METAL-COATED STEEL STRIP
The present invention relates to steel strip that has a corrosion-resistant metal coating that is formed on the strip by coating the strip in a molten bath of coating metal.
The present invention relates particularly but not exclusively to metal coated steel strip that can be cold formed (e.g. by roll forming) into an end-use product, such as roofing products.
The present invention relates more particularly but not exclusively to metal coated steel strip of the type described in the preceding paragraph that has a corrosion-resistant metal coating with small spangles, i.e. a coating with an average spangle size of the order of less than 0. 5mm.
The present invention relates more particularly but not exclusively to metal coated steel strip of the type described above that has a corrosion-resistant metal coating with small spangles and includes an aluminium-zinc-silicon alloy that has a relatively low concentration of silicon and also contains magnesium.
Conventional aluminitua-zinc-silicon alloys used to coat steel strip generally comprise the following ranges in % by weight of the elements aluminium, zinc and silicon:
aluminium: 45.0-60.0;
zinc: 37.0-46.0; and
silicon: 1.2-2.3.
Conventional aluiminium-zinc-silicon alloys may also contain other elements, such as, by way of example, any one or more of iron, vanadium, and chromium, often as

impurities.
Conventionally, an alvraiinium-zinc-silicon alloy coating on steel strip is formed using a hot-dip metal coating method.
In the conventional hot-dip metal coating method steel strip passes through one or more heat treatment furnaces and thereafter into and through a bath of molten aliaminium-zinc-silicon alloy contained in a coating pot. A coating of aluminium-zinc-silicon alloy forms on the steel strip as the strip moves through the bath.
In a widely used conventional method the strip moves downwardly into the bath and around one or more sink rolls in the bath and thereafter upwardly from the bath.
It has also been proposed to provide an opening in a bottom wall of a coating pot and to move strip vertically upwardly through the opening into the bath and thereafter from the bath. This method relies on the use of an electromagnetic plugging means that prevents molten aluminium-zinc-silicon alloy flowing downwardly from the pot via the opening.
The applicant has carried out research and development work to optimize the coxnposition and microstructure of aliiminiimi-zinc-silicon alloys and coatings formed from these alloys on steel strip for given end-use applications and to optimize coating practices for forming such coatings on steel strip.
The present invention was made in the course of research and development work that focused on the impact of silicon in aluminium-zinc-silicon alloys and on coating practices for forming such metallic coatings on steel strip, with a particular objective of achieving low levels
of silicon in the metallic coatings and a secondary objective of forming coatings with small spangles.
The term "small spangles" is understood herein to mean metal coated strip that has spangles that are less than 0.5 mm, preferably less than 0.2 mm, measured using the average intercept distance method as described in Australian Standard AS1733.
The applicant found in the course of the work that reducing the silicon concentration below the conventional 1.2 wt% minimiim mentioned above had advantages in terms of improving corrosion resistance and forming small spangles and disadvantages in terms of growth of an intexmetallic alloy layer of aluminium, zinc and iron between the steel strip and the aluminium-zinc-silicon alloy coating.
It is known that aluminium-zinc-silicon alloy coatings with relatively high aluminium contents (as in the production of GALVALUME ® coated steel) depend on silicon additions to prevent a strongly exothermic reaction during metallic coating in which the entire coatings become an alloy of aluminium, zinc and iron. Such coatings would be highly brittle and commercially useless.
It is also known that without silicon additions the exothermic reaction is so spectacular as to heat steel substrates such that it glows bright red, and on occasion the coating may actually show combustion.
The basis of the present invention is that the applicant has found that growth of the undesirable intermetallic alloy layer can be suppressed by:
(a) the addition of magnesium to the aluminium-zinc-silicon alloy composition; and/or
(b) minimizing the residence time of steel strip in contact with a coating bath.
With regard to point (a), a coated steel strip in accordance with the present invention includes 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 magnesitim.
Preferably the silicon concentration is 0.2-0.5 wt.% and the concentration of magnesium is 0.5-6 wt.%.
Preferably the silicon concentration is at least 0.2 wt.% and less than 1.2 wt.% and the concentration of magnesium is 0.5-1 wt.%.
Preferably the silicon concentration is at least 0.2 wt.%.
In addition to suppressing growth of an intermetallic alloy layer, the magnesium addition to the aluminium-zinc-silicon alloy improves the corrosion resistance of the coating.
Preferably the coating has small spangles, as described herein, i.e. spangles that are less than 0.5 mm, preferably less than 0.2 mm, measured using the average intercept distance method as described in Australian Standard AS1733.
The small spangle size improves the ductility of the coating and compensates for an adverse effect of magnesium on ductility of the coating.
Preferably the magnesium concentration is less
than 8 wt.%.
Preferably the magnesiiim concentration is less than 3 wt.%.
Preferably the magnesitim concentration is at least 0.5 wt.%.
Preferably the magnesiiim concentration is between 1 wt.% and 3 wt.%.
More preferably the magnesium concentration is between 1.5 wt.% and 2.5 wt.%.
Preferably the aluminium-zinc-silicon alloy is a titanium diboride-modified alloy such as described in International application PCT/USOO/23164 (WO 01/27343) in the name of Bethlehem Steel Corporation and contains up to 0.5 wt. % boron as titaniiim diboride. The International application discloses that titanitim diboride mindLmises the spangle size of altiminium-zinc-silicon alloys. The disclosure in the specification of the International application is incorporated herein by cross-reference.
The aluminitim-zinc-silicon alloy may contain other elements.
Preferably the aluminium-zinc-silicon alloy contains strontium and/or calcium.
The strontium and/or calcium addition to the aluminium-zinc-silicon alloy substantially reduces the number of surface defects described by the applicant as "rough coating" and "pinhole - uncoated" defects and condensates for the increased number of such surface defects that appear to be caused by magnesium.
The strontium and the calciiam may be added separately or in combination.
Preferably the concentration of (x) strontium or (ii) calcium or (iii) strontium and calcium together is at least 2ppm.
Preferably the concentration of