Metal-coated steel strip

Abstract

A steel strip having a metal coating on at least one surface of the strip is disclosed. The strip is characterised in that the coating includes aluminium-zinc-silicon alloy that contains magnesium and the coating has small size spangles.

1 . A steel strip having a metal coating on at least one surface of the strip, which is characterised in that the coating includes aluminium-zinc-silicon alloy that contains magnesium and the coating has small spangles, as defined herein. 
     
     
         2 . The strip defined in  claim 1  wherein the magnesium concentration is less than 8% by weight. 
     
     
         3 . The strip defined in  claim 1  or  claim 2  wherein the magnesium concentration is less than 3% by weight. 
     
     
         4 . The strip defined in any one of the preceding claims wherein the magnesium concentration is at least 0.5% by weight. 
     
     
         5 . The strip defined in any one of the preceding claims wherein the magnesium concentration is between 1 and 5% by weight. 
     
     
         6 . The strip defined in any one of the preceding claims wherein the magnesium concentrations is between 1 and 2.5% by weight. 
     
     
         7 . The strip defined in any one of the preceding claims wherein the aluminium-zinc-silicon alloy contains strontium and/or calcium. 
     
     
         8 . The strip defined in  claim 7  wherein the concentration of (i) strontium or (ii) calcium or (iii) strontium and calcium is at least 2 ppm. 
     
     
         9 . The strip defined in any  claim 7  or  claim 8  wherein the concentration of (i) strontium or (ii) calcium or (iii) strontium and calcium is less than 0.2 wt. %. 
     
     
         10 . The strip defined in any one of  claims 7  to  9  wherein the concentration of (i) strontium or (ii) calcium or (iii) strontium and calcium is no more than 50 ppm. 
     
     
         11 . The strip defined in any one of  claims 7  to  10  wherein, in a situation in which the aluminium-zinc-silicon alloy contains strontium and no calcium, the concentration of strontium is in the range of 2-4 ppm. 
     
     
         12 . The strip defined in any one of  claims 7  to  10  wherein, in a situation in which the aluminium-zinc-silicon alloy contains calcium and no strontium, the alloy includes calcium in the range of 4-8 ppm. 
     
     
         13 . The strip defined in any one of  claims 7  to  10  wherein, in a situation in which the aluminium-zinc-silicon alloy contains strontium and calcium, the concentration of strontium and calcium is at least 4 ppm. 
     
     
         14 . The strip defined in any one of the preceding claims wherein the aluminium-zinc-silicon alloy is a titanium boride-modified aluminium-zinc-silicon alloy. 
     
     
         15 . The strip defined in any one of the preceding claims 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 due to contamination in the molten bath. 
     
     
         16 . A method of forming a metal coating on a steel strip which includes the steps of: successively passing the steel strip through a heat treatment furnace and a bath of molten aluminium-zinc-silicon alloy which includes magnesium as defined in any one of the preceding claims, 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 a coating of the alloy with small spangles, as defined herein, on the steel strip.   
     
     
         17 . The method defined in  claim 16  includes controlling the concentration of (i) strontium or (ii) calcium or (iii) strontium and calcium in the molten bath to be at least 2 ppm. 
     
     
         18 . The method defined in  claim 16  or  claim 17  includes controlling the concentration of (i) strontium or (ii) calcium or (iii) strontium and calcium to be less than 0.2 wt. %. 
     
     
         19 . The method defined in any one of  claims 16  to  18  includes the concentration of (i) strontium or (ii) calcium or (iii) strontium and calcium to be less than 150 ppm. 
     
     
         20 . The method defined in any one of  claims 16  to  19  includes controlling the concentration of (i) strontium or (ii) calcium or (iii) strontium and calcium to be no more than 50 ppm. 
     
     
         21 . The method defined in any one of  claims 16  to  20  includes specifying a minimum concentration(s) of strontium and/or calcium in the aluminium that is supplied to form the aluminium-zinc-silicon alloy for the molten bath. 
     
     
         22 . The method defined in any one of  claims 16  to  20  includes periodically dosing the molten bath with amounts of strontium and/or calcium that are required to maintain the concentration(s) at a required concentration. 
     
     
         23 . The method defined in any one of  claims 16  to  22  includes forming the coating with small spangles on the steel strip by adding titanium boride particles (which term includes powders) to the molten bath so that the molten aluminium-zinc-silicon alloy contains titanium boride. 
     
     
         24 . Cold formed products made from the metal coated steel strip defined in any one of  claims 1 - 15 .