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Effect of Steel Slag Basicity on Corrosion Rate of Magnesia Carbon Bricks

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The effect of the basicity of refining steel slag on the erosion rate of protitium carbon bricks was studied when the molten steel temperature was 1600°C and the erosion time was 0.5, 1, 1.5, 2, 2.5, and 3.5 h. The results show that: when the erosion time is less than 3.5 h, the erosion rate of the low-alkalinity steel slag on the protactinium carbon brick is greater; in the same erosion time, the damage index of the low-alkalinity steel slag is greater. The corrosion rate of steel slag with different alkalinity to protactinium carbon bricks shows the same law, that is, the law of increase-decrease-stabilization-increase.

Key words: basicity of steel slag; protactinized carbon brick; erosion rate; damage index; spin dipping method

The establishment of a safe use system centered on refractory materials has an important impact on the quality and cost of steel and the safe production of steel mills. Therefore, in terms of refractory application research, many material science workers have given relevant mathematical models by analyzing the damage causes of related refractory materials, such as Akkurt, etc. MgO-C bricks for tube furnaces under the protection of argon or CO atmosphere The high temperature slag erosion test shows that when the time is prolonged, the temperature rises, the partial pressure of oxygen in the atmosphere increases and the alkalinity of the slag decreases, the erosion of the refractory material is intensified, and the degree of slag erosion is related to the rate of carbon loss. Xu Ping studied the static magnetic field,

Effects of electromagnetic field, Fe content and carbon content in slag on the mechanism of anti-slag erosion of MgO-C refractories. The effect of P-SiAlON content on the slag resistance of MgO-based castables was studied by static crucible method and induction furnace rotary impregnation method under different basicity slag conditions. The research shows that there is a linear relationship between the fractal dimension of the slag erosion interface morphology of the sample and its slag erosion resistance. In actual operation, the damage and failure of refractory materials are mainly judged by the experience and habits of on-site technicians. The basicity of steel slag changes during the actual refining process. At present, the process of ladle refractories changing with service time until failure and dynamic damage needs further study. Therefore, in this work, the influence of steel slag alkalinity on the erosion rate of magnesia-carbon bricks was studied by using the induction furnace rotary impregnation method, and the damage process of magnesia-carbon bricks over time was analyzed, in order to realize the simulation of the damage process of magnesia-carbon bricks. In order to ensure the safe operation of the ladle, reduce the consumption of refractory materials in the ladle, and improve the quality of clean steel, it provides practical guidance and theoretical reference.

Test

Sample brick and sample preparation

The sample brick is a trapezoidal magnesia carbon brick of model YG8 (upper part 180 mm x 90 mm, bottom 160 mm x 90 mm, height 230 mm), physical and chemical indicators: ω( C) = 17%, bulk density 2.87 g cm-3, apparent porosity 3 . 73%, and the compressive strength at room temperature is 7. 78 MPa. The magnesia carbon bricks were cut into 45 mm x 45 mm x 230 mm samples with a cutting machine in the laboratory.

Test process

Clamp the sample on the test machine, then add about 15 kg of rebar (brand HRB400) into the induction furnace, turn on the power, preheat for 10 minutes, increase the power and burn until the steel melts, and then weigh the two kinds of steel respectively. Put 20 g of refined steel slag into the furnace, adjust the power, and measure the surface temperature of the steel slag with an infrared thermometer (model is Ircon, ux60P). When the surface temperature reaches 1 600 C and is stable, clamp the sample with the test machine and immerse it in the molten steel of the induction furnace, stop at a distance of 1 cm from the bottom of the sample (see Figure 1 for the schematic diagram), and then start the speed switch of the induction furnace to make the sample The rotation speed is stable at 30r•min-1. The basicity of refined steel slag 1 is 1.3, and the basicity of refined steel 2 is 2.1. The specific chemical composition is shown in Table 1.

