Discuss the feasibility of using low-carbon magnesia-carbon bricks to replace magnesia-rich dolomite unfired bricks in the ladle working layer in the stainless steel smelting process, and discuss the carbon addition of molten steel and the improvement of the ladle’s thermal insulation. Carbon-magnesia-carbon bricks combined with permanent layer materials with better thermal insulation properties can replace magnesium-rich dolomite unfired bricks for the production of stainless steel.
Key words: low carbon magnesia carbon brick; slag erosion resistance; carbon increase; thermal conductivity
Dolomite-based refractories have been widely used in iron and steel enterprises because of their good high temperature resistance, slag resistance, good volume stability under high temperature and vacuum, and the ability to purify molten steel. At the same time, they also have easy hydration properties. As well as poor thermal stability and cracking problems during use. The molten pool and the bottom of the stainless steel ladle in our factory have been using magnesium-rich dolomite unfired bricks with a carbon content of no more than 5%. Because there is no AOD furnace, the whole process of smelting is carried out in the ladle, and the average residence time of molten steel in the ladle is as long as 6 hours or more. At the same time, considering the cost of electricity consumption, the intermittent production method is adopted to avoid peaks, resulting in the ladle being in the oven for a long time. Bake heat preservation state. Factors such as high temperature, long smelting time, and vigorous argon/nitrogen agitation of double-blown argon bricks lead to low ladle life. After many improvements, although the ladle age has been improved, there are still a large number of cracks in the ladle bricks in the molten pool during hot use, which poses serious safety hazards and limits the increase in the ladle age.
At present, the research on low-carbon magnesia-carbon bricks mainly focuses on binders and carbon raw materials, and some breakthroughs have been made. It is understood that some large stainless steel factories use low-carbon magnesia-carbon bricks instead of magnesium-rich dolomite unfired bricks for the ladle pool and the bottom of the ladle, and the effect is good. Considering that the service life of magnesia-rich dolomite non-fired bricks has reached the bottleneck of use, there is no great potential for improvement, and there are safety hazards, it is decided to try low-carbon magnesia-carbon bricks to replace magnesia-rich dolomite non-fired bricks for ladle melting pools and Package bottom working layer.
Introduction to steelmaking process
The smelting process is a three-stage production model, mainly based on continuous casting slabs, supplemented by die-cast steel ingots of various sizes. The products include various series of low-carbon and low-nitrogen steels, mainly 201/202, 303, 304/304L, 308/309, 316L/316Ti, 318, 2205, 347H, 310, 314, 430, 630, T91, etc. The schematic diagram of the production process is shown in Figure 1, and the schematic diagram of the ladle is shown in Figure 2.
Fig.1 The production process flow diagram
Fig.2 The schematic diagram of the ladle
The equipment parameters are shown in Table 1.
Table 1 The parameters of the main smelting equipment
| Equipment name | Main parameters | Main function |
| Electric furnace | nominal capacity 60T, transformer rated capacity 45MVA | scrap stainless steel/alloy melting |
| LC/LF | nominal capacity 60T,LF transformer rated capacity 6300KVA | 1. Primary decarburization by oxygen blowing at LC station and preliminary adjustment of molten steel composition, tapping temperature 1720-1760℃;2. Adjustment of molten steel temperature/composition at LF station. |
| VOD | nominal capacity 60T,ultimate vacuum ≤15Pa | vacuum blowing oxygen decarburization, vacuum reduction, composition fine-tuning |
Low carbon magnesia carbon brick use analysis
Low carbon magnesia carbon brick slag resistance performance test
In order to understand the feasibility of low-carbon magnesia-carbon bricks in stainless steel production and application, the slag resistance performance test was carried out before the on-line trial, and compared with magnesia-rich dolomite unfired bricks. The tested low-carbon magnesia-carbon bricks use high-purity fused magnesia and composite carbon as the main raw materials, add composite antioxidants and reinforcing agents to the matrix, use phenolic resin as the binder, and bake at 200°C after high-pressure molding. preparation. The tested magnesia-rich dolomite unfired bricks are made of high-purity fused magnesia and calcined dolomite in a rotary kiln as the main raw materials, compound antioxidants and reinforcing agents are added to the matrix, and modified liquid asphalt is used as a binder. Prepared by baking at 250°C. The physical and chemical indicators of the two sample bricks selected are shown in Table 2.
