Main Product Groups
The most common coated steel products are metal coated steels. Their manufacture typically uses cold- rolled steel (although a small percentage is made from hot rolled coil or sheet) which is then coated with zinc or with tin. A further coating is often added to zinc-coated steel in a subsequent process step to produce colour-coated steels.
Zinc Coated Steels
There are several distinct types of zinc coating (or galvanizing). These are
- Continuous hot dip galvanizing: in this process, a cold-rolled coil or sheet in immersed in a pot of molten zinc. A common manufacturing technique for hot dip galvanizing steel is the Sendzimir process (which uses a small quantity of aluminium in the coating). Continuous hot dip galvanizing is presently the most popular method of producing coated steels. Typical applications of these steels include construction (roofs, walls – often as sandwich panels), auto body components (exposed panels), fabrication (metal cabinet frames and walls), agriculture (outhouses, livestock buildings, silos), heating and ventilation (casings, air ducts); and it is very common for the galvanized steel to be further coated (see below) for many of these uses.
- Hot dip batch galvanizing – used for small lots in a non-continuous batch process. Typical products can include lighting poles, antenna poles, radio towers, road barriers, power transmission lines, camera poles, motorway crash barriers, pedestrian bridge systems, garden frames, galvanized wire or mesh, street furniture, advertising poles, boilers, ducts, welded tubes and other profiles, etc. The usual production process involves dipping the shaped steel fabrication into a molten bath, which can be up to 12 metres in length.
- Electrolytic galvanizing – a somewhat popular coating technique producing a zinc coated steel with a coating of 25-100 g/m2 per side – although capital costs, operating costs and market preferences have swung demand away from electro galvanizing towards continuous hot dip galvanizing in recent years.
- Galvalume – which involves application of a 45% Zinc-55% Aluminium alloy. Galvalume is especially well suited to highly corrosive environments [e.g. sea air] and is suitable for roofing, exposed panels, unexposed auto parts, etc. [Galvalume is a registered trademark of BIEC International Inc].
- Galvanneal: here a zinc coating application is followed by heat treatment to form a Zn-Fe alloy (8-15% Fe and 100-120 g/m2 total) coating of very high surface quality. The product is ideal for paint coating and is readily weldable. Steel grades can range from soft deep drawing through to high strength low alloys.
Colour Coated Steel
Organic coated steel (also known as ‘colour coated steel’) is a product made from zinc coated steel – typically, produced from hot dip galvanized steel. Here, the galvanized coil or sheet is painted with an organic coating such as PVC or polyester resin, which imparts much greater surface durability. As a result, the useful life of the steel can be extended for a further 20 years or more, as corrosion is inhibited by the additional protection of the extra organic coating.
Tin plate is popularly used in beverage cans, for food packaging, as well as in the chemical industry [aerosols, paint, oil cans etc]. Despite the name, tinplate actually takes one of two forms:
- One form is thin gauge cold-rolled steel which is electrolytically coated with tin.
- The other form is called tin-free steel, and used a chrome plating instead of tin plating. Producing a less shiny finished product, tin-free steel was introduced some years ago in an attempt to reduce packaging costs when the price of tin rose to high levels. Most tin plated steel today however is still produced as tin plate rather than as tin-free steel.
There are a number of other coatings in the steel marketplace, and these include nickel-zinc coating [favoured by some automotive producers] and terne-coated steel [which uses lead – often for applications such as petrol tanks]. This is therefore a fairly fragmented market, and it is likely that additional coatings for new applications will continue to emerge with time.