Table 1 Chemical composition of two refining steel slcigs

project ω/%
Al2O3 TiO2 SiO2 Fe2O3 CaO MgO MnO2 P2O5
Refined steel slag 1 7.7 0.9 33.1 2.1 43.3 11.2 0.7 0.5
Refined steel slag 2 2.4 1.6 13.1 38.3 28.0 9.0 5.3 2.0

During the test, 50 g of refining slag was added to the induction furnace every 2 hours to keep the composition of steel slag in the furnace stable and the temperature kept constant. The erosion time was set to 0.5, 1, 1.5, 2, 2.5, 3 and 3.5 h, respectively.

Fig. 1 Schematic diagram of rotary dipping method

Performance characterization

Considering the error during cutting and the uncertainty of the slag line layer, mark A, B, C, and D on the 4 surfaces of the sample respectively, measure and record the distance between the 4 surfaces and the bottom surface every Width at intervals of 1 cm. The damage index is used to characterize the damage degree of magnesia carbon bricks. The damage index is the difference in the width of the sample at the same height before and after the erosion test. For example: when the height of the slag line layer of the sample after erosion is 10 cm from the bottom surface, subtract the current width after erosion from the original 10 cm width of surface A before erosion to obtain the double width of surface A (due to the Erosion is that both sides of a surface are eroded at the same time), divided by 2 to get the average width of A surface, that is, the damage index of A surface. Calculate the other three surfaces at 10 cm in turn to obtain the damage index of the four surfaces, sum and divide by 4 to obtain the average damage index at 10 cm, which is the damage index at 10 cm. The specific calculation diagram is shown in Figure 2.

Fig. 2 Schematic diagram of damage index calculation of specimens

The damage degree of the sample is expressed by the damage index y. In Figure 2, H1 is the height from a certain position on the bottom surface of the sample. H1 represents different slag line positions due to different positions, and i represents the surface with different height. When i=1, it represents the A surface of this height, i= 2 represents the B surface at this height, i takes 3 and 4 to represent the C surface and D surface respectively), b is the width of the sample before damage, b’ is the width after damage, the calculation formula of the damage index As shown in (1):

Results and analysis

Erosion Analysis

Calculate the damage index at 10 cm of the sample corroded by refined steel slag 1 and refined steel slag 2, and make it into Fig. 3. At the same time, according to the relationship between the damage index and erosion time, according to the erosion rate v=∆y/∆t (∆y is the absolute value of the damage index difference between two consecutive points, and ∆t is the absolute value of the time difference between two consecutive points) , calculate the erosion rate every two points, and draw the curves in Figure 4 with the values of the erosion rates at two points of the two types of steel slag corresponding to one point.

Fig. 3 Damage index of specimens as a function of time

Fig. 4 Erosion rate of two kinds of steel slag as a function of time

As can be seen from Figure 3: the damage index of two kinds of steel slags with different basicities to magnesia-carbon bricks shows the same law with time, both increase first, then stabilize and finally increase again, and the refined steel slag 1 with lower basicity is first in Stabilization was reached after 1.5 h. Comparing the damage index of the two steel slags in the same time period, it is found that the damage index of the refined steel slag 1 with low alkalinity is larger than that of the refined steel slag 2 with high alkalinity, indicating that the low alkalinity refined steel slag 1 is more serious for the damage of magnesia carbon bricks .

It can be seen from Figure 4 that: at 0.5~1.5h and 2.5~3.5h, the corrosion rate of magnesia carbon bricks by low alkalinity steel slag is greater than that of high alkalinity steel slag; at 1.5~2.5h, although low alkalinity The corrosion rate of steel slag is slightly lower than that of high alkalinity steel slag, but it can be seen from Figure 3 that the stage of 1.5~2.5h is the stage of relatively stable erosion, and the damage index of steel slag with low alkalinity is greater than that of high alkalinity steel slag at <2.5h The steel slag with low alkalinity shows that the average erosion rate of steel slag with low alkalinity is greater than that of high alkalinity steel slag within the same time period. This shows that the corrosion rate of low-basic steel slag to magnesia-carbon bricks is greater than that of high-basic steel slag. Analysis thinks: the raising of basicity of steel slag increases the content of calcium oxide and magnesium oxide in steel slag, and calcium oxide and magnesium oxide can neutralize some acidic oxides in magnesia carbon brick, reduce the saturation degree of calcium oxide and magnesium oxide in steel slag, In order to reduce the corrosion of the aggregate in the magnesia carbon brick by steel slag.

