Corrosion of Nozzle Refractories by Liquid Inclusion in High Oxygen Steels

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1 , pp Corrosion of Nozzle Refractories by Liquid Inclusion in High Oxygen Steels Mun-Kyu CHO 1) and In-Ho JUNG 2) 1) Research Institute of Industrial Science and Technology, Pohang, Republic of Korea. 2) Dept. Mining and Materials Engineering, McGill University, Montreal, Canada. (Received on February 9, 2012; accepted on March 26, 2012) In order to understand the corrosion mechanism of stopper refractories by liquid MnO SiO 2 FeO MnS inclusion in high oxygen steels, the chemical reaction between the liquid MnO SiO 2 FeO MnS slag and several refractories including Al 2O 3 C, Spinel C, Al 2O 3 AlN C and ZrO 2 C were performed at C. It is found that the reduction of the liquid inclusion can occur by C in the refractories to produce Mn Fe Si metallic phase and the degree reduction is important factor influencing to the chemical composition and amount of liquid inclusion. From this experimental and thermodynamic study and in plant test, it is found that ZrO 2 C(13%) refractory has excellent corrosion resistance against high oxygen steel containing liquid MnO SiO 2 FeO MnS inclusion. KEY WORDS: corrosion of refractory; stopper header; liquid inclusions; continuous casting; high oxygen steels. 1. Introduction High oxygen steels with high concentrations of Mn and S typically contain considerable amount of liquid inclusions of MnO MnS FeO SiO 2 during the secondary steelmaking process. Most of continuous casting refractories including submerged entry nozzle (SEN), slide gate nozzle plate (SN plate) and tundish stopper contain a significant amount of carbon and are highly subject to the corrosion by high oxygen steel and liquid inclusions. Several experimental studies have been conducted to investigate the corrosion of the refractories for continuous casting process of high oxygen steels. Nomura et al. 1) reported that spinel-c refractories is superior than Al 2O 3 C or Al 2O 3 SiO 2 C (alumina-mullite-c) refractories for SEN application in high oxygen steels. Wei et al. 2) also examined the corrosion of various refractories including Al 2O 3 C, Al 2O 3 SiO 2 C, spinel, MgO-spinel, MgO and ZrO 2 using a refractory finger test in high oxygen steel, and reported the refractories containing alumina and mullite are more corrosive than the others. Guillo and Simoes 3) investigated the eroded surface of MgO stopper used in the casting of Si Mn killed steels and observed high MnO SiO 2 oxides, possibly originated from liquid inclusion in the steels, were attached on the surface of the stopper. All the previous experimental studies on the corrosion of refractories were performed with molten steels and therefore it was hard to understand the direct influence of liquid inclusion in high oxygen steel on the refractory wear. Recently, Xie et al. 4) performed the corrosion experiments of tundish stopper refractories against liquid inclusion slags. Refractories of Al 2O 3 C, ZrO 2 C and MgO C were dipped in two types of liquid inclusion slags (CaO Al 2O 3 SiO 2 based slag considering Al deoxidation and Ca treatment, and MnO Al 2O 3 SiO 2 based slag considering Mn Si deoxidation without Ca treatment). However, the liquid inclusion composition investigated by Xie et al. does not well represent those in high oxygen steels. A tundish stopper is a special refractory which is used to regulate the discharge of the steel and prevent slag from entering casting mould in the end of casting process. A stopper head contacts the inner wall of the submerged nozzle. The severe wear of the stopper head frequently occurs during the continuous casting of high oxygen steel containing oxygen of over 100 wt ppm and ultra low carbon steel with high contents of Mn and Si. For example, the stopper head made of Al 2O 3 C refractory corroded after series of continuous casting of high oxygen steel is presented in Fig. 1. The original stopper head of hemisphere shape is irregularly Fig. 1. Refractories in continous casting process. (a) Schematic of the stopper system and (b) example of corroded stopper after continous casting of high oxygen steel (Al 2O 3 C refractory) ISIJ

