Crystallization Behavior And Structure Of Mold Fluxes For Continuous Casting

As a kind of silicate material to ensure the surface quality of billet,mold flux plays a vital role in the continuous casting process,which directly affects the production efficiency of steel.As there are many kinds of mold fluxes,it is important to establish the relationship between composition,structure and properties of mold fluxes for the preparation of mold fluxes.At the same time,the heat transfer and lubrication of mold fluxes can be coordinated by controlling their crystallization behavior.The network structure and degree of polymerization?DOP?of mold flux were studied by FT-IR,Raman and NMR.The effects of composition of mould powder on melting temperature,viscosity and crystallization temperature were studied by thermal analysis.XRD,SEM and other characterization methods were used to study the effect of slag composition on the crystalline phase,morphology and crystallization rate,as well as the ultimate effect of these factors on thermal conductivity.Firstly,the effect of Al/Si ratio on the structure and properties of mold flux was studied.With the change of Al/Si ratio,when Al/Si=0.29,the crystallization temperature is the highest,Al/Si ratio continues to increase,and T_p decreased,while the crystallization activation energy increased first and then decreased,and the maximum value appeared at Al/Si=0.36.Structural analysis showed that with the increase of Al/Si ratio,Al continuously replaced Si in the slag network structure.Q²was the main component of the mold flux network structure.Q²?Bridge oxygen number is 2?decreased with the increase of Al/Si ratio,but Q¹?Bridge oxygen number is 1?increased,and NBO/Al decreased with the increase of Al/Si ratio.With the increase of Al/Si ratio,the content of melilite decreased,the content of forsterite increased,and the thermal conductivity of mold flux increased.When Al/Si=0.29,fluorophlogopite layered crystalline phase was precipitated from the slag,and the fluorophlogopite phase increased with the increase of heat treatment temperature.Secondly,the effect of MgO on the structure and properties of mold fulx was studied.With the increase of MgO content in mould powder,the viscosity of mould powder at 1300?tends to decrease.When the content of MgO was less than 10%,the increase of MgO content will lead to the decrease of T_b,and when the content of MgO was more than 10%,it will lead to the increase of T_b.The melting temperature and crystallization temperature of mould powder decrease with the increase of MgO content.The content of akermanite decreased with the increase of MgO content,while the content of forsterite increaseds.The DOP of mold flux decreased with the increase of MgO content.The increase of MgO content promoted the formation of forsterite and inhibits the formation of akermanite.It is not conducive to the formation of melilite at high temperature,but it is conducive to the formation of forsterite.When the content of MgO was less than 12%,the thermal conductivity increased with the increase of the content of MgO,and decreased gradually when the content of MgO is more than 12%.Finally,the effect of K₂O on the structure and properties of mold flux was studied.With the increase of K₂O content in the flux,the melting temperature of the mold flux K₂O=5%was the highest,and then the melting temperature decreased slightly.T_p decreases continuously,and there is an obvious exothermic peak of kalsilite in the mold flux when K₂O=5%.The theoretical values of NBO/T and?_corrorr almost remain unchanged with the change of K₂O content,which was consistent with the results of FT-IR and Raman analysis.With the increase of K₂O content,melilite decreased slightly,but kalsilite increased gradually.In addition,layered fluorophlogopite was precipitated from K₂O=1%and 2%slag.The thermal conductivity of mold flux decreased with the increase of K₂O content after heat treatment at 1173 K and 1223 K,but the trend is opposite when the heat treatment temperature exceeded 1223 K.