Analysis and prevention of edge cracking phenomenon during hot rolling of non-oriented electrical steel sheets

In this study, a microstructural investigation was conducted on the edge cracking phenomenon in non-oriented electrical steel sheets during hot rolling. Particular emphasis was placed on the role of iron oxides and ferrite grain shape in the edge cracking. Detailed microstructural analyses of the sheets showed that highly elongated, fine grains were predominantly found in the edge region which did not recrystallize. The hardness of this region was higher than that of the central region composed of equiaxed grains where dynamic recrystallization occurred. Various oxides were observed in this edge region, and initiated intergranular or cleavage cracks when the edge regions were impacted by side guide rolls. These oxides were identified to be Si- and Mn-containing iron oxides which might have formed during continuous casting or hot rolling. In addition to edge cracks, line scabs were found in a linear mode parallel to the rolling direction on the surface of the edge-cracked region, probably due to the exposure to an oxidation atmosphere. Thus, in order to prevent the edge cracking, the minimization of iron oxides and highly elongated grains in the edge region and the careful control of the spacing between steel sheets and side guide rolls were suggested.