PURPOSE: To form hardly releasible black film of iron tannate on the surface of steel sheet or hoop, by treating the steel sheet or hoop with aqueous iron tannate soln. contg. inorg. nitrate before heat treatment.

CONSTITUTION: To an aqueous soln. of iron tannate, made by dissolving iron into an aqueous soln. of tannin or tannic acid 2W15% concn., is added 0.2W5% inorg. nitrate e.g. nickel nitrate. Then the aqueous soln. is painted on thesurface of steel sheet or hoop by roll coating or spray coating, and the metal is dried with hot air at about 100°C, if necessary after it is roll squeezed. As a result, hardly releasible black film having high adhesive strength is formed easily and in a short time. This blackening treatment is effective in the following heat treatment.