Investigation Of Low Carbon Slide Gate Refractories Strengthened By In–situ Synthesized Non–oxide Bonding Phases

In order to preventing carbon pickup in clean steel in the steel continuous casting process,it is necessary to develop high performance low–carbon slide gate refractories.Graphene nanosheets with excellent mechanical properties have shown the capability in reducing carbon content while ensuring thermal shock stability and slag erosion resistance of slide gate refractories.The present work studies low carbon alumina–zirconia–carbon（Al₂O₃–ZrO₂–C）slide gate refractories prepared by adding graphene nanosheets.The optimum condition of preparation of the cheaper graphene nanosheets obtained by ball–milling method was confirmed.The properties,phase composition and microstructure of low carbon Al₂O₃–ZrO₂–C slide gate refractories are researched.The reinforcing mechanism of low carbon slide gate refractories by in–situ synthesis of SiC whiskers is discussed.The effect of submicron silicon powders on properties of low carbon slide gate refractories is furtherly investigated.The condition of preparation of the expanded graphite fabricated by microwave method was confirmed by orthogonal design,and the expanded graphite has the largest expansion volume of 201 mL·g^–1.Graphene nanosheets were prepared by ball–milling method using spherical corundum and expanded graphite as raw materials.The character and micmorphology of graphene nanosheets were characterized by x–ray diffraction（XRD）,scanning electron microscope（SEM）,energy disperse spectroscope（EDS）,atomic force microscope（AFM）and transmission electron microscopy（TEM）.The results of XRD show that the（002）diffraction peak intensity of graphene nanosheets prepared by ball milling method is close to that of the commercially available graphene nanosheets,therefore the graphite sheets can be thinned by ball milling.The results of SEM and EDS show that graphene nanosheets are dispersed averagely in composite powders,and the thickness of some graphene nanosheets’edge warping can be up to 3 nm.The results of TEM show that graphene nanosheets are translucent sheets with certain warpage,and the edge thickness of graphene nanosheets are about 20 layers’thickness of graphene.The results of AFM show that the transverse dimension of graphene nanosheets is larger than 1μm,and the thickness of the whole graphene nanosheet is about 8.7 nm.Low carbon Al₂O₃–ZrO₂–C slide gate refractories was prepared with graphene nanosheets as carbon source.The properties,phase composition and microstructure of the fabricated refractories were examined.The results showed that graphene nanosheets could effectively improve cold crushing strength and thermal shock resistance of slide gate refractories by filling the micro pores and promoting the formation of silicon carbide whisker between particles.However,when the addition amount of graphene nanosheets is more than the amount needed for pore filling,the remaining graphene nanosheets exist outside of the pores,which blocks the direct bonding of particles in the slide gate refractories.As a result,it is difficult to density low carbon slide gate refractories,and the mechanical properties of them are poor.After adding 1.5wt%graphene nanosheets/Al₂O₃ composite powders to partly substitute carbon black,the slide gate refractories showed excellent oxidation resistance while having better mechanical properties and thermal shock stability.Effect of submicron silicon on the structure and properties of low carbon slide gate refractories was explored.The results show adding appropriate amount of submicron silicon into the low carbon slide gate refractories can not only make the refractories denser,but also promote the formation of SiC whiskers.With the increase of submicron silicon content,the cold crushing strength and modulus of rupture of the low carbon slide gate refractories are improved.However,when the addition amount of submicron silicon exceeds the critical value,it is difficult to compact the slide gate refractories during the forming process,resulting in decrease of mechanical strength.After adding1.5wt%submicron silicon powders instead of 4wt%micron silicon powders,the oxidation resistance of the slide gate refractories remains little change.Besides,the cold crushing strength and modulus of rupture of slide gate refractories with 1.5wt%submicron silicon powders is better than that of slide gate refractories with 4wt%micron silicon powders.