Cold Rolling of Steels Oct08

Cold Rolling of Steels...

Cold Rolling of Steels  The primary purpose of cold rolling of steels is to reduce the thickness of the hot rolled steel strips (normally in the range of 1.5 mm to 5 mm) into thinner thicknesses (usually in the range of 0.12 mm to 2.5 mm) which cannot be normally achieved during hot rolling in a hot strip mill. Besides reduction in thickness cold rolling is done for improving the surface finish of steels, for improving the thickness tolerances, for offering a range of ‘tempers’, for improving the physical characteristics, and for preparing the strip for surface coating. Cold rolling makes the cold rolled sheets a much improved product. Cold rolled steel products offer good control of thickness, shape, width, surface finish, and other special quality features that compliment the need for highly engineered end user applications.  To meet the various end user requirements, cold rolled sheets are metallurgically designed to provide specific attributes such as high formability, deep drawability, high strength, high dent resistance, good magnetic properties, weldability, enamelability, and paintability etc. Cold rolling of hot rolled steel strips is done below the recrystallization temperature normally at room temperature. In cold rolling process, usually no heat is applied to the hot rolled strip before rolling.  However, frictional energy at the contact surfaces of the strip being rolled gets converted into heat. This heat may increase temperature of the strip being rolled in rapid adiabatic process to a level of 50 deg C to around 250 deg C. During cold rolling process the reduction in thickness is due to the plastic deformation which occurs by means of dislocation movement. Steel gets hardened because of the buildup of these dislocations. This increases strength and strain hardening upto 20 %. These dislocations reduce the ductility of the...

Acid Regeneration for Spent Hydrochloric Pickle Liquor Sep20

Acid Regeneration for Spent Hydrochloric Pickle Liquor...

Acid Regeneration for Spent Hydrochloric Pickle Liquor  Pickling is a process which consists of chemical removal of scale (surface oxides) and other dirt from steel by immersion in aqueous acid solution. During the pickling process, acid reacts with scale as well as base steel to produce dissolved metal salts. The pickling process generates a considerable quantity of spent pickle liquor (SPL) containing the dissolved salts as well as residual free acid. This SPL is either to be disposed of after chemical neutralization with alkali or regenerated. Carbon steel is usually pickled either by sulphuric acid (H2SO4) or hydrochloric acid (HCl). But hydrochloric acid is now preferred over sulphuric acid due to the following advantages. It consistently produces a uniform light gray surface on high carbon steel. Probability of over pickling is much less. Iron concentrations can be as high as 13 %. Rinsing is facilitated because of high solubility of iron chloride. It is safer to handle when compared with sulphuric acid. Pickling is carried out at lower operating temperatures  Hydrochloric acid regeneration refers to process for the reclamation of bound and unbound HCl from metal chloride solutions such as ferrous chloride. Regenerated acid has no adverse effect on metal cleaning efficiency compared to virgin acid.  It pickles as efficiently as virgin acid. There are several processes for the regeneration of spent hydrochloric pickle liquor. A number of different process routes are available. The most widely used is based on pyrohydrolysis, hydro thermal and adiabatic absorption of hydrogen chloride in water, a process invented in the 1960s. Distillation process Distillation process has been used where there is a significant level of free acid is remaining in the SPL. Purified HCl, at the azeotropic concentration of about 15 % w/w is recovered from the ‘overs’ while the concentrated...

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...