According to the law of deformation of steel ingot when being rolled said invention defines the relationship of ingot-rolling shrinkage depth to folded waste product and oblique side of convex bottom of the ingot to included angle of bottom side of the ingot, and gives out a designing formula. Using said method can quick and accurately design out a concave underpan matched with the ingot form. For cast convex bottom ingot, at the same time of cutting off shrinkage waste product, the folding waste product also can be cut off, so that the tail-cutting rate is less than 1.5%, and the yield of finished billet and surface quality also are greatly raised. |