Metal-coated steel strip

Abstract

A method of forming an Al—Zn—Si—Mg alloy coating on a steel strip includes 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. The method also includes controlling conditions in the molten coating bath and downstream of the coating bath so that there is a uniform Al/Zn ratio across the surface of the coating formed on the steel strip. An Al—Zn—Mg—Si coated steel strip includes a uniform Al/Zn ratio on the surface or the outermost 1-2 μm of the Al—Zn—Si—Mg alloy coating.

1 .- 11 . (canceled) 
     
     
         12 . A method of forming an Al—Zn—Si—Mg alloy coating on a steel strip having no visual apparent ash mark defect, the method comprising:
 dipping steel strip into a bath of molten Al—Zn—Si—Mg alloy and controlling a Ca concentration of the molten Al—Zn—Si—Mg alloy to be at least 100 ppm and less than 200 ppm, 
 forming a coating of the alloy on exposed surfaces of the steel strip to form a coated steel strip, and 
 controlling a rate of cooling of the coated steel strip after the coated steel strip leaves the bath to be greater than 10° C./s and less than 40° C./s while the coated strip temperature is between 400° C. and 510° C., to produce a uniform Al/Zn ratio across a surface of the coating. 
 
     
     
         13 . The method of  claim 12 , wherein a variation in the Al/Zn ratio between two or more independent areas on a surface of the coating is less than 0.1. 
     
     
         14 . The method of  claim 12 , comprising controlling the Ca concentration of the molten Al—Zn—Si—Mg alloy to be at least 120 ppm. 
     
     
         15 . The method of  claim 12 , comprising controlling the Ca concentration of the molten Al—Zn—Si—Mg alloy to be less than 180 ppm. 
     
     
         16 . The method of  claim 12 , comprising controlling a Mg concentration of the molten Al—Zn—Si—Mg alloy. 
     
     
         17 . The method of  claim 16 , comprising controlling the Mg concentration of the molten Al—Zn—Si—Mg alloy to be at least 1.8% by weight. 
     
     
         18 . The method of  claim 12 , wherein the Al—Zn—Si—Mg alloy comprises more than 1.8% by weight Mg. 
     
     
         19 . The method of  claim 12 , wherein the Al—Zn—Si—Mg alloy comprises less than 3% by weight Mg. 
     
     
         20 . The method of  claim 12 , wherein the Al—Zn—Si—Mg alloy comprises less than 2.5% by weight Mg. 
     
     
         21 . The method of  claim 12 , wherein the Al—Zn—Si—Mg alloy comprises more than 1.2% by weight Si. 
     
     
         22 . The method of  claim 12 , wherein the Al—Zn—Si—Mg alloy comprises less than 2.5% by weight Si. 
     
     
         23 . The method of  claim 12 , wherein the Al—Zn—Si—Mg alloy comprises:
 Zn: 30% to 60%; 
 Si: 0.3% to 3%; 
 Mg: 1.8% to 10%; and 
 Balance: Al and unavoidable impurities. 
 
     
     
         24 . The method of  claim 12 , wherein the Al—Zn—Si—Mg alloy comprises:
 Zn: 35% to 50%; 
 Si: 1.2% to 2.5%; 
 Mg: 1.8% to 3.0%; and 
 Balance: Al and unavoidable impurities. 
 
     
     
         25 . The method of  claim 12 , comprising taking a sample from the bath of molten Al—Zn—Si—Mg alloy and measuring the Ca concentration of the molten Al—Zn—Si—Mg alloy. 
     
     
         26 . The method of  claim 12 , comprising taking a sample from the bath of molten Al—Zn—Si—Mg alloy and measuring a Mg concentration in the molten coating bath. 
     
     
         27 . The method of  claim 12 , wherein the coating has uniform surface/sub-surface distribution of Mg 2 Si in a microstructure of the coating. 
     
     
         28 . The method of  claim 12 , comprising controlling the cooling rate of the coated strip to be greater than 15° C./s while the coated strip temperature is between 400° C. and 510° C. 
     
     
         29 . The method of  claim 12 , comprising controlling the cooling rate of the coated strip to be less than 35° C./s while the coated strip temperature is between 400° C. and 510° C. 
     
     
         30 . The method of  claim 12 , wherein the coated strip has a coating mass of 50 g/m 2  to 200 g/m 2 . 
     
     
         31 . The method of  claim 12 , wherein the Al—Zn—Si—Mg alloy is maintained molten in the coating bath at a temperature in a range of 595° C. to 610° C.