In order to solve the problem of serious corrosion when casting calcium-treated steel, the traditional aluminum-zirconium carbon sliding plate using zirconium mullite as raw material has a high content of SiO2. Use partially stabilized ZrO2 (PSZ) to replace the zirconium mullite raw material, add tabular corundum, activated α-Al2 O3, carbon black, Si powder, B4C powder and thermosetting phenolic resin, and burn at 1300℃, 1400℃ and 1500℃ respectively. A low silicon aluminum zirconium carbon slide was made. The slag resistance, thermal expansion coefficient, expansion rate and strength loss rate after thermal shock of the samples were measured. The results show that the appropriate firing temperature for low silicon aluminum zirconium carbon sliding plate is 1300~1400℃. The performance of the specimen fired at this temperature is excellent, and its thermal shock resistance and slag resistance are better than the traditional aluminum-zirconium carbon slide plate using zirconium mullite as raw material.
Keywords: zirconia, aluminum zirconium carbon slide, strength, slag resistance, thermal shock resistance
Sliding nozzle is one of the key refractory materials in steelmaking production. Its use conditions are harsh and it needs to keep sliding during the steel pouring process, so it has strict requirements on its material and performance. The skateboard is a key component of the sliding nozzle, and its quality directly determines the performance of the sliding nozzle. At present, commonly used aluminum-zirconium carbon skateboards are generally made of corundum, zirconium-mullite, carbon-containing raw materials and various additives. It is made with phenolic resin as a binder and undergoes reduction firing, asphalt impregnation, carbonization and post-finishing. Although the aluminum zirconium carbon sliding plate has excellent thermal shock resistance, high temperature wear resistance and slag iron corrosion resistance when used on general steel types. However, with the advancement of steelmaking technology and changes in market demand, the smelting proportion of calcium-treated steel has increased year by year, and the traditional aluminum-zirconium carbon slide shows obvious incompatibility when casting this type of steel. The main reason is that SiO2 in zirconium mullite, the main raw material used in traditional aluminum-zirconium carbon slides, will react with Al2O3 and CaO at high temperatures to form yellow feldspar-like low-melting materials, which will cause serious erosion of the slides when casting this type of steel. In order to solve this problem, this work is based on the traditional aluminum zirconium carbon sliding plate using zirconium mullite, and uses partially stabilized ZrO2 (PSZ) raw material to replace the fused zirconium mullite raw material. In order to reduce the SiO2 content in the sliding plate, a new low-silicon aluminum-zirconium carbon sliding plate was developed, laying the foundation for casting molten steel with harsh operating conditions such as calcium-treated steel.
The test uses tabular corundum, active α-Al2 O3 (brand name CL-370C), partially stabilized ZrO2 (PSZ), carbon black (brand name N330) as the main raw materials, and Si powder (w (Si) = 98.21%) and B4C powder (w (B4 C) = 95. 35%, w (Fe) = 0..13%, d50 = 29. 2μm) is an additive, thermosetting phenolic resin (solid content is 80.57%, residual carbon content is 56. 39%, viscosity of 16.5 Pa·s, moisture content of 3.21%) was used as the binding agent, and a low-silicon aluminum-zirconium carbon sliding plate was developed. The chemical composition of the main raw materials is shown in Table 1, and the test formula is shown in Table 2. At the same time, a traditional aluminum-zirconium-carbon fired skateboard produced by a well-known domestic skateboard manufacturer using zirconium mullite as raw material (after asphalt impregnation and retort treatment) was selected as a comparison sample to compare performance indicators.
Table 1 Chemical composition of main raw materials used in the test (w)
|Raw material||Al₂O₃||ZiO₂||Fe₂O₃||CaO||MgO||SiO₂||N a₂O||Burning reduction|
Table 2 Test formula
|raw material||tabular corundum||Active α-Al₂O₃||RSZ||carbon black|
Note: The formula also contains 5% Si powder, 1% B4 C powder, and 5% phenolic resin.
The fine powder is first premixed, and then mixed with corundum particles and resin in a forced mixer. After the material was trapped for 24 hours, it was formed into a sample <50 mm × 50 mm on a 100t hydraulic press with a forming pressure of 50 MPa. It is used to test bulk density, apparent porosity, compressive strength and slag resistance test; it is formed into a 230 mm × 114 mm × 40 mm sample on a friction press and used to test flexural strength and thermal shock resistance.
