Numerical Analysis Of Flow,Heat Transfer,Solute Transport Behavior And Solidification Structure In Continuously Cast Bloom

Heavy rail steel has been widely applied in high rails,and its quality has significantly depended on the processing of bloom continuous casting.The key factors of producing this kind of high quality steel are to control the solidification structure,fluid flow,heat transfer,and solute transport behaviors.In this paper,a series of studies on the control of solidification structure,optimization of turbulent behavior and improvement of centerline segregation during continuous casting process of heavy rail U71 Mn bloom with cross section of 380mm×280mm were conducted to supply a certain theoretical basis for the casting technology of heavy rail steel with high quality.The heat transfer behavior and solidification structure evolution under different secondary cooling conditions and superheat were simulated by the Cellular Automaton-Finite Element(CAFé)model.The results showed that the simulated results of the solidified shell,solidification structure distribution and equiaxed grain size are basically consistent with the nail shooting,macro etching experiments and OM observations;errors in the shell thickness are controlled within 4%.Compared to the slow cooling condition,the solidification end of the bloom is 2.46 m longer while the length of the bloom center mushy zone increases by approximately 1.46 m under super-slow cooling condition.However,the bloom surface and corner temperatures are higher,and the degree of temperature rise when entering air-cooling zone is much smaller,which can reduce the chance of bloom surface cracks.Furthermore,the solidification structure shows little change under these two secondary cooling conditions.When the superheat increases from 15 K to 40 K,the ratio of center-equiaxed grain decreases from 44.6% to 20.5% and the average grain radius increases from 1.025 to 1.128 mm.A step size increase in the superheat of 5K causes an increase of only about 0.19 m in the solidification end length,and an increase of 3K in the surface temperature.The superheat for the high rail U71 Mn steel can be controlled within 20 K based on the premise of the fluency of pouring process.The fluid flow,level fluctuation,heat transfer,solidification and solute transport behaviors and their interactions under different structures of submerged entry nozzle(SEN)and parameters of in-mold electromagnetic stirring(M-EMS)were systematically investigated in the turbulent zone of continuous casting bloom were solved simultaneously by a multiphysics numerical model with considering the effects of M-EMS.The results showed that the maximum level fluctuation can be decreased to 4.5mm,the local solidified shell-thinning phenomena can be eliminated on both the wide and narrow sides,the chance of breaking out is avoided,the removal of floating non-metallic inclusions can be enhanced,and the temperature difference between the surface and corners can be alleviated when applying the diagonally-installed four-port SEN.Moreover,the degree of negative segregation both around the wide sides and narrow sides are decreased,and the distribution of solute element in the initial solidified shell is relatively uniform.After loading with M-EMS,the liquid level fluctuation enlarged to 6.2mm,and the distribution of temperature,solidified shell,and solute is more uniform in the EMS effective zone.With the increase of EMS current intensity(from 450 A to 600A),the stirring effect and tangential velocity at the solidification front both around the center of the wide and narrow sides increases,the liquid fluctuation increases from 5.3mm to 6.2mm while the negative segregation in the EMS effect zone deteriorates causing the reduced corner segregation.In the meanwhile,the growth velocity of the solidified shell thickness in the EMS effective zone decreases,and the bloom surface temperature increases.The M-EMS with a current intensity of 600 A may be more suitable during the bloom casting.With the decrease of the distance from EMS center to meniscus,the height of level fluctuations and temperature of bloom surface increases,while the thickness of solidified shell at computational outlet and the impact depth of liquid steel decreases.The proper M-EMS center should be at approximately 0.42 m below the meniscus.To reduce the centerline segregation and improve the internal quality of the bloom,the effects of secondary cooling conditions,M-EMS and final EMS(F-EMS)on the centerline segregation of the bloom were investigated by the multiphysics numerical model based on the metallurgical outputs of the turbulent zone and combined with the distribution of solidification structure simulated by CAFé model.The results showed that the simulated solute distribution agree well with the measured results.The M-EMS has less effect on the heat and solute transport behavior at the secondary cooling and air cooling zone.The simulated solidification end under slow and super slow secondary cooling conditions are 17.9m and 20.5m,respectively,while the solute transport behavior are nearly the same.When the current intensity of F-EMS varys from 300 A to 600 A,the tangential velocity of the solidification front at the bloom center increases from 0.013m/s to 0.023m/s,the liquid fraction decreases from 0.7856 to 0.7256.A step size increase in the current intensity of 100 A causes an additional temperature decrease of 2.4K in the bloom center.When the current intensity locates in the range of 300A~400A,the electromagnetic force cannot cause negative segregation in the F-EMS zone,while the obviously decreased solute concentration distributed homogeneously at the mushy zone.Therefore,to effectively reduce the centerline segregation and further improve the inner quality of the bloom,the optimized current intensity of F-EMS should be controlled within 300A~400A.