2 eroded, which causes difficulty in accurate control of molten metal flow during casting process and eventually induces casting defects of the steel products. The erosion of the stopper is considered as the result of the corrosion of stopper refractories by liquid inclusions in the molten steel. As the gap between stopper and SEN collar is controlled tightly narrow during casting, there is a high chance for liquid inclusion to attach on and react with stopper refractory materials. In particular, as high oxygen steels contains a large amount of liquid oxide inclusions, the build-up of liquid inclusion around stopper head can be inevitably more severe. The purpose of the present study is the investigation of the influence of liquid inclusion (high MnO SiO 2 FeO MnS) in high oxygen steels on the corrosion of various refractories used for continuous casting stopper materials. In order to understand the complex chemical reactions between the inclusion and refractories, thermodynamic calculations using the FactSage thermodynamic software 5) were also performed. 2. Experimental As-delivered refractory materials were baked at C inside coke powders for 1 hr to decompose binders and evaporate volatile species. The baked refractories were cut into the plate of (t) mm in size and the surfaces of the refractory plates were polished. The chemical compositions of the refractories used in the present study are listed in Table 1. The reagent grade MnO, SiO 2, FeO and FeS powders were mixed and pre-melted under Ar atmosphere to prepare liquid inclusion material. FeS can be transformed to MnS by the chemical reaction FeS + MnO FeO + MnS during melting. The solidified inclusion material was cut into disk shape of 10 (OD) 10 (t) mm. It was confirmed, by the investigation of the microstructure, that there was nearly no pre-reduction of the oxide during the inclusion sample preparation. The chemical reaction experiments between the premelted inclusion and baked refractories were carried out in a horizontal resistance furnace under purified Ar atmosphere. The disk shaped inclusion material was put on top of the plate shaped refractory material, then the specimen was placed in the furnace. The temperature of the furnace was increased with 10 C / min heating rate up to C and maintain for 1 hr. After the experiment, the specimen was taken out from the furnace and air cooled. The reacted Table 1. Chemical compositions of the refractories investigated in the present study. wt% Al 2O 3 C Al 2O 3 AlN C Spinel L C Spinel H C ZrO 2 C Al 2O MgO ZrO 2 85* C SiC 2 2 AlN 30 etc. 1 * CaO stabilized ZrO 2. specimens were cut and mount for SEM/EPMA analysis. All chemical analyses for the reaction interfaces of the specimens were carried out using EPMA. Analyzed compositions for each refractory sample are listed in Table Thermodynamic Analysis In the present study, the FactSage thermodynamic software 5) was used for the thermodynamic calculations for the inclusion formation in high oxygen steels and chemical reactions between the inclusion and various refractories. For the inclusion formation, FTmisc-FeLq database, and FToxid Table 2. Chemical compositions of various phases after the reaction between liquid inclusion and refractories, determined by EPMA. Oxide (mole %) Metal/sulfide (atomic %) # MnO SiO 2 FeO Al 2O 3 MgO CaO ZrO 2 Fe Mn Si S Fig Fig Fig Fig ISIJ 1282

3 database were used for molten steel and inclusions (solid/ liquid oxide and oxy-sulfide), respectively. For the chemical reactions between liquid inclusion and refractories, FToxid and FACT53 database were used for the refractories, inclusions and gases. FSStel database (liquid) was used to simulate the reduction of liquid inclusion to metallic phase containing high Mn and Si. The important phases for the inclusion and refractories used in the present study are: Liquid oxy-sulfide (FToxid-Slag): MnO FeO Fe 2O 3 MgO Al 2O 3 SiO 2 ZrO 2 MnS FeS MgS etc. Spinel (FToxid-Spin): MgAl 2O 4 MnAl 2O 4 FeAl 2O 4 with excess Al 2O 3. Monoxide (FToxid-Mono): MgO MnO FeO Fe 2O 3 etc. Sulfide (FTmisc-MeS): MnS FeS Other solid phases: Al 2O 3, ZrO 2, AlN, SiC, C, etc. 4. Results and Discussion 4.1. Liquid Inclusion The chemical composition of high oxygen steel investigated in the present study is Fe 0.08C 1.2Mn 0.3S 0.01Si O minor P, Bi, etc. in wt%. Due to high sulfur and oxygen content, oxy-sulfide inclusions can form at ladle by Mn Si deoxidation and at tundish due to temperature drop. The calculated inclusions are plotted in Fig. 2. When the concentrations of all other alloying elements are fixed, the amount of inclusion forming in the molten steel can be varied with oxygen content. Thermodynamic