Investigation Of Slag Resistance Of Lightweight Al₂O₃-MgO Castables

Serving as an indispensable equipment for molten steel undertaking,transporting and refining,ladle is essential for the metallurgical industry.For its lining material,two problems often arise.Firstly,traditional ladle lining refractories tend to cool down the molten steel due to their high thermal conductivity.Secondly,the service life of lining material is shorten due to severe thermal spalling damage,leading to an higher amount of inclusions in liquid steel and adversely affacting casting and the quality of final products.Microporous aggregates with large amount of micropores are expected to overcome these problems due to their low bulk density and thermal conductivity,as well as good thermal shock resistance,and the key to their successful application in refining ladle working layers is to ensure their slag resistance.In this paper,corundum-spinel castables commonly used in ladle as working layers are chosen,and mainly three aspacts of work were carried out.Firstly,for lightweight Al₂O₃-MgO castables matrix,a particle dense packing model was established to design its particle size distribution?PSD?,and a comprehensive study of the normal and high temperature properties of the castable with different matrix PSD was carried out.For the microporous aggregates,the effects of aggregates' microstructure on slag resistance of lightweight Al₂O₃-MgO castables was studied by adopting the induction furnace dynamic slag corrosion test,and the results was compared with that of a common castable contain tabular corundum aggregates.Seconly,the sample with the best performance were selected to study the slag resistance and corrosion mechanism of lightweight Al₂O₃-MgO castables when facing slag with different compositon and alkalinity,the effects of atmospheric condition was also investigated.Then the alkali earth oxide SrO was used to adjust the viscosity,meling and crystallization properties of a tenary Al₂O₃-CaO-SiO2 refining slag,and its effects on the slag corrosion bebaviour was examined.At last,a comprehensive mathematical model of slag corrosion on lightweight Al₂O₃-MgO castables was established and the slag corrosion simulation was carried out by using numerical method,the simulation results were compared with the experiment.The main conclusions are as follows:?1?For lightweight Al₂O₃-MgO castables,the appropriate q value for matrix particle dense packing is about 0.28,with which the castable' properties reached a favorable compromise among sintering characteristics,strength,and resistance against both slag corrosion and penetration.The sample with q = 0.25 has significantly larger matrix crystal grain after sintering due to its high amount of micropowders in the castable green body,resulting to the highest resistance against slag corrosion?matrix dissolved in slag?.However,since the excess volume shrinkage of the matrix during sintering caused the detachment of matrix from aggregates,cracks formed at the matrix-aggregates boundary and caused deterioration of castables' slag penetration resistance.The castable matrix with small pore size and large grain favour the formation of dense CA6 layer at the slag–matrix interface,which can protect the material from futher slag corrosion.?2?Under dynamic corrosion condition,aggregate microstructural parameters such as grain size dp,sphericity as and apparent porosity e affect the dissolution rate of aggregate into slag by changing the slag-aggregate contact interface area and the thickness of diffusion boundary layer.The integrated effects of aggregates crystal grain size,crystal grain sphericity and aggregates apparent porosity on the slag corrosion?dissolution?resistances can be defined as “the slag dissolution resistance based on material microstructure – Rm”,which can be deduced as ???.Aggregates with large and flake-like crystal grain show lower dissolution rate,which can improve the castables' slag corrosion resistance.?3?Under dynamic slag corrosion condition,for slag with different composition system,thermodynamic play a leading role in the slag resistance of castable.Among them,Al₂O₃-CaO-SiO2 slag show the most serious corrosion on lightweight Al₂O₃-MgO castables,of which both aggregate and the matrix suffered from serious direct corrosion.Al₂O₃-CaO Slag and Al₂O₃-CaO-SiO2-MgO slag corroded castable more mildly,and mainly indirect corrosion happened for them due to the generation of solid phases CA6 and periclase at the corrosion interface.For Al₂O₃-CaO-SiO2 slag with Ca/Si ratio 0.42,0.84,and 1.12,the effect of Marangoni-convection induced corrosion were more dominating than thermodynamic factors.When Ca/Si=0.84,the castable was corroded seriously due to a joint effects of thermadynamic corrosion and a relatively strong Marangoni convection.For Ca/Si=0.112,slag has similar Marangoni effect but weaker thermodynamic corrosion ability compared with Ca/Si=0.84,and the overall corrosion was lighter.For slag with Ca/Si=0.42,the most unremarkable corrosion happened due to a slightest Marangoni convection.?4?The corrosion of converter slag on lightweight Al₂O₃-MgO castable is closely related to the ambient atmosphere.Under P?Ar?=1.0 atm,slag corrosion is alleviated but infiltration is intensified.Atmosphere mainly affects the valence and chemical state of some transition-metal ions such as Fe and Mn in the slag: With P?O2?= 0.21 atm,Mn and Fe are present in the form of +2 and +3 valence ions and are partially involved in the formation of spinel solid solutions.Since the formation of MgAl2O4 solid solution,MnFe2O4 solid solution at the slag-castable interface consumed Fe and Mn ions largely,the relatively content of silicate increased,causing a high slag viscosity and weaker slag infiltration.With P?Ar?=1.0 atm,however,Fe2O3 in the slag is reduced into iron?Fe?,and the slag liquid phase is reduced by large amounts,leading to significantly reduced corrosion of castable.Corrosion reaction products under P?Ar?=1.0 atm are mainly MgAl2O4 and CA6?5?A proper substitution ratio of CaO by SrO?SrO/?SrO+CaO?? 0.32?can lead to a relatively lower liquidus temperature without significantly increasing the viscosity of refining slag.The main phases identified are Ca3Si2O7 and?Ca,Sr?7Mg?SiO4?4 with a Sr/Ca ratio 0,0.2,about 0.5 and about 1.0 respectively according to different SrO content SrO/?SrO+CaO??mol?= 0.0,0.16,0.32 or 0.48.The effects of SrO on the corrosion behaviour of Al₂O₃-CaO-SiO2 refining slag is mainly reflected in the fact that SrO can accelerate the crystallization rate of?Ca,Sr?Al12O19 and Al10.1Mg0.91O17Sr0.92 at the interface position during the reaction of molten slag and the castable components,and the continuing corrosion of castable by slag was inhibitted.?6?A comprehensive mathematical model of slag corrosion on lightweight Al₂O₃-MgO castables with flow,temperature and reaction coupled was established through the combination of the level-set two phases flow model,thermodynamic database and UDF functions of heat thransfer and chemical reaction.The calculated results show significantly milder slag corrosion than penetration.The corrosion pruducts were CA2 and CA6 respectively,of which the former mainly distributed at the matrix while the later only at aggregates.Calculated composition of the reaction products and their distribution characteristics from numerical simulation are in accordance with that of an actual corrosion test.The correctness of the model was verified.