Metal-coated steel strip

Abstract

A method of forming an Al—Zn—Si—Mg alloy coating on a strip that includes dipping strip into a bath of molten Al—Zn—Si—Mg alloy and forming a coating of the alloy on the strip, with the Al—Zn—Si—Mg alloy containing in % by weight: Al: 2 to 19%, Si: 0.1 to 2%, Mg: 1 to 10%, and Zn: 80 to 97%, and with the bath having a molten metal layer and a top dross layer on the metal layer, and the method including providing Ca in the composition of the bath to minimise the top dross layer in the molten bath.

1 . A method of forming an Al—Zn—Si—Mg alloy coating on a strip that includes dipping the strip into a bath of molten Al—Zn—Si—Mg alloy and forming a coating of the alloy on the strip, with the Al—Zn—Si—Mg alloy containing in % by weight: Al: 2 to 19% , Si: 0.1 to 2% , Mg: 1 to 10% , and Zn: 80 to 97% , and with the bath having a molten metal layer and a top dross layer on the metal layer, and the method including providing Ca in the composition of the bath to minimise the top dross layer in the molten bath. 
     
     
         2 . The method defined in  claim 1  wherein the Al—Zn—Si—Mg alloy contains other elements that are present as deliberate alloying additions or as unavoidable impurities. 
     
     
         3 . The method defined in  claim 1  wherein the composition of the bath includes more than 50 ppm Ca. 
     
     
         4 . The method defined in  claim 1  wherein the composition of the bath includes more than 100 ppm Ca. 
     
     
         5 . The method defined in  claim 1  wherein the composition of the bath includes more than 200 ppm Ca. 
     
     
         6 . The method defined in  claim 1  wherein the composition of the bath includes less than 2000 ppm Ca. 
     
     
         7 . The method defined in  claim 1  includes adding Ca by way of specific additions of Ca compounds on a continuous or a periodic basis. 
     
     
         8 . The method defined in  claim 1  includes adding Ca in Al and/or Zn ingots that are provided as feed materials for the bath. 
     
     
         9 . The method defined in  claim 1  and further including controlling the composition of the bath to minimise the top dross layer in the bath by periodically monitoring the concentration of Ca that is in the bath and adding Ca as required to maintain the bath composition for the element. 
     
     
         10 . The method defined in  claim 1  wherein the Al—Zn—Si—Mg alloy includes more than 8% by weight Al. 
     
     
         11 . The method defined in  claim 1  wherein the Al—Zn—Si—Mg alloy includes less than 15% by weight Al. 
     
     
         12 . The method defined in  claim 1  wherein the Al—Zn—Si—Mg alloy includes more than 0.3% by weight Mg. 
     
     
         13 . The method defined in  claim 1  wherein the Al—Zn—Si—Mg alloy includes more than 2% by weight Mg. 
     
     
         14 . The method defined in  claim 1  wherein the Al—Zn—Si—Mg alloy includes less than 5% by weight Mg. 
     
     
         15 . The method defined in  claim 1  wherein the Al—Zn—Si—Mg alloy includes more than 0.15% by weight Si.