Annealing

Coils used for the manufacture of a car body must not only have a bright smooth surface but must also be soft enough for bending,rolling,shaping and pressing operations,and so the hardness of cold rolled coils to be used for car bodies must be reduced by annealing. If annealing were carried out in an open furnace this would destroy the bright surface of the coil and therefore oxygen must be excluded or prevented from attacking the metal during the period of heating the coils.
The normal method of annealing the coils is box annealing. The coils are stacked on a furnace base, covered by an inner hood and sealed. The atmos- phere is purged with nitrogen and hydrogen to eliminate oxygen. A furnace is then placed over the stack and fired to heat the steel coils to a temperature of about 650°C for around 24 hours,depending upon charge size and steel grade.Car Repair Talk's forum.