Magnesia-carbon bricks are alkaline refractory materials made of magnesia and carbon materials, using asphalt or resin as binders. It has the multiple advantages of high refractoriness of magnesia refractories and carbon refractories that are difficult to be wetted by slag and molten steel. Because of its high refractoriness, strong slag erosion resistance, excellent thermal shock resistance and many other excellent characteristics, it is It is widely used in the steel industry, such as converter furnace mouth, electric furnace slag line part, ladle working layer and other harsh environments such as high temperature, severe mechanical erosion, and severe slag erosion. (Picture 1)
Although magnesia-carbon bricks have the above-mentioned many advantages, the carbon in the bricks is easily oxidized in high temperature and oxidizing atmosphere, which leads to the deterioration and looseness of the brick structure, and finally erodes, peels and fails. Therefore, it is necessary to add various antioxidants to inhibit carbon oxidation and improve the quality of magnesia-carbon bricks, such as adding boron carbide, metal aluminum powder, metal silicon powder and their mixtures.
Test materials and sample preparation
Using large crystal fused magnesia (particle size distribution of 5~3mm, 3~1mm, 1~0mm, <0.088mm) as aggregate, adding flake graphite, high temperature asphalt (particle size <0.088mm), liquid phenolic resin, Boron carbide, metal aluminum powder, and metal silicon powder are made into samples. The chemical composition of the raw materials is listed in Table 1. (Figure II)
The raw materials are added to the mixer and mill in a certain order according to the proportion in Table 1 and mixed uniformly, and then press-molded on a 1000t friction brick press. Prepare 5 types of standard samples numbered MT1~MT5.
Performance test and result discussion
2.1 Bulk density, apparent porosity and normal temperature compressive strength test
The sample was dried at 110°C for 6 hours, and dried to 200°C for 12 hours. Determine the bulk density, apparent porosity and compressive strength at room temperature of the sample. The specific test method refers to the national standard GB/T2997-2015, GB/T5072-2008, and the test results are shown in Figure 1. (Picture 3)
It can be seen from the test results that with the compound addition of boron carbide, metal aluminum powder, metal silicon powder, metal aluminum powder and metal silicon powder, the volume density of the sample decreases, and after the metal silicon powder is added separately, the sample The volume density of the sample reaches the lowest; and after the composite addition of metal aluminum powder and metal silicon powder, the volume density of the sample has risen. The reason for the above-mentioned regular changes in sample volume density is: the volume density of metal silicon powder<the volume density of boron carbide<the volume density of metal aluminum powder, so with the addition of various antioxidants, the sample volume density is not The added sample showed a downward trend.
With the compound addition of boron carbide, metal aluminum powder, metal silicon powder, metal aluminum powder and metal silicon powder, while the volume density of the sample decreases, the apparent porosity shows an upward trend. With the compound addition of boron carbide, metal aluminum powder, metal silicon powder, metal aluminum powder and metal silicon powder, the room temperature compressive strength of the sample shows a decreasing trend, and with the increase of the amount of metal silicon Compressive strength at room temperature also shows a downward trend. This is because with the addition of antioxidants, the volume density of the sample decreases, the porosity increases, and the solid-solid bond inside the sample weakens, resulting in a decrease in compressive strength at room temperature.
2.2 High temperature flexural strength test
Heat the sample to 1400 ℃, keep it for 30 minutes, and test its high temperature bending strength. The specific test method refers to the national standard GB/T3002-2017, and the test results are shown in Figure 2.
It can be seen from the test results that with the compound addition of boron carbide, metal aluminum powder, metal silicon powder, metal aluminum powder and metal silicon powder, the high-temperature flexural strength of the sample generally shows an upward trend, resulting in this result The reason is:
(1) Samples with anti-oxidant boron carbide, in which boron carbide is preferentially oxidized by C, which greatly reduces the internal oxygen partial pressure PO 2 of the material, which protects C from a large amount of oxidation; at the same time, after boron carbide is oxidized , Generate liquid B2O3, form a layer of liquid film on the surface of the sample, block the material pores, reduce the diffusion rate of O2 on the surface of the sample, thereby reducing the degree of oxidation of C; In addition, liquid B2O3 has good wettability to MgO , The two can easily react to form trimagnesium borate (3MgO · B2O3), and trimagnesium borate can form a dense protective layer, which can further seal the pores on the surface of the magnesia carbon brick and prevent the intrusion of O2, thereby protecting the oxidized boron carbide of C from passing The above method prevents the oxidation of C in the magnesia carbon brick, reduces the pores left by the oxidation of C at high temperature, strengthens the solid-solid bond at high temperature, and thereby improves the high temperature flexural strength of the magnesia carbon brick.
