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 p H 5-9. Particular embodiments focus on Al—Zn—Si—Mg alloys that contain the following elements in % by weight: Zn: 30 to 60, Si: 0.3 to 3, Mg: 0.3 to 10, and Balance Al and unavoidable impurities.

1 . A method of forming an Al—Zn—Si—Mg alloy coating on a steel strip to form a coating of an Al—Zn—Mg—Si on a steel strip, the method including 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, with the cooling step including controlling the pH of cooling water to be in a range of pH 5-9. 
     
     
         2 . The method defined in  claim 1  wherein the cooling step includes controlling the pH of cooling water to be less than 8. 
     
     
         3 . The method defined in  claim 1  wherein the cooling step includes controlling the pH of cooling water to be less than 7. 
     
     
         4 . The method defined in  claim 1  wherein the cooling step includes controlling the pH of cooling water to be greater than 6. 
     
     
         5 . The method defined in  claim 1  wherein the cooling step includes controlling the temperature of cooling water to be in a range of 25-80° C. 
     
     
         6 - 10 . (canceled) 
     
     
         11 . The method defined in  claim 1  wherein the cooling step includes controlling cooling water temperature to be greater than 30° C. 
     
     
         12 . (canceled) 
     
     
         13 . The method defined in  claim 1  wherein the cooling step includes controlling the pH by adding acid to the cooling water. 
     
     
         14 . The method defined in  claim 1  wherein the strip cooling step includes controlling the chemistry of the cooling water. 
     
     
         15 . The method defined in  claim 1  wherein the strip cooling step includes controlling the chemistry and the pH by adding acid to the cooling water. 
     
     
         16 . The method defined in  claim 1  wherein the cooling step includes a water quench step. 
     
     
         17 . The method defined in  claim 1  wherein the cooling step includes a closed loop in which water is circulated through a circuit that supplies water to the coated strip and collects and cools water and returns the cooled water for cooling the coated strip. 
     
     
         18 . The method defined in  claim 1  wherein the cooling step includes an open loop in which cooling water is supplied from a cooling tower to the coated strip and collected and recirculated through the cooling tower. 
     
     
         19 - 20 . (canceled) 
     
     
         21 . The method defined in  claim 1  wherein the Al—Zn—Si—Mg alloy includes more than 0.3% by weight Mg. 
     
     
         22 - 26 . (canceled) 
     
     
         27 . The method defined in  claim 1  wherein the Al—Zn—Si—Mg alloy includes more than 1.2% by weight Si. 
     
     
         28 . (canceled) 
     
     
         29 . The method defined in  claim 1  wherein the Al—Zn—Si—Mg alloy includes the following ranges in % by weight of the elements Al, Zn, Si, and Mg:
 Zn: 30 to 60% 
 Si: 0.3 to 3% 
 Mg: 0.3 to 10% 
 Balance Al and unavoidable impurities. 
 
     
     
         30 . The method defined in  claim 1  wherein the Al—Zn—Si—Mg alloy includes the following ranges in % by weight of the elements Al, Zn, Si, and Mg:
 Zn: 35 to 50% 
 Si: 1.2 to 2.5% 
 Mg 1.0 to 3.0% 
 Balance Al and unavoidable impurities. 
 
     
     
         31 . An Al—Zn—Mg—Si alloy coated steel strip produced by the method defined in  claim 1 . 
     
     
         32 . (canceled)