From the relationship between the mass ratio of CaO and SiO2 and the combination of magnesia refractories, it can be seen that when the mass ratio of CaO to SiO2 in the system is < 1.87, there will be low melting point substances, and the initial melting temperature will be lower, which will seriously affect the refractory performance of magnesia refractories. ; When the mass ratio of CaO to SiO2 is ≥1.87, the refractory performance will not be significantly reduced due to the formation of high refractory minerals. After the carbon in the magnesia-carbon brick is oxidized, the interface between the steel slag and the magnesia-carbon brick forms MgO-CaO-SiO2, which conforms to the low melting point CMS when the mass ratio of CaO to SiO2 is 0.93~1.87; when the mass ratio of CaO to SiO2 is ≥1.87, it is formed High melting point C2S. The basicity of the refining slag 1 is 0.93-1.4, and more low melting point substances are produced, which can promote the dissolution of magnesia particles and accelerate the falling off of magnesia particles.

Fig. 5 XRD patterns of surfaice of specimens corroded by refining steel slcig

Phase and microstructure analysis

The XRD pattern of the surface at 1 cm of the sample after erosion by refining steel slag is shown in Figure 5. It can be seen from Figure 5 that the erosion of magnesia-carbon bricks by low-alkalinity steel slag does produce low-melting point CMS, and the appearance of low-melting point materials promotes the dissolution of magnesia particles and accelerates the shedding of magnesia particles. As shown in Figure 6. It can be seen from Figure 6(a) that the interface between the steel slag and the magnesia-carbon brick is very clear, and the steel slag has obvious penetration on the surface of the magnesia-carbon brick. This may be due to the pores left by the oxidation of surface carbon or the large pores on the surface itself to allow steel slag to penetrate. It can be seen from Figure 6(b) that the magnesia-carbon brick itself is not very dense, and there are large gaps between the particles. The existence of these gaps leaves space for the penetration and erosion of steel slag into the magnesia-carbon brick, which accelerates the Oxidation of magnesia carbon bricks.

Fig. 6 SEM photographs of magnesia carbon brick specimens after erosion

In order to further prove the existence of CMS, 1 cm of the eroded sample of refining slag 1 was taken for scanning electron microscope observation (see Figure 7) and energy spectrum analysis. In the SEM photos, it is found that the particle distribution in the erosion area is small and numerous, and cracks appear on the surface of the large particles, and the cracks penetrate each other. The appearance of cracks connects the erosion area to the outside world, further accelerating the oxidation of carbon. The micro-area energy spectrum analysis results (*) of point 1 are: C 46.36%, O 37.41%, Mg 4.08%, Al 2.31%, Si 4.83%, Ca 5.01%. It is proved that CaMgSiO4 is generated in the erosion area.

Fig. 7 SEM photograph of magnesiacarbon brick after surface erosion

Conclusion

(1) When less than 3.5 h, the erosion rate of low-alkalinity steel slag to magnesia-carbon bricks is greater; at the same time, the damage index of low-alkalinity steel slag is greater.

(2) Under the conditions of alkalinity 1.3 and 2.1, the corrosion rate of steel slag to magnesia carbon bricks shows the same law, that is, the law of increase-decrease-stabilization-increase.

(3) Low-alkalinity steel slag generates low melting point CMS at the erosion interface, and the appearance of low melting point promotes the dissolution of magnesia particles, thereby accelerating the shedding of magnesia particles.

