Method of producing metal-coated steel strip

Abstract

A method of forming a coating of an Al—Zn—Si—Mg alloy on a steel strip to form an Al—Zn—Mg—Si coated steel strip is disclosed. The method includes the steps of dipping steel strip into a bath of molten Al—Zn—Si—Mg alloy and forming a coating of the alloy on exposed surfaces of the steel strip and cooling the coated strip with cooling water. The cooling step includes controlling the pH of cooling water to be in a range of pH 5-9. Particular embodiments focus on Al—Zn—Si—Mg alloys that contain the following elements in % by weight: Zn: 2 to 19, Si: 0.01 to 2, Mg: 1 to 10, and Balance Al and unavoidable impurities.

1 .- 35 . (canceled) 
     
     
         36 . A method to reduce precipitate formation on cooling water heat exchangers in a metal coating line, wherein the metal coating line is for forming a coating of an Al—Zn—Si—Mg alloy on a metal strip, wherein the Al—Zn—Si—Mg alloy includes the following ranges in % by weight:
 Al: 2 to 19% 
 Si: 0.01 to 2% 
 Mg: 1 to 10% 
 Balance Zn and unavoidable impurities, 
 the method including: 
 a) suppressing the alkalinity of cooling water by controlling the pH of the cooling water to within the range of pH 5-9 to reduce the dissolution of the alloy coating in the cooling water. 
 
     
     
         37 . The method of  claim 36 , wherein prior to step (a), a native oxide layer has formed on the surface of the alloy coating. 
     
     
         38 . The method of  claim 37 , wherein the native oxide layer includes magnesium oxide, aluminium oxide, and oxides of other elements of the Al—Zn—Si—Mg alloy coating. 
     
     
         39 . The method of  claim 36 , wherein step (a) includes continuously monitoring and controlling the pH of the cooling water. 
     
     
         40 . The method of  claim 36 , wherein step (a) includes controlling the pH by adding acid to the cooling water. 
     
     
         41 . The method of  claim 36 , further including:
 b) continuously monitoring and controlling the temperature of the cooling water.   
     
     
         42 . The method of  claim 36 , wherein the metal strip is a steel strip. 
     
     
         43 . The method of  claim 36 , wherein step (a) includes controlling the pH of the cooling water to be less than 8. 
     
     
         44 . The method of  claim 36 , wherein step (a) includes controlling the pH of the cooling water to be less than 7. 
     
     
         45 . The method of  claim 36 , wherein step (a) includes controlling the pH of the cooling water to be greater than 6. 
     
     
         46 . The method of  claim 41 , wherein step (b) includes controlling the temperature of the cooling water to be less than 70° C. 
     
     
         47 . The method of  claim 41 , wherein step (b) includes controlling the temperature of the cooling water to be less than 60° C. 
     
     
         48 . The method of  claim 41 , wherein step (b) includes controlling the temperature of the cooling water to be less than 55° C. 
     
     
         49 . The method of  claim 41 , wherein step (b) includes controlling the temperature of the cooling water to be less than 50° C. 
     
     
         50 . The method of  claim 41 , wherein step (b) includes controlling the temperature of the cooling water to be greater than 40° C.