Steel Cold Rolled Sheet Electrolytic Cleaning Process
Although the electrolytic cleaning process every cold-rolled coil prior to annealing, improvements to the sheet cleanliness at the 5-Stand and to the annealing equipment and process have allowed many products to be heat-treated in the ‘mill clean’ condition. The cleaner pulls strip through a caustic bath that is maintained at temperatures around 200 degrees Fahrenheit.
Oil residues from the 5-stand are removed and a thin film of silicate is plated onto the strip to deter the wraps from sticking to one another during anneal. The bath incorporates an ‘electrolytic cleaner unit’ that applies 7200V in a reverse electro-plating process, attracting oils and iron ‘fines’ (dust) off of the strip. Rotating brushes scrub both surfaces before the steel is rinsed in water and hot-air-dried.
A small 3-roll leveler, called a flattener, at the entry end straightens out the steel, removing ‘coil set’. The headend of the next coil to be cleaned is over-lapped with the tail of the previous coil and the two are either spot-welded or, for heavier gauges, crimped (‘stitched’) to one another.
At the exit end, a pair of bridle rolls allow the exit reel to hold the strip under tension while coiling to produce a straight side-wall and prevent the coil from collapsing. Various tension control strategies allow the wind tension to be adjusted as the coil is formed in order to produce a tightly wound coil without contributing to annealing stickers by applying unnecessarily high pressure between the wraps. Coils are upended to an eye-vertical orientation in preparation for the annealing stack, then staged between the cleaner and annealing bases with a craneoperated magnet until they can be incorporated into a charge.