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 Al—Zn—Si—Mg alloy coating on the strip, 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: at least 1.5 and less than 2.5%; 
 
 (b) cooling the coated strip; and 
 (c) heat treating the coated strip to improve the ductility of the coating, with the heat treating step comprising heating the strip from a lower temperature to a hold temperature of 150-300° C., holding the coated strip at the hold temperature for a period of time up to 45 minutes, and slow cooling the coated strip at a cooling rate of 40° C./hr or less from the hold temperature to a temperature of 100° C. or less. 
 
     
     
         2 . The method defined in  claim 1  wherein the hold temperature is less than 275° C. 
     
     
         3 . The method defined in  claim 1  wherein the Al—Zn—Si—Mg alloy comprises 55% aluminum. 
     
     
         4 . The method defined in  claim 1  comprising slow cooling the coated strip from the hold temperature to a temperature of 100° C. or less at a cooling rate of 30° C./hr or less. 
     
     
         5 . The method defined in  claim 1  wherein the silicon concentration is less than 3.0 wt. %. 
     
     
         6 . The method defined in  claim 1  wherein the aluminium concentration is at least 45 wt. %. 
     
     
         7 . The method defined in  claim 1  wherein the Al—Zn—Si—Mg alloy does not contain deliberate additions of chromium and/or manganese.