The samples were dried at 180°C for 24h, then placed in a SiC sagger, covered with flake graphite, and fired at 1300°C, 1400°C, and 1500°C respectively, with a holding time of 8 hours. In order to simplify the test process, the samples were not subjected to asphalt impregnation and retort treatment.
After burning, the sample is tested according to national standards for volume density, apparent porosity, normal temperature compressive strength, normal temperature flexural strength, high temperature flexural strength (1400°C 0.5 h), thermal expansion coefficient and linear expansion rate (test temperature is 100 ~ 1300 ℃), and use X-ray diffractometer to measure the phase composition.
The static crucible method was used to measure the slag resistance of traditional slide plates and low-silicon slide samples after being burned at 1300°C and 1400°C. The slag erosion crucible is drilled from a sample <50 mm, with the hole size <10 mm×25 mm. The crucible is filled with slag and placed in an electric furnace, and the test is conducted under an oxidizing atmosphere. The test conditions are 1600°C for 3 hours. The slag used for the test was taken from the steel pouring tailings of a steelmaking plant. Its chemical composition (w) is as follows: Al2O3 10.48%, Fe2O3 3.80%, SiO2 28.20%, TiO2 1.21%, CaO 33.22%, MgO 12.23% , K2O 0. 08%, Na2O 0. 06%. After the slag resistance test, cut the crucible longitudinally along the center line to measure the slag erosion depth.
The air-cooling method was used to measure the thermal shock resistance of the samples after burning at 1400°C. After the test electric furnace is heated to 1100°C, the sample is placed and kept warm for 30 minutes, then the sample is taken out of the furnace and forced to cool to ambient temperature with a fan. After 3 cycles of this cycle, the thermal shock resistance is characterized by the flexural strength loss rate of the sample after thermal shock (equal to the difference in flexural strength before and after thermal shock/flexural strength before thermal shock × 100%).
Results and discussion
Physical properties of the sample
The physical property test results of the samples are shown in Table 3. It can be seen from Table 3 that as the firing temperature increases, the apparent porosity of the low silicon sliding plate increases, and the bulk density, compressive strength, normal temperature flexural strength and high temperature flexural strength all show a trend of first increasing and then decreasing. The bulk density and strength of the sample after burning at 1400°C are both at their maximum values. It can be predicted that after the asphalt impregnation treatment, the strength of this sample can be greatly improved, reaching the level of traditional aluminum zirconium carbon slide plates and meeting the usage requirements.
Table 3 Test results of physical performance indicators of samples
|sample||Firing temperature/℃||Volume density (g.cm-3)||Apparent porosity/%||Compressive strength/MPa||Room temperature flexural strength/MPa||High temperature flexural strength/MPa|
Note: The test sample is a low-silicon aluminum-zirconium carbon sample that has not been impregnated with asphalt and retorted. The comparison sample is a traditional finished aluminum-zirconium carbon slide plate made of zirconium-mullite as raw material that has been impregnated with asphalt and retorted. The same below.
Analysis shows that the main factors affecting the strength of the sample are the sintering temperature and the introduced Si powder. First, in order to form a certain ceramic bond and achieve a certain strength at room temperature and high temperature after the sliding plate is fired, the appropriate firing temperature must be controlled. Comprehensive analysis shows that the appropriate sintering temperature for low silicon aluminum zirconium carbon sliding plates is 1300-1400°C. Secondly, the introduction of Si powder can improve the strength of the sample. From the XRD analysis results (see Figure 1), it is found that elemental Si still remains in the sample after burning at 1300°C, while the presence of elemental Si is no longer detectable in the sample after burning at 1500°C. This shows that during the firing process at 1300~1500℃, solid-solid reaction and gas-solid reaction occurred in the sample, and the reaction products are SiO2, SiC, C and nitrogen carbides. These reaction products can fill the pores and enhance the bonding force between the matrices, thereby improving the strength of the sample.
