Influence Of Thermoelectric Magnetic Effect On The Columnar To Equiaxed Transition And Solidification Shrinkage During Solidifying Steel

Adopting electromagnetic field technique to eliminate the segregation and shrinkage porosity during the solidification stage of steels is one of the most attractive subjects for metallurgists.With the development of super conducting technology,high static magnetic fields（10 T）in rather large space can be produced easily.In addition,based on the Seebeck effect,a thermoelectric Lorentz force,named as thermoelectric magnetic force（TEMF）,is resulted from interaction between a thermoelectric current（TEC）near the liquid/solid interface of non-magnetic or feeble-magnetic materials and the static magnetic field.The TEMF can impose on the melts,dendrites and equiaxed grains in the mushy zone,which is commonly called thermoelectric magnetic（TEM）effect.This effect under the static magnetic field without the skin effect has open many new possibilities in controlling the solidification process of steels.Through further investigation of influence of the TEM effect on the columnar to equiaxed transition（CET）and solidification shrinkage in directionally solidified steel,theoretical basis and practical guidance for application of the static magnetic field to improve quality of steel ingots have been obtained.（1）A new formation mechanism for the CET promoted by the TEM effect is proposed based on comprehensive analysis of the studies on the CET and TEM effect.On the one hand,this mechanism mainly explains the reasons for the secondary dendrites detached from the primary dendrites form the fragments under an axial static magnetic field（ASMF）from microcosmic point of view.On the basis of the catastrophic elastic remelting mechanism,the stress(10^-1 MPa),localized into the secondary dendrite necks,induced by the TEM effect can serve as the driving force for causing the dendrites to the fragments in approximately 10^-1 s.On the other hand,the TEMC（10³μm/s）existed in the interdendritic spacing transports a part of the fragments from the interdendritic spacing to the region ahead of the solidification front,especially along the edge of the sample.（2）The increase in the magnetic field intensity and temperature gradient or the decrease in the growth speed contributes to the CET of directionally solidified 20 steel and GCr15 steel.Columnar dendrites degenerate and transform into equiaxed dendrites at the edge of the sample.With a relatively low growth speed（5μm/s）under ASMF,columnar dendrites become equiaxed dendrites.Applying 4 T magnetic field intensity can form equiaxed grains below the pulling rate of 20μm/s at about 100 K/cm temperature gradient.This cooling rate（0.2 K/s）is in the range of the actual cooling rates of the steel(10^-1～10⁰ K/s).This phenomenon mainly is caused by the TEM effect.（3）With the application of ASMF,the temperature gradient,the growth speed and magnetic field intensity are the most crucial parameters which govern the solidification microstructure of directionally solidified GCr18Mo steel.The CET is enhanced with the increases in the magnetic field intensity and temperature gradient and the decrease in the growth speed.Combined with the experimental results,a numerical simulation was proposed to explore the mechanism of the CET with ASMF.The TEMF affects the CET ranging from the formation of the TEMC,transport of the fragments in the melts to detachment of dendritic side arms.Applying 5 T magnetic field intensity can form equiaxed grains below the cooling rate of 0.3 K/s at temperature gradient of 104 K/cm.（4）The increase of the TEM effect can improve the uniformity of distribution of composition in the directionally solidifying GCr15 steel and GCr18Mo steel.This result can explain the influence of ASMF on the CET and the grain refinement of GCr18Mo steel in another way.（5）The variation in the three-dimensional morphology and amount of the porosity was characterized quantitatively via an X-ray tomographic approach.The amount of the pores first decreases and then increases with increasing intensity of ASMF.At experimental conditions,the optimization magnetic field intensity is 1 T.The interfacial shape distribution plots show that the shape of the shrinkage porosity changed from tortuous adjusted by the remaining space between the dendrites to elliptic modified by feeding liquid steel under ASMF.On the basis of both experimental and simulated results,we believe that adjusting the magnitude of the TEMC properly can lead to sufficient feeding,thus reducing the formation of shrinkage porosities.