Table 2 Composition comparison of the physical and chemical indicators between Low-carbon MgO-C brick and magnesium dolomite unburned brick
| type | Low carbon magnesia carbon brick | Magnesia rich dolomite unfired brick |
| MgO(%) | 91.15 | 60.8 |
| CaO(%) | 1.25 | 37.4 |
| C(%) | 5.8 | 3.6 |
| Apparent porosity(%) | 2.9 | 12.8 |
| Bulk density (g/cm3) | 3.15 | 2.94 |
| Compressive strength (Mpa) | 112 | 96.2 |
| Thermal conductivity (W/m·k) | 5.0 | 3.23 |
The slag resistance test of the two sample bricks was carried out by the static crucible method, the test condition was 1650 °C × 3 h, the test steel slag was VOD steel slag, and its chemical composition was shown in Table 3.
Table 3 The chemical composition of VOD slag
| chemical composition | SiO2 | Al2O3 | Fe2O3 | CaO | MgO | MnO |
| content/% | 2.94 | 19.82 | 0.3 | 63.34 | 6.69 | 0.17 |
It can be seen from the chemical composition in Table 3 that its main components are CaO and Al2O3. The main phases of the steel slag are identified by X-ray diffractometer as CaO and dodecacalcium heptaaluminate (7Al2O3∙12CaO, A7 C12), because the melting point of A7 C12 is at Between 1415 °C and 1495 °C, it is a low-viscosity liquid phase in the molten steel refining process, which has strong corrosion and permeability to refractory materials.
The cross-sectional photos of the two sample bricks after the static crucible slag test are shown in Figure 3. The measured erosion widths of the low-carbon magnesia-carbon brick and the magnesia-rich dolomite unfired brick are 1.10 mm and 1.05 mm, respectively, indicating the erosion resistance of the two samples Basically the same. In addition, observing the residues in the crucibles of the two sample bricks, it was found that the amount of residues in low-carbon magnesia-carbon bricks was more than that of magnesium-rich dolomite unfired bricks, that is, low-carbon magnesia-carbon bricks had better The anti-slag penetration performance is inseparable from the greater density of low-carbon magnesia-carbon bricks.
Fig.3 The sectional photos of Low-carbon MgO-C brick and magnesium dolomite unburned brick after slag resistance test
The comparison test shows that the low-carbon magnesia-carbon brick can be tried on the ladle.
Trial of low carbon magnesia carbon bricks
For example, the bottom of the 60T ladle and the part of the molten pool have an average ladle age of 17.5 times, while the ladle age of the magnesium-rich dolomite unfired brick in the same period is 14.4 times, an increase of 3.1 times. Dolomite unfired bricks are raised by 15 mm. Both safety and package age have been improved.
Cracks can be observed during online use of magnesium-rich dolomite unfired bricks, and the cracks will further expand during medium repair/overhaul, and there are slag inclusions in the cracks, indicating that the cracks are not surface cracks during online use; low carbon There is no cracking problem found in the online use of magnesia carbon bricks, but cold shrinkage cracks will appear during the cooling process of the off-line intermediate repair/overhaul, and the maximum crack width is about 10 mm. Comparing the two, it can be seen that the safety of low-carbon magnesia-carbon bricks is higher than that of magnesia-rich dolomite unfired bricks, but the cracking problem of low-carbon magnesia-carbon bricks during the cooling process of the off-line makes the ladle unable to be repaired. Difficult to meet flexible production needs.
According to the analysis, after the carbon content in low-carbon magnesia-carbon bricks is reduced, the thermal conductivity of the bricks decreases and the elastic modulus increases, which makes the thermal shock resistance of the bricks worse, and makes the slag and molten steel and low-carbon magnesia-carbon bricks The wettability of the low-carbon magnesia-carbon brick is enhanced, and the permeability of the slag and molten steel becomes poor [6], which leads to cracking problems in the use of low-carbon magnesia-carbon bricks. However, if the carbon content is too high, it will cause problems such as carburization of molten steel, and it is difficult to meet the production needs of low-carbon stainless steel [7].
Refractory manufacturers supplying low-carbon magnesia-carbon bricks have subsequently improved antioxidants and magnesia, which has significantly reduced cracking problems during off-line cooling. Using the improved low-carbon magnesia-carbon bricks, no large cracks appeared during the offline cooling process, and no spalling problems occurred. The average life of the ladle has reached 18.4 times, which is 27.7% higher than before the improvement.
Carbon increase analysis of low carbon magnesia carbon bricks
Table 2 shows that the carbon content of magnesia-rich dolomite unfired bricks is distributed between 3.0%-4.0%, and the carbon content of low-carbon magnesia-carbon bricks is distributed between 5.5%-6.5%. Stone unfired bricks may have more carburization problems. While testing low-carbon magnesia-carbon bricks, track the amount of carbon added to molten steel from VOD breaking through to the continuous casting sampling interval without adding high-carbon alloys. The data is shown in Figure 4.
