Microstructure Tailoring And Properties Investigation Of Low Carbon MgO/Al₂O₃-C Refractories For Continuous Casting

High-end products and greenization preparation are the important technological development direction in iron and steel industry.For this reason,the life and function of the refractories are put forward higher requirement,especially for carbon-containing refractories.At the same time of longevity,it is hoped to reduce the carbon content in order to reduce the pollution of molten steel.Low-carbon refractories have become the hot spot of development.Continuous casting is an important process in steel production.The service behavior of functional refractories such as long nozzle,monolithic stopper and submerged nozzle has an important effect on steel billet quality and efficient operation.Functional refractories are usually prepared from Mg O-Al₂O₃-C system components,its low carbonization is an urgent technical problem to be solved in the production of high quality steel.In this paper,the carbonization performance of the binder was studied by controlling the composition and structure of binder.The physical and chemical conditions of in-situ formation of high content carbon nanofibers were investigated.The mechanism of obtaining high graphitization bonding carbon and high content carbon nanofibers was clarified.The effect of the in situ formation nano component carbon and bonding carbon on the properties of low carbon refractories was studied.By simulating the thermodynamic conditions in service environment,combined with thermodynamic calculation,phase diagram analysis and experimental design,the in-situ gas phase and gas-solid formation mechanisms of MA（magnesia-alumina）spinel were clarified in carbon containing refractories.The influence of gas-gas and gas-solid formation of spinel on the structure and properties of low carbon refractories was studied by controlling spinel formation mode.Meanwhile,the effect of secondary spinel on the properties and structure of low carbon refractories was studied.Through the tailoring of bonding carbon,spinel formation and secondary spinel,combined with the synergy optimization of components carbon and additives,the composition design principle and new technology of micro-structure regulation of carbon-containing refractory were put forward which provided the theoretical support for the research and development of high-performance low-carbon refractory materials.The main research contents and results are as follows:（1）The structure of the bonding carbon was optimized by using different binder carbon sources and nickel nitrate catalyst precursor.The carbonization behavior of different binders（carbores pitch,mesophase pitch,high temperature pitch and different phenolic resins）was studied.Mesophase pitch and high molecular weight resin had high carbon yield.By adding carbores pitch or mesophase pitch into high molecular weight resin,the overall carbon yield of mixed binder was improved due to optimizing the structure of binders.In addition,pitch and resin could form porous structure,the activity of nickel catalyst could keep for a long time due to suitable carbon-rich gases concentration in the pores in order to promote the growth of carbon fibers.The nickel catalyst changed the structure and morphology of the bonding carbon,promoted the transition of amorphous carbon to crystalline carbon and improved the carbon yield and graphitization degree of bonding carbon resources after heat treatment.Low-carbon Al₂O₃-C samples were prepared by using the binding-carbon precursor solution after composition optimization as the binder,then embedded in coke and treated at 800^oC,1000^oC and 1200^oC respectively.The thermal shock resistance of the samples was improved significantly after 1000^oC or1200^oC heat treatment.High molecular weight phenolic resin,mesophase pitch or carbores pitch and nickel catalyst worked together to make that the low carbon Al₂O₃-C sample had excellent strength properties and high thermal shock resistance over a wide temperature range.After 1200^oC heat treatment,the CMOR,HMOR and thermal shock resistance of the samples were improved synergistically.In addition,nitrogen heat treatment atmosphere promoted the formation and growth of nano carbon fiber and further improved the thermal shock resistance of Al₂O₃-C material.（2）By simulating the gas phase conditions inside the Mg O-Al₂O₃-C refractories during continuous casting process and combining with thermodynamic analysis,it is shown that gas partial pressure of CO,O₂and Mg could meet the gas phase formation and stable existence conditions of MA spinel in the Mg O-Al₂O₃-C refractories under service environment,and nitrogen could not affect the formation of the MA spinel.The gas-gas and gas-solid forming processes of MA spinel were analyzed experimentally under embedding carbon atmosphere.Mg gas was formed by carbon thermal reaction,then reacted with alumina（gas-solid）and gas containing aluminum（gas-gas）to generate MA spinel.When the gas-gas and gas-solid reactions coexisted,the production of MA spinel was higher than single gas-gas reaction.By introducing carbon-coated alumina powders into Mg O-C materials,the direct contact between magnesia and alumina was prevented,thus the solid phase reaction of MA spinel formation was inhibited.Meanwhile,the MA spinel was formed through gas phase and gas-solid reaction as far as possible.The results showed that the carbon-coated alumina powder was in the form of aggregates with a diameter of 400-800μm.These existed in the Mg O-C matrix with internal porous structure after heat treatment.After heat treatment at 1100^oC,XRD could detect the apparent spinel phase in the（UA-MC）Mg O-C sample with the addition of un-coated alumina.However,the weak spinel diffraction peak appeared in the Mg O-C sample（CCA-MC）withthe addition of carbon-coated alumina after 1250^oC heat treatment.The micrograph showed that the spinel layer was formed on the periphery of the carbon-coated alumina agglomerate in the CCA-MC sample.The dense spinel layer prevented the diffusion of Mg gas into the agglomerate.So,there was still residual alumina after 1550^oC heat treatment.CCA-MC samples had lower porosity,better mechanical properties and thermal shock resistance.In addition,the average thermal expansion coefficient of CCA-MC samples was significantly lower than that of UA-MC samples.There was no sharp expansion in the CCA-MC sample during the thermal expansion test,while the UA-MC samples showed sharp expansion from 1175^oC to 1375^oC.Therefore,carbon-coated alumina powder could improve the volumetric stability of low carbon Mg O-C refractories.（3）The structure and properties of low carbon spinel-C refractories were studied by adopting optimized bonding carbon,adjusting the composition ratio of different types graphite,introducing additives and different content alumina or magnesia.An appropriate amount of expanded graphite could disperse the binder more evenly,and the porous structure could promote the growth of nano carbon fiber.The reasonable combination of flake graphite superfine graphite and expanded graphite could significantly improve the strength and thermal shock resistance of low carbon spinel-C refractories.An appropriate amount of alumina dissolved into spinel could improve the bonding strength of the sample,but the excessive expansion could also destroy the continuity of the matrix.The line and mass change rates of the samples with magnesia were significantly higher than those with alumina,and the in-situ formation of spinel whiskers could significantly improve the strength of the material.The addition of Al-Si-B₄C improved the oxidation resistance of spinel-C material and provided samples with excellent bending strength over a wide temperature range.The thermal shock resistance of the sample was improved at the expense of slag corrosion resistance with the addition of Si.Adding proper amount of alumina or magnesia could promote the formation of spinel and secondary spinel that absorbed Fe O and Ca O to form solid solution improving slag corrosion resistance.The in situ spinel,secondary solid solution spinel and other in situ formation ceramic phases gave the low carbon spinel-C refractories excellent comprehensive properties.