Annealing of Cold Rolled Steel Nov09

Annealing of Cold Rolled Steel...

Annealing of Cold Rolled Steel The cold rolling of steel is done at temperatures below the recrystallization temperature. During cold rolling process the reduction in thickness is due to plastic deformation which occurs by means of dislocation movement. Steel gets hardened because of the buildup of these dislocations. These dislocations reduce the ductility of cold rolled steel making it useless for forming operation. To recover the ductility, cold rolled steels need to undergo an annealing process for the relieving of the stresses that have buildup within the microstructure during the process of cold rolling. Annealing consists of heating of the steel to above the recrystallization temperature, soaking at that temperature and then cooling it. Heating of the steel during annealing facilitates the movement of iron items, resulting in the disappearance of dislocations and formation and growth of new grains of various sizes. It has three different stages namely (i) stress relief, (ii) recrystallization, and (iii) grain growth. During stress relief which takes place around 480 deg C to 500 deg C, the atoms move only small distances, pushed and pulled by the surrounding atoms into a configuration in which the internal stresses are reduced but the boundary between the crystals remains unchanged. The stage of recrystallization takes place at around 550 deg C and during this stage new crystals begin to form at the boundary of the original rolled grain. These crystals grow roughly into spheres, realigning atoms from the cold rolled grains until their boundaries meet up with those of other newly formed grains. Once the cold worked grains are fully consumed, the steel is fully recrystallized. In the third stage of grain growth the steel gets softened as the grains consume other newly formed crystals and grow in size. This stage takes...