The production process for metallic coated and organic coated steel begins with the pickling or cleaning of hot-rolled coils in order to remove the iron oxide scale which is formed during hot rolling of steel.
Hydrochloric acid is used for pickling, and the spent acid is regenerated in an acid regeneration plant and re-used making the whole process environmentally friendly.
After this process, the product is referred to as HRPO (“hot-rolled pickled and oiled”).
The Cold Rolling Mill trims the HRPO coils to the required thickness, with the percentage reduction ranging from 50 – 90%. After this process, the product is referred to as “cold-rolled” or “full-hard steel”.
The continuous hot-dip galvanizing line has an in-line annealing furnace, with an atmosphere comprised of a mixture of nitrogen and hydrogen to avoid oxidation of the steel, followed by a molten zinc pot to galvanize the steel. The in-line furnace softens the steel through an annealing process, while the molten zinc pot, along with an air wiping mechanism, controls the application and mass of zinc coated on the steel. The product after zinc coating is referred to as galvanized steel.
The zinc coating provides corrosion resistance to the steel through barrier protection. Galvanized steel is available in different coating designations based on the zinc coating mass, and is available with regular spangle and mini-skin-passed spangle surfaces. While galvanized steel with a regular-spangle surface finds applications in many industries, including ducting, the product is not suitable for applications which require painting. Mini-skin-passed spangle, which has a uniform surface texture, is the most suitable product for painting applications.
The color coating line includes an online pretreatment section to clean the galvanized steel, followed by the application of an organic primer coating and an organic finish coating. Each coating is followed by curing in an oven, which finally yields pre-painted galvanized steel. The primer coating provides corrosion resistance and the finish coating provides color and aesthetics.
Pre-painted galvanized steel has many uses in the construction industry. It also has applications in products such as home appliances, garage doors and automotive parts.