Surface defect evolution in hot rolling of high-Si electrical steels

Surface defects on metal strip products can have their root cause created in the hot rolling process by cavities and indents entrapped scale. The initial size and aspect ratio of surface features are critical parameters that determine whether they will be eliminated by the rolling process. The present research investigates the effects of the initial cavity geometry and the evolution mechanisms in a single hot rolling pass for a high-silicon electrical steel substrate. Laboratory hot-rolling experiments were carried out on blocks with open cavities of different geometries machined into the surface. The final geometry of the longitudinal and transversal profiles of the deformed cavities was analysed from metallurgical cross-sections. For a given initial cavity width, the width of the resulting oxide-filled defect is inversely related to the initial depth of the machined cavity. It has also been observed that the depth of defects increases linearly with the original depth to width aspect ratio of the initial cavities. Cavities with a depth of less than 1 mm essentially disappeared after the rolling. The final length is only dependent on its initial length in the rolling direction. The percentage reduction of cross-sectional area of the defects is found to increase with the initial depth and width of the cavities.