(2) When the magnesia carbon brick is heated, the metal aluminum powder reacts with C and CO to form carbides, and re-aggregates C, and finally produces Al4C3, Al2O3, MA and other high melting point substances, which will cause volume expansion. , So that the brick body is densified, forming a ceramic bond, thereby improving the high temperature strength of the sample.
(3) When metal silicon powder is at 600 ℃ or 1000 ℃, preferential C reacts with oxygen to form SiO (g) and SiO2 (s). The generated SiO2 blocks some pores of the material and effectively improves the oxidation resistance of magnesia carbon bricks. As the temperature rises, SiO2(S) reacts with MgO to produce high melting point M2S, accompanied by a certain volume expansion, which causes the material to be densified, thereby improving the high temperature strength of the magnesia carbon brick.
(4) The effect of adding the mixture of metal aluminum powder and metal silicon powder on the flexural strength of the magnesia carbon brick is the result of the combined effect of the above two factors.
2.3 Heating permanent line change rate test
Heat the sample to 1600 ℃, keep it for 120 minutes, and test its heating permanent linear change rate. The specific test method refers to the national standard GB/T5988-2007, and the test results are shown in Figure 3. (Picture 7)
It can be seen from the test results that with the addition of antioxidants, the heating permanent linear change rate of the sample shows an irregular trend. The reasons for this result are:
(1) Samples with anti-oxidant boron carbide, on the one hand, after the anti-oxidant boron carbide is oxidized, liquid phase B2O 3 and trimagnesium borate (3MgO ·B2O3) are generated. The formation of both promotes the sintering of the sample and is beneficial The sample volume is reduced; on the other hand, the magnesia used in the sample has a larger expansion coefficient, which causes the sample to undergo thermal expansion when subjected to high temperatures. When the latter effect is greater than the former effect, the sample swells slightly.
(2) The addition of the anti-oxidant metal aluminum powder makes the sample heated, MgO reacts with Cat high temperature, and the generated CO is reduced by Al to produce C deposition and Al2O3, and Al2O3 reacts with MgO to form The secondary spinel with high refractory and subsequent volume expansion, so with the increase of the amount of metal aluminum powder added, the linear change rate of the sample after firing at 1600°C increases significantly.
(3) The addition of silicon metal powder causes the sample to change in two aspects: on the one hand, the metal silicon powder is oxidized to SiO2, which promotes the sintering of the sample; on the other hand, SiO2 reacts with MgO to form high melting point M2S. There is a certain volume expansion, and the sample itself has an expansion effect when subjected to high temperatures. The expansion characteristic of the sample is greater than the shrinkage characteristic of the sample, so the sample expands slightly.
2.4 Anti-slag performance test
The slag erosion resistance test adopts the static crucible method. Weigh 10g steel slag (electric furnace slag, its alkalinity is 4.85, and its chemical composition is listed in Table 2), put it into the blind hole of the crucible sample, and raise it to 1600°C at a heating rate of 5-10°C/min in the electric furnace, and keep it warm After 3h, cool to room temperature and take out the sample. Cut the cooled sample into two parts longitudinally from the middle, and measure the amount of hole expansion after the bottom of the slag hole is eroded by the slag (the smaller the value, the stronger the slag erosion resistance). The specific test method refers to the national standard GB/T8931-2007, and the test results are shown in Figure 4. (Figure 8)
It can be seen from Figure 4 that with the addition of various antioxidants, the slag corrosion resistance of the samples has decreased. Only the MT4 sample with silicon powder added alone does not decrease significantly. According to the analysis of the bulk density and apparent porosity of the magnesia-carbon brick with the addition of antioxidants, it can be seen that the slag erosion resistance of the magnesia-carbon bricks is reduced compared with that without samples. The reaction temperature range of aluminum powder is wide. Al starts to form Al4C3(△V=50%) and Al2O3(△V=30%) at 660 ℃, and is formed at higher temperature (>1400 ℃) MgAl2O4 (△V=6.9%) makes the sample structure compact, which has the function of sealing pores and preventing slag penetration, so its anti-slag corrosion performance is not much different from that of samples without antioxidants. .
(1) After adding antioxidants, the bulk density and normal temperature compressive strength of magnesia-carbon bricks are reduced.
(2) When metal aluminum powder and metal silicon powder are added together, the high temperature flexural strength of the sample reaches the maximum value of 17.6MPa.
(3) The introduction of anti-oxidant boron carbide and metal silicon powder causes the sample to expand slightly, which is beneficial to improve the high-temperature volume stability of the sample.
(4) The sample with anti-oxidant metal silicon powder has good comprehensive performance.