The effect of the basicity of refining steel slag on the erosion rate of protitium carbon bricks was studied when the molten steel temperature was 1600°C and the erosion time was 0.5, 1, 1.5, 2, 2.5, and 3.5 h. The results show that: when the erosion time is less than 3.5 h, the erosion rate of the low-alkalinity steel slag on the protactinium carbon brick is greater; in the same erosion time, the damage index of the low-alkalinity steel slag is greater. The corrosion rate of steel slag with different alkalinity to protactinium carbon bricks shows the same law, that is, the law of increase-decrease-stabilization-increase.

Key words: basicity of steel slag; protactinized carbon brick; erosion rate; damage index; spin dipping method

The establishment of a safe use system centered on refractory materials has an important impact on the quality and cost of steel and the safe production of steel mills. Therefore, in terms of refractory application research, many material science workers have given relevant mathematical models by analyzing the damage causes of related refractory materials, such as Akkurt, etc. MgO-C bricks for tube furnaces under the protection of argon or CO atmosphere The high temperature slag erosion test shows that when the time is prolonged, the temperature rises, the partial pressure of oxygen in the atmosphere increases and the alkalinity of the slag decreases, the erosion of the refractory material is intensified, and the degree of slag erosion is related to the rate of carbon loss. Xu Ping studied the static magnetic field,

Effects of electromagnetic field, Fe content and carbon content in slag on the mechanism of anti-slag erosion of MgO-C refractories. The effect of P-SiAlON content on the slag resistance of MgO-based castables was studied by static crucible method and induction furnace rotary impregnation method under different basicity slag conditions. The research shows that there is a linear relationship between the fractal dimension of the slag erosion interface morphology of the sample and its slag erosion resistance. In actual operation, the damage and failure of refractory materials are mainly judged by the experience and habits of on-site technicians. The basicity of steel slag changes during the actual refining process. At present, the process of ladle refractories changing with service time until failure and dynamic damage needs further study. Therefore, in this work, the influence of steel slag alkalinity on the erosion rate of magnesia-carbon bricks was studied by using the induction furnace rotary impregnation method, and the damage process of magnesia-carbon bricks over time was analyzed, in order to realize the simulation of the damage process of magnesia-carbon bricks. In order to ensure the safe operation of the ladle, reduce the consumption of refractory materials in the ladle, and improve the quality of clean steel, it provides practical guidance and theoretical reference.

Test

Sample brick and sample preparation

The sample brick is a trapezoidal magnesia carbon brick of model YG8 (upper part 180 mm x 90 mm, bottom 160 mm x 90 mm, height 230 mm), physical and chemical indicators: ω( C) = 17%, bulk density 2.87 g cm-3, apparent porosity 3 . 73%, and the compressive strength at room temperature is 7. 78 MPa. The magnesia carbon bricks were cut into 45 mm x 45 mm x 230 mm samples with a cutting machine in the laboratory.

Test process

Clamp the sample on the test machine, then add about 15 kg of rebar (brand HRB400) into the induction furnace, turn on the power, preheat for 10 minutes, increase the power and burn until the steel melts, and then weigh the two kinds of steel respectively. Put 20 g of refined steel slag into the furnace, adjust the power, and measure the surface temperature of the steel slag with an infrared thermometer (model is Ircon, ux60P). When the surface temperature reaches 1 600 C and is stable, clamp the sample with the test machine and immerse it in the molten steel of the induction furnace, stop at a distance of 1 cm from the bottom of the sample (see Figure 1 for the schematic diagram), and then start the speed switch of the induction furnace to make the sample The rotation speed is stable at 30r•min-1. The basicity of refined steel slag 1 is 1.3, and the basicity of refined steel 2 is 2.1. The specific chemical composition is shown in Table 1.

