Metal-coated steel strip

Abstract

A steel strip having a coating of an aluminium-zinc-silicon alloy on at least one surface of the strip is disclosed. The strip is characterised in that the aluminium-zinc-silicon alloy contains less than 1.2 wt. % silicon and also contains magnesium. A method of forming a coating of an aluminium-zinc-silicon alloy on a steel strip is also disclosed. The method includes moving steel strip upwardly through a coating pot containing a bath of an aluminium-zinc-silicon alloy and having an opening in a bottom wall of the pot and forming a coating of the alloy on the strip. The method is characterized by minimizing residence time of steel strip in contact with the aluminium-zinc-silicon alloy bath in the pot.

1 . A steel strip having a coating of an aluminium-zinc-silicon alloy on at least one surface of the strip which is characterised in that the aluminium-zinc-silicon alloy contains less than 1.2 wt. % silicon and also contains greater than 1 wt. % magnesium. 
     
     
         2 . The steel strip defined in  claim 1  wherein the silicon concentration is 0.2-0.5 wt. %. 
     
     
         3 . The steel strip defined in  claim 1 -wherein the silicon concentration is at least 0.2 wt. % less than 1.2 wt. %. 
     
     
         4 . The steel strip defined in  claim 1  wherein the silicon concentration is at least 0.2 wt. %. 
     
     
         5 . The steel strip defined in  claim 1  wherein the coating has small spangles. 
     
     
         6 . The steel strip defined in  claim 1  wherein the magnesium concentration is less than 8 wt. %. 
     
     
         7 . The steel strip defined in  claim 1  wherein the magnesium concentration is less than 3 wt. %. 
     
     
         8 . The steel strip defined in  claim 1  wherein the magnesium concentration is between 1 and 3 wt. %. 
     
     
         9 . The steel strip defined in  claim 1  wherein the magnesium concentration is between 1.5 and 2.5 wt. %. 
     
     
         10 . The steel strip defined in  claim 1  wherein the aluminium-zinc-silicon alloy is a titanium diboride-modified alloy. 
     
     
         11 . The steel strip defined in  claim 1  wherein the aluminium-zinc-silicon alloy contains strontium and/or calcium. 
     
     
         12 . The steel strip defined in  claim 11  wherein the concentration of (i) strontium or (ii) calcium or (iii) strontium and calcium together is at least 2 ppm. 
     
     
         13 . The steel strip defined in  claim 11  wherein the concentration of (i) strontium or (ii) calcium or (iii) strontium and calcium together is less than 0.2 wt. %. 
     
     
         14 . The steel strip defined in  claim 11  wherein the concentration of (i) strontium or (ii) calcium or (iii) strontium and calcium together is less than 100 ppm. 
     
     
         15 . The steel strip defined in  claim 11  wherein the concentration of (i) strontium or (ii) calcium or (iii) strontium and calcium together is no more than 50 ppm. 
     
     
         16 . The steel strip defined in  claim 11  wherein the aluminium-zinc-silicon alloy does not contain vanadium and/or chromium as deliberate alloy elements—as opposed to being present in trace amounts for example as unavoidable impurities due to contamination in the molten bath. 
     
     
         17 . A method of forming a coating of an aluminium-zinc-silicon alloy on a steel strip includes moving steel strip upwardly through a coating pot containing a bath of an aluminium-zinc-silicon alloy and having an opening in a bottom wall of the pot and forming a coating of the alloy on the strip and is characterized by minimizing residence time of steel strip in contact with the aluminium-zinc-silicon alloy bath in the pot. 
     
     
         18 . The method defined in  claim 17  wherein the residence time is less than 0.75 seconds. 
     
     
         19 . The method defined in  claim 18  wherein the residence time is less than 0.5 seconds. 
     
     
         20 . The method defined in  claim 17  wherein the residence time is at least 0.2 seconds. 
     
     
         21 . The method defined in  claim 18  wherein the aluminium-zinc-silicon alloy contains less than 1.2 wt. % silicon and optionally is a magnesium containing alloy. 
     
     
         22 . The method defined in  claim 17  wherein the aluminium-zinc-silicon alloy contains magnesium. 
     
     
         23 . The method defined in  claim 17  also includes the steps of: successively passing the steel strip through a heat treatment furnace
 and the bath of molten aluminium-zinc-silicon alloy, and: 
 (a) heat treating the steel strip in the heat treatment furnace; and 
 (b) hot-dip coating the strip in the molten bath and forming the coating of the alloy with small spangles on the steel strip. 
 
     
     
         24 . A steel strip having a coating of an aluminum-zinc-silicon alloy on at least one surface of the strip which is characterized in that the aluminum-zinc-silicon alloy contains less than 1.2 wt. % silicon and also contains greater than 1 wt. % and less than 2.5 wt. % magnesium.