calculations show that liquid inclusion can form in the entire temperature and oxygen concentration ranges considered in the present study. According to the calculations (Fig. 2(a)), liquid inclusion can form at C when oxygen content in the molten steel becomes higher than 122 wt ppm, and the amount of the liquid inclusion increases linearly with increasing oxygen content. Once the inclusion is generated, the composition of inclusion is constant regardless of total oxygen content (Fig. 2(b)): 62MnO 25SiO 2 7FeO 5MnS 1FeS in wt%. When total oxygen content is fixed to be 200 wt ppm, the inclusion can begin to form even at C and the amount of liquid inclusion can increase linearly with decreasing of temperature (Fig. 2(c)). The composition of liquid inclusion (Fig. 2(d)) is also almost independent with temperature except that MnO concentration slightly increases with decreasing temperature. When small amount of Al is considered in the calculation, MnO SiO 2 Al 2O 3 MnS inclusion can be calculated. The liquid inclusion calculated in the present study agrees with the inclusion observed in real plant steel sample. As mentioned earlier, the liquid inclusion of 62MnO 25SiO 2 7FeO 5MnS 1FeS was prepared for simulating the chemical reaction between the inclusion and refractories. Fig. 2. Calculated inclusions formed in high oxygen steel (Fe 0.08C 1.2Mn 0.3S 0.01Si O minor P, Bi, etc. in wt%) during continuous casting process. (a) and (b): amount and composition of liquid inclusion varied with total oxygen content in steel at C, (c) and (d) amount and composition of liquid inclusion in molten steel containing 200 wt ppm oxygen varied with temperature ISIJ

4 4.2. Chemical Reaction between Liquid Inclusion and Refractories Al 2O 3 C Refractory Figure 3 shows the microstructure of Al 2O 3 C refractory after reaction with liquid inclusion. The amount of carbon in the refractory is just about 3 wt% with additional 2 wt% of SiC. During the experiment the inclusion material was well-wetted on the surface of the refractory. A large hole in the middle of original inclusion region was generated due to CO gas formation and small holes are also found near the interface of the refractory and inclusion. The microstructure analysis for the remaining inclusion shows that there are noticeable amount of Fe Mn metals (99Fe 1Mn in at.%) reduced from the original liquid inclusion of MnO FeO SiO 2. This reduction process should be induced by the carbon in the refractory. In the microstructure at the interface between the refractory and inclusion, liquid MnO SiO 2 FeO, MnO FeO mangano-wustite and MnAl 2O 4 phase are observed. MnAl 2O 4 layer appears on the surface of Al 2O 3 refractory grains, and MnAl 2O 4 particles were also found in the middle of liquid MnO SiO 2 FeO phase. Small amount of metallic Fe Mn and MnS particles are also observed at the interface. From the microstructure in Fig. 3, it is found that small amount of inclusion was reduced to metallic Fe Mn phase by carbon. But majority of liquid inclusion could be well wetted on the de-carburized area of Al 2O 3 refractory and form interface of MnAl 2O 4 layer along the reaction boundary. Strangely, no significant amount of Al 2O 3 was detected in the penetrated liquid inclusion. However, the existence of MnAl 2O 4 particles found in the middle of liquid MnO SiO 2 FeO phase could be interpreted as the results of solidification of MnO SiO 2 FeO Al 2O 3 during air cooling. MnS particles distributed in liquid MnO SiO 2 FeO phase also seem to be form during the solidification of the liquid inclusion Al 2O 3 AlN C Refractory Figure 4 shows the microstructure of Al 2O 3 AlN C refractory after the reaction with liquid inclusion. Interestingly, a large metallic Mn Fe Si (49Mn 40Fe 11Si in at.%) droplet is found in the middle of inclusion region. A large hole which is presumed to be generated by CO gas bubble is also found next to the metallic droplet. Remaining liquid inclusion is wetted on the surface of metallic droplet and refractory. The wetting behavior of the inclusion on the Al 2O 3 AlN C refractory is less than Al 2O 3 C refractory, but remaining liquid inclusion could be wetted well enough with the refractory to produce chemical reaction with refractories. The microstructure at the reaction interface is rather different from that of Al 2O 3 C. Small Al 2O 3 particles are distributed uniformly in liquid MnO SiO 2 Al 2O 3 (34MnO 48SiO 2 18Al 2O 3 2MgO in mole %) matrix. Metallic Mn Si (89Mn 11Si in at.