Experimental Research And Theoretical Analysis On Mold Powder Entrainment Behavior At Steel–Slag Interface In Continuous Casting Mold

With the improving of control level of steel cleanliness in modern continuous casting,the issue of mold powder entrapping into molten steel becomes more and more important.However,understanding of the entrainment mechanisms is still in qualitative or semi-quantitative stage due to the complicated flow condition in casting mold.Unfortunately,the demand of accurate controlling of technological conditions during modern continuous casting does not reduce.Hence,understanding of the mechanisms of varous types of mold powder entrainment behaviors and their dynamics evolution process,which can guide to more reasonable technological adjustment in actual produce process,is a key way to enhance the quality of continuous casting slab.In this paper,for the mold powder entrainments caused by shear flow,bubble impacting or emulsification which would usually occurs in slab mold with large argon flow rate,theoretical analysis and experimental study method have been applied to describe the issues above quantificationally.The following results are obtained:The wave theory has been introduced to explain the slag entrainment mechanism caused by shear stress of surface molten steel from narrow face.Based on the interfacial wave of liquid–liquid flow and combining the Bernoulli‘s equation and the kinematic,dynamic boundary conditions,a criterion of critical condition for limited place viscous KHI(Kelvin–Helmholtz Instability)model is developed.Then a small hydraulic experiment has been carried out to verify the accuracy of the theoretical model.Moreover,the PIV(Particle Image Velocimetry)method is used to test the critical velocity of interface broken in various physical properties.After comparing the effects of different slag entrainment models,the model developed in current work was applied to the steel–slag interface of actual casting mold to predict the critical criterion for mold powder entrainment.In steel–slag system,the critical steel velocity for slag entrainment by shear stress is 0.26m/s to 0.54m/s.The phenomenon of bubble swarm impacting the liquid–liquid interface is studied by the hydraulic experiment.The process of this phenomenon can be summarized as: the bubble swarm firstly impacts the liquid–liquid interface,and then some liquid banding,which is still connect with liquid pool,will generate in the pool of light phase,after that,by the effect of the dragging from liquid flow of bubble swarm,the liquid banding will finally divorce from light phase liquid pool and become entrainment at critical gas flow rate.Basic on the experimental phenomenon,an entrainment model caused by bubble impacting is established by analyzing the force balance at critical entrainment condition.After that,the dynamics cause of this type of entrainment is analyzed and the critical criterion has been obtained.By comparing the results from the theoretical model and the experiment,the model has been corrected.Based on the application of the modified model to steel–slag system,the maximal argon flow rate(11NL/min to 93NL/min),in which condition the entrainment caused by bubble impacting will not occur,has been obtained in given technologic mold.The effects of bubble and physical properties on the emulsification have been studied by using the CCD camera and professional image processing software to record and analyze the experimental phenomenon.Increases of interfacial tension,viscosity rate and density rate are benefited for decreasing the emulsification droplet size.Quantitative models to describe the size of emulsification droplet have been gotten by fitting the experimental data.Moreover,the formation of emulsification droplets in continuous phase has been analyzed and the effects of emulsification layer on the liquid–liquid interface property have been discussed.In the end,the effects of emulsification droplet,caused when bubbles cross the steel–slag interface at different situations of argon flow rate and physical properties,on entrainment in casting mold have also been studied.Through the N-S equation,and the dynamic,kinematics boundary conditions,the criterions for liquid–liquid two phases interface behavior completely similarity has been developed by using the equation analysis method.Combining those similarity criterions above and experimental data which described the critical situation of entrainment,a dimensionless number group related to the entrainment behavior is derived.Taking this dimensionless number group and Froude number into consideration,a similarity method whith has few limits for mold powder entrainment behavior of steel–slag interface has obtained.At last,this similarity method has been verified by comparing the interfacial conditions of actual steel–slag system and water model.Based on the similarity theory,a water model experimental apparatus for continuous casting mold has been built up in which both the flow field and steel–slag interface entrainment behavior are similar.The relation between various steel–slag interface character parameters is studied.By analyzing the effects of mainly influent factors on interface character parameters without/with argon flowing,character parameters related to the critical situation of entrainment has been obtained.After calculation by these character parameters,the following conclusions have been obtained: in conventional slab casting mold without argon flowing,in order to prevent the entrainment caused by shear pressure,the F value needs to be controlled in the range of 3.5N/m to 6.2N/m;in the slab casting mold with argon flowing,in order to prevent the entrainment caused by emulsification of bubble,the maximum argon flow rate needs to be controlled in the range of 2.3NL/min to 7.1NL/min.