetration layer, which was most likely caused by the penetration of liquid slag. The EDS results show MgO·Al2O3 generated in the reaction layer and it was formed by the reaction between MgO in the refining slag and Al2O3 in the refractory, which is the main component in the reaction layer. Some researchers [4,27] pointed out that the presence of MgO·Al2O3 at the interface has some hindrance to the penetration of slag. Meanwhile, trace amounts of CaAl4O7 were also detected in the reaction layer, but its generation mechanism is different from the CA6 crucible. The Al2O3 in the refractory and CaO in the slag reacted to form CaAl12O19 first, and then the CaAl12O19 continued to react with CaO in the slag to form CaAl4O7 in the reaction layer. Compared with the CA6 crucible, EDS results (area 3) show that the content of Mg in the reaction layer increased obviously and the amount of Ca decreased. It is confirmed again that the main phase was the MgO·Al2O3 and the content of CaAl4O7 was limited in the reaction layer. Meanwhile, it can be noticed that the liquid slag phase was found in the reaction layer, and its content is more than that in the CA6 crucible. In the penetration layer, except for the Al2O3 particles, the CaAl12O19 is also found in this area, proving that the Ca in the refining slag gradually penetrated into the refractory through the pores or cracks and reacted with the refractory. In addition, some of slag phase of Al2O3-CaO-SiO2-MgO was found in the penetration layer, proving that the reaction layer does not have an advantage in preventing slag penetration. The EDS results of area 4 show that the content of Mg and Si in the penetration layer is more than that of CA6 and CA6-Al2O3 crucibles, indicating the Al2O3 crucible has the worst slag corrosion resistance of the three crucibles. Figure 6 is the element distribution result of the slag-crucible interface. It can be seen that the content of Al in the crucible was more than that in slag and Ca was detected in the slag at higher amounts. The element of Ca in the refractory mainly came from the penetration of the refining slag because of the absence of CaO in the raw material. A small amount of Mg and Si were found in the refractory, combined with the EDS result of area 4 in the penetration layer, which was most likely caused by the penetration of liquid slag.   Figure 7 shows the SEM image of the CA 6 -Al 2 O 3 crucible after the slag corrosion. From left to right are the slag layer, reaction layer, penetration layer, and original brick layer, respectively. From Figure 6, the reaction layer was generated between the refining slag and CA 6 -Al 2 O 3 crucible, and its width was about 200 µm, which was wider than that of the CA 6 crucible and thinner than that of the Al 2 O 3 crucible. The microstructure of the reaction layer was denser compared to the original brick layer. In the reaction layer, CaAl 4 O 7 is found from the EDS results, and it is quite possibly generated by the reaction between CA 6 in the refractory and CaO in the refining slag. Of course, a tiny part of CaAl 4 O 7 may also come from the multistep reaction of Al 2 O 3 in the refractory and CaO in the refining slag. In the CA 6 crucible, the CaAl 4 O 7 can inhibit further corrosion of the refining slag. However, the content of CaAl 4 O 7 in the CA 6 -Al 2 O 3 crucible was lower than that in the CA 6 crucible due to the limit of raw materials, which is one of the reasons for the wider reaction layer than that of the CA 6 crucible. In the reaction and penetration layer, a similar position to that of the CA 6 and Al 2 O 3 crucibles was selected for EDS analysis. The results show the content of Ca in the reaction layer is less than that in the CA 6 crucible but more than in the Al 2 O 3 crucible, so it can be deduced that the amount of CaAl 4 O 7 in the reaction layer is between the other two crucibles. In the penetration layer, the amount of Mg and Si was decreased compared with the Al 2 O 3 crucible, indicating the slag corrosion resistance of the CA 6 -Al 2 O 3 crucible is better than the Al 2 O 3 crucible but worse than the CA 6 crucible.

