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 - 18 . (canceled) 
     
     
         19 . An Al—Zn—Si—Mg alloy coated steel strip comprising:
 a substrate, wherein the substrate is a steel strip; and 
 a coating of an Al—Zn—Si—Mg alloy on one or more surfaces of the steel strip, the coating having a surface with no visual apparent ash mark defect; 
 wherein the coating is formed from an Al—Zn—Si—Mg alloy that 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: more than 1.8 and less than 3.0%, 
 Balance: A1, unavoidable impurities and at least 100 ppm and less than 200 ppm of Ca; and 
 
 wherein the coating of the Al—Zn—Si—Mg alloy has a uniform Al/Zn ratio across the surface of the coating such that a variation in an Al/Zn ratio between two or more independent areas on the surface of the coating is less than 0.1. 
 
     
     
         20 . The Al—Zn—Si—Mg alloy coated steel strip defined in  claim 19 , wherein the outermost 1-2 μm of the Al—Zn—Si—Mg alloy coating has a uniform Al/Zn ratio such that a variation in an Al/Zn ratio between two or more independent areas of the outermost 1-2 μm of the Al—Zn—Si—Mg alloy coating is less than 0.1. 
     
     
         21 . The Al—Zn—Si—Mg alloy coated steel strip defined in  claim 19 , wherein the Al—Zn—Si—Mg alloy includes less than 2.5% by weight Mg. 
     
     
         22 . The Al—Zn—Si—Mg alloy coated steel strip defined in  claim 19 , wherein the Al—Zn—Si—Mg alloy includes less than 180 ppm Ca. 
     
     
         23 . The Al—Zn—Si—Mg alloy coated steel strip defined in  claim 19 , wherein the Al—Zn—Si—Mg alloy includes more than 120 ppm Ca. 
     
     
         24 . The Al—Zn—Si—Mg alloy coated steel strip defined in  claim 19 , wherein the Al—Zn—Si—Mg alloy includes more than 1.2% by weight Si. 
     
     
         25 . The Al—Zn—Si—Mg alloy coated steel strip defined in  claim 19 , wherein the Al—Zn—Si—Mg alloy includes less than 2.5% by weight Si. 
     
     
         26 . The Al—Zn—Si—Mg alloy coated steel strip defined in  claim 19 , wherein the Al—Zn—Si—Mg alloy includes:
 Zn: 35 to 50%, 
 Si: 1.2 to 2.5%, 
 Mg: more than 1.8 and less than 3.0%, 
 Balance: A1, unavoidable impurities and at least 100 ppm and less than 200 ppm of Ca. 
 
     
     
         27 . The Al—Zn—Si—Mg alloy coated steel strip defined in  claim 19 , wherein the coating of the Al—Zn—Si—Mg alloy has a microstructure and the coating of the Al—Zn—Si—Mg alloy has uniform surface/sub-surface distribution of Mg 2 Si in the microstructure. 
     
     
         28 . The Al—Zn—Si—Mg alloy coated steel strip defined in  claim 19 , wherein the Al—Zn—Si—Mg alloy coated steel strip has a coating mass of 50-200 g/m 2 . 
     
     
         29 . A profiled wall and roofing sheet that has been roll formed or press formed or otherwise formed from the Al—Zn—Si—Mg alloy coated steel strip defined in  claim 19 .