When the deformed metal is heated at a higher temperature, the elongated （or flattened） and fractured grains become newly uniform, fine equiaxed crystals by being re-nucleated and grown due to an increase in atomic diffusion capacity. This process is called recrystallization. After the deformed metal is recrystallized, the strength and hardness of the metal are significantly reduced, and the plasticity and toughness are greatly improved, and the work hardening phenomenon is eliminated. At this time, the internal stress disappears completely, and the physical and chemical properties are substantially restored to the level before the deformation. The lattice type of the new crystal grains formed by recrystallization is the same as that of the pre-deformation and post-deformation lattice types.
2. Recrystallization temperature
The temperature at which the deformed metal recrystallizes is a temperature range, not a constant temperature. The recrystallization temperature generally refers to the lowest recrystallization temperature （T again）, and is usually the lowest temperature at which a cold deformed metal having a large deformation amount （70% or more） can be completely recrystallized after heating for one hour. Said. The minimum recrystallization temperature has the following relationship with the melting point of the metal:
T again = （0.35 ~ 0.4） T melting point
The temperature unit in the formula is the absolute temperature （K）. The minimum recrystallization temperature is related to the following factors:
（1） Pre-deformation degree The relative deformation amount of plastic deformation before metal recrystallization is referred to as pre-deformation degree. The greater the degree of pre-deformation, the more crystal defects the metal have, the more unstable the tissue, and the lower the minimum recrystallization temperature. When the degree of pre-deformation reaches a certain size, the minimum recrystallization temperature of the metal tends to a certain stable value.
（2） The higher the melting point of the metal, the higher the minimum recrystallization temperature.
（3） Impurities and alloying elements The impurities and alloying elements, particularly high melting point elements, hinder atomic diffusion and grain boundary migration, and can significantly increase the minimum recrystallization temperature. For example, high purity aluminum （99.999%） has a minimum recrystallization temperature of 80 ° C, while industrial pure aluminum （99.0%） has a minimum recrystallization temperature of 290 ° C.
（4） Heating rate and holding time Recrystallization is a diffusion process that takes a certain amount of time to complete. Increasing the heating rate causes recrystallization to occur at higher temperatures, while the longer the holding time, the lower the recrystallization temperature.
3. Grain size of crystal grains after recrystallization
The grain size affects the strength, ductility and toughness of the metal, so the production of grain size after recrystallization is highly valued, especially for steels and alloys that have no phase change. The main factors affecting the grain size after recrystallization annealing are the heating temperature and the degree of pre-deformation.
（1） Heating temperature The higher the heating temperature, the stronger the atomic diffusion ability, the easier the grain boundary migrates and the faster the grain growth.
（2） The influence of the degree of deformation of the pre-deformation is mainly related to the uniformity of the metal deformation. The more uneven the deformation, the larger the grain after recrystallization annealing. The degree of deformation is small, which is insufficient to cause recrystallization and the crystal grains are unchanged. When the degree of deformation reaches 2 to 10%, a small number of grains in the metal are deformed, and the deformation distribution is very uneven. Therefore, the number of crystal nuclei generated during recrystallization is small, and the grain size is greatly different, which is very favorable for the process of engulfing the grains. Growing up quickly, the result is extremely coarse grains. The degree of deformation that causes the crystal grains to grow abnormally is called the critical deformation degree. Plastic deformation processing within the critical deformation range should be avoided as much as possible in production. After the critical deformation degree exceeds, as the deformation degree increases, the deformation of the crystal grains becomes more intense and uniform, and the recrystallized core becomes more and more, so the crystal grains after recrystallization become smaller and smaller. However, when the degree of deformation is too large （about ≥90%）, the crystal grains may grow abnormally again, and it is generally considered to be constructed by deformation woven.
Since recrystallization after metal heating after plastic deformation can eliminate the work hardening phenomenon and restore the plasticity and toughness of the metal, the recrystallization annealing process is often used in production to restore the plastic deformation property of the metal, so as to continue the deformation process. For example, when producing iron-chromium-aluminum electric resistance wire, after cold drawing to a certain degree of deformation, hydrogen protection and recrystallization annealing are performed to continue cold drawing to obtain a finer wire. In order to shorten the processing time, the recrystallization annealing temperature actually employed is higher by 100 ° C to 200 ° C than the minimum recrystallization temperature of the metal.