ELECTROMAGNETIC BRAKE TRIALS IN CONTINUOU S-CASTING MOLD AT SOLLAC-DUNKERQUE

Analysis of the origins of surface aspect defects observed on steel sh eet (spalled and blistered inclusionary lines) shows a close correlati on between these defects and the ones observed on slabs (sub-shell and internal inclusionary clusters, cracks), themselves controlled by hyd rodynamics and by the behavior of the meniscus in the continuous casti ng mold. The electromagnetic brake (EMBR) makes it possible to modify the hydrodynamic and surface state of the liquid metal. Sollac Dunkerq ue was chosen as pilot site to develop and test the EMBR. Industrial e quipment was thus installed on strand 6 for its continuous caster No. 23From December 1991 to July 1993, ten series of tests made it possibl e to treat nearly 50 000 tons of steel. The electromagnetic brake inst alled at Sollac Dunkerque is made of four electromagnetic poles which generate, in the mold, a static, crosswise magnetic field whose action creates volume (Laplace) forces with direction opposite to the one of the initial speed in the steel jets leaving the nozzle. Application o f the electromagnetic brake induces a reduction, within a ratio of 2 t o 3, of the metal recirculation speed under the meniscus and the fluct uations of this same speed. In some casting conditions (for example, f or large formats > 1700 mm), an inversion of the speed direction at th e meniscus may appear when the EMBR is applied. The hydrodynamic modif ications induced by the electromagnetic forces lead to a temperature r ise of the meniscus of 8 to 10 degrees C on the average. A decrease wi th a ratio of about 1.5 to 3 on the meniscus level standard deviation appears when the EMBR is applied. The incidense of the EMBR on slab ch aracteristics is assessed by three values:- the internal inclusionary cleanness characterized by the Midas method, - the size of the solidif ication hooks, - the presence of surface craks for peritectonic grades . The average inclusionary cleanness in the slab evolves with respect to the intensity of the EMBR. The brake brings an improvement to inter nal inclusionary cleanness in the axis and at one fourth the width for slabs with width greater than 1330 mm, while at the slab edges improv ement only appeared for widths greater than 1640 mm. A slight decrease in the size of the solidified hooks appeared in the IFs grade. In med ium carbon grades, the rate of slabs showing surface cracks is only sl ightly decreased. On the IFS grade, application of the electromagnetic brake leads to an improvement of about 65% for large widths in terms of spalled and blistered defect presence and about 30% in terms of dow ngraded coils. On extra-soft grades, the improvement in terms of downg raded slabs is not measurable because the rate of downgraded reference slabs is very low and even null. The electromagnetic brake is an effi cient tool for modifying the thermo-hydrodynamic state of the liquid m etal in the mold.