Coated strip steel is a composite material: a steel strip substrate, normally cold-rolled or hot-rolled low-carbon steel, carrying one or more layers of organic, inorganic or metallic coating. The coating is not decorative only. It is selected to change the surface behaviour of the strip, raising corrosion resistance, adding wear resistance, providing a defined appearance or delivering a specific function such as electrical conductivity or thermal insulation. Coated strip feeds the construction, appliance, automotive and packaging industries, usually through continuous coil-coating lines and roll-forming equipment.
What Coated Steel Strip Is
Three variables define the finished strip: the substrate, the coating chemistry and the coating mass or thickness. The substrate fixes the mechanical properties and the forming route; the chemistry fixes the chemical, optical and thermal performance; the coating mass fixes the service life. Two coils of the same steel, the same paint colour and the same nominal thickness can differ by several years of outdoor life if the metallic coating mass differs, which is why coating mass belongs in the specification alongside the paint system. Coated strip is supplied in coil, in cut lengths and in slit widths, and its forming behaviour is governed by the adhesion between coating and substrate rather than by the steel alone.
Organic Coated Steel Strip
Organic coatings are applied as liquid paint or laminated film on a continuous coil-coating line, where the primer and top coat are cured at controlled peak metal temperature before the strip is recoiled. The binder system determines the balance between appearance, flexibility and outdoor durability.
| Binder | Key characteristics | Typical use |
|---|---|---|
| Polyester | Good colour range and formability, moderate weather resistance | Interior panels, appliances, general cladding |
| Polyvinylidene fluoride | Excellent colour and gloss retention outdoors | Architectural curtain wall and facades |
| Epoxy | High chemical and abrasion resistance | Container linings, equipment housings |
| Acrylic | Good clarity and flexibility | Decorative and functional films |
Textured finishes such as wood grain and stone grain are printed before the top coat is applied, and the print layer must be protected against abrasion during forming. Organic coated strip is normally specified as a primer plus top coat system, and where severe forming is involved the coating has to survive the same bend radius as the substrate without cracking or loss of adhesion.
Metallic Coated Steel Strip
Metallic coatings are applied by continuous hot-dip immersion or by electrodeposition. Each coating brings a different balance of corrosion protection, heat resistance and forming behaviour:
Zinc coating: the workhorse of corrosion protection. The zinc acts as a sacrificial anode, so cut edges and scratches remain protected after forming.
Aluminium coating: resists high-temperature oxidation and is used for exhaust components and heat-exposed parts.
Aluminium-zinc alloy coating: combines barrier protection with sacrificial protection, giving good durability in roofing and ducting.
Tin coating: used for food-contact packaging, where formability, weldability and a non-toxic surface are required at the same time.
Metallic coated strip is generally weldable, but the coating changes the welding process. Zinc and aluminium coatings affect electrode life, current settings and coating integrity around the weld, so resistance spot welding parameters have to be requalified whenever a coated grade replaces an uncoated one.
Inorganic and Composite Coated Steel Strip
Inorganic coatings are vitreous or ceramic in nature and are chosen where temperature resistance or chemical resistance is the limiting requirement. Enamel-coated strip, produced by firing a vitreous layer onto the steel, offers high hardness, resistance to chemical attack and a surface that can be cleaned aggressively, which is why it appears in oven liners, chemical equipment and high-temperature flues. Composite systems combine a metallic layer with an organic top coat, for example a zinc or aluminium-zinc layer beneath a paint or anti-fingerprint film. The result combines barrier and sacrificial corrosion protection with a controlled appearance, and it is increasingly specified for architectural panels and automotive body panels.
Key Performance Properties, Test Methods and Selection
Coated strip is qualified by measurable properties rather than by description. The table below lists the properties that are normally written into a purchase specification, together with the standard test method and a typical acceptance level.
| Property | Test method | Typical acceptance level |
|---|---|---|
| Coating adhesion | Cross-cut test, ISO 2409 | Class 0 or 1, no flaking at the incision |
| Corrosion resistance | Neutral salt spray, ASTM B117 or ISO 9227 | No red rust after 500 h on a zinc-coated strip |
| Weather resistance | Fluorescent UV exposure, ASTM G154 | Limited colour and gloss change after extended exposure |
| Surface hardness | Pencil hardness, ASTM D3363 | 2H to 3H for a fully cured architectural coating |
| Formability of coating | T-bend test, ASTM D4145 | 2T without cracking, where T is the coating thickness |
| Impact resistance | Falling-weight impact test | No loss of adhesion at the specified energy |
Salt-spray hours are frequently quoted out of context. A result obtained on a flat, undamaged panel does not predict the behaviour of a bent, sheared and drilled component, so tests should be run on samples formed and cut in the same way as the finished part. Selection should follow the service environment first: corrosivity category and expected outdoor exposure decide the metallic coating mass, the required forming severity decides the binder, and the appearance requirement decides whether a printed or a plain finish is acceptable.
FAQ
Q: What is the difference between coated strip steel and painted steel sheet?
In coated strip the coating is applied on a continuous coil line under controlled conditions and forms part of the product specification. Painting after forming is a separate operation with different adhesion, curing and appearance controls.
Q: How is coating thickness or mass specified?
Metallic coatings are specified by coating mass per unit area, for example a zinc coating designation, while organic coatings are specified by dry film thickness together with the complete paint system.
Q: Which test shows whether a coating will survive bending?
The T-bend test, ASTM D4145, is the standard method. A result of 2T without cracking indicates adequate flexibility for the specified bend radius.
Q: Can coated strip steel be welded?
Yes, but the coating changes the process. Zinc and aluminium coatings affect electrode life and current settings, so welding parameters must be requalified and the coating in the weld zone is usually removed before welding and repaired afterwards.
Q: How long will a coated strip last outdoors?
Service life depends on the coating system, the coating mass and the corrosivity category of the environment. A fluorocarbon top coat over a heavy metallic coating retains appearance for many years in a moderate atmosphere, while the same paint over a light coating will show edge corrosion much sooner.
Q: What is a composite coated strip?
It combines a metallic layer with an organic top coat, so the barrier protection of the paint is combined with the sacrificial protection provided by the zinc or aluminium-zinc layer beneath it.