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 controlling the conditions in the molten bath to minimise the top dross layer in the molten bath. In particular, the method includes controlling top dross formation by including Ca and/or Sr in the coating alloy in the bath.

1 .- 24 . (canceled) 
     
     
         25 . A method of minimizing a top dross layer in a molten bath containing a layer of molten Al—Zn—Si—Mg alloy, the method comprising:
 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 from a heat treatment furnace into the molten bath wherein the molten alloy forms a coating of the alloy on the strip; 
 removing the coated strip from the molten bath including passing the coated strip through a top dross layer on the molten bath; 
 monitoring a Ca concentration and/or a Sr concentration in the molten bath; and 
 controlling the composition of the molten bath by adding Ca and/or Sr to the bath to achieve: 
 (a) a concentration of Sr of more than 150 ppm and less than 1250 ppm; or 
 (b) a concentration of Ca of more than 150 ppm and less than 500 ppm; or 
 (c) a combined concentration of Sr and Ca of more than 150 ppm, wherein the concentration of Sr is less than 1250 ppm and the concentration of Ca is less than 500 ppm; 
 
       to minimize the top dross layer in the molten bath. 
     
     
         26 . The method of  claim 25 , 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. 
     
     
         27 . The method of  claim 25 , wherein controlling the composition of the bath comprises maintaining a concentration of more than 200 ppm Ca in the bath. 
     
     
         28 . The method of  claim 25 , wherein controlling the composition of the bath comprises maintaining a concentration of more than 200 ppm Sr in the bath. 
     
     
         29 . The method of  claim 25 , wherein controlling the composition of the bath comprises maintaining a concentration of more than 1000 ppm Sr in the bath. 
     
     
         30 . The method of  claim 25 , wherein the Ca and/or Sr are added on a continuous or a periodic basis or by inclusion of Ca and/or Sr in Al and/or Zn ingots that are provided as feed materials for the molten bath. 
     
     
         31 . The method of  claim 25 , wherein the molten Al—Zn—Si—Mg alloy comprises more than 1.3% by weight Mg. 
     
     
         32 . The method of  claim 25 , wherein the molten Al—Zn—Si—Mg alloy comprises more than 1.5% by weight Mg. 
     
     
         33 . The method of  claim 25 , wherein the molten Al—Zn—Si—Mg alloy comprises more than 2.5% by weight Mg. 
     
     
         34 . The method of  claim 25 , wherein the molten Al—Zn—Si—Mg alloy comprises the following ranges in % by weight:
 Al: 53%; 
 Zn: 43%; 
 Si: 1.5%; and 
 Mg: 2%. 
 
     
     
         35 . The method of  claim 25 , wherein controlling the composition of the bath comprises maintaining a concentration of more than 200 ppm of Sr and Ca in the bath. 
     
     
         36 . The method of  claim 25 , wherein the mass of top dross layer generated is less than 400 grams per 15 kg of molten alloy after 2 hours. 
     
     
         37 . The method of  claim 25 , wherein the mass of top dross layer generated is less than 200 grams per 15 kg of molten alloy after 3 hours.