Abstract: A tool for forming parting grooves in a flat, plastically deformable workpiece, especially a continuously cast steel slab in a manner which seals the internal porosity of the slab in the regions which become exposed upon division, has a working portion in the form of a ridge which is most preferably in the form of a circumferential flange on a roll; and the cross-sectional shape of the ridge or flange is that of a highly convex tip between two gently convex side faces, and optionally, or essentially if either or both of the tip and the side faces are instead straight, the angle at which the side faces are mutually inclined is 20.degree. to 60.degree., the width of the tip is a given porportion of the thickness of the workpiece, and the width of the tip is specifically related to the said angle of inclination of the side faces. The height of the side faces, and their curvature and that of the tip, are also preferably within set limits.