| The center segregation,a typical continuous casting(CC)strand internal quality defect,is closely related to the macroscopic transport phenomena such as flow,heat transfer,solidification and solute transportation during the CC process.Aiming to reduce the center segregation and improve the internal quality of the CC strand furtherly,a series of studies on the macroscopic transport phenomena and the formation of center segregation in the CC process are carried out by means of numerical simulation.To clarify the macroscopic transport phenomena in CC mold under electromagnetic stirring condition,the flow-thermal-solute transportation coupled model in the region of billet mold and mold electromagnetic stirring(M-EMS)model were established.The influence of mold curvature was considered sufficiently in the modeling process.The results indicate that the macroscopic physical field in calculation domain presents an asymmetric distribution due to the influence of mold curvature.With the electromagnetic stirring intensity increased,the swirling flow of molten steel in the mold is enhanced and the superheat dissipation of molten steel is accelerated,but the element segregation at the outlet of the model could also become deteriorated.That the installation position of stirrer is "too high" or "too low" will both lead to a bad effect on the metallurgical for MEMS indicating that it is more reasonable for the stirrer to be installed in the middle of the mold.By measuring the secondary dendrite arm spacing(SDAS),the value of mushy zone parameter(Amush)was calculated,and Amush interpolation was loaded into the three-dimensional billet CC model including solidification end point to investigate the effect of dendrite structure on billet center segregation.The results show that solute element continues to accumulate in the center of the billet during molten steel solidification,because of which center segregation forms at the end of billet solidification.During continuous casting,the smaller the Amush,the more serious the element segregation in the billet.Once there is electromagnetic force in the CC system,the "solute washing effect" of melt flow on the solidification front would be strengthened,which intensifies the enrichment of solute element in the center of billet.In addition,the fine solidifying structure in the center of the billet can significantly reduce the center segregation.It would be most beneficial to improve the center segregation of the billet by controlling the length of the fine dendrite region at 20 mm,within which the SDAS is below 75 ?m.A hybrid model of three-dimensional and longitudinal two-dimensional continuous casting slab was developed.In the simulation process,the changes of element equilibrium distribution coefficient and diffusion coefficient caused by phase evolution during molten steel solidification were fully considered.Based on the hybrid model,the formation mechanism of slab center segregation was studied,and the influence of casting speed on macroscopic transport phenomena and center segregation in CC process was analyzed.The results show that in the turbulent flow region,there is an obvious correlation between the distribution of solute element and the flow field.The "solute washing effect" of melt flow on the solidification front is the main reason for the element negative segregation near the surface of the slab.In the natural convection region,the redistribution of solutes at the liquid-solid interface makes the solidification front on both sides of the slab show obvious element-enriched bands,which eventually converge at the end of solidification forming center segregation.Higher casting speed will strengthen the "solute washing effect" in the turbulent flow region and delay the solidification process of molten steel,which will result in the negative segregation on the slab surface and the positive segregation at the slab center.Combined with the simulation results of this model and the industrial validation,the reasonable casting speed should be 0.8 m/min. |
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