Pickling of scale formed on hot rolled strip of carbon steel Apr23

Pickling of scale formed on hot rolled strip of carbon steel...

Pickling of scale formed on hot rolled strip of carbon steel During the hot rolling or heat treatment of steel, oxygen from the atmosphere reacts with the surface iron to form a crust that is made up of oxides of iron. This crust is known as scale and need to be removed before steel is further processed in cold rolling mill. Non removal of scale will have the following detrimental effects. Scale not only give bad appearance to the product but also accelerate corrosion During cold rolling of the strip scale patches affects the reduction with the possibility of the skidding of rolls. Effective scale removal is essential for the success of not only for cold rolling but also of subsequent annealing and coating operations. During cold rolling and annealing the scale will produce a dirty surface and cause the rusting of the strip During coating of the strip, presence of scale causes poor to total adhesion failure. Fig 1 shows hot rolled strip surface as well as pickled surface.   Fig 1 hot rolled surface and pickled surface Scale and its origin The normal scale found on hot rolled strip is blue/grey in colour and covers the entire strip surface. This scale is generated during rolling in the last stands of the finishing mill, across the run out table (ROT) and during cooling of the coil. It is composed of three well defined layers of iron oxides. Adjacent to the steel is the thickest layer consisting of wustite having an approximate composition of FeO. The intermediate layer consists of magnetite (Fe3O4) while the outermost layer is hematite (Fe2O3). The thickness of these layers will depend on the temperature of the strip at the exit of the finishing mill, temperature of the coiling and...