Effet Of Adding Methods Of ZrO₂-nitride On Properties Of Al₂O_3-C Refractories

In order to avoid carbon pickup to steel, the production of clean steel calls for lowcarbon and carbon-free refractories increasingly. However, reducing carbon of Al₂O₃-Crefractories will result in thermal shock resistance （TSR） and slag corrosion resistancedecrease. Therefore, how to ensure the properties of Al₂O₃-C refractories when reducingcarbon content is the current hot topic. Owing to their excellent slag corrosion resistanceand good TSR, ZrO₂and nitrides were co-added in Al₂O₃-C refractories in this work.The effect of adding methods of ZrO₂-nitride on composition, structure and propertiesof Al₂O₃-C refractories was researched. And so, low carbon Al₂O₃-C refractories used inclean steel production were developed.At present, it has been reported that ZrO₂-nitride composite powders prepared bycarbothermal reduction and nitridation （CRN） of raw material zircon are added Al₂O₃-Crefractories: the addition of can ensure the properties of Al₂O₃-C refractories whenreducing carbon content. The introduction of ZrO₂-nitride composite powders remainsunsatisfactory due to complex process and high heat treated temperature. To tackle withthese shortcomings, a new approach of introduction of zirconia-nitride by in situcarbothermal reduction and nitridation process from zircon is put forward.According to thermodynamic analysis, using zircon powders （-320mesh） as rawmaterial, ZrO₂-nitride composite powders in large scale were synthesized in industrialnitriding furnace by the method of CRN of zircon. Using tabular alumina as aggregates,and tabular alumina fines, reactive alumina ultra-fines, aluminum powders, siliconpowders, carbon black, flake graphite, B4C, ZrO₂-nitride composite powders and zirconfines as matrix, low carbon Al₂O₃-C refractories are prepared. Influences of ZrO₂-nitridecomposite powders and zircon powders respectively addition at0%,3%,5%and7%onproperties, composition and structure respectively were investigated. Furthermore,influences of heatreated tempreture and atmosphere on main properties of Al₂O₃-Crefractories with5%ZrO₂-nitride composite powders addition were also researched. The thermodynamic analysis shows that the stability domains of reaction productshave relations to reaction temperature, the partial pressure of CO in the furnace andcarbon proportion. The CRN of zircon may lead to the formation of different oxide-nitride composites such as ZrO₂-Si₂N₂O, ZrN-Si₂N₂O, and ZrN-Si₃N₄by controllingthese factors. The XRD results of ZrO₂-nitride composite powders synthesized inindustrial furnace shows that the relative content of ZrO₂and nitrides reachs about40%-50%and50%respectively, of which the mainly phases of the nitrides are:Zr₇O₈N₄（10%-20%）, Si₂N₂O （20%） and ZrN（10%）.Oxidation resistance and TSR of Al₂O₃-C refractories are improved obviously bythe addition of the synthesized ZrO₂-nitride composite powder, while the hot modulusof rupture （HMOR） remains unchanged and then decreases. Permanent linear change,cold crushing strength （CCS） and cold modulus of rupture （CMOR） can be clearlyimproved with the addition of ZrO₂-nitride composite powders and when the compositepowders addition is5%, the performance is optimal. Lowering heating temperature andchanging heat treated atmosphere have little effect on properties of Al₂O₃-C refractorieswith5%addition of ZrO₂-nitride composite powders.The Al₂O₃-C refractories in situ forms ZrO₂-nitride as expected throughcarbothermal reduction and nitridation of zircon at1500℃. With the increasing ofzircon, oxidation resistance of Al₂O₃-C refractories is improved obviously, the thicknessof oxidation layer decreases from7.94mm without zircon addition to2.71mm with5%zircon addition. HMOR of Al₂O₃-C refractories reaches the maximum15MPa when thezircon addition is5%. With the increasing of zircon, residual CMOR ratio appearsincreasing, the apparent porosity and bulk density change little, CMOR and CCSincrease.The results and achievements deserve to be a reference to the development of lowcarbon Al₂O₃-C refractories and synthesis of ZrO₂-nitride composite powders, and toprovide new ideas for low carbon refractories used in clean steel production.