Figure 1 XRD patterns of samples burned at 1300℃ and 1500℃
Slag resistance of the sample
Figure 2 shows the static crucible slag resistance test results of the sample. It can be seen that the low silicon aluminum zirconium carbon slide samples (codenamed C2 and C4 respectively) after firing at 1300°C and 1400°C have good slag erosion resistance. It is superior to the traditional aluminum zirconium carbon slide sample (codenamed ZM), and as the firing temperature increases, the corrosion resistance of the sample also increases, that is, the slag resistance of the sample after burning at 1400°C is better than that after burning at 1300°C of. The use test at TG Company also confirmed that the developed low-silicon aluminum-zirconium carbon sliding plate has a smooth surface without obvious pull marks after being used twice when casting billets; However, after one use of traditional aluminum-zirconium carbon skateboards under the same conditions, there will be obvious erosion on the skateboard surface.
Qiu Wendong et al. studied the decomposition of zirconium mullite in traditional aluminum zirconium carbon skateboards and found that the fused zirconium mullite in the skateboards decomposed to varying degrees after use. The reaction of mullite was as follows:
3Al2 O3·2SiO2 + 2C→3Al2O3+ 2SiO (g)+ 2CO (g)
3Al2 O3·2SiO2+ 2CO→3Al2O3+ SiO (g)+ 2CO2(g)
The volatilization of SiO(g) and the decomposition of mullite transform into columnar and bone-like corundum crystals, forming a porous structure. It destroys the original dense eutectic structure composed of mullite and baddeleyite, making the material’s structure loose, greatly reducing its strength and corrosion resistance, and accelerating the damage of the skateboard. By replacing the zirconium-mullite raw material with partially stabilized ZrO2 raw material, the loose structure caused by the decomposition of mullite can be avoided. Therefore, under the same conditions, the corrosion resistance of low-silicon aluminum-zirconium carbon skateboards is better than that of traditional aluminum-zirconium carbon skateboards.
However, it should be noted that Si powder is generally the preferred additive for low-silicon aluminum-zirconium carbon skateboards (the same is true for traditional aluminum-zirconium carbon skateboards), but this will have a certain impact on the corrosion resistance of the skateboards. In order to reduce this effect and further improve the corrosion resistance of the skateboard, the composite use of Si powder and metallic Al powder can be considered, but Si powder should be the main component.
Figure 2 Erosion test results of the sample
Linear expansion rate, linear expansion coefficient and strength loss rate after thermal shock
The test results of the linear expansion rate and average linear expansion coefficient of the test sample and the comparison sample are shown in Table 4. It can be seen from Table 4 that the thermal expansion rate and average linear expansion coefficient of the low-silicon aluminum-zirconium carbon sliding plate sample from room temperature to each test temperature point are lower than those of the traditional aluminum-zirconium carbon sliding plate. It can be speculated that its thermal shock resistance is better than that of traditional aluminum zirconium carbon skateboards.
The flexural strength of the low-silicon aluminum-zirconium carbon slide sample after burning at 1400℃ is 1.2 MPa before thermal shock, and the flexural strength after being air-cooled at 1100℃⇌ for three times is 18.6 MPa. The strength loss rate after thermal shock is 3.1%, which shows that the strength loss rate after thermal shock is small and the thermal shock resistance is good. This is related to the fact that the Si powder added to the material melts during the firing process, producing an appropriate amount of liquid phase to absorb thermal stress.
Table 4 Linear expansion rate and average linear expansion coefficient of the sample after burning at 1400 ℃
|Average linear expansion coefficient ×106/℃-1||test sample||126||10.0||9.6||8.7||8.9||8.6||8.1||7.4||7.0||6.5||6.0||5.5||5.1|
|Linear expansion rate/%||test sample||0.10||0.18||0.27||0.33||0.43||0.50||0.55||0.58||0.62||0.64||0.64||0.66||0.66|
It uses tabular corundum, active α-Al2O, partially stabilized ZrO2 (PSZ), and carbon black as the main raw materials, with Si powder and B4C as additives. Thermosetting phenolic resin is used as a binding agent, and after being fired at 1300-1400°C, a low-silicon aluminum-zirconium carbon skateboard with excellent performance can be prepared. Its thermal shock resistance and slag resistance are better than traditional aluminum zirconium carbon skateboards using zirconium mullite.