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Damage to the working surface of the sliding nozzle slide under negative pressure conditions

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Abstract: The surface of the sliding nozzle slide plate after ladle use is analyzed to study the reasons for the loosening of the working surface structure. The slide plate is made of Al2O3-zrO2-C material fired at high temperature. When heated under negative pressure, an obvious loosening layer can be formed, which is consistent with the thermodynamic calculation results. When heated under Ar atmosphere or N2 atmosphere, a slight loosening phenomenon can be observed. Compared with heating under negative pressure conditions, the degree of loosening is lighter; when heated under a buried carbon atmosphere, there is almost no loosening phenomenon. Through the heating test under negative pressure conditions, the loosening reaction of the slide gate plate working surface tissue can be reproduced, which is also an effective method to evaluate the surface damage of the slide gate plate.

Keywords: slide gate plate; working surface; negative pressure; loosening; thermodynamic calculation

1 Introduction

Sliding nozzle slides for ladles and tundishes pay attention to the thermal peeling resistance and oxidation resistance of the material, and most of them use Al2O3-C materials. The typical damage to the sliding nozzle slide is the damage to the working surface, which is often called roughening on the stroke surface. There have been many studies in the previous literature on the morphology of hair pulling. Among them, regarding the loosening of the stroke surface structure and the peeling off of the molten layer caused by decarburization and oxidation, most studies have shown that this is due to the occurrence of O2, Mn and other components and inclusions in the molten steel at the interface of the sliding nozzle and slide plate during the casting process. caused by reaction. Mizobuchi et al. studied the problem of increased damage to the working surface of the sliding plate when casting ultra-low carbon aluminum-killed steel (Al-K steel). They observed the reaction between various steel types and Al2O3-C materials while controlling the atmosphere in the electric furnace, especially ultra-low carbon aluminum-killed steel. The reaction between low-carbon aluminum-killed steel and the working surface of the Al2O3-C sliding plate found that during the reaction between the Al2O3 particles in the Al2O3-C sliding plate and C, gas phases such as Al(g) and CO(g) were generated and escaped, forming Loose layer. Chifeng et al. studied the damage state of the sliding nozzle slide when casting steel types with different dissolved oxygen concentrations. They found that when casting steel types with low dissolved oxygen concentrations, the Al2O3 particles and C in the working surface of the slide disappeared, forming a loose layer, and formed Penetration of slag components such as Fe and CaO can be observed in the loose layer. Chifeng et al. used a high-frequency induction furnace to study the reaction between molten steel containing almost no dissolved oxygen and a sliding nozzle slide. The results showed that Al2O3 particles and C disappeared in the working surface to form a loose layer. In this study, the working surfaces of several used skateboards were further observed and analyzed, and it was found that use under negative pressure conditions aggravated the loosening of the skateboard working surfaces. This article reports on this research.

2 Analysis of slide gate plate after use

The sample used in this study is the sliding nozzle slide A used on the ladle of a steel plant. Its original physical and chemical performance indicators are shown in Table 1. The slide plate is made of Al2O3 -zrO2 -C material fired at high temperature and can be widely used in tundish and ladle. The appearance photos of the upper and lower slide plates after 6 heats (a total of 764 minutes) are shown in Figure 1. The steel slag is obviously attached to the upper slide plate. The microstructure photos near the nozzle hole of the working surface are shown in Figure 2. It can be observed in both the upper and lower sliding plates that steel slag has penetrated into the working surface to a depth of 1500 μm, and the lower sliding plate has a metamorphic layer about 300 μm thick from the working surface. However, the working surface remains slidable. The Al2O3 particles and C near the working surface of the upper sliding plate have disappeared, and the structure has become loose; a metamorphic layer of zrO2-mullite aggregate can be observed in the loose layer. It can be inferred that the generation of the loose layer leads to tissue peeling, and then stroke surface roughening damage occurs. After long-term casting use, the upper slide plate is more likely to have structural loosening than the lower slide plate.

