Combustion Synthesis Of AlB₂-containing Composite Powders And Its Effects On The Anti-oxidant Properties Of Low-carbon Refractories

Carbon containing refractories are widely used in converters,ladles,electric furnaces,AOD furnaces,continuous casting and other steelmaking equipment because of their high slag corrosion resistance and thermal shock stability.However,there are some problems in the use of carbon-containing refractories,such as high carburizing,high heat release resulting in the furnace shell deformation,and comsuming more flake graphite resources.Therefore,it is of singnificance for research and development of high performance low-carbon refractories to improve the quality,efficiency and green prodction of ferrous metallurgical enterprises.The preparation and application of high performance anti-oxidant is one hotspot in the research field of low-carbon refractories.In this study,low-cost AlB₂-containing composite powders were prepared by combustion synthesis method,and their effects on the oxidation resistance of low-carbon refractories were studied.The main research contents and the results are as follows:?1?The adiabatic temperature of Al-B₂O₃,Al-KBF₄ and Al-Na₂B₄O₇ systems were calculated.As the initial temperature was 298 K,the adiabatic temperatures of the three systems were 2553 K,1891 K and 1586 K,respectively.According to Mozhanov criterion?1800 K?,the combustion wave can propagate in a self-sustaining manner in Al-B₂O₃ and Al-KBF₄ systems,while it would propagate only if the initial temperature is higher than 623 K in Al-Na₂B₄O₇ system.The combustion products of Al-KBF₄system comprise of Al B₂ with hexagonal columnar shape and KAlF₄ with tetragonal prism and hollow needle leaf shape,and the content of AlB₂ is 5 wt%.The combustion products of Al-Na₂B₄O₇ system consist of hexagonal AlB₂,tetragonal?-Al₂O₃,needle-rod NaAl₇O₁₁/NaAl₅O₈ and unreacted spherical particles Al,and the content of AlB₂ is 5 wt%.While the combustion products of Al-B₂O₃ system include hexagonal AlB₂,tetragonal?-Al₂O₃,and unreacted spherical particles Al.The results show that the content of AlB₂ in Al-B₂O₃ system is highest?9 wt%?between Al-B₂O₃,Al-KBF₄ and Al-Na₂B₄O₇ systems.Therefore,the best boron source for combustion synthesis of AlB₂-containing composite powders is B₂O₃.?2?The phase composition and the micro-morphology of combustion synthesized products in Al-B₂O₃ system were analyzed.Combining with the TG-DSC curve and XPS results,the combustion synthesis process was analyzed,then the reaction mechanism of combustion synthesis of AlB₂-containing composite powders in Al-B₂O₃system was proposed as dissolution-precipitation mechanism.Al and B₂O₃ melted successively with the increase of temperature to form Al melt-B₂O₃ melt.As the temperature was 800?,Al and B₂O₃ reacted to form Al₂O₃ melt and B,releasing heat,which made the temperature rise.Then Al₂O₃ gradually precipitated from Al melt-Al₂O₃ melt during the decrease of temperature,while the element B dissolved in Al melt reacted with Al to form AlB₁₂,then,at peritectic reaction temperature which was transformed into AlB₂ at high cooling rate.?3?The effects of Al-Mg,Al-Si and Al-La alloys on the adiabatic temperature,phase composition and microstructure of AlB₂-containing composite powders based on Al-B₂O₃ system were investigated.The adiabatic temperature decreased with the addition of alloy,and the more content the alloy added,the more the adiabatic temperature decreased.When Al-Si alloy and Al-La alloy were added,respectively,the diffraction peaks of AlB₂ in the product decreased obviously,and SiO₂ and LaB₆ phases appeared,respectively.When Al-Mg alloy was added,besides hexagonal columnar AlB₂,AlB₂ nanowires were found in the products.The diffraction peaks of?-Al₂O₃ and AlB₂ in the product increased and then decreased with the increase of Al-Mg alloy.The results show that Al-Mg alloy has the best promoting effect on the combustion synthesis of AlB₂-containing composite powders in Al-B₂O₃ system,and the optimum content of Al-Mg alloy is 12 wt%.?4?The effect of Al-Mg alloy on the AlB₂ nanowires in the combustion synthesized AlB₂ containing composite powders was further studied,and the synthesis mechanism of AlB₂ nanowires was discussed.With the increase of Al-Mg alloy,the amount of AlB₂ and Al₂O₃ in the product increased and then decreased,while the amount of Al decreased and then increased with the increase of Al-Mg alloy.The amount of AlB₂ nanowires depends on the content of Al-Mg alloy.As the content of Al-Mg alloy is 12 wt%,the amount of AlB₂ nanowires is highest,and the amount of AlB₂ in the composite powders is 12 wt%.The gas phase Al-Mg,AlO,Al₂O and B₂O₂generated during the combustion synthesis process could react with each other,and then deposite to solid B and form AlB₂ nanowires.The growth process is controlled by the gas-solid growth mechanism model.?5?AlB₂-Al-Al₂O₃ composite powder,Al powder and Al/Si composite powder were used as anti-oxidants of low-carbon magnesia-carbon bricks and low-carbon alumina-carbon bricks,respectively.Their anti-oxidant effects were compared,and the anti-oxidant mechanism of AlB₂-Al-Al₂O₃ composite powder was revealed.The results show that with addition of AlB₂-Al-Al₂O₃ composite powder,the oxidation resistance was the best for both of low-carbon magnesia-carbon bricks and low-carbon alumina-carbon bricks and they had high mechanical properties at room temperature and high temperature.In the oxidation zone of low-carbon magnesia-carbon bricks and low-carbon alumina-carbon bricks with AlB₂-Al-Al₂O₃ composite powder,Mg₃B₂O₆phase with the melting point 1360?and Al₁₈B₄O₃₃ phase with the melting point1103?were formed,respectively,they were melt at 1400?.However,the oxidation products of Al/Si and Al anti-oxidants are MgAl₂O₄,Mg₂SiO₄,Al₄C₃ and Al₂SiO₅,and they can not melt at 1400?.Therefore,AlB₂-Al-Al₂O composite powder is superior to Al/Si and Al anti-oxidants in filling pores and blocking gas channels.It is concluded that AlB₂-Al-Al₂O₃ composite powder is a potential anti-oxidant with high performance and low cost for low-carbon refractories.