| Segregation is a common quality problem in the process of steel continuous casting.Serious segregation is usually accompanied by defects such as porosity,shrinkage cavity,and inclusion aggregation,which harms the quality and performance of the slab.Studying the segregation characteristics of solute components in the multicomponent system of steel and exploring the factors affecting solute redistribution during solidification are helpful to reveal the forming mechanism of segregation.This is of great significance to the control of segregation and the improvement of slab quality in steel production.In the present thesis,to systematically study the solute partition coefficient(ki),which is an important physical parameter related to segregation,a calculation model of ki for steel solidification considering phase transition and multi-component interaction is established.Taking high-sulfur steel as the research object,the influence of solidification temperature and phase composition on ki is discussed;the quantitative effect of phase transition path of solidification on ki is analyzed;the influence mechanism of inclusion precipitation on ki is revealed;a method for calculating the ki of multicomponent steel with composition fluctuation is proposed.Besides,a modified microsegregation model which considering inclusions precipitation is established to deeply study the microsegregation phenomenon of high-sulfur steel solidification.According to the specific composition of molten steel,the ki which varies with temperature and phase composition is determined and applied in the modified microsegregation model.Moreover,a three-dimensional macrosegregation model coupling inclusions precipitation is developed to investigate the precipitation distribution of MnS inclusion in the mushy zone and its quantitative effect on macrosegregation for the slab continuous casting of high-sulfur steel.Outside of this,the segregation behavior of carbon(C)atoms at the solidification interface of fcc Fe-C alloy and its dragging effect on interface migration are explored by molecular dynamics simulation.The main results are summarized as follows:(1)There is a lack of ki data for Fe-based multicomponent alloy(steel)solidification.In the present study,the ki for the steel with the different compositions are obtained,and the quantitative effect of phase transition path of solidification on ki is analyzed.According to the difference in C content or phase transition,it was considered that there were four kinds of phase transition paths for high-sulfur steel solidification.The variations of?and?phase proportions in the mushy zone under different solidification paths were analyzed.The influence of phase composition and temperature on ki was discussed,that is,k C,k P,and k S were mainly affected by phase composition,k Si was mainly influenced by temperature,while the k Mn was determined by both phase composition and temperature.The C content in steel influenced the phase transition path of solidification(i.e.temperature range and phase composition of solidification),and thus affected ki.When[C]?0.07 wt%and[C]?0.48 wt%,the solidified phases were respectively only?and?phases during solidification,and the C content has a small effect on the kCave,kPave,and kSave(kiave is the average value of ki in the whole solidification stage of molten steel).Specifically,the kCave,kPave,and kSave were around0.150,0.328,and 0.036 for[C]?0.07 wt%,and around 0.325,0.167,and 0.018 for[C]?0.48 wt%,respectively.But,for the steel with 0.07?0.48 wt%C,the?and?phases appeared successively during solidification.With the decrease of C content(or the increase of?phase proportion),the kCave increased linearly while kPave and kSavedecreased linearly.(2)ki is influenced by steel composition during solidification.The inclusion precipitation will affect the component concentration of molten steel,thus affecting ki.The effect of MnS precipitation on ki in the solidification process of high sulfur steel is studied,and the influence trend of Mnand S content on ki is discussed.With the increasing Mncontent,kMnave and kSave increased,kCave,kSiave,and kPave decreased,but the S content has an opposite effect on each kiave.For the steel with high S content([S]?0.05 wt%,[Mn]=1.30 wt%),with the decrease of solidification temperature,k C and k Sboth decreased first and then increased,that is,there was a turning point.Besides,the change rate of each ki would increase or decrease suddenly at a certain point during solidification,which was caused by MnS precipitation.