Table 1 Performance of slide gate plate materials

projectindex
chemical components/% 
Al2O379
zO210
siO24
F.C.5
Bulk density/(g .cm-3)3 . 35
Apparent porosity/%6.3
Room temperature flexural strength/Mpa29
High temperature flexural strength (1400℃)/Mpa23
Linear expansion rate (1500℃)/%1 . 00

Figure 1 Photo of the appearance of slide A after using 6 heats

Figure 2 Microstructure photos of slide plate A after using 6 heats

3. Mechanism analysis of loosening of working surface structure

The reason for the loosening of the working surface structure: the oxygen in the air that invades between the upper and lower sliding plate mating surfaces oxidizes the carbon in the sliding plate material. The dissolved oxygen and inclusions in the molten steel react with the slide gate plate material, causing the slide gate plate to oxidize, decarburize and loosen its structure; in addition, there are also reactions inside the skateboard material that cause its structure to become loose.

The oxygen in the air reacts with the carbon in the refractory material, causing the carbon to vaporize and escape, causing the structure of the refractory material to become loose.

C( S ) +O2 ( g) = CO( g)↑ (1)

Similarly, the dissolved oxygen in the molten steel reacts with the carbon in the refractory material, causing the carbon to vaporize and escape, and the carbon in the refractory material dissolves into the molten steel, which causes the structure of the refractory material to become loose.

C( S ) + [O] = CO( g)↑ (2)

C( S) =[C] (3)

The reaction inside the refractory material should be the reaction between components such as Al2 O3, ZrO2 and mullite in the refractory material and the carbon component. For example, when Al2O3 reacts with carbon, Al(g), Al2O(g), and CO(g) are formed and released. The reaction formula is as follows:

Al2 O3 ( S ) + 3C(S) = 2Al(g)↑+ 3CO(g)↑ (4)

Al2 O3 ( S ) + 2C(S) = Al2O(g)↑ + 2CO(g)↑ (5)

zrO2 ( S ) + 2C(S) = ZrC(S ) + 2CO(g)↑ (6)

Al6Si2O13(S)+6C(S)=3Al2O3(S)+2SiC(S)+4CO(g) (7)

For slide A after use, the disappearance of Al2O3 and carbon components and the loosening of the organizational structure observed under the microscope can be explained by reaction equations (4) and (5). The metamorphic phenomenon of ZrO2-mullite particles can be explained by reaction equations (6) and (7).

The schematic diagram of the loosening of the slide gate plate structure is shown in Figure 3. When casting when the slide plate is not fully open, a non-filled area of molten steel will appear near the nozzle hole of the upper slide plate, which will generate negative pressure. Fluent software was used to simulate the thermal fluid, and the results showed that the non-filled area was almost a vacuum. For the skateboard in the negative pressure zone, the Al(g), Al2O(g) and CO(g) produced by the above reactions (4)~(7) accelerate the dissipation, thereby promoting the progress of these reactions. In particular, the long casting time and the small opening of the slide plate (the working surface area in the negative pressure zone is large) are the main factors that aggravate the damage to the upper slide plate.

Figure 3 Schematic diagram of the formation of negative pressure area and loose layer of slide gate plate

4 Thermodynamic calculation verification

4.1 Al2 03 and C coexistence conditions

Using the thermodynamic calculation software Factsage, the above reaction conducted under negative pressure conditions was calculated and verified. Calculation conditions: Assume that the temperature near the working surface is 1550°C. Calculate the amount of various thermodynamically stable phases present in the system when the Al2O3 and C coexistence system (Al2O3 – C system) reaches equilibrium under different system pressures at 1550°C. . Taking reaction (4) as an example, assuming that the amount of C in the starting material is 1 mol and the amount of Al2O3 is 0.33 mol, the relationship between the amount of each component in the Al2O3-C system and the system pressure is shown in Figure 4. It can be seen from the figure that both Al2O3 and C exist stably in the pressure range of 0. 01 ~ 1 atm. However, when the pressure is less than 0. 01 atm, it decomposes to generate CO (g), Al (g) and Al2O (g), and the lower the system pressure, the greater the extent of the reaction. It can be seen that the lower the system pressure, the easier it is to form a loose layer. In other words, the reaction between Al2O3 and C is more likely to occur on the upper sliding plate working surface in the negative pressure zone, thus exacerbating the structural loosening of this part. At different temperatures, the relationship between the amount of CO (g) generated in the system and the system pressure is shown in Figure 5. It can be seen from the figure that the higher the temperature, the smaller the degree of negative pressure required to generate CO (g), that is, high temperature promotes the decomposition reaction of Al2O3.

