Multi-Dimensional Multi-Scale Characteristics Of Solidification Structure And Its Relationship With Segregation In High Carbon Steel Billets

During solidification of alloys,redistribution of solutes occurs between liquid and solid phases leading to non-uniformity of composition.Segregation of solute elements is an inherent characteristic of alloy solidification.Due to large sizes,macro/semi–macro segregation in the billets are considered more harmful to finished steel properties,as they cannot be eliminated even with prolonged heat treatments.In order to reduce macro/semi-macro segregation,most of the existing studies have been conducted by adjusting continuous casting parameters such as superheat and secondray cooling strength and/or using techniques such as electromagnetic stirring and mechanical reduction.However,the quantitative description of solidification structure and segregation morphology and their intrinsic relationship is insufficient in the previous research.Consequently,the thesis focuses on the multi–dimensional multi–scale characterization of the solidification structure and segregation morphology of the billets and the intrinsic relationship between them,taking 82B cord steel,70 steel and GCr15 bearing steel as examples,thus providing theoretical and technical guidance for the fine control of the high-quality steel billets.The thesis starts with the quantitative characterization method of solidification structure and segregation morphology of the billets,innovatively introduces fractal theory and combines traditional Euclidean geometry method to carry out multi–scale characterization of solidification structure and segregation morphology.Furthermore,the mechanism of the influence of solidification parameters and liquid flow on the solidification structure of the billets was investigated from the morphology perspective,according to the comparison of solidification structure under different solidification parameters and processes.Moreover,to break through the limitations of traditional two-dimensional analysis,the solidification structure of different sections in the billets were studied and the solidification structure of the billets was reconstructed in three dimensions based on the serial sectioning method.The flow behavior of molten steel in three-dimensional structure was studied.Finally,a quantitative equation for the intrinsic relationship between the solidification structure and the segregation morphology was constructed.The main research contents are summarized as follows:（1）Fractal dimension can effectively quantitatively characterize the multi–scale（from micron to millimetre）features of solidification structure and segregation morphology.Simultaneously,it describes different types of solidification structure in a certain zone of the billet with unidirectional grains and equiaxed grains without a classification.It was found that the dendrite growth process has a phenomenon of the increasingly complicated dendrite overall morphology（ICDOM）,which is mainly depends on the degree of constitutional undercooling at the front of the solidifying interface.Negative–correlation is shown between fractal dimension calculated by perimeter–area method(D_PA)and cube root of local solidification time（the fitting coefficient is 0.79）.This finding indicates that D_PAcan be used as a parameter for estimating local solidification time of the billet in which the measurement of SDAS is difficult.In addition,the fractal dimension not only is a suitable geometric parameter to describe the complexity of the solidification structure,also embodies the entropy value in the solidification system.（2）The cooling rate（R）at the stage of the superheat elimination and liquid–solid phase transformation has an important influence on the solidification structure.Secondary dendrite arm spacing（SDAS）decreases linearly,and the fractal dimension calculated by box–counting method(D_Box)increases first and then remains unchanged with increasing of R at the stage of the superheat elimination.SDAS decreases in the form of a power function,and D_Boxincreases first and then decreases with increasing of R at the stage of liquid–solid phase transformation.In addition,from the morphology perspective,the superheat and R affect the macro/semi–macro segregation of the billet by jointly affecting the fractal dimension and dendrite arms spacing,resulting in the change of the flow resistance of the enriched solute liquid phase.At low superheat,the solidification structure has a small permeability and the liquid has a high resistance to flow.At higher cooling rates,SDAS are smaller and the solidification structure is more complex,the liquid phase has a high resistance to flow.（3）Through the comprehensive comparison of the solidification structure obtained by the high temperature confocal laser scanning microscope（CLSM）and actual continuous casting billets,it is found that the liquid flow can increase SDAS and also lead to an increase in the solute concentration gradient at the solidified interface,thus making the dendrite morphology more complex.Furthermore,compared to CLSM,the permeability of the solidification structure in continuous casting billets is greater,and the flow resistance of the liquid is smaller,making it easier to produce macro/semi–macro segregation.（4）The method for calculating the permeability of an actual metallic alloy based on dendrite arm spacing,fractal dimension and segregation area ratio is effective.Comparison of permeability values at different locations and sections of the billets,it was found that the flow of liquid in the columnar zone is stronger in the casting direction,the flow of liquid in the CET zone is stronger along the direction from the surface to center of the billet,and the flow of liquid in the equiaxed zone is stronger along the direction from inner to outer arc of the billet.Combined with the simulation results of molten steel in the three-dimensional reconstructed structure,it is found that the flow resistance of the liquid phase in the equiaxed dendrite zone is smaller than that in the columnar dendrite zone.This result shows that when controlling the macrosegregation of the billets by increasing the proportion of equiaxed dendrites,the three-dimensional characteristics and liquid flow behavior of the solidification structure in the equiaxed dendrites region should be considered at the same time.（5）Quantitative expressions for solidification structure morphology parameters(D_Boxand SDAS)and macro/semi-macro segregation of cast billets were constructed by correlating the segregation area ratio,fractal dimension and secondary dendrite arm spacing(R_seg=a-b·D_Box+c·λ₂).This equation compensates for the shortcomings of the traditional method,which reflects macro/semi-macro segregation of the cast billets only through the SDAS or the equiaxed dendrite ratio.By combining this quantitative equation with the influence of the continuous casting parameters on the solidification morphology,a more accurate quantitative prediction of the occurrence of macro/semi-macro segregation defects in the cast billet can be achieved.