PURPOSE: To decrease heat input to rolls and to improve the protecting effect of the rolls by using a fluid film contg. an inorg. compd. and having poor heat conductivity or a powder-contg. film at the roll bite under hot rolling thereby suppressing the contact heat conduction of the roll bite.

CONSTITUTION: Pulverous powder of an inorg. compd. which has ≤1μ average grain size has no corrosiveness to iron and steel including cast iron and cast steel and other metals and functions as a material having poor heat conductivity is prepd. Said powder is so supplied as to be drawn into the roll bite under hot rolling. The contact heat conduction in the roll bite is suppressed by any of the endothermic reaction with melting of said pulverous powder, the fluid film having poor heat conductivity by melting and the heat insulating effect by the film contg. said pulverous powder. The above-described inorg. compd. is, for example, NaPO3, KPO3 and CaCO3 or the mixture thereof and the supply of such compd. after mixing with a rolling mill lubricant is equally satisfactory.