Corrosion of CA 6 -Al 2 O 3 Crucible
CaAl4O7 may also come from the multistep reaction of Al2O3 in the refractory and CaO in the refining slag. In the CA6 crucible, the CaAl4O7 can inhibit further corrosion of the refining slag. However, the content of CaAl4O7 in the CA6-Al2O3 crucible was lower than that in the CA6 crucible due to the limit of raw materials, which is one of the reasons for the wider reaction layer than that of the CA6 crucible. In the reaction and penetration layer, a similar position to that of the CA6 and Al2O3 crucibles was selected for EDS analysis. The results show the content of Ca in the reaction layer is less than that in the CA6 crucible but more than in the Al2O3 crucible, so it can be deduced that the amount of CaAl4O7 in the reaction layer is between the other two crucibles. In the penetration layer, the amount of Mg and Si was decreased compared with the Al2O3 crucible, indicating the slag corrosion resistance of the CA6-Al2O3 crucible is better than the Al2O3 crucible but worse than the CA6 crucible.  Figure 8 is the element distribution result of the CA6-Al2O3 crucible. From Figure 8, the content of Al in the crucible was more than that in slag and Ca was detected in the refractory at higher amounts. The Mg and Si were also detected in the crucible and the content was less than that of the Al2O3 crucibles, which was also confirmed by the EDS result of area 6, proving the slag resistance of the CA6-Al2O3 crucible was worse than the CA6 crucible but better than Al2O3 crucible.   Figure 8, the content of Al in the crucible was more than that in slag and Ca was detected in the refractory at higher amounts. The Mg and Si were also detected in the crucible and the content was less than that of the Al 2 O 3 crucibles, which was also confirmed by the EDS result of area 6, proving the slag resistance of the CA 6 -Al 2 O 3 crucible was worse than the CA 6 crucible but better than Al 2 O 3 crucible.
between CA6 in the refractory and CaO in the refining slag. Of course, a tiny part of CaAl4O7 may also come from the multistep reaction of Al2O3 in the refractory and CaO in the refining slag. In the CA6 crucible, the CaAl4O7 can inhibit further corrosion of the refining slag. However, the content of CaAl4O7 in the CA6-Al2O3 crucible was lower than that in the CA6 crucible due to the limit of raw materials, which is one of the reasons for the wider reaction layer than that of the CA6 crucible. In the reaction and penetration layer, a similar position to that of the CA6 and Al2O3 crucibles was selected for EDS analysis. The results show the content of Ca in the reaction layer is less than that in the CA6 crucible but more than in the Al2O3 crucible, so it can be deduced that the amount of CaAl4O7 in the reaction layer is between the other two crucibles. In the penetration layer, the amount of Mg and Si was decreased compared with the Al2O3 crucible, indicating the slag corrosion resistance of the CA6-Al2O3 crucible is better than the Al2O3 crucible but worse than the CA6 crucible.  Figure 8 is the element distribution result of the CA6-Al2O3 crucible. From Figure 8, the content of Al in the crucible was more than that in slag and Ca was detected in the refractory at higher amounts. The Mg and Si were also detected in the crucible and the content was less than that of the Al2O3 crucibles, which was also confirmed by the EDS result of area 6, proving the slag resistance of the CA6-Al2O3 crucible was worse than the CA6 crucible but better than Al2O3 crucible. Through the above analysis, it can be found that the CA 6 crucible shows the best slag corrosion resistance, followed by the CA 6 -Al 2 O 3 crucible, and the Al 2 O 3 crucible is the worst. The reaction mechanism of the CA 6 and Al 2 O 3 crucible with the refining slag is shown in Figure 9. It can be seen from Figure 9a-c that the CA 6 particles reacted with the refining slag to form CaAl 4 O 7 that can fill the pores or the gaps between the particles. Due to the high viscosity of CaAl 4 O 7 , the slag-refractory interface will be denser and has a positive effect on inhibiting the slag corrosion. At the same time, it should be noticed that the probability of CA 6 particles entering the molten steel is greatly reduced due to the formation reaction layer, so the introduction of exogenous inclusions from the refractory is greatly reduced. In our recent work, it has been proved that the reaction product of the CA 6 refractory and refining slag can decrease the number and size of the inclusions. For the Al 2 O 3 crucible, in addition to reacting with the Al 2 O 3 particles at the interface, the refining slag also reacts with the inside Al 2 O 3 crucible by penetrating into the refractory through the pores and the gaps between the Al 2 O 3 particles, as shown in Figure 9d. During the corrosion process, the Al 2 O 3 particles gradually dissolve or fall off into the refining slag, causing the more serious corrosion of the crucible, which is shown in Figure 9e,f. In addition, the fall-off particles can enter the molten steel and increase the number of inclusions in steel, which has a negative influence on the quality of steel production. Regarding the CA 6 -Al 2 O 3 crucible, the CaAl 4 O 7 is generated faster and the content is more due to the CA6 raw materials compared with the Al 2 O 3 crucible. Therefore, the slag corrosion resistance of the CA 6 -Al 2 O 3 crucible is better than that of the Al 2 O 3 crucible.