Fig.4 Low-carbon MgO-C brick and magnesium dolomite unburned brick increase of carbon for the molten steel
It can be seen that the carbon addition of low-carbon magnesia-carbon bricks to molten steel is slightly higher than that of magnesium-rich dolomite unfired bricks, but the carbon addition of most furnaces is below 50ppm, which can basically meet the needs of low-carbon steel types. production needs.
Ladle insulation problem
The thermal conductivity of low-carbon magnesia-carbon bricks is 5.0 W/m·k, which is much higher than that of magnesia-rich dolomite unfired bricks (3.23 W/m·k). The phenomenon that the shell of the ladle is red. In order to protect the ladle shell and improve the safety of use, it is necessary to improve the thermal insulation performance of the ladle. The thermal insulation layer of the front ladle of low-carbon magnesia-carbon bricks is a layer of lightweight thermal insulation bricks with a thickness of 32 mm, and the permanent layer is a layer of fired magnesia bricks with a thickness of 64 mm. In order to improve the thermal insulation performance, it is considered to use thermal insulation bricks High-alumina castables and pyrophyllite bricks with better performance are matched with magnesia-silicon lightweight insulation boards. The thermal conductivity of fired magnesia bricks, high-alumina castables and pyrophyllite bricks is shown in Table 4.
Table 4 Thermal conductivity of different insulation materials at different temperatures (W/m·k)
| T(°C) | 300 | 400 | 500 | 600 | 700 | 800 | 900 | 1000 | 1100 | 1200 |
| Burnt magnesia brick | 8.49 | 7.37 | 6.36 | 5.48 | 4.71 | 4.07 | 3.56 | 3.15 | 2.87 | 2.7 |
| High alumina castable | 1.89 | 1.93 | 1.98 | 2.02 | 2.07 | 2.11 | 2.155 | 2.2 | 2.25 | 2.29 |
| wax stone brick | 1.09 | 1.1 | 1.12 | 1.13 | 1.15 | 1.16 | 1.18 | 1.19 | 1.21 | 1.22 |
It can be seen from Table 4 that the thermal conductivity of magnesia bricks is the highest, indicating that the improvement has certain feasibility. To this end, the following five groups of comparative tests were developed, and the test schemes and descriptions are shown in Table 5.
Table 5 The narrative of test program
| Scheme number | Insulation | permanent layer | working layer |
| 1 | 32mm thick lightweight thermal insulation brick | 64mm thick fired magnesia brick | Low carbon magnesia carbon brick |
| 2 | 10mm thick lightweight insulation board | 86mm thick high aluminum castable | Low carbon magnesia carbon brick |
| 3 | 32mm thick lightweight thermal insulation brick | 64mm thick fired magnesia brick | Magnesium-rich dolomite unfired brick |
| 4 | 10mm thick lightweight insulation board | 30mm thick pyrophyllite brick and 64mm thick high alumina brick | Low carbon magnesia carbon brick |
| 5 | 10mm thick lightweight insulation board | 30mm thick pyrophyllite brick and 64mm thick high alumina brick | Magnesium-rich dolomite unfired brick |
The comparison method is to measure the temperature change curve of the ladle shell at the same molten pool position after the ladle is poured. The measuring instrument is the same infrared temperature measuring gun (XT-672), and the change curve is shown in Figure 5.
Fig.5 Temperature distribution of the ladle shell
From the analysis of the chart data, it can be seen that the temperature of the steel ladle shell is the highest in the scheme 1 before the thermal insulation performance is not improved, and the temperature of the steel ladle shell in the scheme 2 and scheme 3 is basically the same. The temperature is about 50°C lower. Considering issues such as safety, service life, thermal insulation performance, and cost, it was decided to use 10mm lightweight insulation boards for the insulation layer, and 30mm thick pyrophyllite bricks and 64mm thick high alumina bricks for the permanent layer.
Conclusion
Comprehensive data analysis shows that low-carbon magnesia-carbon bricks combined with high-alumina castables and pyrophyllite bricks with better thermal insulation performance can replace magnesia-rich dolomite bricks for smelting low-carbon stainless steel. :
(1) After using low-carbon magnesia-carbon bricks, the average ladle age of the ladle is significantly improved, which reduces the cost of refractory materials for ladle bricks;
(2) Compared with magnesia-rich dolomite unfired bricks, low-carbon magnesia-carbon bricks can add 10-20ppm more carbon to molten steel, which can meet the needs of stainless steel smelting;
(3) During the use of low-carbon magnesia-carbon bricks, shrinkage and cracking are significantly reduced compared with magnesia-rich dolomite unfired bricks, which significantly improves the safety of ladle use.