%) and MnS phase are also observed. MnAl 2O 4 phase which is observed at the interface of Al 2O 3 C refractory is rarely observed. Compared with the Al 2O 3 C refractory, the more reduction of liquid inclusion happened by carbon and AlN, which left a large metallic droplet above the refractory. Therefore, the small amount of liquid inclusion can remain to react with the refractory. Interestingly, no FeO was detected in the liquid inclusion penetrated into the refractory, but noticeable amount of Al 2O 3 was dissolved in the penetrated liquid inclusion MgAl 2O 4 C Refractory Spinel refractories containing both low (4 wt%) and high (13 wt%) amount of carbon were investigated in the present study. As can be seen in Fig. 5, the wetting behavior of liquid inclusion on MgAl 2O 4 C refractories were significantly changed with carbon content. The liquid inclusion was relatively well wetted on MgAl 2O 4-low C refractory, while no significant wetting behavior of liquid inclusion on the MgAl 2O 4-high C refractory was observed. The wetting behavior and liquid inclusion penetration into the refractories seem to be highly related to the reduction of inclusion by carbon in the refractories. The MgAl 2O 4-low C refractory produced a large metallic particle in the middle of liquid inclusion sample, as shown Fig. 3. Microstructure of the Al 2O 3 C refractories reacted with liquid inclusion. Overview of the cross-section of the refractory, and the zoom-up views of liquid inclusion region and reaction interface between liquid inclusion and the refractory. Fig. 4. Microstructure of the Al 2O 3 AlN C refractory reacted with liquid inclusion. Overview of the cross-section of the refractory and the zoom-up view of reaction interface between liquid inclusion and the refractory ISIJ 1284

5 Fig. 5. Microstructure of the MgAl 2O 4 C refractories ( low carbon = 4 wt% and high carbon = 13 wt%) reacted with liquid inclusion. Overview of the cross-section of the refractories and the zoom-up views of reaction interfaces between liquid inclusion and the refractories. Fig. 6. Microstructure of the ZrO 2 C refractory reacted with liquid inclusion. Overview of the cross-section of the refractory (metallic droplet is detached during the post-sample treatment) and the zoom-up view of reaction interface between liquid inclusion and the refractory. in Fig. 5. But there is still a large amount of liquid inclusion remained after the reduction by carbon. The remaining liquid inclusion penetrated well into the de-carburized refractory and reacted with MgAl 2O 4 grain to form (Mg,Mn) Al 2O 4 spinel phase where MnO is about 2 3 mole % and liquid 18MnO 50SiO 2 23MgO 9Al 2O 3 (in mole %) phase. Many pores can be also observed in the liquid inclusion area and the reaction area between liquid inclusion and MgAl 2O 4 refractory, which should be generated by CO gas bubbles. In the case of MgAl 2O 4-high C refractory, the original liquid inclusion was almost completely transformed to a metallic droplet Fe Mn Si (48Fe 32Mn 20Si in at.%) as shown in Fig. 5(c). The reaction interface shows a sharp distinction between metallic phase (originally liquid inclusion) and spinel refractory. Small amount of MnO oxide is reacted with spinel at the interface to form (Mg,Mn)Al 2O 4 spinel solution (MnO < 7 mole %), and small amount of metallic Mn Fe Si (94Mn 4Fe 2Si in at.%) phase is also observed. This microstructure tells that most of liquid inclusion was already reduced to metallic phase by C in refractory before it reacted with spinel refractory. Small amount of liquid inclusion could penetrate into spinel refractory but it was immediately reduced by C after the penetration ZrO 2 C Refractory The microstructure of ZrO 2 C refractory after the reaction with liquid inclusion is shown in Fig. 6. Like MgAl 2O 4 C refractory above, no significant wetting of liquid inclusion happened on the refractory surface. The surface of the refractory is very flat and no noticeable chemical reaction is observed. Unfortunately, metallic droplet on the surface of the refractory like in Fig. 5 was detached during the sample treatment. The metallic droplet composition was almost identical to that of MgAl 2O 4-high C case. Small amount of Fe Mn Si metallic phase can be observed near the interface between inclusion and the refractory. Among all other refractories, the ZrO 2 C refractory showed the