Recrystallization annealing is a heat treatment method after cold forming. Like stress-relief annealing, normalizing or soft annealing, this method belongs to the annealing processes.
The procedure is carried out at a temperature in the recrystallization annealing range. For steel, this is normally between 550 and 700 °C, i.e. recrystallization annealing is carried out without structural change. Recrystallization annealing is usually performed after the individual forming stages during cold rolling or drawing of wires and sheets of steels.
Cold forming results in an increase in strength, but reduces the formability of the steels. During further forming processes, the probability of crack formation thus increases, making multi-stage forming processes difficult to implement. However, many steels must undergo several forming processes during production until they reach their final state. For this reason, the deformed crystals of a deformed microstructure must be brought back to their original position before each multi-stage forming process. Recrystallization annealing is helpful here.
Recrystallization annealing is usually carried out at annealing temperatures between 550 and 700 °C. The process is therefore performed at a temperature below the PSK line, as in stress relief annealing. In contrast to normal annealing or in some cases soft annealing, no lattice transformation occurs in this process, although the two annealing methods mentioned above also have a recrystallization effect. During recrystallization annealing, grains can form anew, since the grain boundaries migrate due to diffusion processes. When the grains return to their original shape, the material regains its ductility.
The size of the recrystallized grains is determined by three factors. In addition to the annealing temperature and duration, the degree of deformation of the grains before the process is decisive. If there is only a low degree of deformation, a rather coarse-grained recrystallization structure is formed. On the other hand, the microstructure is recrystallized in a rather fine-grained manner if a high degree of deformation with fine, elongated crystals predominates.
The different processes
Three different methods are used industrially for annealing steel strip.
A closed furnace is used for batch annealing. A few rolls of steel strip are placed in this furnace. The annealing time can exceed several days. However, the heating and cooling speeds are limited. The temperatures in this process range from 620 to about 700 °C. Higher temperatures can be achieved by coiling the wire. In these cases, however, the edge of the steel strip must be scrapped.
Another method is continuous annealing. After the strip has been unwound, it passes through a furnace several 100 m long. This is divided into different temperature zones. Limited by the length of the furnace, the annealing time for continuous annealing is limited to 10 minutes. The temperatures reach 950 °C in this method and can be even higher in the production of electrical steel sheets.
Until 1940, continuous annealing was carried out under normal atmosphere, which resulted in scaling of the surface. For this reason, the annealed workpieces had to be descaled in a pickling bath. Annealing under normal atmosphere is still practiced today.
Bright annealing of steel is the most modern process of recrystallization annealing. It also takes place in a continuous furnace. In contrast to continuous annealing, the steel strip is treated under a protective gas atmosphere, which eliminates the need for pickling after the passage through the furnace. The protective gas obtained from partially burnt gas is under a slight overpressure. This successfully prevents the ingress of oxygen. The heat in the continuous furnaces is usually generated by heating with gas, but in rare cases also electrically. Bright annealing can be used not only for steel but also for non-ferrous metal alloys such as brass.
Recrystallization annealing of steel
Recrystallization annealing of steels leads to the elimination of the effects of cold forming. However, it does not cause α-γ transformation of the crystal lattice. If the degree of deformation is between 5 and 15 %, a coarse grain is produced, since the nucleus count and thermodynamic driving force are low. In this situation, normal annealing of the steel is recommended. The desired fine grain is produced at a degree of deformation of more than 20 %.
Recrystallization annealing is used for fine grain size and is normally carried out just above the A1 temperature (PSK line). The treatment takes about 10 minutes for thin parts and is carried out at a temperature of about 700 °C. Thicker parts are treated for about one hour at a temperature of 600 to 650 °C.
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