CA6 refractory and refining slag can decrease the number and size of the inclusions. For the Al2O3 crucible, in addition to reacting with the Al2O3 particles at the interface, the refining slag also reacts with the inside Al2O3 crucible by penetrating into the refractory through the pores and the gaps between the Al2O3 particles, as shown in Figure 9d. During the corrosion process, the Al2O3 particles gradually dissolve or fall off into the refining slag, causing the more serious corrosion of the crucible, which is shown in Figure 9e,f. In addition, the fall-off particles can enter the molten steel and increase the number of inclusions in steel, which has a negative influence on the quality of steel production. Regarding the CA6-Al2O3 crucible, the CaAl4O7 is generated faster and the content is more due to the CA6 raw materials compared with the Al2O3 crucible. Therefore, the slag corrosion resistance of the CA6-Al2O3 crucible is better than that of the Al2O3 crucible.

Thermodynamic Simulation of Corrosion of Crucibles by Refining Slag
The results of the thermodynamic simulation of the corrosion process of the CA6 crucible are shown in Figure 10a. The corrosion process can be divided into the following steps according to the variation of X.

Thermodynamic Simulation of Corrosion of Crucibles by Refining Slag
The results of the thermodynamic simulation of the corrosion process of the CA 6 crucible are shown in Figure 10a. The corrosion process can be divided into the following steps according to the variation of X.
When 0.7 < X < 0.6, the spinel phase appeared in the system at X = 0.66 through the reaction of Equation (5) When 0.6 < X < 0.47, the content of the CaAl 4 O 7 decreased sharply and the liquid slag increased rapidly. When X = 0.47, the content of CaAl 4 O 7 was zero. When 0.47 < X < 0.14, two phases of liquid slag and spinel were in the system, and the content of spinel decreased to zero at X = 0.14. When 0.14 < X < 0, the liquid slag was the only phase in the system.
The results of the thermodynamic simulation of the Al 2 O 3 crucible and CA 6 -Al 2 O 3 crucible are shown in Figure 10b,c, respectively. The trends of the thermodynamic simulation results for the Al 2 O 3 and CA 6 -Al 2 O 3 crucibles are roughly similar to the CA 6 crucible. However, significant differences existed in the beginning stage of the corrosion. For the Al 2 O 3 crucible, when 1 < X < 0.76, the Al 2 O 3 converted to CaAl 12 O 19 gradually by Equation (7), and the Ca 2 Mg 2 Al 28 O 46 can also be generated through Equation (8). When X = 0.76, the phase of CaAl 4 O 7 generated in the system and its content reached the maximum of 34.18% when X = 0.48, which is lower than that of the CA 6  formed at X = 0.54 and 0.86, respectively. Therefore, the CaAl 4 O 7 generated fastest and the amount was the most during the corrosion process of the CA 6 crucible, which had a great benefit on the slag resistance corrosion.

