PRODUCTION OF STEEL SHEET ALLOYED ON ONE SIDE TO DIFFERENT THICKNESS

PURPOSE:To suppress recrystallization of spangle and to obtain a titled treated steel sheet having a lustrous and smooth surface by subjecting the thin plating side of a steel strip subjected to differential thickness plating to an alloying heat treatment to reflow the thick plating side then cooling quickly the thick plating side. CONSTITUTION:Both surfaces of a steel strip 1 emerging from a galvanizing bath cell 3 are subjected to differential thickness plating in which the difference in coating weight is two times or above by adjusting gas wiping nozzles 4, 5. The strip is then passed through an alloying heat treating furnace 2 to alloy the surface layer part on the thin plating side and to reflow the pure zinc in the surface layer part on the thick plating side. The strip 1 is quickly cooled while the zinc in the surface layer part on the thick plating side is in a molten state. A method of water quenching, etc. in which the strip 1 is passed through the inside of a water tank 6 is used as a means for the quick cooling. The ultrasmooth surface appearance having luster is obtd. by such treatment without recrystallization of the spangle on the surface of the pure zinc remaining on the thick plating side.