Figure 4 The relationship between the amount of each component in the Al2O3-C system and the system pressure at 1550°C

Figure 5 The relationship between the production amount of CO (g) and the system pressure in the Al2 O3 -C system at different temperatures.

4.2 Zr02 and C coexist

Calculate the amount of various thermodynamically stable phases present in the system when the ZrO2 and C coexistence system (ZrO2-C system) reaches equilibrium under different system pressures at 1550°C. Taking reaction (6) as an example, assuming that the amount of C in the starting material is 1 mol and the amount of ZrO2 is 0.33 mol, the relationship between the amount of each component in the ZrO2-C system and the system pressure is shown in Figure 6. It can be seen from the figure that both ZrO2 and C exist stably when the system pressure is 1 atm, but when the system pressure is less than 0.5 atm, the two react to form ZrC. It can be seen that the reaction between ZrO2 and C under negative pressure causes C to disappear and lead to the loosening of the material structure. At different temperatures, the relationship between the amount of CO (g) generated in the ZrO2-C system and the system pressure is shown in Figure 7. It can be seen from the figure that the higher the temperature, the smaller the degree of negative pressure required to generate CO(g), that is, high temperature promotes the reaction between ZrO2 and C.

Figure 6 The relationship between the amount of each component and the system pressure in the ZrO2-C system at 1550℃

Figure 7 The relationship between the production amount of CO(g) and system pressure in ZrO2-C system at different temperatures.

4.3 Coexistence of mullite and C

Calculate the amount of thermodynamically stable phase present in the system when the coexistence system of mullite and C (mullite-C system) reaches equilibrium under different system pressures at a temperature of 1550°C. Taking reaction (7) as an example, assuming that the amount of C in the starting material is 1 mol and the amount of mullite is 0.46 mol, the relationship between the amount of each component in the mullite-C system and the system pressure is shown in Figure 8. It can be seen from the figure that mullite undergoes a decomposition-reduction reaction and is converted into Al2O3 and SiC at a pressure of 1atm. This decomposition-reduction reaction intensifies when the pressure is less than 1 atm, and SiO(g), Al(g) and CO(g) are generated when the pressure is less than 0.01 atm, further aggravating the loosening of the material. That is to say, mullite undergoes a decomposition-reduction reaction with C under negative pressure conditions and is converted into gas phase components of C, Si, and Al and then escapes, resulting in a loosening of the material structure. At different temperatures, the relationship between the production amount of CO (g) in the mullite-C system and the system pressure is shown in Figure 9. It can be seen from the figure that the higher the temperature, the smaller the degree of negative pressure required to generate CO(g), that is, high temperature promotes the reduction reaction of Al2O3.

For Al2O3 -SiO2 -ZrO2 -C materials, the reactions of Al2O3, mullite, ZrO2 and C are as follows: the lower the system pressure and the higher the temperature, the easier it is to react and the easier it is to form a loose layer; In addition, the order of reduction and decomposition reactions from easy to difficult is mullite, ZrO2, and Al2O3.

Figure 8 The relationship between the amount of each component and the system pressure in the mullite-C system at 1550℃

Figure 9 The relationship between the production amount of CO (g) and system pressure in the mullite-C system at different temperatures.

5 Loose layer formation test

In order to verify the conclusion that the negative pressure condition derived from the above thermodynamic calculation promotes the reduction reaction of Al2O3, mullite and ZrO2 and thereby promotes the formation of a loose layer on the surface of the slide gate plate, a heating test was conducted on slide gate plate A under negative pressure: Put the sample into a horizontal tube furnace, reduce the pressure in the furnace tube to 400pa (0.004atm) and maintain it, and then heat it up to 1500℃ at a heating rate of 300℃.h-1. After 5 hours of heat preservation, stop heating but continue to maintain a negative pressure of 400pa until the sample cools down. In addition, heating tests at 1500°C for 5 hours were conducted in Ar atmosphere, N2 atmosphere and buried carbon atmosphere respectively. In the heating tests of Ar atmosphere and N2 atmosphere, the furnace tube must be decompressed and inflated three times before heating. (Ar or N2) operation.

