| In the current study,the effect of electromagnetic stirring(EMS)on the macrosegregation of the continuous casting(CC)bloom of a 20CrMnTi gear steel was investigated through numerical simulation and industrial trials.Firstly,the effect of the gap between the copper mold and the solidified shell on the EM field,the fluid flow,the temperature,the shell thickness,the removal of inclusions,and slag entrainment within the mold region was investigated using numerical simulation.The distribution of the magnetic induction intensity for the case with the gap was identical to the case without the gap,however,the distribution of the induced current density was different,resulting in a different distribution of the EM force.With the consideration of the gap,the EM force along the casting direction was nearly parabolic and the maximum value was near the stirrer center,the speed of the fluid flow near the solidification front in the mold region was 0.21 m/s and the removal fraction of inclusions was higher and the slag entrainment rate was approximately 0.0144 kg/s.Without the consideration of the gap,two peaks of the EM force along the casting direction existed,one 40 mm above the stirrer center and the other near the mold exit,the fluid flow speed near the solidification front in the mold region was 0.14 m/s,and the net slag entrainment rate was around 0.0091 kg/s.In order to accurately simulate the fluid flow and heat transfer in the mold under EMS,the gap should be mandatorily considered.Secondly,the effect of the EMS on the internal quality of the CC bloom,including the solidification structure,the macrosegregation,the removal of inclusions,and slag entrainment of the CC bloom was investigated through industrial trials.With the current intensity of mold EMS(M-EMS)increasing from 0 A to 390 A,the fraction of the equiaxed zone increased from 20.4%to 25.7%,however,it had little effect on the center porosity,center shrinkage,and center segregation;moreover,the degree of the carbon segregation was worsened from 1.01 to 0.89 in the subsurface of the bloom,from 1.07 to 1.14 in the columnar-toequiaxed-transition(CET)zone.Compared to that without M-EMS,the bloom with 390 A M-EMS had much less number density and less area fraction of inclusions while more large inclusions from slag entrainment.With the current intensity of the final EMS at the solidification end of the bloom(F-EMS)increasing from 0 A to 500 A,the fraction of the equiaxed zone,the negative segregation in the subsurface,and the positive segregation in the CET zone changed little while the center porosity and the center shrinkage cavity were improved significantly and the degree of the center positive segregation decreased from 1.48 to 1.30.For the bloom CC with side-port submerged entry nozzle,the EMS scheme should be weak or no MEMS+strong F-EMS to improve or even eliminate the negative segregation in the subsurface of the bloom,to decrease or even avoid the occurrence of slag entrainment,to improve the positive segregation in the CET zone and the bloom center,to improve the center porosity and shrinkage cavity.Then,a three-dimensional macrosegregation model in a straight mold zone was established and the effect of M-EMS on the transfer phenomena of the molten steel was investigated.With the current intensity of M-EMS increasing from 290 A to 490 A,the maximum speed near the solidification front increased from 0.162 m/s to 0.264 m/s,and the temperature distribution of molten steel and shell thickness became more uniform.However,the negative segregation in the subsurface of the bloom deteriorated and the content of carbon of the negative segregation was worsened from 0.176%to 0.167%.Compared to that without M-EMS,the bloom with a 390 A M-EMS had 2.0%more removal of inclusions while an increased slag entrainment of 0.0123 kg/s.Next,the fluid flow,heat transfer,solidification,and macrosegregation for a three-dimensional full-length curved bloom continuous casting process was calculated using default models built in the commercial software.In default models,the mushy zone was regarded as a porous zone without difference among the columnar zone and the equiaxed zone,and the Lever rule was used to model the element segregation.This model was able to predict the negative segregation in the subsurface and the positive segregation in the bloom center.The minimum content of carbon in the subsurface of the bloom was 0.179%,and the maximum content of carbon in the bloom center was 0.291%.However,the negative segregation after the CET zone was hardly able to be predicted,which was observed in the bloom of industrial trials.Finally,an improved three-dimensional macrosegregation model for the fulllength curved continuous casting bloom was established.In the improved model,the columnar zone and equiaxed zone were distinguished in the calculation of fluid flow,heat transfer,and solidification by adding new terms in the conservation equations that were solved by user-defined subroutines.The columnar zone and the equiaxed zone were distinguished according to the cooling rate on the iso-surface of a 0.8 liquid fraction,where it was the columnar zone if the cooling rate was higher than the critical cooling rate of 0.03℃/s,otherwise,it was the equiaxed zone.Governing equations of the porous zone were used for the columnar zone and the mushy zone constant was non-isotropic and different in three directions.In the equiaxed zone,if the solid fraction was higher than 0.5,it was the porous zone where dendrite arms of equiaxed crystals contacted each other forming solid networks and equiaxed crystals hardly moved freely with the liquid phase,and governing equations of the porous zone were used while the mushy zone constant was isotropic in three directions.If the solid fraction was less than 0.5,it was the free-floating zone where equiaxed crystals moved freely with the liquid phase,for which governing equations of the free-floating zone were used while the molecular viscosity of the steel depended on its solid fraction.To save calculation time,the calculation of the fluid flow,heat transfer,and solidification was closed after reaching convergence,and only the species equation was solved to calculate the element segregation.The Lever rule was used.Compared to the default model,since free-floating dendrites were considered in the improved model,in addition to predicting the negative segregation in the subsurface of the bloom and the positive segregation in the bloom center,the negative segregation after the CET zone was also able to be predicted.With a 0.42 m/min casting speed and a 29℃ superheat,the calculated carbon was 0.170%,0.190%,and 0.268%in the subsurface of the bloom,after the CET zone and the bloom center,respectively. |
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