Table 1 Chemical composition of two refining steel slcigs

project ω/%
Al2O3 TiO2 SiO2 Fe2O3 CaO MgO MnO2 P2O5
Refined steel slag 1 7.7 0.9 33.1 2.1 43.3 11.2 0.7 0.5
Refined steel slag 2 2.4 1.6 13.1 38.3 28.0 9.0 5.3 2.0

During the test, 50 g of refining slag was added to the induction furnace every 2 hours to keep the composition of steel slag in the furnace stable and the temperature kept constant. The erosion time was set to 0.5, 1, 1.5, 2, 2.5, 3 and 3.5 h, respectively.

Fig. 1 Schematic diagram of rotary dipping method

Performance characterization

Considering the error during cutting and the uncertainty of the slag line layer, mark A, B, C, and D on the 4 surfaces of the sample respectively, measure and record the distance between the 4 surfaces and the bottom surface every Width at intervals of 1 cm. The damage index is used to characterize the damage degree of magnesia carbon bricks. The damage index is the difference in the width of the sample at the same height before and after the erosion test. For example: when the height of the slag line layer of the sample after erosion is 10 cm from the bottom surface, subtract the current width after erosion from the original 10 cm width of surface A before erosion to obtain the double width of surface A (due to the Erosion is that both sides of a surface are eroded at the same time), divided by 2 to get the average width of A surface, that is, the damage index of A surface. Calculate the other three surfaces at 10 cm in turn to obtain the damage index of the four surfaces, sum and divide by 4 to obtain the average damage index at 10 cm, which is the damage index at 10 cm. The specific calculation diagram is shown in Figure 2.

Fig. 2 Schematic diagram of damage index calculation of specimens

The damage degree of the sample is expressed by the damage index y. In Figure 2, H1 is the height from a certain position on the bottom surface of the sample. H1 represents different slag line positions due to different positions, and i represents the surface with different height. When i=1, it represents the A surface of this height, i= 2 represents the B surface at this height, i takes 3 and 4 to represent the C surface and D surface respectively), b is the width of the sample before damage, b’ is the width after damage, the calculation formula of the damage index As shown in (1):

Results and analysis

Erosion Analysis

Calculate the damage index at 10 cm of the sample corroded by refined steel slag 1 and refined steel slag 2, and make it into Fig. 3. At the same time, according to the relationship between the damage index and erosion time, according to the erosion rate v=∆y/∆t (∆y is the absolute value of the damage index difference between two consecutive points, and ∆t is the absolute value of the time difference between two consecutive points) , calculate the erosion rate every two points, and draw the curves in Figure 4 with the values of the erosion rates at two points of the two types of steel slag corresponding to one point.

Fig. 3 Damage index of specimens as a function of time

Fig. 4 Erosion rate of two kinds of steel slag as a function of time

As can be seen from Figure 3: the damage index of two kinds of steel slags with different basicities to magnesia-carbon bricks shows the same law with time, both increase first, then stabilize and finally increase again, and the refined steel slag 1 with lower basicity is first in Stabilization was reached after 1.5 h. Comparing the damage index of the two steel slags in the same time period, it is found that the damage index of the refined steel slag 1 with low alkalinity is larger than that of the refined steel slag 2 with high alkalinity, indicating that the low alkalinity refined steel slag 1 is more serious for the damage of magnesia carbon bricks .

It can be seen from Figure 4 that: at 0.5~1.5h and 2.5~3.5h, the corrosion rate of magnesia carbon bricks by low alkalinity steel slag is greater than that of high alkalinity steel slag; at 1.5~2.5h, although low alkalinity The corrosion rate of steel slag is slightly lower than that of high alkalinity steel slag, but it can be seen from Figure 3 that the stage of 1.5~2.5h is the stage of relatively stable erosion, and the damage index of steel slag with low alkalinity is greater than that of high alkalinity steel slag at <2.5h The steel slag with low alkalinity shows that the average erosion rate of steel slag with low alkalinity is greater than that of high alkalinity steel slag within the same time period. This shows that the corrosion rate of low-basic steel slag to magnesia-carbon bricks is greater than that of high-basic steel slag. Analysis thinks: the raising of basicity of steel slag increases the content of calcium oxide and magnesium oxide in steel slag, and calcium oxide and magnesium oxide can neutralize some acidic oxides in magnesia carbon brick, reduce the saturation degree of calcium oxide and magnesium oxide in steel slag, In order to reduce the corrosion of the aggregate in the magnesia carbon brick by steel slag.

