Numerical And Physical Simulation On The Separation Process Of Non-Metallic Inclusions At Steel-Slag Interface In A Ladle

Quickly and efficiently removing inclusion has always been the hotspot and keystone in the secondary refining process.The removal efficiency for different state inclsuions is still disputed.Particularly,the standpoint that the solid inclusion is removed more easily in ladle has not been accepted in widely.Therefore,based on the physical modelling,the motion behaviours of solid inclusions and liquid inclusions at the steel-slag interface are investigated in detail.Meanwhile,in order to comprehend the phenomenon that solid inclusions are removed more easily than liquid inclusions in ladle,two mathematical models are established.to describe the separation processes of solid and liquid inclusions at the steel-slag interface quantitatively,respectively.The main research contents and conclusions are summarized as follows:(1)A physical model on the removal behavior of inclusion in argon stirred ladle is carried out,in order to investigate the effect of argon diameter,flowrate and molten steel flow on the removal rate of non-metallic inclusions in detail.The results indicate that the floatation of inclusions to the steel-slag interface is mainly due to the rising movement of liquid steel,not the adhesion of small bubble.The changing inclusions removal rate with argon blowing rate is caused by the flow field below the steel-slag interface,not bubble diameter changing.With the argon flow rate is soft,less than 0.1 Nm3/h,the steel flow is almost parallel to the steel-slag interface,and the inclusions float to the steady steel-slag interface with the help of buoyancy force.In the moderate flow rate(0.2 to 0.5 Nm3/h),the slag absorbing inclusions rely on the slag droplet around the open eye.When the argon blowing operation for deoxidizing and alloying,more than 0.6 Nm3/h,some slag droplets containing some gas bubbles can be formed.Then the manner of slag absorbing inclusion is similar to that of the middle flow rate.Compared with the moderate flow rate,there are longer contact time and more contact area between inclusions and slag in the soft flow rate,resulting in better-inclusion removal rate in soft flow rate than that in moderate flow rate.Meanwhile,compared with the middle flow rate,due to lighter and more droplets in the deoxidizing and alloying,inclusions can be removed more easily.The inclusions are captured by these slag bubbles and then removed into top slag as well.When the gas flow rate increases to a certain value,the absorption of inclusions at the steel-slag interface becomes the controlling step of inclusion removal,and the removal result becomes worse.Furthermore,apart from molten steel flow field below the steel-slag interface,the slag property is also very important for the inclusions removal.(2)The physical modeling on the separation process of solid and liquid inclusions at the steel-slag interface is carried out.Effects of the inclusion geometry(shape and size),slag viscosity,and interfacial tension between phase(water,oil and particle)on the separation process are investigated in detail.It is found that the motion behaviours of solid and liquid inclusions are quite different.The molten steel film between inclusions and the interface is the reason for the phenomenon.There is no film during the solid inclusions moving at the interface.But a thin steel film forms when liquid inclusions are separated at the steel-slag interface.The film results in liquid inclusions remaining at the interface.Liquid inclusions enter into the slag phase once the film ruptures.The effect factor on different state inclusions at the interface is different.The slag viscosity and released interfacial free energy of the system of steel,slag and solid inclusions are the most important factor for solid inclusions at the interface.Meanwhile,the steel-slag interfacial tension decides the residence time of liquid inclusions at the steel-slag interface.Furthermore,the physical simulation can only describe the process qualitatively.(3)A mathematical model is established to describe the separation process of the solid(octahedral and plate-like)inclusions at the steel-slag interface.Compared with the previous mathematical models,the interfacial resistance force formed by the deformation of the steel-slag interface is considered in the model.Meanwhile,based on the model,the paper investigates the effects of viscosity and density of slag,geometry(shape and size)and density of solid inclusions,interfacial tension betweenphases(steel,slag and inclusions)in detail.The results show that the separation time of solid inclusions at the steel-slag interface is very short(less than 10-3 s).That is to say,solid inclusion can pass through the interface once they contact the slag phase.Without the consideration of chemical dissolution,the solid inclusion may be remian at the interface.Meanwhile,the released interfacial free energy of the system of steel,slag and solid inclusions is the driving force for the separation process of solid inclusions at the interface.However,the released interfacial free energy is insufficient to make solid inclusions remain at the interface.Furthermore,the Gibbs free energy released during the dissolution of inclusions is much larger than the interfacial free energy.It means that it is sufficient to make solid inclusions separate from the interface.Therefore,solid inclusions can be removed quickly at the steel-slag interface once they contact with the interface.(4)A mathematical model for the separation process of liquid inclusions at the steel-slag interface is established to describe the thinning of steel film between liquid inclusions and interface.Meanwhile,an expression,t=567.11?M/(Rd??g?MS1.5),for residence time of liquid inclusions at the steel-slag interface is developed,t is residence time of liquid inclusion at the interface(s),?? is difference between densities of steel and liquid inclusions(kg/m3),Rd is radius of liquid inclusions(m),?M is steel viscosity(Pa-s),?MS is interfacial tension between steel and slag(N/m),and g is gravity acceleration(m/s2).The result shows that the residence time of liquid inclusion is far longer than the separation time of solid inclusion at the steel-slag interface,which is the main reason for the solid inclusion being removed easily.Size of liquid inclusions and interfacial tension between the steel and slag are the main factors for the residence time of liquid inclusions at the steel-slag interface.Increasing the diameter and interfacial tension can obviously shorten the residence time of liquid inclusions at the steel-slag interface.