MANUFACTURE OF GALVANNEALED STEEL SHEET

PURPOSE:To develop a galvannealed steel sheet excellent in low temp. pitting resistance, corrosion resistance and galvannealed stickness by controlling an Fe-Al reaction developed at the initial stage of galvanizing, at the time of applying the galvannealing treatment to the surface of the steel sheet. CONSTITUTION:At the time of applying the galvanizing treatment by dipping the steel sheet into a hot dipping zinc bath, the temp. of the steel sheet is made to be at >=490 deg.C to promote the Fe-Al reaction at the initial stage, and in order to increase Al concn. in the galvanized film, the Al concn. in the galvanized film and the Al concn. in the hot dipping zinc bath are adjusted so as to become (the Al concn. in the galvanized film)-(the Al concn. in the hot dipping zinc bath) >=0.2wt.%. Further, by making >=150 PPI of the surface roughness of the steel sheet after adjusting the galvanized film by uniform alloying reaction to impart the fine ruggedness, an anchor effect is improved. In such a manner, the formation of Fe-Al layer is promoted and uniform alloying reaction is executed, and galvanized interface adhesion is improved by concentrating Al in the galvanized film and the peeling between the galvanized layer and the base steel sheet such as pitting is prevented.