Numerical Simulation Of Macrosegregation In Continuously Cast Billet Of 20CrMnTi Gear Steel

Macrosegregation is an inevitable solidification defect of continuously cast steel,and hot rolling process,the subsequent process of continuous casting,can't eliminate the defect,because of the solubility difference between liquid phase and solid phase,and the serious macrosegregation has a great effect on the mechanical property and service life.During continuous casting,the reasonable flow field distribution of molten steel is conducive to uniform molten steel temperature and solute composition,improve the quality of billet,and due to the advantages of non-contact,pollution-free and the superior metallurgical function of grain refinement and macrosegregation elimination,electromagnetic stirring technology has become one of the widely used process in continuous casting process.In order to comprehensively study the macrosegregation phenomenon in the continuous casting process,a 2D/3D hybrid geometric mesh model and a multi-field coupled model,coupling electromagnetic field with fluid flow,heat transfer,solidification and solute transport,are established by the drawing software GAMBIT and a finite volume element software ANSYS FLUENT respectively,and the magnetic flux density of the electromagnetic stirrer center,the surface temperature,and the C solute content of the billet were measured using a CT-3 Tesla meter,an infrared thermometer,and a carbon-sulfur analyzer,respectively,to verify the electromagnetic field models,heat transfer model,and solute transport model in this study.Finally,based on the continuous casting production process parameters of a 160 mm×160 mm 20CrMnTi gear steel produced in a domestic factory to study the effects of electromagnetic stirring(M+F-EMS)on the fluid flow,heat transfer,solidification and solute transport of molten steel during billet continuous casting process,which is of great significance for optimizing the process parameters of continuous casting production.The result shows that:(1)When the M-EMS is on,the flushing effect and the high-temperature molten steel moved up by horizontal swirling flow limit temporarily the growth of the shell,and the shell thickness at the mold exit becomes thinner with the increase of current density.when the current intensity is 0 A,150 A,200 A and 240 A,the thickness of the solidified shell at the crystallizer outlet is 15.7 mm,11.8 mm,11.0 mm,and 10.6 mm.While the upward movement of the high temperature region is beneficial to accelerate the heat dissipation of the molten steel in the mold region,which has a positive effect on the rapid solidification of the molten steel after the mold.(2)When the M-EMS is on,the penetration depth of injecting stream from the SEN decreases,and the flow velocity of molten steel at the solidification front increases.Thus,the wash effects of fluid flow on initial solidified shell become intensive and a negative segregation of carbon appears at the billet subsurface,which becomes more significant with the increase of current density.(3)Compared without considering the thermal-solute buoyancy,the mass fraction of carbon in billet center increased from 0.200%to 0.235%under the influence of the thermal-solute buoyancy,indicating that the thermal-solute buoyancy is one of the causes of center segregation of carbon during continuous casting billet.(4)Compared without F-EMS,F-EMS promotes the horizontal rotation of the liquid core molten steel in the electromagnetic stirring region,accelerates the decrease of the liquid fraction,and raises the mass fraction of carbon in the center of billet along the casting direction significantly.After the F-EMS installation location,the porosity of the porous medium is reduced,and the increase rate of the solute content in the center of billet is greatly reduced,and finally the purpose of reducing the center segregation of the slab is achieved.And the frequency is 7.58 Hz,when the current intensity of FEMS is 0 A and 300 A,the carbon segregation in the center of billet is 1.23 8 and 1.174,respectively.(5)F-EMS can reduce the center segregation of carbon.Within a certain range,the center segregation of carbon decreases with the current intensity increase,after increasing to a certain extent,with the current intensity increase and increase.Among them,the frequency is 7.58 Hz,when the current intensity increases from 300A,350A to 400 A,the center segregation of carbon decreases first and then increases,and the optimal current intensity of F-EMS is 350 A.