From the relationship between the mass ratio of CaO and SiO2 and the combination of magnesia refractories, it can be seen that when the mass ratio of CaO to SiO2 in the system is < 1.87, there will be low melting point substances, and the initial melting temperature will be lower, which will seriously affect the refractory performance of magnesia refractories. ; When the mass ratio of CaO to SiO2 is ≥1.87, the refractory performance will not be significantly reduced due to the formation of high refractory minerals. After the carbon in the magnesia-carbon brick is oxidized, the interface between the steel slag and the magnesia-carbon brick forms MgO-CaO-SiO2, which conforms to the low melting point CMS when the mass ratio of CaO to SiO2 is 0.93~1.87; when the mass ratio of CaO to SiO2 is ≥1.87, it is formed High melting point C2S. The basicity of the refining slag 1 is 0.93-1.4, and more low melting point substances are produced, which can promote the dissolution of magnesia particles and accelerate the falling off of magnesia particles.

Fig. 5 XRD patterns of surfaice of specimens corroded by refining steel slcig

Phase and microstructure analysis

The XRD pattern of the surface at 1 cm of the sample after erosion by refining steel slag is shown in Figure 5. It can be seen from Figure 5 that the erosion of magnesia-carbon bricks by low-alkalinity steel slag does produce low-melting point CMS, and the appearance of low-melting point materials promotes the dissolution of magnesia particles and accelerates the shedding of magnesia particles. As shown in Figure 6. It can be seen from Figure 6(a) that the interface between the steel slag and the magnesia-carbon brick is very clear, and the steel slag has obvious penetration on the surface of the magnesia-carbon brick. This may be due to the pores left by the oxidation of surface carbon or the large pores on the surface itself to allow steel slag to penetrate. It can be seen from Figure 6(b) that the magnesia-carbon brick itself is not very dense, and there are large gaps between the particles. The existence of these gaps leaves space for the penetration and erosion of steel slag into the magnesia-carbon brick, which accelerates the Oxidation of magnesia carbon bricks.

Fig. 6 SEM photographs of magnesia carbon brick specimens after erosion

In order to further prove the existence of CMS, 1 cm of the eroded sample of refining slag 1 was taken for scanning electron microscope observation (see Figure 7) and energy spectrum analysis. In the SEM photos, it is found that the particle distribution in the erosion area is small and numerous, and cracks appear on the surface of the large particles, and the cracks penetrate each other. The appearance of cracks connects the erosion area to the outside world, further accelerating the oxidation of carbon. The micro-area energy spectrum analysis results (*) of point 1 are: C 46.36%, O 37.41%, Mg 4.08%, Al 2.31%, Si 4.83%, Ca 5.01%. It is proved that CaMgSiO4 is generated in the erosion area.

Fig. 7 SEM photograph of magnesiacarbon brick after surface erosion

Conclusion

(1) When less than 3.5 h, the erosion rate of low-alkalinity steel slag to magnesia-carbon bricks is greater; at the same time, the damage index of low-alkalinity steel slag is greater.

(2) Under the conditions of alkalinity 1.3 and 2.1, the corrosion rate of steel slag to magnesia carbon bricks shows the same law, that is, the law of increase-decrease-stabilization-increase.

(3) Low-alkalinity steel slag generates low melting point CMS at the erosion interface, and the appearance of low melting point promotes the dissolution of magnesia particles, thereby accelerating the shedding of magnesia particles.

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