| In continuous casting of steel,surface defects arising in the mold severely affect production efficiency and steel quality.The complex and continuous behaviors in the mold,including fluid flow,heat transfer,chemical reactions and multiple phase transformations,which combined and interacted,are difficult to predict and control.The development of multiphysics thermo-fluid and solidification models plays a significant role in addressing these coupled issues since it can provide a good description of natural phenomena occurring in the mold.The model featured the evolution of heat transfer and solidification from cast-start to steady-state in combination with actual operating conditions,including the formation of meniscus and slag films.Furthermore,the model described fluctuations in the pressure,heat flux,and slag film thicknesses during the oscillation cycles of mold and the coupling effect of these variables on slag flow and shell growth.The model provided quantitative analysis regarding the influence of meniscus shape and slag films related to the casting speed on slag consumption and oscillation mark formation.The results show that a large amount of liquid slag infiltrates into the gap as the shell is withdrawn at casting speed of 0.3 m/min,which creates the initial.meniscus topography.The meniscus profile tends to bulge up at a higher casting speed,while the size of slag rim decreases.Increasing in casting speed leads to thiner slag films and higher heat flux.Large fluctuations of heat flux are found before forming a steady structure of slag films throughout the mold.The predicted flux pressure is decided by the combined effect of mold velocity and liquid film thickness.The presence of slag rim enhances the nonuniform pressure flow near the meniscus during mold oscillation.Increases in pressure and heat flux occur in the negative strip time while the mold and slag rim move downwards approaching the lowest position,which squeezes the slag flow to be divided into two tributaries,promoting the slag infiltration and the initial shell solidification.Positive slag consumption occurs during the period from the end of a positive strip time to the beginning of the next positive strip time.Development of high Mn and high Al steel has exhibited noticeable product performance such as high tensile strength,extraordinary ductility,and prominent work hardening ability,which is widely used in wear-resistant steel,non-magnetic steel,lightweight of automobile steel,and ocean platform steel.However,a range of issues with the poor casting performance are found during casting process due to the interfacial reaction between molten steel and mold flux,which changes slag composition and deteriorates lubrication and heat transfer between solidifying shell and mold,inducing severe depressions,cracks,and even breakout.In order to investigate the interfacial reaction phenomena between the continuous casting molten steel with high Mn and high Al content and CaO-SiO2 based mold slag,a series of laboratory experiments were carried out,where various factors which may affect the composition evolution in mold slag were taken into account.Based on the results obtained experimentally,the effect of property evolution of mold slag on lubrication and heat transfer in the mold during the reaction process was elucidated.The application of mold slag for high Mn and high Al steel was successfully carried out in industry trials.The interfacial reaction in continuous casting process for high Mn steel is mainly governed by the Mn content in molten steel,while slag basicity could suppress this reaction to some extent.The couple effect of MnO accumulation and SiO2 reduction induces a remarkable decline in viscosity and break temperature of mold slag.Crystallization temperature shows a downward trend after the first rise.The formation of Ca4Si2O7F2 takes place first in all slag samples.A small amount of non-faceted CaF2 crystals sandwiched between Ca4Si2O7F2 crystals is observed.The morphology of cuspidine changes from small faceted shape with regular spacing to larger interconnected block,and the number of cuspidine crystals decreases with MnO accumulation.In mold fluxes with higher MnO content,the micro tephroite crystals precipitate at lower temperature,while the crystallization of CaF2 is suppressed.Higher heat flux across the slag films is detected due to the thinner solid slag film,where crystalline layer decreases and glassy layer increases.During the continuous casting for high A1 steel,slag viscosity increases faster in the cooling process with increasing in Al2O3 content after interfacial reaction,and no obvious break point is detected.Meanwhile,both liquidus temperature and crystallization temperature of mold slag show a downward trend after the first rise.Accordingly,heat flux across the slag films in the mold changes in the opposite trend.As the content of Al2O3 increases,the formation of Ca4Si2O7F2 with rod like is sparsely arranged in slag film,where a large amount of lacklustre NaAlSiO4 crystals and a few CaF2 microcrystals sandwich between Ca4Si2O7F2 crystals.With Al2O3 concentration increasing to 28.39%,the crystallization of Ca4Si2O7F2 disappears.Instead,the blocky Ca2Al2SiO7 crystals are precipitated,while both the amount and size of NaAlSiO4 and CaF2 increase.In order to figure out the composition difference between solid and liquid film in plant practice for high Mn steel casting,a reaction experiment was carried out to investigate the composition profile of MnO and SiO2 in slag layer.Based on experimental results,a rheological model of mold slag for high Mn steel was developed to describe the non-uniform infiltration behavior of mold slag during oscillation cycles.Moreover,the effect of uneven slag films on slab lubrication and heat transfer in the mold was discussed.The results show that concentrations of MnO and SiO2 over the flux layer show opposite stair-step distribution,which consists of reaction layer and non-reaction layer.When the reacting thickness is 5 mm,a thicker liquid film is formed between the slab and the mold.Inhomogeneity of composition in solid film causes large fluctuations in heat flux in the mold,resulting in irregular depressions on slab surface.With reacting thickness of 10 mm,reacting slag fills in the gap to generate a thinner solid film,which rises overall heat flux in the mold.Since mold oscillation enhances the mixture of liquid slag in the meniscus,the liquid slag flows fast within 80 mm from the mold wall,which leads to a high consumption rate.On the contrary,liquid slag with low velocity stays longer in liquid pool beyond 80 mm from the mold wall.Slag consumption for high Mn steel increases 90?120%than that for low carbon steel. |
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