The present invention relates to controlling surface defects, as described hereinafter, in steel strip that has a corrosion-resistant metal coating that is formed on the strip by hot-dip 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 particularly but not exclusively to metal coated steel strip having an aluminum-zinc-silicon alloy coating that can be cold formed (e.g., by roll forming) into an end-use product, such as roofing products. The applicant is interested particularly in aluminum-zinc-silicon alloy coated steel strip that is sold in Australia under the registered trade mark Zincalume and in other countries under the registered trade mark Galvalume.
The present invention also relates particularly but not exclusively to metal coated steel strip having an aluminum-zinc-silicon alloy coating with small spangle size, i.e., a coating with an average spangle size of the order of less than 0.5 mm. Coated steel strip products with larger spangle size do not tend to show the generally small defects because the defects are camouflaged by the appearance of the spangle pattern.
The term “aluminum-zinc-silicon alloy” is understood herein to mean alloys comprising the following ranges in weight percent of the elements aluminum, zinc and silicon:
Aluminum: 50-60
Zinc: 37-46
Silicon: 1.2-2.3
The term “aluminum-zinc-silicon” alloy is also understood herein to mean alloys that may or may not contain other elements, such as, by way of example, any one or more of iron, vanadium, chromium, and magnesium.
In the conventional hot-dip metal coating method, steel strip generally passes through one or more heat treatment furnaces and thereafter into and through a bath of molten coating metal, such as aluminum-zinc-silicon alloy, held in a coating pot. The furnaces may be arranged so that the strip travels horizontally through the furnaces. The furnaces may also be arranged so that the strip travels vertically through the furnaces and passes around a series of upper and lower guide rollers. The coating metal is usually maintained molten in the coating pot by the use of heating inductors. The strip usually exits the heat treatment furnaces via an outlet end section in the form of an elongated furnace exit chute or snout that dips into the bath. Within the bath, the strip passes around one or more sink rolls and is taken upwardly out of the bath. After leaving the coating bath, the strip passes through a coating thickness control station, such as a gas knife or gas wiping station, at which its coated surfaces are subjected to jets of wiping gas to control the thickness of the coating. The coated strip then passes through a cooling section and is subjected to forced cooling. The cooled strip may thereafter be optionally conditioned by passing the coated strip successively through a skin pass rolling section (also known as a temper rolling section) and a tension leveling section. The conditioned strip is coiled at a coiling station.
The present invention is concerned particularly but not exclusively with minimizing the presence of particular surface defects on steel strip that has been hot dip coated with an aluminum-zinc-silicon alloy.
The particular surface defects are described by the applicant as “rough coating” and “pinhole-uncoated” defects. Typically, a “rough coating” defect is a region that has a substantial variation in coating over a 1 mm length of strip, with the thickness varying between 10 micron thick and 40 micron thick. Typically, a “pinhole-uncoated” defect is a very small region (<0.5 mm in diameter) that is coated.