Stirring strands via traveling-wave magnetic fields to increase the equiaxed crystal ratio of stainless-steel slab castings

Electromagnetic stirring of strands by a traveling-wave magnetic field is a cutting-edge continuous casting technology for eliminating the columnar crystal structure that tends to develop in stainless- and/or silicon-steel slab castings. The common ridging defect on the surface of ferritic stainless strip products has been found to be closely related to the well-developed as-cast columnar crystal structure. To explore the various electromagnetic properties of the traveling-wave magnetic fields applied to the secondary cooling zone of a slab casting strand, we used the segmented computational domain method to develop a coupled math model to analyze the electromagnetic, fluid flow, heat transfer, and solidification behaviors, which had been previously determined in an electromagnetic measurement experiment to be a valid approach. The modeling analysis results regarding the traveling-wave magnetic fields show that molten steel stirring has some effect on the end of the slab strand. We also found that the intensity of the magnetic induction when using a box-type electromagnetic stirrer (B-EMS) is much greater on the inside of the strand than on the outside, as compared with its symmetric behavior when applying a roller-type electromagnetic stirrer (R-EMS). At an electrical power of 400 kW and frequency of 7 Hz, the current intensity of the R-EMS is higher than that of the B-EMS by 75 A, achieving a more efficient stirring effect for promoting equiaxed crystal nucleation in front of the solidified shell. In casting experiments in a stainless-steel slab caster, both the B-EMS and R-EMS are found to inhibit the growth of columnar crystals through nucleation of the heads of the dendrites, which realizes an equiaxed crystal ratio of the slab casting 45% higher than its threshold value. In addition, an R-MES with two pairs of rollers using inverse thrust EMS forces can produce an equiaxed crystal ratio 17% higher than that achieved by the B-EMS, and can thus be used in the casting production of ferritic stainless steels to obtain final strip products with no ridging defects.