ON THE MECHANISM OF HALFWAY CRACKS AND MACROSEGREGATION IN CONTINUOUSLY CAST STEEL SLABS

Background/Aims: In the continous casting process (CCP) it is common t o observe one or more types of macrosegregation such as ''centre-line segregation'', ''V-segregation'', ''inverse segregation'' and recently ''fluctuated segregation''. In a recent paper and part I, a mathemati cal model of heat transfer, strain and segregation has been presented. The present paper extends this model to include the effects of mechan ical strain in the secondary cooling zones under various cooling condi tions, and applies it to explain the mechanism of halfway cracks and m acrosegregation formation in the continous casting process. Methods: T he effect of inelastic thermo-mechanical strain on the formation of ma crosegregation in continously cast slabs has been made by mathematical modelling and metallographic examination. The mathematical model has been formulated for a one-dimensional case to examine the solidificati on of low alloy carbon steel. The inelastic thermo-mechanical strain h istory has been predicted in the first part of this two-part paper, wh ich serves as input to the solidification model. The metallographic st udy combined macrosegregation analysis and microstructural measurement s of slab samples collected during the industrial trials. Slab samples have been examined metallographically to verify the solidification mo del by measuring the carbon distributions and the dendrite arm spacing s as a function of distance from the surface. Results and Conclusions: Good agreements were obtained between the predicted and measured macr osegregation distribution and between the computed and measured dendri te arm spacings. The findings of this study indicate unambigously that macrosegregation in continously cast steel slab is quite sensitive to thermo-mechanical interdendritic strain, particularly in the secondar y cooling zones. The influence of the inelastic interdendritic strain function in the mushy zone on the macrosegregation is considered more important than the influence of its absolute value. The peritectic rea ction close to the solidus front significantly affects the macrosegreg ation criteria where the observed change in the macrosegregation is as sociated with the various cooling regimes between roll pairs.