The appearance photos of the sample before and after the heating test are shown in Figure 10. It can be seen from the figure that the color of the sample surface is black before heating; after heating under negative pressure, Ar atmosphere or N2 atmosphere, the color of the sample surface has changed to gray, indicating that the C on the sample surface has disappeared; After heating under carbon-embedded conditions, the color of the sample surface still remains black, indicating that C on the sample surface still exists.

The mass change rate and linear change rate of the sample after the heating test, as well as the volume density and apparent porosity of the sample before and after heating are shown in Table 2.

Figure 10 Appearance photos of the sample before heating and after heating under different atmosphere conditions

Table 2 Properties of samples before heating and after heating under different atmosphere conditions

projectNegative pressure heatingAr medium heatingN2 medium heatingburied carbon heatingbefore heating
Quality change rate/%-2 . 86-0 . 75       -1 . 190 . 65 
Line change rate/%0 . 220 . 48         0 . 100 . 58 
Bulk density/(g .cm-3)3 . 263 . 30        3 . 313 . 34       3 . 36
Apparent porosity/%13.611.1         10.89.2        6.7

It can be seen from the data in Table 2 that the samples showed mass reduction after heating under negative pressure, Ar atmosphere and N2 atmosphere conditions. This should be caused by the reaction causing the material components to convert into the gas phase and then escape. The sample showed an increase in mass after the buried carbon was heated, which should be caused by the reaction of elements such as elemental Al and elemental Si contained in the material. Moreover, compared with the samples heated under Ar atmosphere and N2 atmosphere, the mass reduction rate and volume density of the samples heated under negative pressure conditions were significantly smaller, while the apparent porosity was significantly larger. This indicates significant dissipation of material components. The microstructural photos of the samples heated under different atmosphere conditions are shown in Figure 11.

Figure 11 Microstructure photos of samples heated under different atmosphere conditions

It can be seen from the figure that after heating under negative pressure conditions, the sample has a loose layer about 300 μm thick from the surface to the inside. In the loose layer, more voids are formed due to the disappearance of Al2O3 particles and C, and zrO2-mullite is obviously metamorphosed. This verifies the conclusion drawn from the previous thermodynamic calculation that “the reaction between oxide aggregate and carbon under negative pressure conditions can easily lead to the loosening of the material structure”, and also confirms the structural loosening of the sliding plate working surface after actual use. After heating in Ar atmosphere, some zrO2-mullite inside the sample was modified, but no Al2O3 particles were found to disappear. Although a 50 μm thick continuous oxide layer is formed on the surface of the sample, it is only formed by the reaction between a very small amount of Al(g) escaping inside the sample and a very small amount of oxygen on the surface of the sample. The condition of the sample after heating in N₂ atmosphere is the same as that of the sample after heating in Ar atmosphere. After carbon-embedded heating (that is, heating in a CO atmosphere), the ZrO₂-mullite in the sample did not deteriorate, and the material structure almost did not become loose.

