Effect Of Shear Stress On Viscosity,crystallization And Heat Transfer Behaviors Of Continuous Casting Mold Fluxes

Mold fluxes are vital important functional materials in the continuous casting process of steels,which play an extremely important role in the success of the continuous casting process and the enhancement of strands quality.Currently,the successful continuous casting process of most steel grades can be achieved using the conventional mold fluxes.However,there are still problems for mold fluxes in the application of two typical steel grades:1)the ultra-low carbon steel mold fluxes,which has the contradiction between slag entrapment and lubrication,related to the viscosity of mold fluxes;2)the peritectic steel mold fluxes,which has the contradiction between heat transfer and lubrication,related to the crystallization and heat transfer performance of mold fluxes.Combined with the force conditions of mold fluxes subjected in the mold,researchers found that the shear-thinning properties of non-Newtonian fluids can solve the above problems.Therefore,it is necessary to study the influence of shear stress on the rheology,crystallization and heat transfer performances of mold fluxes,which can provide theoretical guidance for the development of non-Newtonian mold fluxes.The non-Newtonian mold fluxes with shear-thinning properties were firstly prepared by using Al₂O₃.Then,the relationship between the shear-thinning behavior and structure of mold fluxes was studied by high-temperature viscometer,Raman spectroscopy,and²⁷Al and ²⁹Si nuclear magetic resonance technology.Besides,the influene of Al₂O₃,basicity(Ca O/Si O₂),metal oxide and F content on the molten slag structure and shear-thinning behavior under shear stresses were also considered.in detail.Finally,the slag film heat flow simulator,ultraviolet visible near infrared spectrophotometer and confocal laser scanning microscope were adopted to analyze the corresponding relationship between different shear stresses and crystallization and heat transfer performance of mold fluxes.The main conclusions are summarized as follows:(1)Through the influence analysis of Al₂O₃ content,basicity,metal oxide and F content on the shear-thinning behavior of molten slag,the influence law of common components in mold fluxes on the shear-thinning performance of molten slag was established,and the non-Newtonian mold fluxes with a strong shear-thinning property was obtained.If Al₂O₃ content was 14%,basicity is 0.9,molten slags possesed a strong shear-thinning behavior.The shear thinning rate(?)was 36.074%,the reduction value for activation energy of viscosity(?Ea)was 20.49 k J·mol^-1,and flow behavior index(n)was0.7681.If Al₂O₃ content and basicity increased,the shear-thinning property first enhanced and then weakened.If Na₂O or Mg O is substituted for Li₂O,and Mg O is substituted for Na₂O at the same amount,?and?Ea increased,n decreased,and the shear-thinning performance boosted.Besides,as the F content increased,the shear-thinning performance strengthened.(2)The mechanism of shear-thinning behavior was proposed based on the research of structure transformation history of molten slag during the shear-thinning process.In non-Newtonian mold fluxes,Qⁿ_Si(m Al)structure could be formed by Al₂O₃.Among them,Q³_Si(1Al)and Q²_Si(1Al)structure were depolymerized by shear stresses to form Q²_Si(0Al)and Q¹_Si structure,which was the most fundamental reason for the shear-thinning behavior of molten slags.By regulating the content of Al₂O₃,basicity,metal oxide and F content,the degree of shear-thinning behavior could be controlled by adjusting the content of each structural unit in Qⁿ_Si(m Al)structure.With the increase of Al₂O₃ content,basicity and F content,Q³_Si(1Al)and Q²_Si(1Al)first increased and then decreased,[Al O₃F]and Q³_Si(2Al)increased.In addition,the influence of shear stresses on structure of molten slag would be further enhanced by[Al O₃F]structure formed by F through the Al-F bond.(3)The crystallization performance of mold fluxes increased with an increase of shear stresses.The influence of shear stresseses on the crystallization properties of non-Newtonian mold fluxes was stronger than that of conventional mold fluxes.If the shear stresses increased,the crystallization rate increased,the activation energy of crystallization decreased.The increase value of crystallization rate and the decrease value of crystallization activation energy for the non-Newtonian mold fluxes were larger than those of conventional mold fluxes.As the shear stresses increased,the crystal morphology changed from columnar and dendritic to massive and planar,the crystal size decreased,the crystal content increased.In addition,the mineral phase of crystals was not affected by shear stresses.The break temperature was a little affected by shear stresses.(4)The horizontal heat transfer between strands and mold was controlled by the heat transfer performance of solid slag film and air gap layer.The influence of shear stresses on the heat transfer performance of non-Newtonian mold fluxes was greater than that of conventional mold fluxes.If the shear stresses increased,compared with those of conventional mold fluxes,the reduction value of the heat flux and radiation heat transfer for the solid slag film of non-Newtonian mold fluxes were larger,the increase value of slag film thickness and the reduction value of characteristic time were smaller.The ability of controlling heat transfer was enhanced.The lubrication performance was improved.As the shear stresses increased,the surface roughness(Ra)of solid slag films increased,and the increments in surface roughness(?Ra_?)of the non-Newtonian mold fluxes were22.01 um and 21.09 um,being much larger than that of 12.84 um for the conventional mold fluxes.The heat transfer performance of air gap layer was enhanced.