What does the region of strain hardening cause?

The working of the metals below their  recrystallisation  temperature is known as cold working process. Due to cold working the strength and hardness of the metal increases. Due to this process, the dislocations inside the ,metal crystals become entangled with each other and hence it become tough for the dislocations to move, hence hardening of the material atoms takes place .Example of cold working processes-cold rolling, cold forging, cold extrusion etc.

Strain hardening (also called work-hardening or cold-working) is the process of making a metal harder and stronger through  permanent plastic deformation. When a metal is plastically deformed, dislocations move and additional dislocations are generated continuously. The more dislocations within a material, the more they will interact and get tangled. This will result in a decrease in the mobility of the dislocations and a strengthening of the material of the metal. This type of strengthening is commonly called cold-working. The plastic deformation must occurs at a temperature low enough that atoms cannot rearrange themselves, hence cold working is preferred for strain hardening. In strain hardening region less amount of deformation or strain occurs even if high amount of stress is developed.