Metal-coated steel strip

Abstract

A coating of an Al—Zn—Si—Mg alloy on a steel strip that is applied by a hot dip process and is subsequently heat treated to improve the ductility of the coating.

1 . A method of forming a coating of a corrosion-resistant Al—Zn—Si—Mg alloy on a steel strip that comprises:
 (a) passing the steel strip through a hot dip coating bath that contains Al, Zn, Si, and Mg and optionally other elements and forming an alloy coating on the strip, and   (b) heat treating the coated strip to improve the ductility of the coating.   
     
     
         2 . The method defined in  claim 1  comprises heat treating the coated strip at a hold temperature of at least 150° C. 
     
     
         3 . The method defined in  claim 1  comprises heat treating the coated strip at a hold temperature of at least 200° C. 
     
     
         4 . The method defined in  claim 1  comprises heat treating the coated strip at a hold temperature of less than 300° C. 
     
     
         5 . The method defined in  claim 1  comprises heat treating the coated strip at a hold temperature of less than 275° C. 
     
     
         6 . The method defined in  claim 1  comprises holding the coated strip at the hold temperature for up to 30 minutes. 
     
     
         7 . The method defined in  claim 1  comprises slow cooling the heat treated coated strip from the hold temperature to a temperature of 100° C. or less. 
     
     
         8 . The method defined in  claim 1  comprises slow cooling the heat treated coated strip from the hold temperature to a temperature of 100° C. or less at a cooling rate of 40° C./hr or less. 
     
     
         9 . The method defined in  claim 1  comprises slow cooling the heat treated coated strip from the hold temperature to a temperature of 100° C. or less at a cooling rate of 30° C./hr or less. 
     
     
         10 . The method defined in  claim 1  wherein the Al—Zn—Si—Mg alloy comprises the following ranges in % by weight of the elements aluminium, zinc, silicon, and magnesium:
 Aluminium: 40 to 60%   Zinc: 40 to 60%   Silicon: 0.3 to 3%   Magnesium 0.3 to 10%   
     
     
         11 . The method defined in  claim 10  wherein the magnesium concentration is less than 8 wt. %. 
     
     
         12 . The method defined in  claim 10  wherein the magnesium concentration is at least 0.5 wt. %. 
     
     
         13 . The method defined in  claim 10  wherein the magnesium concentration is between 1 wt. % and 3 wt. %. 
     
     
         14 . The method defined in  claim 10  wherein the silicon concentration is less than 3.0 wt. %. 
     
     
         15 . The method defined in  claim 10  wherein the aluminium concentration is at least 45 wt. %. 
     
     
         16 . The method defined in  claim 10  wherein the Al—Zn—Si—Mg alloy does not contain deliberate additions, i.e. additions above concentration levels that would be regarded as impurity levels, of chromium and/or manganese. 
     
     
         17 . A method of forming a painted, metal coated steel strip that comprises:
 (a) passing the steel strip through a hot dip coating bath that contains Al, Zn, Si, and Mg and optionally other elements and forming an alloy coating on the strip,   (b) heat treating the coated strip to improve the ductility of the coating;   (c) slow cooling the heat treated coated strip from the hold temperature to a temperature of 100° C. or less; and   (d) forming a coating of a paint on the cooled heat treated coated strip.