Research On Formation Mechanisms Of IF Steel Surface Defects Through Numerical Simulation

Currently,IF steel has been extensively applid in automotive industry.However,the surface defect of IF steel is a key problem during its producting process,and has not been efficiently controlled.Sliver defect always appears during IF steel rolling process.Given this,the papper conducts a series of studies on the particle(large inclusion and argon bubble)transient motion,entrapment,slag entrainment mechanisms during IF steel continuous casting process and the evolution of inclusion positions in slab during IF steel hot rolling process.A three dimentional(3D)LES numerical model coupling molten steel transient flow,heat transfer and particle motion,entrapment has been constructed.In the model,the momentum exchange between molten steel and bubbles is performed by two-way coupling.The effects of the argon bubble injection on the molten steel flow,the motion and capture of inclusions have been studied.The predicted results indicate that argon bubble injection obviously changed the flow pattern of the molten steel and the inclusion motion in the liquid pool.Consequently,the inclusion entrapment in the solidified shell was also changed.The particle distributions in the slab surface layer have been revealed.In addition,measurements of bubbles and large inclusions in samples obtained from a plant have been performed,by methods of optical microscope examination and galvanostatic electrolysis.The results of this work show a favourable agreement between the numerical model predictions and the measurements.Furthermore,the predicted results of the inclusion capture in the case with argon bubble injection are more compatible with the measurements than those in the case without argon bubble injection.By method of couping the applied magnetic field of EMBr into the above model,this paper has studied the effects of EMBr on the transient molten steel flow,particle motion and entrapment during continuous casting process.The results indicate that EMBr can efficiently control the turbulent flow of molten steel,make the horizontal velocity of molten steel at the mold top surface vary in a reasonable range.Moreover,EMBr can reduce the entrapment ratios of large inclusions and argon bubbles by the initial solidified shell.However,EMBr make more inclusions and argon bubbles move into the deep positions of liquid pool.This paper has developed a transient multiphase LES model.In the model,the multiphase(steel-slag-air)flow was simulated through VOF method,argon bubbles were tracked using DPM method.The slag entrainment mechanisms are different under different conditions.In the case without argon injection and EMBr,three slag entrainment mechanisms were found:shear flow,Von Karman vortex,and level fluctuation.Nevertheless,when argon was injected,in addition to the above mechanisms,one extra mechanism was predicted that was bubble interaction.Additionally,bubble interaction was the unique slag entrainment mechanism in the case with argon injection and EMBr.The impact of argon bubbles on the steel-slag interface resulted in exposed slag eye.In the case without EMBr,the exposed slag eye phenomenon was intermittent.However,the phenomenon was persistent when EMBr was applied.A 3D numerical model has been constructed for the IF steel hot rolling process by finite element method.Through node tracking method,the positions of large inclusions in IF steel slab surface layer(50mm)have been tracked during hot rolling process.At last,the distribution of these inclusions in the hot rolled plate has been revealed.The results indicate that,for both the no scarfing slab and scarfing slab,the large inclusions in their outer arc surface layer would be gathered in 1 mm surfec layer of the hot rolled plate.Moreover,for both the no scarfing slab and scarfing slab,the large inclusions in their inner arc surface layer would be gathered in 1.5 mm surface layer of the hot rolled plate.After hot rolling process,large inclusions in slab narrow face generated an inclusion concentrating zone at the edge region of hot rolled plate.By method of galvanostatic electrolysis,measurements of large inclusions in samples obtained from hot rolled plate in a plant have been performed.With regard to the distribution of large inclusions in hot rolled plate,the trend of the experimental results is similar to that of the predicted results.The scope of the paper is to provide some theoretical guidances for IF steel production basing on these research.