Abstract: A body of steel heated to a high temperature of at least 300.degree. C., e.g. a slab coming from a hot-rolling stage, is marked with alphanumerical characters by training a jet of molten metallic material, preferably consisting at least predominantly of aluminum or copper, onto a face of that body from a nozzle which is two-dimensionally displaceable parallel to that face under the control of a programmable microprocessor. The nozzle is part of a working head to which the marking material is fed in the form of a wire whose leading end is atomized by an electric arc generated between that end and an adjoining counterelectrode. The working head is mounted on a carriage movable toward and away from the steel body to be marked.