(3)The quantitative effect of composition fluctuation of steel grade on ki is investigated,and a method to calculate the ki for the solidification of multicomponent melt with composition fluctuation is proposed.Taking 1215 high-sulfur steel as an example,the quantitative relationship between ki,temperature,and component content is obtained.Results showed that the composition fluctuation in 1215 steel has a small effect on k C,k P,and k S,but a relatively large effect on k Mn and k Si.The k C,k Si,k Mn,k P,and k S were respectively in the ranges of 0.147?0.153,0.58?0.76,0.73?0.65,0.32?0.34,and 0.038?0.035.Furthermore,the fluctuation of Mnand S content has the largest effect on each ki,followed by the fluctuation of C content,and the fluctuation of Si and P content has the least effect.(4)The effects of solute partition coefficient,inclusion precipitation,and phase transition on the microsegregation for high-sulfur steel solidification are systematically explored,and the influence mechanism of the peritectic reaction on microsegregation is emphatically discussed.At different solidification stages of high-sulfur steel,the difference in ki,which was caused by the change in phase composition and temperature,has an important influence on the variation trend of solute microsegregation concentrations,especially at the end of solidification.The percentage difference of microsegregation concentration calculated by variable ki and constant ki for solutes Si,P,C,Mn,and S,were-35.5,-25.0,14.0,14.8,and 43.0%,respectively.The Mnmicrosegregation concentration first increased and then decreased while the S microsegregation concentration increased monotonously during solidification due to the combined effects of segregation and MnS precipitation.Compared with the case of without considering MnS precipitation,the final Mnand S microsegregation concentrations for the case of considering MnS precipitation could be reduced by 88.3and 74.9%,respectively.For the steel with 0.07?0.16 wt%C,the peritectic reaction would occur at the end of solidification,which led to the difference of?(and?)phase fractions under different C content,thus affecting the kiand microsegregation.As a result,with the increasing C content,SRC and SRMn decreased rapidly,SRP increased rapidly,and SRS decreased first and then increased(SRi is the microsegregation ratio of solute i).But the peritectic reaction that occurred in the early stage of solidification has little effect on microsegregation.(5)The interaction between macrosegregation and inclusion precipitation in the mushy zone of slab continuous casting(1530 mm×190 mm)for high-sulfur steel is revealed.The MnS precipitation and its improvement effect on macrosegregation are investigated.During the continuous casting process,a large number of MnS inclusions would precipitate in the core of the slab when the casting distance was 17?19 m.The position with the largest amount of MnS precipitates was around 15 mm away from the slab centerline(width direction).Compared with the case of without considering MnS precipitation,the macrosegregation ratios of solutes Mnand S on the slab centerline for the case of considering MnS precipitation decreased from 1.00?1.02 to 0.97?1.01 and from 1.03?1.05 to 0.93?1.04,respectively.In particular,for the steel with S-0.35 wt%,the decrease percentages of Mnand S macrosegregation were 2?4 and 5?10%,respectively.The larger the S content in the steel,the more the MnS precipitates,the larger the area of MnS precipitation in the slab,and the more significant effect of the MnS precipitation on Mnand S macrosegregation.(6)The interface kinetic coefficients(?)for the solidification and melting of fcc Fe-(0?0.5wt%)C alloy are estimated by molecular dynamics simulation.The diffusion activation energy(QC)of C atoms in the region of the solid-liquid interface for solidification is calculated using the Debye-Waller factor(<u2>).The internal relations among interface migration,C segregation,and interface morphology are analyzed.For the case of melting,the effect of C content on the?was small,and?=18.1?19.1cm/s·K.But for the case of solidification,the?decreased monotonically with the increasing C content due to the segregation and dragging effect of C atoms at the solid-liquid interface,and?=9.6?17.9 cm/s·K.Besides,there was critical undercooling(?TP)for the interface migration in the case of solidification,and the?TP increased monotonically with the increasing C content.Moreover,QC increased monotonically with increasing C content.That is,the higher the C content,the lower the diffusion ability of C atoms at the interface,and the stronger the C j,resulting in the greater dragging effect of C atoms on the interface migration.As a result,the?decreased.More importantly,the smooth solid-liquid interface was not conducive to the interface migration,and the smooth interface was often accompanied by serious non-uniform distribution of C atoms. |
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