Annealing of cold rolled steel is defined as a process of softening the material, metal, or an alloy to make it less brittle by heating to a particular temperature ,maintain it at that temperature for particular duration and ,Cooling it slowly to a particular temperature.
It can be defined as a heating of a material above re crystallization temperature ,maintains at suitable
Temperature and then cooling .
Annealing of cold rolled steel
Cold rolling occurs with the metal below its re crystallization temperature( usually at room temperature).
During cold rolling process grain structure is deformed and became closer due to reduction in thickness.
Due to this steel become hardened and more brittle.
It will reduce the ductility of cold rolled steels and make it useless for end use or forming operation.
Annealing is used for
Softening of cold rolled steel
Relieving the internal stress developed during cold rolling process
Develop desired mechanical properties suitable for intended applications and Bright Surface Finish .
STAGES OF ANNEALING
Process of relieving internal stresses induced during cold rolling takes place in three stages
Physical and mechanical properties ( Hardness, ductility prop.) which suffered changes during cold working tend to recover to their original values. Occurs at the initial phase of annealing at lower temperatures.
In this internal stress are reduced. it will take at temperature 480 deg to 500 deg C.
It is a Process of nucleation of new strain free crystals (grains) which replaces all deformed crystals of the cold worked material. Takes place at a temperature known as re crystallization temperature .it takes place around 550 deg C .
Increase in average size of crystals/grains on further annealing subsequent to the completion of re crystallization . Grain size has direct effect on the mechanical properties. Hence it is controlled to achieve optimum properties. it will takes place at the time of soaking.
Heating the cold rolled steel to the re crystallization 550 To 700 deg C temperature.
Soaking at this temperature for sufficient time for homogenization and necessary changes to occur.
Cooling at a predetermined rate
Temperature, soaking time , cooling rate depending on the desired mechanical properties
In order to meet high demand on the cold rolled surface annealing is take place under protective
Gas atmosphere consist of
Generally low percentage of hydrogen are used because it is quite flammable and it is considerably
more expensive than nitrogen.
Other types of gases such as methane and the like can be used for annealing process.