Fabrication And Characterization Of Microporous Magnesia And Properties Of Its Magnesia Bsed Castables

Continuous-casting tundish serves as an essential smelting vessel for steel purity,the refractrories of which directly contact molten slag and steel.By appropriately configuring the refractories for flow control such as the weir,dam,turbulent inhibitor and filter,the flow field of liquid steel in tundish can be effectively improved and purified steel can be achived,which plays an important effect on steel-quality improvement.Due to the characteristic properties that protecting steel against pollution and cleaning steel,magnesia based refractories not only have become dominated working-lining refractories in tundish,but also the development trend of the refractories for flow control as mentioned above.With the increase of smelting fuctions and time in tundish,the service life and properties of magnesia based refractories,especially the magnesia based castables for flow control in tundish are directly related to the stable operation of continuous-casting process and metallrugical effect.However,magnesia based castables still have several shortcomings,such as poor slag penetration resistance and low thermal shock resistance,which need to be overcome urgently under the background of increasing demand on the smelting of high-quality steel.In recent years,microporous refractories have developed rapidly,which possess outstanding slag resistance,adequate mechanical strength and thermal shock resistance owing to their structure characteristics such as low apparent porosity,high closed porosity and small pore size.Hence,the farbrication of microporous magnesia has great significance on improving properties of magnesia based unshaped refractories used in tundish.For solving problems above,in this paper,wetting grinding was firstly adopted to form pores by in-situ decomposition method.Then nano-sized particles were used to adjust pore structure and intergranular phases.Then microporous magnesia was farbricated after high-temperature sintering.The effects of raw material types and particle size,sintering temperature,nano-sized Mg O,Al₂O₃and ZrO₂on the sintering properties and microstructure of microporous magnesia were studied.And the sintering model was built to clarify the mechanism of micro-pore formation and intergranular-phase transformation.The slag corrosion process was studied to explore the slag resistance mechanism.Ultimately,considering the service condition of refractories used in tundish,self-bond microporous magnesia castables were prepared using microporous magnesia,and the impact of microporous magnesia on the properties was studied.Based on the research above,the conclusions were as followed:（1）Compared to brucite,taking magnesite as raw material(d₅₀:～4.41μm)and with the decline of its particle size,the diffusion rate and sintering kinetic force increased,the grain boundary migration was accelerated and the pores could be more easily sealed,the fabricated microporous magnesia obtained higher closed porosity（increased by～5.8times）,lower pore size（<10μm）.At the lower temperature（1700°C）,grain bonding degree of samples decreased,with higher apparent porosity（9.4%）and larger pore size.However,the excessive temperatue（1850°C）led to the remove of closed pores.The appropriate sintering temperature was 1780°C,where microporous magnesia possessed the highest closed porosity（respectively increased by～57%,～7%compared to that at1700°C and 1850°C）,lowest apparent porosity（respectively decreased by～56%,～27%compared to that at 1700°C and 1850°C）while smallest pore size（median pore size:～5.35μm）.（2）The introduction of nano sized Mg O formed micro-nano double-scale sintering,which provided higher sintering kinetic force.The sintering pressure difference and unit-molar free energy were respectively improved by～43 and～48 times.The grain boundary migration was accelerated,improving the grain size（increased by～30%）and closed porosity（increased by～25%）,and decrease pore size（decreased by～6%）.The introduced Al₂O₃led to the generation of intergranular Mg Al₂O₄spinel,improved grain bonding strength,and significantly decreased pore size（decreased by～37%）.While excessive spinel（Al₂O₃addition>3wt%）played a pinning effect on the grain boundary migration,decreasing closed porosity（decreased by～62%）,and suppressed the direct bonding between Mg O grains.（3）Introduced nano-sized ZrO₂reacted with Ca O impurities in magneiste to generate intergranular Ca ZrO₃phases,decreasing the content of intergranular Ca O-Si O₂-Mg O low-melting-point phases,enhance the direct-bonding degree between grains,which was beneficial for the improvement of mechanical properties and slag penetration resistance.The in-situ reaction and cation vacancies increased grain boundary migration and grain growth,meanwhile arose closed porosity while decreased apparent porosity and pore size.Sample possessed optimal properties with 0.75 wt%addition.The bulk density was 3.33 g/cm³,closed porosity was 6.2%,apparent porosity was 2.1%and median pore size was 3.57μm.（4）The mechanism of intergranular-phase transformation indicated that mixing homogeneity in wetting grinding and the direct contact between ZrO₂and Ca O impurities had a crucial effect on the formation of intergranular Ca ZrO₃.Ca O were prior to Si O₂to react with ZrO₂for generating Ca ZrO₃at the grain boundaries.When 20 wt%brucite was substituted for magnesite,the ZrO₂particles contacted tightly with Mg O crystallites decomposed from brucite thus prevented from reacting with Ca O impurities in magnesite and Ca ZrO₃would not be generated,wihe utilized the high-activity Mg O microcrystallines for the cubic stabilization.The formation mechanism of micro-pore suggested that at temperature of 1700°C and above,nano-size ZrO₂accelerated grain boundary migration,and promoted the transition from intergranular pores to intragranular pores.The grain growth was promoted,and the surrounding grain number of single pore was decreased below six.The dihedral angle of pore was higher than 120°and the concave interface was formed between pores and grains.Then driven by curvature potential,the pores shrank and pore size decreased.Meanwhile,the stability of pores increased and pore aggregation was decreased.（5）Slag penetration calculation results of magnesia indicated that the dissolution rate decreased with the decline of grain boundary radius diameter,in favor of the accumulation of solid phases precipitating in the slag corrosion process at grain boundary,which reduced penetration rate.With the increase of slag Ca O/Si O₂ratio,the saturated solubility and dissolution rate of magnesia decreased,slag resistance was improved.Smaller pore size of microporous magnesia was beneficial for the supersaturation precipitation of solid phases,and led to the formation of interfacial isolation layer,effectively inhibiting corrosion and penetration of slag.Meanwhile since the interfacial energy and dihedral angle between microporous magnesia and slag were much higher,the penetration depth of slag along grain boundaries was significantly reduced.Under static magnetic field.The Lorentz force was generated and decreased velocity of slag flow and diffusion,forming“electromagnetic damping”effect,which effectively suppressed slag penetration,and decreased the wetting of slag on microporous magnesia.Microporous magnesia possessed excellent slag resistance。（6）Compared with common fused magnesia castables,in microporous magnesia castables,the aggregates had stronger combination with matrix.Ca ZrO₃improved grain direct-bonding strength,which increased mechanical strength.The micro-closed pores effectively relieved thermal stress in thermal shock process and decreased the formation of cracks.And Ca ZrO₃suppressed the crack propagation,which was mainly transgranular propagation and had higher thermal stress fracture resistance factor.The residual retention rate after 3 thermal shock cycles was 73.4%.The thermal shock resistance significantly increased compared with fused magnesia castable（38.9%）.Owing to the higher interfacial energy between slag and microporous magnesia,the slag dihedral angle increased（66.4°）.Ca ZrO₃could block the slag penetration,which led to the increase of slag viscosity and the dissolution of grains was inhibited.The slag penetration index was 53.3%lower and slag penetration resistance was significantly improved.