least refractories wear by the liquid inclusion Corrosion Mechanism of Refractories by Liquid MnO SiO 2 FeO Inclusion Based on the present experimental data, it can be summarized that the corrosion of continuous casting refractories by liquid inclusion can occur in two stage processes: (i) reduction of liquid inclusion by carbon in refractories and (ii) the wetting and chemical reaction of the remaining liquid inclusion with refractory mineral phases. In order to understand the reduction of liquid inclusion by carbon, thermodynamic calculations were performed and the results are presented in Fig. 7. Two types of calculations were performed: (i) assuming the reduced metal can be continuously in equilibration with remaining liquid inclusion (solid lines in Fig. 7), and (ii) assuming the reduced metal cannot participate in further reaction with remaining liquid inclusion during the reduction process because the metal can be physically almost separated from the remaining liquid inclusion (dashed lines in Fig. 7). The reality should be in between these two calculation conditions. According to the calculations, FeO can be firstly reduced and MnO and SiO 2 can be reduced afterward. The concentration of the resultant liquid metal is, therefore, changed with the degree of reduction (amount of carbon reacted). The more reduction of liquid inclusion by carbon, the higher concentrations of Mn and Si in liquid metal. Thus, the composition of metallic phase can give a reasonable estimate for the degree of reduction of liquid inclusion. In low carbon refractories such as Al 2O 3 C, the degree of reduction of liquid inclusion can be less than those of other refractories containing high amount of C. The degree of reduction by Al 2O 3 C refractory can be estimated by the composition of metallic phase (97 99 Fe-1 3 Mn in at.%) in liquid inclusion, which can be formed by low degree of reduction according to the calculation in Fig. 7. MgAl 2O 4 low C refractory produced a large metallic droplet (80Fe 15Mn 5Si in at.%), but there is still a large amount of remaining liquid inclusion penetrated into refractories. It is hard to explain why such a large metallic droplet can be ISIJ

6 Fig. 7. Reduction of 100 g of liquid inclusion (MnO SiO 2 FeO MnS in wt%) with increasing amount of carbon. (a) Amount of liquid inclusion and metal, (b) composition of liquid inclusion and (c) liquid metal. Solid lines were calculated with the assumption that the reduced metal can be continuously in equilibration with remaining liquid inclusion and dashed lines were calculated with the assumption that reduced metal cannot participate in further reaction with remaining liquid inclusion during the reduction process. Fig. 8. Variation of liquid inclusion composition depending on the degree of reduction by carbon contained in refractory. Fig. 9. Refractory corrosion mechanism by liquid inclusion in high oxygen steels. formed on MgAl 2 O 4 refractory but not on Al 2 O 3 refractory with the same amount of C. The refractories containing high carbon always produce a large metal droplet (40 48Fe 32 49Mn 11 20Si at.%) on the surface the refractories. Thus, the amount of carbon in refractories can determine the degree of reduction of liquid inclusion. AlN in Al 2 O 3 AlN C refractory can also contribute to the reduction of liquid inclusion. Although AlN itself can be thermally stable, the chemical reaction with oxides can produce Al 2 O 3 by 2AlN + 3FeO(MnO)(l) = Al 2O 3 + 3Fe (Mn) + N 2(g), and 4AlN + 3SiO 2(l) = 2Al 2O 3 + 3Si + 2N 2(g). Al 2O 3 formed in this way may be easily dissolved into liquid inclusion to form MnO SiO 2 Al 2O 3 liquid oxide as shown in Fig. 4. In Fig. 8, the two different paths of liquid inclusion compositions depending on the degree of reduction are presented on the calculated ternary phase diagram of the MnO Al 2O 3 SiO 2 system at 1550 C. The possible corrosion mechanism of the refractories by liquid inclusion in high oxygen steels is presented in Fig ISIJ 1286

