Microstructure Control And Properties Of Silicon-containing Ceramic Phase Composite MgO-C Refractory Prepared By Nitriding Method

MgO-C refractories,as key basic materials for iron and steel smelting,are widely used as lining brick of converter furnace,furnace wall of electric arc furnace,and brick of ladle slag line.However,traditional MgO-C refractories are difficult to meet the technological advancement requirements of modern clean steel smelting,and its low-carbonization has become a development trend.The decrease of the graphite content in MgO-C refractories can inevitably reduce the thermal shock resistance and slag corrosion resistance of the material,and seriously damage its high temperature performance and service life.Therefore,it is of great significance for the development of refractory and metallurgical industry to explore new ways of preparing high-performance low-carbon MgO-C refractories.In this paper,Si powder is used as the raw material for in-situ formation of silicon-containing ceramic phases in MgO-C refractories prepared by nitridation,the effects of nitriding temperature,catalyst addition amount and loading method on the formation mechanism and morphology evolution of silicon-containing ceramics are investigated;and then the effect of c-Zr N nanopowder synthesized via molten-salt method at low temperature on the formation,morphology evolution of silicon-containing ceramic phase in MgO-C refractories and mechanical properties is studied;finally,based on the excellent properties of Si C whiskers,such as high tensile strength,high thermal conductivity and low thermal expansion coefficient,Si C whiskers are synthesized from the resource-rich biomass rice husk,and the catalytically synthesized Si C whiskers are used in MgO-C refractories to explore their effects on the structure and properties of refractories.The following conclusions can be drawn:（1）Si₃N₄composite MgO-C refractories were prepared by the nitridation of Si powder-supported catalyst.The generated silicon-containing ceramic phases included Si C,Mg₂Si O₄,Mg Si N₂and Si₃N₄.For refractories without catalyst,when the nitriding temperature was 1350-1450℃,the increase of nitriding temperature promoted the transformation ofα-Si₃N₄intoβ-Si₃N₄through liquid phase,and the formed columnarβ-Si₃N₄was uniformly distributed and pinned in the matrix;when the nitriding temperature was 1400℃,the material exhibited excellent mechanical strength,and its cold modulus of rupture and cold crushing strength were 15.2 MPa and 88.2 MPa,respectively.Compared with refractories without catalyst,the formation ofβ-Si₃N₄could be effectively controlled by adding 1 wt%ferric nitrate at the same temperature（1400℃）,and good Si₃N₄morphology and higher content of columnarβ-Si₃N₄were formed inside the material,the cold modulus of rupture,cold crushing strength,and oxidation resistance of refractories were increased by 13%,4%and 9%,respectively.（2）In the nitridation reaction of Si powder-phenolic resin co-supported catalyst to prepare the powder,when the addition amount of ferric nitrate was 5-15 wt%of the mass of Si powder,Si@Si C@Si₃N₄powders were synthesized in which the inner shell is a Si C plate and the outer shell is a Si₃N₄whisker,and the growths of Si₃N₄whiskers with Fe-droplets at the tips and roots followed the vapor-liquid-solid（VLS）and solid-liquid-gas-solid（SLGS）growth mechanisms,respectively.On this basis,silicon-containing ceramic phase composite MgO-C refractories,based on high temperature nitriding,were prepared by introducing Si powder-phenolic resin co-supported catalyst.Both the increase of nitriding temperature and the increase of catalyst content could promote the formation of Mg_（g）,which reacted withα-Si₃N₄to form Mg Si N₂,and the morphology of Mg Si N₂gradually evolved from a tapered morphology to a blocky morphology.When the addition amount of ferric nitrate was 1wt%and the nitriding temperature was 1400℃,the main product of generated silicon-containing ceramic phase was Mg Si N₂,which coexisted with granular and long tapered morphologies,and a large amount of short hexagonal columnarβ-Si₃N₄was formed,they synergistically improved the property of MgO-C refractories.Compared with refractories without catalyst（1400℃）,the cold modulus of rupture,cold crushing strength and corrosion resistance were increased by 6%,16%and 13%,respectively.（3）c-Zr N nanopowders were synthesized by using molten salt-assisted reduction nitridation.Pure c-Zr N nanopowders were obtained under the conditions that the molar ratio of raw material Zr O₂/Mg was 1:3,the molar ratio of molten salt system Mg Cl₂?6H₂O/Na Cl was 1:2,temperature was 1000-1100℃,and holding time was 3 h.The introduced c-Zr N nanopowders acted as a"catalyst"in the preparation of MgO-C refractories by reactive nitridation,the Zr N powders formed a chemical bond with the generated ceramic phases（e.g.,α-Si₃N₄and Mg Si N₂）,and promoted the growth of silicon-containing ceramic phases（e.g.,Mg₂Si O₄andβ-Si₃N₄）by mutual conversion reaction with the intermediate products Zr O₂or Zr Si₂.With the addition of Zr N increased to 3-5 wt%,the morphology of Mg₂Si O₄gradually evolved from flocculent whisker to flake.Compared with refractories without Zr N,the density and strength of refractories with 5 wt%Zr N were significantly improved,and its cold crushing strength was increased by 54%.（4）Si C whiskers were synthesized by using rice husk powder as raw material under Ar gas.The synthesis conditions of Si C whiskers with high crystallinity were as follows:Synthesis method of catalytic method（without molten salt）,nickel nitrate content of 2 wt%,heat treatment at 1400℃for 3 h.Compared with the synthesis of Si C from rice husk without catalyst,the addition of 2 wt%nickel nitrate could significantly reduce the formation temperature of Si C,the synthesized whisker morphology was a linear shape with Ni-droplets at tips,and had high crystallinity.Both noncatalytic-molten salt method and catalytic-molten salt method（2 wt%nickel nitrate）could not effectively reduce the formation temperature of Si C synthesized from rice husk,their morphologies were bead-like and flocculent,and these whisker were covered with thick amorphous Si O₂.（5）Si C whiskers synthesized from rice husk powder（2 wt%nickel nitrate,1400℃for 3 h）were introduced into the preparation of MgO-C refractories by reactive nitridation.When the addition amount of Si C powder was 1 wt%,the cold modulus of rupture and cold crushing strength were increased by 32%and 17%,respectively,compared with the refractories without Si C powder.The synthesized Si C whiskers resupported catalyst(denoted as Si C_Ni)were introduced into MgO-C refractories for nitriding preparation,this whisker evolved into chain-bead morphology,which enhanced its bonding strength with the matrix,and the introduced catalyst also promoted the formation of hexagonal columnarβ-Si₃N₄and flaky Mg₂Si O₄.When the addition of Si C_Niwas 1 wt%,the material showed excellent mechanical properties,and its cold modulus of rupture,cold crushing strength were 11.0 MPa and 68.8 MPa,respectively.The composite introduction of 3 wt%Si C_Niand Al₂O₃could optimize the internal structure of the material,the columnarβ-Si₃N₄with stepped distribution,and the mosaic structure formed between granular and flaky Mg₂Si O₄,synergistically strengthened and toughened the material,which gave the material excellent mechanical properties and thermal shock resistance,the oxidation resistance and corrosion resistance of material were significantly improved,and its cold modulus of rupture,oxidation resistance and corrosion resistance were increased by 46%,38%and15%,respectively.