The thermodynamic software FactSage was used to calculate the amount of each stable phase in the system when the material with the initial composition as shown in Table 1 reaches equilibrium when heated at 1500°C under different atmosphere conditions. When heating under negative pressure conditions, it is assumed that the system pressure is 0.001atm. When heating under Ar atmosphere and N₂ atmosphere conditions, the total amount of the initial components of the refractory material is set to 100% (w), assuming that the amounts of Ar and N₂ are sufficient and calculated as more than 1% (w). When heating under buried carbon (CO atmosphere) conditions, the total amount of the initial components of the refractory material is also set to 100% (w), and it is also assumed that CO (g) is sufficient and calculated with more than 1% (w). The calculation results are shown in Figure 12. It can be seen that during heating under negative pressure conditions, Al₂O₃(s), SiC(s), ZrC(s), CO(g), SiO(g), Al(g) and Al₂O(g) are calculated as stable phases. . Due to the reduction reaction of C, mullite, ZrO₂, and Al₂O₃ in the refractory materials, gas phases such as CO (g), SiO (g), and Al (g) are generated, so the total solid phase is reduced to 86.8% (w) , and the total amount of gas phase is as high as 13.2% (w). It is precisely because these gas phases escape outward that the mass reduction rate of the sample is greater after heating. The disappearance of Al₂O₃ particles and C in the loose layer on the surface of the sample, as well as the metamorphism of ZrO₂-mullite observed from the microstructural photos, are consistent with this calculation result. When heated in Ar atmosphere, Al₂O₃(s), mullite (s), C(s), SiC(s), ZrC(s), Ar(g), CO(g) and SiO(g) are stable phase calculation. Due to the reaction of C, a small amount of mullite, and ZrO₂ in the refractory material, the total amount of the solid phase is 99.3% (w), and the total amount of the gas phase is only 1.7% (w). The test result that the sample mass was only slightly reduced after heating was confirmed. During heating under N₂ atmosphere, Al₂O₃(s), ZrO₂(s), C(s), Si₂Al₈O₉N₈(s) are calculated as stable phases. Due to the reaction of C and mullite in the refractory material, the total content of the solid phase is 98.7% (w), while the total content of the gas phase is only 2.3% (w), which confirms the test result that the sample mass is slightly reduced after heating. . For heating under buried carbon (CO atmosphere), Al₂O₃(s), mullite (s), ZrO₂(s), C(s) and CO(g) are calculated as stable phases. Since C in the refractory material hardly reacts, the total amount of the solid phase is 100% (w), while the total amount of the gas phase is 1.0% (w), which proves that the amount of the gas phase has not increased. This is consistent with the test results that the sample mass did not decrease after heating, and almost no internal tissue deterioration and looseness were observed in the microstructure photos.

The above results show that the heating test under negative pressure conditions can reproduce the phenomenon of structural loosening of the working surface of the sliding nozzle slide, and therefore can effectively evaluate the loosening mechanism of the sliding nozzle slide.

Figure 12 Relative content of each stable phase in the sample before heating and when heating reaches equilibrium at 1500°C under different atmospheres

6 Conclusion

Through the analysis of the working surface of the slide gate plate after use, it was found that the Al₂O₃ particles and C disappeared near the working surface, and the ZrO₂-mullite aggregate deteriorated and became loose. In particular, a negative pressure zone is formed in the non-filled part near the nozzle hole of the upper slide plate. During the casting process, the reaction between the Al₂O₃ particles and ZrO₂-mullite aggregate in the working surface structure of the slide plate close to the negative pressure zone and the carbon in the refractory material is intensified. , the generated gas phase escapes outward, causing the structure of the slide gate plate working surface to become loose.

From a thermodynamic point of view, the smaller the pressure and the higher the temperature, the easier it is for the aggregate to react with carbon, leading to loosening. The reduction and decomposition reactions in order from easy to difficult are mullite, ZrO₂, and Al₂O₃.

The test on the formation of surface loosening layer under various heating conditions shows that heating under negative pressure can form an obvious loosening layer, which is consistent with the thermodynamic calculation results. When heated under Ar atmosphere or N₂ atmosphere, a slight loosening phenomenon can be observed. Compared with heating under negative pressure conditions, the degree of loosening is lighter; when heated under a buried carbon atmosphere, there is almost no loosening phenomenon. Through the heating test under negative pressure conditions, the loosening reaction of the slide gate plate working surface tissue can be reproduced, which is also an effective method to evaluate the surface damage of the slide gate plate.

LMM YOTAI established in 2007. Our production technology comes from Japanese Yotai. As an experienced and international player in the refractories industry. We have succeeded in expanding both the breadth of its product range and the depth of its services. From raw material selection, refractory portofio & optimization, installation & services & recycle of used refractories on site to further reduce client’s Opex & Capex in refractory consumption per ton steel output, meanwhile improve product quality of client.

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