Annealing of cold rolled steel - Steel casting foundry USA

Annealing of cold rolled steel

Cold rolling of steel:

Cold rolling of steel is a process of treating steel below its recrystallization temperature. This method is used to get a uniform thickness, increase thickness tolerances, and improve strength and a smooth surface finish through strain hardening. Though cold rolling has these advantages, the ductility of the cold rolled steel is reduced due to the dislocations that are formed due to cold rolling. In order to achieve good ductility, it is necessary that cold rolled steel must undergo a process called annealing. This reduces the stress in the steel that was formed during the cold rolling process.

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Annealing:

Annealing is the process of relieving the internal stresses in the steel that was built up during the cold rolling process. In this process, the cold rolled steel is heated above its recrystallisation temperature by soaking the steel at that temperature and then cooling it. Annealing is done in a controlled gas atmosphere consisting of nitrogen and hydrogen gases in various compositions. This is done to prevent surface rusting resulting from oxidation.

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The three stages of annealing are:

1. Stress relief :
This stage takes place at a temperature around 480 C to 500C. During this stage, the atoms in the steel move around causing a reduction of internal stresses.
2. Recrystallisation :
This stage takes place at a temperature around 500-550 C. Though the internal stresses are removed in the stress relief, new crystals are not formed. To change this, the steel undergoes recrystallization where new crystals start to form. This realigns the atoms and the newly formed grains unite with the cold rolled grains.
3. Grain growth :
This stage happens during soaking. The grains increase in number and size as they consume the newly formed crystals.

There are two types of annealing, Batch Annealing, and Continuous Annealing.

Batch annealing :

The interior cover of the base unit is filled with protective gases to prevent oxidation of the steel surface. The heating hood is positioned and the cold rolled steel coils are placed one above the other. The interior cover is placed on top of the steel. The heat from the burner is circulated to the surface of the steel. The steel coils remain soaked in this temperature. Then the cooled hood is placed to cool the steel to room temperature.

Continuous annealing :

In this process, a decoiler and two loopers are involved. The steel coil is placed at the decoiler. The movement is maintained during heat treatment. Two sets of rolls are present in the looper. The entry looper moves to and fro to feed the steel coil into the heat treating section. The exit looper receives the steel coil and thus the continuous movement is maintained during heat treatment. After soaking, the steel roll is allowed to cool down at room temperature. Continuous annealing gives a smooth finish, uniform surface, and shape to the steel. This process requires less processing time and is preferred for high productivity. This is also cost effective.e.