Hot Rolling, Cold Rolling, Galvanising And Pre-finished Steel

Hot rolling

Hot rolling

Steel slabs are heated to a temperature of around 1225C to1400C. In the Hot Rolling Mill the thickness of the steel is reduced by several rolling stands. Rolling the steel gives a finer grain structure improving toughness and tensile strength.

Pickle lines

The surface of the strip will be covered in a layer of iron oxide (scale) after hot rolling. This needs to be removed before despatch to the customer or further processing. Pickling acid tanks remove the mill scale from the surface of the strip.

The most commonly used pickling acid is HCI, at around 15% concentration and at a temperature of approximately 80C. Rinse tanks then apply a hot/cold water wash to remove excess acid from the strip surface.

Cold rolling

Cold rolling is carried out to improve surface finish and to closely control mechanical properties. It is easier to achieve a flat product during cold rolling. It is easier to achieve a tighter tolerance on strip gauge during cold rolling. During the process the steel undergoes deformation below its recrystallisation temperature.

Strip will be harder and brittle due to cold rolling process and so may need heating to increase ductility. This steel is used for applications in the automotive industry, drums, radiators and tubes, as well as being further processed for use in the construction industry as cladding and roofing.

Galvanising

This is where the strip is coated with a thin layer of metallic coating to improve corrosion performance. The metallic coating acts as sacrificial anode to protect the steel. The steel strip is continuously hot-dip coated in a bath of molten zinc

n.b. Hot Dipped Galvanising = HDG

Pre-finished steel is generally made up of these layers

* Topcoat

* Primer

* Pre-treatment

* Metallic coating

* Backing coat

* Steel substrate

which have an aesthetic or functional performance. There are four main families of pre-finished steel products which include Plastisols, Polyesters, Polyurethane and PVDFs.

by: Gareth Hoyle

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