PRODUCTION OF STEEL SHEET ALLOYED ON ONE SIDE TO DIFFERENTIAL THICKNESS

PURPOSE:To obtain a titled steel sheet with which spangle and zinc sag do not arise 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 oxidizing electrolytically the surface layer on 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 is then passed through a slow cooling zone 7, an air cooling zone 8 and water quench 9, by which the strip is cooled. Such strip 1 is conducted into an electrolytic pickling cell 10 where only the one surface on the thick plating side is electrolytically pickled to dissolve and remove the excessive coating in the surface layer part of the plating. The strip is finally passed through a finishing device 12, by which the surface on the thick plating side is finished to uniform and smooth surface having luster.