Mechanisms of White Band Formation in Continuously Cast Slab Induced by Strand Electromagnetic Stirring

To elucidate the formation mechanism of white band induced by strand electromagnetic stirring (S-EMS) during slab continuous casting process, the microstructural features in the columnar grain region, white band region, and equiaxed grain region are firstly characterized using optical microscopy, electron backscatter diffraction, and original position analysis (OPA-200). The results indicate that during the transition of the solidified structure from columnar grains to white bands and then to equiaxed grains, the secondary dendrite arm spacing initially decreases from approximately 90 to 60 μm, and then increases again to 130 μm. As the current intensity increases, the crystallographic orientation of the grains, particularly the <111> orientation, becomes more pronounced, and the grain size further decreases. During the transformation from columnar grains to white bands to equiaxed grains, the carbon content exhibits a trend of first decreasing from 0.175 to 0.163 pct, and then increasing again to 0.177 pct. Later, our previous volume-averaged three-phase solidification model coupled with an S-EMS electromagnetic field model is adopted to investigate the effect of electromagnetic stirring intensity and secondary cooling intensity on the white band region. The numerical results show that the application of S-EMS is beneficial for the elimination of the slab centerline segregation, but it can cause white band with negative solute segregation when the current intensity is excessive. Higher current intensity and larger molten steel flow velocity promote negative segregation, while increasing secondary cooling water flow reduces negative segregation near the white band. Finally, the mechanism of white band formation in the slab induced by the S-EMS is proposed. The forced convection generated by S-EMS is the primary factor affecting the dynamic solute washing in the mushy zone, and determining the brightness of the white band. The extent of the abrupt change in cooling rate after the stirring zone determines the width of the white band. Moreover, equiaxed crystals exhibit a stronger resistance to segregation due to their higher fluidity and enhanced capability to redistribute solute elements.