Research On The High-Temperature Behavior Of Al₄SiC₄ In MgO-C System And Development Of Low-carbon MgO-C Brick

In recent years,with the increasing demand for ultra-low carbon steel and clean steel smelting,higher requirements have been placed on MgO-C bricks.In order not to add carbon to molten steel,as well as considering the cleanliness of molten steel,smelting heat loss and service life,reducing carbon content has become an important direction for the development of MgO-C refractories.However,the reduction of carbon content will cause a series of problems,such as easy oxidation,reduced corrosion resistance,and reduced service life.Al4SiC4 has excellent oxidation resistance and corrosion resistance,and its physical and chemical properties are stable at room temperature.The introduction of it into low-carbon MgO-C refractory materials is expected to make up for the related performance degradation of MgO-C bricks caused by the reduction of graphite content,and obtain excellent performance.The thesis first uses Al,Si and carbon black powders as raw materials to synthesize pure Al4SiC4 powder by solid-phase sintering,and conducts thermodynamic calculations and process analysis of its synthesis mechanism;the high-temperature oxidation resistance and stability under air and MgO-C systems were studied.When the reaction temperature reaches 1500℃,relatively pure Al4SiC4 powder can be obtained.The oxidation of Al4SiC4 powder under air conditions starts at 850℃;when the temperature is below 1200℃,the oxidation of Al4SiC4 is mainly due to the first oxidation of the Al on the surface of Al4SiC4,which leads to the reduction of the A1 content on the Al4SiC4 surface and the immigration of internal A1 to outside.The external migration causes the collapse and deterioration of the Al4SiC4 structure;during this evolution,the Si is more stable than the A1 and maintains the SiC structure without being oxidized;when the temperature is higher than 1200℃,Al and Si element was obviously oxidized,and the content of SiO2 continued to increase,resulting in weight increasing,and at the same time the oxidation product further formed mullite.Thermodynamic calculations show that in the MgO-C system,as the temperature rises,the partial pressure of CO(g)in the system continues to increase,and the partial pressure of O2(g)continues to decrease;under this partial pressure,AlSiC4 will is oxidized,a series of reactions occur.When the reaction temperature is lower than 1400℃,the oxidation product of AlSiC4 tends to form Al2O3-Al6Si2O13-C system;and when the temperature continues to rise,the oxidation product of AlSiC4 tends to Al2O3-SiC-C system.Through the analysis of magnesia carbon bricks with AlSiC4 added at 1400～1600℃,it is found that the amount of MgAl2O4 in the sample and the size of its crystal grains have increased,and the diffraction peak of SiC appears at the oxidation temperature of 1600℃,which is accordance with thermodynamics.The formation of MgAl2O4 in the sample further strengthens the mechanical properties of the MgO-C system,reduces porosity,and improves oxidation and corrosion resistance.In view of the synthesis cost of Al4SiC4 and the positive effect of the formation of MgAl2O4 on the performance of low-carbon MgO-C bricks,the microstructure of low-carbon MgO-C bricks should be studied before the introduction of different contents of Al4SiC4.The optimization experiment is to control the microstructure and slag penetration resistance of magnesia-carbon bricks through the spinel phase.For this reason,α-Al2O3 powder,tabular corundum and fused MgAl2O4 are used as additives to finally optimize.When the addition amount ofα-Al2O3 micropowder is 4 wt%,the low-carbon magnesia-carbon brick shows the best performance.And on this basis,Al4SiC4 was introduced as an additive into low-carbon magnesium-carbon refractories.When the addition amount of Al4SiC4 is 8 wt%,the oxidation resistance and slag erosion resistance of low-carbon magnesia-carbon bricks have been improved.The low-carbon MgO-C bricks with Al4SiC4 added were industrially prepared and applied to a 210-ton ladle refining furnace in a domestic steel plant for industrial application tests.Although the carbon content of the low-carbon magnesia-carbon brick in this test is only 3 wt%,it achieves the similar use effect of the traditional magnesia-carbon brick with a carbon content of 12 wt%.The resistance to slag erosion and penetration is very good,and the service life is up to 50 times.Low-carbon magnesia-carbon bricks with Al4SiC4 have great application potential and commercial value in converters,refining ladle,etc.