Crystal Structure Analysis of the Refractories
The results of the EDS analysis and thermodynamic simulations show that the CA 6 crucible can rapidly generate a higher amount of CaAl 4 O 7 by reacting with the refining slag, while the Al 2 O 3 crucible generates the CaAl 4 O 7 through a series of reactions. In this section, the crystal structure of the CA 6 and Al 2 O 3 is analyzed, and the reason for the difference in the CaAl 4 O 7 generation rate and quantity between the CA 6 and Al 2 O 3 is also explained. Figure 11 shows the crystal structure of the CA 6 and Al 2 O 3 . It can be seen that one Al and five O combined to form the double pyramid module in the mirror layer of the CA 6 crystal structure, as shown in the area circled red dotted line. The double pyramid module is the active site of CA 6 and is unstable during the reaction process. When the refining slag reacts with the crucible, the active site will be destroyed quickly, resulting in the rapid collapse of the CA 6 crystal structure to form the denser CaAl 4 O 7 . The crystal structure of CaAl 4 O 7 is more stable and can effectively inhibit the further corrosion of slag, which is one of the reasons for the excellent slag corrosion resistance of the CA 6 crucible. Al 2 O 3 crystal is an octahedral structure and more stable compared to CA 6 , so the reaction rate with Ca in the refining slag is very slow, resulting in a low content of CaAl 4 O 7 in the reaction layer and poor corrosion resistance. More importantly, Al 2 O 3 particles with high stability may directly enter the refining slag and molten steel, resulting in the generation of exogenous inclusions, which has a negative impact on the control of inclusions. At this point, the CA 6 refractory can well avoid this problem. So, the excellent slag corrosion resistance of the CA 6 crucible has a great application prospect in the ladle refining process, especially for the smelting clean steel. resistance of the CA6 crucible has a great application prospect in the ladle refining process, especially for the smelting clean steel.

Conclusions
Three crucibles (CA6 crucible, Al2O3 crucible, and CA6-Al2O3 crucible) were selected to investigate the corrosion resistance of the refining slag through laboratory experiments and thermodynamic simulations. The following conclusions were obtained.
(1) The three crucibles show different slag corrosion resistance, CA6 crucible has the best corrosion resistance, followed by the CA6-Al2O3 crucible. The Al2O3 crucible shows the worst slag corrosion resistance.
(2) The addition of CA6 to the raw materials has a positive effect on improving the slag corrosion resistance of the Al2O3 crucible.
(3) The generation of high melting point CaAl4O7 is the critical factor for inhibiting the further corrosion of the CA6 and CA6-Al2O3 crucible. The CaAl4O7 was also detected in the Al2O3 crucible, but Al2O3 in the refractory reacts with CaO in the refining slag to produce CaAl12O19 firstly, and then the CaAl12O19 reacted with slag to form CaAl4O7. Therefore, the generation of CaAl4O7 in Al2O3 crucible is slower than that of CA6 crucible and the amount generated is relatively less, which results in a worse slag corrosion resistance of Al2O3 crucible compared to CA6 crucible and CA6-Al2O3 crucible.
(4) When the refining slag reacts with the crucible, the double pyramid module of Figure 11. The crystal structure of CA 6 , CA 2, and Al 2 O 3 .

Conclusions
Three crucibles (CA 6 crucible, Al 2 O 3 crucible, and CA 6 -Al 2 O 3 crucible) were selected to investigate the corrosion resistance of the refining slag through laboratory experiments and thermodynamic simulations. The following conclusions were obtained.
(1) The three crucibles show different slag corrosion resistance, CA6 crucible has the best corrosion resistance, followed by the CA 6 -Al 2 O 3 crucible. The Al 2 O 3 crucible shows the worst slag corrosion resistance.
(2) The addition of CA 6 to the raw materials has a positive effect on improving the slag corrosion resistance of the Al 2 O 3 crucible.
(3) The generation of high melting point CaAl 4 O 7 is the critical factor for inhibiting the further corrosion of the CA 6 and CA 6 -Al 2 O 3 crucible. The CaAl 4 O 7 was also detected in the Al 2 O 3 crucible, but Al 2 O 3 in the refractory reacts with CaO in the refining slag to produce CaAl 12 O 19 firstly, and then the CaAl 12 O 19 reacted with slag to form CaAl 4 O 7 . Therefore, the generation of CaAl 4 O 7 in Al 2 O 3 crucible is slower than that of CA 6 crucible and the amount generated is relatively less, which results in a worse slag corrosion resistance of Al 2 O 3 crucible compared to CA 6 crucible and CA 6 -Al 2 O 3 crucible.
(4) When the refining slag reacts with the crucible, the double pyramid module of CA 6 will be destroyed quickly, resulting in the rapid collapse of the CA 6 crystal structure to form the denser CaAl 4 O 7 , which is an essential reason for the excellent slag corrosion resistance. At the same time, it also avoids CA 6 particles entering the molten steel to introduce exogenous inclusions, so CA 6 has great application potential in ladle refining and clean steel smelting.