7 After the inclusion is attached to refractories, the inclusion can be partially reduced by C directly from refractories or the dissolved C (highly concentrated in steel near the refractories) in high oxygen steels. The degree of reduction can be mainly dependent on carbon content in refractories. The remaining inclusion (MnO SiO 2) can attack de-carburized region of refractories. The metallic phase produced by reduction (in particular, the large metal droplet on high C refractories) can be washed away by being dissolved into surrounding liquid steel. New liquid inclusion can accumulated on the pre-reacted surface of the refractories and the similar reactions can occur repeatedly. The following three factors can be important in the refractory corrosion in high oxygen steels, if the refractories have all similar microstructure (size of oxide refractory grains, porosity, etc.): (i) Amount of remaining liquid inclusion after the (ii) reduction by C in refractories The chemical solubility of refractory materials to the remaining liquid inclusion (iii) The wetting tendency and penetration force of the remaining liquid inclusion into the refractory (the penetration force can be related to the viscosity of liquid inclusion) The more reduction of liquid inclusion by carbon remains the less liquid inclusion to react with refractories. Therefore, in general, the high C containing refractory can be less corrosive and have longer life time. The chemical solubility (stability) of refractories to the remaining liquid inclusion can be determined by phase diagram between refractories and liquid inclusion. The ternary phase diagrams of refractory oxide MnO SiO 2 systems at C were calculated for the various refractory oxides, Al 2O 3, MgO, MgAl 2O 4 and ZrO 2 in Fig 10. As mentioned above, the MnO/SiO 2 ratio in the inclusion remained after the reduction is depending on the degree of reduction (mainly C content in refractory): (wt%mno)/(wt%sio 2) in the remaining inclusion can be about 3 and 1.5 (or even below 1.0) after low and high degree of reduction, respectively. If the de-carburized zone in refractories was already developed, the remaining liquid inclusion can readily penetrate and react with oxide refractory grains. The viscosity of liquid inclusion can increase with the decrease of basicity MnO/SiO 2. However, the viscosity of the MnO SiO 2 based liquid inclusion is increasing but not significant when the basicity (wt%mno)/(wt%sio 2) of the inclusion varies from 3 to 1.5. Therefore, factors (i) and (ii) would be most important parameters for the corrosion of refractories by liquid inclusions in high oxygen steels. Fig. 10. Thermodynamic stability of the refractory components against liquid inclusion (MnO SiO 2) at C. The arrows (gray arrow represents high degree of reduction, and empty arrow represents low degree of reduction) in phase diagrams represent the possible amount of dissolution of each refractory component ISIJ

8 According to the calculated phase diagrams in Fig. 10, Al 2O 3 C refractory can be weak after both low and high degree of reduction. In case of the spinel-c refractory, the chemical stability of spinel phase can be weaker after high reduction than low reduction but the amount of remaining liquid inclusion after high reduction is much lower. Therefore, the corrosion tendency is lower at high reduction condition (high carbon spinel refractory) as can be seen in the present experimental study shown in Fig. 5. MgO C refractory can be more stable than Al 2O 3 C and Spinel C refractory but it can produce (Mg,Mn) 2SiO 4 olivine silicate as reaction product after high reduction. ZrO 2 refractory is very stable after high reduction of initial liquid inclusion, but less stable at no or less reduction of liquid inclusion. In the case of Al 2O 3 AlN C refractory, both C and AlN can contribute to the reduction of liquid inclusion. C can reduce the oxide inclusion directly to metallic phase with formation of CO gas. As a result, the composition of the MnO SiO 2 based liquid inclusion interfaced with oxide refractory can be changed from high MnO side to low MnO side and Al 2O 3 refractory can be dissolved into MnO SiO 2 liquid until its saturation. On the other hand, AlN can reduce the oxide inclusion to metallic phase like C but simultaneously form Al 2O 3 and N 2 gas. Therefore, the dissolution of Al 2O 3 refractory matrix into the remaining inclusion can be reduced. In the present study, the Al 2O 3 AlN refractory with high C (15%) was tested. If the C content in the Al 2O 3 AlN refractory is low, AlN will be reacted actively with liquid inclusion with the production of N 2(g). Of course, the resultant chemical reaction will be similar to that of Al 2O 3 low C refractory except the generation of N 2(g), which can hinder the chemical reaction of remaining liquid inclusion and the refractory by N 2 pore formation at the