Numerical Simulation Research Of Solidification Process During Vertical Continuous Casting

The internal defects such as center segregation and center porosity often occur in large size continuous casting billets,which seriously damage the mechanical properties of the products,while the formation of equiaxed crystal region in the billet has a significant influence on the occurrence of the above defects.Due to a lack of the intuitive understanding of the phenomenon of columnar to equiaxed transition(CET),a numerical simulation study of the above process was carried out.In this paper,a multiphase solidification model considering the macroscopic growth process of equiaxial and columnar crystal structures was established,and the CET transformation and macrosegregation formation in continuous casting solidification process were predicted,and at the end the influence of the maximum grain density and different molten steel super heat was analyzed.The mechanism of the solidification microstructure evolution and macroscopic segregation formation was revealed.The main contents are as follows:(1)Based on Eulerian-Eulerian approach and metal solidification theory,A three-phase mixed liquid-columnar-equiaxed solidification model was established.In this model,an Eulerian treatment is used for each phase,which considering the liquid,equiaxed and columnar phase as spatially interpenetrating continua.The mass,momentum,species and enthalpy conservation equations are solved for each phase.In addition,a conservation equation of equiaxed grain density is defined and solved.Compared with the single-phase enthalpy-porous medium solidification model,the multi-phase solidification model divides the mass transfer from liquid phase to solid phase during solidification into the growth of equiaxed crystal and columnar crystal,which are calculated according to different growth mechanisms.Then the CET phenomenon is predicted by simulating the competitive growth of the different crystal structure in the solidification process.(2)The solidification process of molten steel in a continuous casting mould was numerically simulated using the above model.Considering a continuous casting round billet,the process phenomenon and macroscopic solute transport during solidification were studied,which considered columnar crystal growth,equiaxial crystal nucleation,equiaxial grain precipitation and thermo-solute buoyancy flow.The process of CET transformation and macrosegregation in billet were revealed.The simulation results show that due to the decrease of temperature gradient and the sedimentation of equiaxial crystals,the columnar crystal tips gradually stop growing and CET transformation occurs.Due to the sedimentation of grains,the solute enriched liquid is discharged upward to form backflow,thus forming negative segregation in the central equiaxial crystal region and positive segregation near the CET transition position.(3)The influence of different liquid steel superheat was analyzed.With the increase of the inlet liquid steel superheat,the equiaxial crystal nucleation process is inhibited,the equiaxial crystal rate near the ingot wall decreased significantly,but the degree of segregation does not show a clear change.There was no significant change in the equiaxial crystal rate in the central region of the ingot because the difference in the superheat of the inlet melt had no significant influence on the solidification rate and equiaxial crystal settlement when the solidification was close to the central region.(4)The maximum nucleation density parameters on the model predicted results are analyzed,because the nucleation process in the molten pool is affected by many factors,which in the present model can not be fully considered,parameter analysis is carried out for different maximum nucleation densities.The simulation results show that as the maximum nucleation density increases,the equiaxial zone proportion increase,and the center equiaxial grain diameter decrease.