Numerical Simulation Of Solidification Microstructure And Macrosegregation In Continuous Casting Round Bloom Of Alloyed Steel

The surface defects and internal defects such as subcutaneous negative segregation,center segregation and center porosity frequently occur in the process of continuous casting of large size round bloom,which has a great effect on the mechanical property and service life of the final product,because it is difficult to be eliminated by the thermos-menchanical posttreatments due to the large diffusion length required.Due to the continuous casting process is pretty complicated and its’ characteristic of low internal visibility,it is very difficult to investigate the melt convection and solute transport behavior in the mush zone through the ingot solidification experiments and plant trails.In addition,the numerical simulation methods of mixture model and the CA model are restricted by the solidification microstructure evolution and computing power.For this reason,based on Eulerian-Eulerian method and theory analysis and metals solidification,a 2D/3D multiphase solidification model coupling with turbulence flow,interphase heat transfer,interphase mass transfer,microstructure evolution,interdendritic solute diffusion and electromagnetic field is established,and the magnetic flux density of the electromagnetic stirrer center,the surface temperature of the bloom,the carbon solute content of the bloom and the distribution of solidification structure were measured using a CT-3 Tesla meter,an infrared thermometer,a carbon-sulfur analyzer and pickling experiment,respectively to verify the accuracy of electromagnetic field model,heat transfer model,interdendritic solute diffusion model and interphase mass transfer model.Different from the solidification model of ingot production,the multiphase model in this paper considered the influence of turbulence and the steady state of the continuous casting process on the solidification process.The turbulence term and electromagnetic force term are introduced in the calculation of the momentum equation;In the enthalpy equation,the latent heat is distributed according to the volume fraction;The improved Gaussian nucleation formula and columnar tip tracking model are used to simulate the nucleation and growth process of nondendritic equiaxed phase or non-dendritic columnar phase.This paper focuses on the formation mechanism of subcutaneous negative segregation,positive segregation at 3/4R position,center segregation and CET solidification microstructure evolution,and analyzed the feasibility of solutions such as swirl nozzle,mold electromagnetic stirring(M-EMS),and final electromagnetic stirring(F-EMS)for the macrosegregation problem,which is of great significance for expanding the non-dendritic equiaxed phase region and optimizing the parameters of continuous casting.The main conclusions are as follows:(1)In this paper,the problem of subcutaneous negative segregation on the surface of the bloom is studied,and it is believed that the negative segregation of the subcutaneous surface of the bloom is formed by the washing mechanism of the solute.The solute enriched in the solidification front will be washed away by the horizontal swirling flow formed by the mold electromagnetic stirring,which promotes the precipitation of solute in the solid-phase caused by the selective crystallization,that leads to the formation of subcutaneous negative segregation.In the same way,the use of swirling nozzles will also impact the initial solidified shell area,and together with the M-EMS,cause the appearance of the secondary subcutaneous negative segregation.However,with the proper swirl nozzle structure and electromagnetic stirring parameters,the upper swirling flows obstructive effect on the solute enrichment area of the meniscus is weakened,so that the top solute elements can be supplemented downwards and the subcutaneous negative segregation problem is reduced.It is calculated that the swirl nozzle can effectively reduce the generation of subcutaneous negative segregation under the current intensity of 300A,and is beneficial to the improvement of the heat-affected zone.At the same time,the jet impacted velocity and the steel-slag interface fluctuation are within a reasonable range.(2)In this paper,the formation mechanism of CET transition and the distribution of positive segregation zone at 3/4R position are studied,and the formation of CET transition is investigated to be the result of the combined effect of hard transition mechanism and soft transition mechanism.The hard transition mechanism believes that as the cooling intensity decreases,the liquidus line and the columnar solidification front position gradually separate,causing a large number of equiaxed begin to nuclear at the solidification front,which hinders the growth of columnar and finally completes the CET transformation.The soft transition mechanism believes that as the solidification progresses,the solute precipitated in the solid phase is enriched at the solidification front,which reduces the liquidus temperature of the solidification front and hinders the growth of columnar crystals.During the CET transformation process,the equiaxed grown at the solidification front of the columnar hinder the diffusion of the solute elements precipitated at the solidification front of the columnar to the center of the bloom,which eventually leads to the appearance of a positive segregation zone at the position of 3/4R.(3)This paper studies the problem of center segregation at the end of solidification.It is believed that the occurrence of center segregation is due to the different solubility of solute elements between solid phase and liquid phase.During the solidification process,the solute precipitated from the solid phase accumulates in the liquid phase,and finally forms center segregation at the end of solidification.The existence of backflow was found in the electromagnetic stirring process at the end of solidification,which caused the horizontal annular flow transformed to the vertical agitating flow,helpful to solve the central segregation problem.The appearance of backflow is affected by the thickness of the liquid core and the strength of electromagnetic stirring,and it is difficult to achieve continuous casting with small size and weak electromagnetic stirring strength.According to the research on the influence of different electric stirring positions on the center segregation,it is suggested that a steel plant should move the end electromagnetic stirring position back by 3.1m to work out the center segregation.