Method of forming an alloy coating on a strip

Abstract

A hot dip method of forming an Al—Zn—Si—Mg alloy coating on a strip is disclosed. The method includes control-ling the conditions in the molten bath to minimise the top dross layer in the molten bath. In particular, the method includes con-trolling top dross formation by including Ca and/or Sr in the coating alloy in the bath.

1 . A method of minimizing a top dross layer in a molten bath containing a layer of molten Al—Zn—Si—Mg alloy external of an outlet snout of a heat treatment furnace that extends into the molten bath, the method including:
 providing molten Al—Zn—Si—Mg alloy in a molten bath, the alloy comprising the following ranges in % by weight:
 Al: 45 to 60% 
 Zn: 35 to 50% 
 Si: 1.2 to 2.5% and 
 Mg: 1.0 to 3.0%; 
 
 passing a steel strip into the molten bath downwardly through an outlet snout of a heat treatment furnace that extends into the molten bath and then through the molten alloy with the molten alloy forming a coating of the alloy on the strip; 
 removing the coated strip from the molten bath by passing the coated strip upwardly through a top dross layer external of the outlet snout; 
 monitoring a Ca concentration and/or a Sr concentration in the molten bath; and 
 controlling the composition of the molten bath by including Ca in an amount which is more than 100 ppm and less than 500 ppm, and by including Sr in an amount that is more than 100 ppm and less than 1250 ppm in the molten bath to minimize the top dross layer in the molten bath external of the outlet snout. 
 
     
     
         2 . The method defined in  claim 1  wherein controlling the conditions in the molten bath minimizes entrainment of any one or more of molten metal, gas, and intermetallic particles in oxide films in the top dross layer. 
     
     
         3 . The method defined in  claim 1  including controlling the composition of the bath to include more than 150 ppm Ca. 
     
     
         4 . The method defined in  claim 1  including controlling the composition of the bath to include more than 200 ppm Ca. 
     
     
         5 . The method defined in  claim 1  including controlling the composition of the bath to include more than 150 ppm Sr. 
     
     
         6 . The method defined in  claim 1  including controlling the composition of the bath to include more than 200 ppm Sr. 
     
     
         7 . The method defined in  claim 1  including controlling the composition of the bath to include less than 1000 ppm Sr. 
     
     
         8 . The method defined in  claim 1  wherein the Ca and Sr are added on a continuous or a periodic basis or by inclusion of Ca and Sr in Al and/or Zn ingots that are provided as feed materials for the molten bath.