reaction interface. Therefore, in general, Al 2O 3 AlN C refractory can be less corrosive than Al 2O 3 C, if the carbon contents are same for both refractories. If liquid inclusion in high oxygen steel contained a certain amount of Al 2O 3 (Al can be dissolved in molten steel from top slag, ladle refractories and impurity from ferro-alloy, which can change the inclusion composition in return), the thermal stability of various oxide refractories can be different from the calculated results in Fig. 10. For example, Al 2O 3 C refractory can be more stable than MgO C refractory against the MnO SiO 2 Al 2O 3 slag, as reported by Xie et al. 4) because the liquid slag can be readily saturated with Al 2O 3 with small amount of Al 2O 3 dissolution but a large amount of MgAl 2O 4 should be dissolved until the slag is saturated with (Mg,Mn)Al 2O 4. In real continuous casting process, the influence of molten steel cannot be neglected. As Nomura et al. 1) reported from the microstructure of the Al 2O 3 C refractories dipped in molten steel (Fe 0.3Mn 0.004C 0.001Si 0.06O in wt.%), the solute elements such as Mn can reacted with the refractory oxide. For example, Nomura et al. interpreted the results as two stage processes: (i) the formation of (Mn,Fe)Al 2O 4 by the reaction of Al 2O 3 + Mn (or Fe) + 2O (Mn,Fe)Al 2O 4 and (ii) the modification of the oxide products by (Mn,Fe)O inclusion to produce MnO FeO Al 2O 3 liquid phase on the surface of Al 2O 3 C refractory. Even though this corrosion mechanism is considered for the present steel grade, the highly concentrated solutes Mn (1.2 wt%)and Si (0.01 wt%) in high oxygen steel can form liquid MnO SiO 2 refractory phase on the surface of refractories in the early stage and then this liquid oxide can react with liquid MnO SiO 2 inclusion. The resultant stability of refractories can be similar to the calculations in Fig In Plant Tests Al 2O 3 AlN C and ZrO 2 C refractories were tested for the stopper head refractories at Posco, Pohang work, Korea. The corrosion resistance of the refractories was evaluated by two casting process indexes typically used for stopper: MLAC (Mould Level Auto Control) and average profile distance during the casting. Both indexes show that the stopper of ZrO 2 C is superior to that of Al 2O 3 AlN C. The number of continuous casting heats for high oxygen steels can be improved with the employment of ZrO 2 C stopper by 20 50% more compared to the casting heats with Al 2O 3 AlN C stopper. 5. Summary In order to understand the corrosion mechanism of stopper refractories by liquid MnO SiO 2 FeO MnS inclusion in high oxygen steels, the chemical reaction between the liquid MnO SiO 2 FeO MnS slag and several refractories including Al 2O 3 C, Spinel C, Al 2O 3 AlN C and ZrO 2 C were performed at C. Thermodynamic analyses using the FactSage thermochemical software were also performed to understand the chemical reactions between liquid inclusions and refractories. The refractories with small amount of C partially reduced the inclusion, and the remaining liquid inclusion penetrated into the refractory easily to induce a noticeable chemical corrosion. On the other hand, the refractories containing high amount of C reduced a large quantity of the liquid inclusion to metallic Mn Fe Si and remained only a small amount of MnO SiO 2 of which composition was largely shifted toward SiO 2 rich side. As a result, the corrosion of refractories containing high C by liquid inclusion could be significantly reduced. Among the various refractories, ZrO 2 C showed the highest corrosion resistance against liquid MnO SiO 2 FeO MnS inclusion. In real plant test, ZrO 2 C(13 wt%) refractory showed an excellent corrosion resistance against high oxygen steels containing liquid MnO SiO 2 FeO MnS inclusion, which improved the continuous casting of high oxygen steels. REFERENCES 1) O. Nomura, S. Uchida and W. Lin: Shinagawa Tech. Rep., 42 (1999), 31. 2) W. Lin, O. Nomura and S. Uchida: CAMP-ISIJ, 11 (1998), ) P. Guillo and J. Simoes: Proc. of 3rd Int. Conf. on Continuous Casting of Steel in Developing Countries, The Chinese Society for Metals, Beijing, China, (2004), ) D. Xie, C. Garlick and T. Tran: ISIJ Int., 45 (2005), ) C. W. Bale, E. Belisle, P. Chartrand, S. A. Decterov, G. Eriksson, K. Hack, I.-H. Jung, Y.-B. Kang, J. Melancon, A. D. Pelton, C. Robelin and S. Peterson: Calphad, 33 (2009), ISIJ 1288

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