Numerical Study On The Porosity And Macrosegregation In Continuously Cast Billet

The key to successfully impove properties of productes is the control of internal quality during strand continuous casting.The center segregation and porosity remain the focus of studies among various internal flaws.Given this,the paper performs a series of studies on the formation and control of center segregation and porosity,aiming to effectively suppress the flaws in billets.The formation of porosity in billets was analyzed using a mathematical model,which predicted the size of macro-porosity(void)to be about 2.52 mm under the research conditions of this work.On this basis,plant trials were conducted to study the influence of heavy reduction on porosity in billets.The results show that the heavy reduction can effectively alleviate the porosity in continuously cast billets,and the large reduction and high solid fraction promote the alleviation,which is superior to that abtained with soft reduction.In this study,the heavy reduction at the position with corresponding solid fraction high than 0.7 and a reduction large than 10 mm will reduce the porosity degree from 1.5 to below 0.5.This work develops the 3D macrosegregation model,which coupling the fluid flow,heat transfer,and solute transport on the basis of continuum model to predict the distribution of solute elements.The results indict the similar distribution of the different solute elements.Affected by the convection of molten steel and solute redistribution,the maximum segregation degree of 1.36 was predicted at the central region and a negatively segregated minima besides while the solidification completes.For the bow-type caster,the maximum segregation degree was not predicted at the billet center influenced by the thermo-solutal buoyancy,a certain of solute enrichment occures at the upper region of billet in mold.The solute-enriched liquid would emerge at the solidification front as affected by the ongoing solute redistribution.As the solidifictaion proceeds,the solute-enriched liquid concentrates at the central region of billets,resulting in the compositional inhomogeneity,arising at the outer side with a shift of 8 mm.Also,the microsegregation model applied in the macrosegregation simulation results in the differently-predicted segregations.The Voller-Beckermann model was decided to be more appropriate compared to the others in the simulation of macrosegregation.In the simulation of electromagnetic stirring,the influence of fluid flow on the calculation of electromagnetic force was considered,and some comparisons were performed between the new MHD model and the traditional.For the in-mold electromagnetic stirring,the results show that large Lorentz force(less than 1700 N/m3)appeared below the nozzle affected by the magnetic field,while no value was predicted by the traditional model.But nearly no influence occurs for the final electromagnetic stirring(FEMS)because of the small velocity.The new MHD model in comprsion to the result of traditional calculation case predicts stronger level fluctuation(3 mm for the new model and 1.5 mm for the traditional model),thus,the new MHD model will be more applicable in the determination of EMS parameters.The mathematical model coupling fluid flow,heat transfer,solute transport,and electromagentism was developed basing on the new MHD model.Using the model,the influence of final electromagnetic stirring on the solute distribution in billets were investigated.The results show that uniform distribution of solute resulted from the FEMS appears in the mushy zone,and the maximum concentration of solute C was reduced from 1.062 to 1.002.The decrease in segregation degree(from 1.368 to 1.315)indicates a certain improvement was obtained for the macrosegregation using the FEMS,and the position of FEMS will contribute to the improvement.In this study,it is appropriate that the corresponding size of mushy zone at the position of FEMS accounts for 30?40%of the billet section.