Numerical Investigation Of Macrosegregation Formation And Influence Mechanisms Of External Fields During Continuous Casting Process

Due to the thermosolutal flow,grain sedimentation,or solid deformation,the solute rejected from solid dendrite transports with liquid flow,leading to strand macrosegregation formation.Because the continuous casting process is complex and the detection capacity is limited,the transport behavior in the mushy zone cannot be deep understood in the ingot solidification experiments and plant trails.The influence of multi-factors on solute distribution cannot be considered by the continuum model.Besides,the formation mechanism of macrosegregation has not been thoroughly investigated and many different theories are presented.Moreover,the effects of mechanical reduction and electromagnetic stirring(EMS)on center segregation evolution have not been fully obtained.Therefore,the improvement of strand inner quality is unstable and the key parameters are still determined by plant trails.In this paper,the multiphase solidification model is built to investigate multi-factors on solidification structure evolution and macrosegregation formation,such as solidification shrinkage,thermal shrinkage,grain sedimentation,and thermal flow.Subsequently,multiphase solidification model is coulped with reduction model to investigate the effect of reduction amount,reduction zone,reduction mechanism,and slab stretch on the center segregation improvement of Q345 steel.Besides,the multiphase solidification model is coulped with electrmagnetic model.The fluid flow,grain transport,and solute redistribution in the continuously casting billet with M-EMS and F-EMS are also studied.The influences of stiring parameters on SWRH82B steel solidification structure and solute segregation are investigated in detail.The main conclusions are as follows:(1)Based the multiphase solidification model,the influences of the thermal shrinkage,the solidifcation shrinkage,the grain sedmentation and thermal flow on the solute transport in the continuous casting process are investigated.With the effect of solidification shrinkage,the liquid steel moves faster to compensate for volume shrinkage.The solute enriched liquid also transports from columnar front to the root region,leading to the formation of center negative segregation.As the thermal shrinkage is considered,the solid dendrite contracts from the center part to the slab surface.Because less latent heat needs to dissipate near the solidification end,the center temperature decreases sharply and the solid phase contracts intensively,leading to the liquid phase squeezed out.As the enriched liquid moves toward the slab center,the positive segregation accompanied by negative segregation in the periphery part come into being.With the effects of grains sedimentation and thermal flow,the equiaxed grains transport from the inner arc side to the external arc side,resulting in the asymmetrical distrbution of equiaxed zone.The grains sedimentation also promotes the negative segregation at the external side and the positive segregation near the CET point at the inner side.Besides,it is found that the grain sedimentation and the thermal flow only influence the solute transport before equiaxed grains impinging with each other,while solidification and thermal shrinkage still affect the solute redistribution in the later stage.(2)The influence of reduction amount,reduction zone,reduction mechanism,and slab stretching on the center segregation evolution is numerical investigated.The result shows that the enriched liquid steel transporting downwards will be supressed with the reduction amount increasing from 2 mm to 5 mm,which is benefit to the center segregation reduction.The optimal reduction zone should cover the solidification start and end position,where the center liquid fraction is between 0.95 and 0.01.The mechanical reduction applied in the earlier stage strongly influences the solidification end movement,while that in the later stage obviously affects center segregation evolution.Due to the center temperature decreasing sharply,the solid phase still contracts intensively to slab surface and the center porosity forms in the subsequent cooling stage,which cannot be improved by the soft reduction technology.Besides,it is found that the slab stretching in the reduction zone is another important factor influencing the center segregation improvement.(3)The fluid flow,grains nucleation,and solute transport in the billet continuous casting process with mold electromagnetic stirring(M-EMS)are simulated.The results show that the liquid steel from the submerged entry nozzle is forced to move around in horizontal direction and the penetration of the stream is reduced obviously with M-EMS applied.In the upper part of the mold zone,the vertical vertex is enhanced and liquid steel impinges the steel-slag interface directly,which changes the fluid flow behavior near the meniscus.In the lower part of the mold zone,the secondary flow is induced and the nucleated grain near the columnar front is transported to the liquid pool,which promotes the columnar to equiaxed transaction(CET).As the current intensity increases from 100 A to 250 A,the stirring velocity rises quickly and the supherheat dissipates rapidly,result in center grain density increase.Due to the limited increase of grain density in the liquid pool,the equiaxed phase ratio doesn't rise too obviously with the current intensity above 200 A.Besides,the negative segregation near the strand surface becomes more serious with the stirring intensity further increasing.(4)The influences of final electromagnetic stirring(F-EMS)parameters on billet center segregation improvement are investigated in-depth.The results show that in the F-EMS zone,the solute redistribution and equiaxed grains transport are enhanced by the rotational flow,which is benefit for the center segregation decreasing.It's also found that the billet center segregation improvement is more affected by the current intensity and stirring pool width,compared with the stirring velocity.As the stirring pool width is 73 mm,the center segregation declines continuously with current intensity increasing.That's because a large amount of latent heat still remains in the liquid pool and the center temperature decreases quickly with the fluid flow.As the liquid pool width decreases to 61 mm,less latent heat needs to dissipate in the later solidification,so the center segregation improvement is more obvious by F-EMS.However,with higher current intensity used,the center segregation turns to rise reversely.It becomes more serious with stirring pool width further decreasing to 43 mm,attributed to the limitted room for the rejected solute to dilute.As the stirring pool width is 25 mm,the positive segregation has already formed and solute can still concentrate with the weak stirring,deteriorating the center segregation.The optimized current intensity is 400 A and the stirring pool width should be 61 mm for F-EMS.Comparied with alternative stirring mode,the center temperature decreases to a lower level and center segregation improves more obviously with the continuous stirring mode applied.