Annealing Process – Cold Rolled Steel Sheet
Annealing process. The furnaces use pure hydrogen gas to carry heat from the inner shell to the steel, preventing oxidation and cleaning the steel by volatizing any oil and sweeping away the iron fines left by the 5-stand. The primary function of the annealing process, however, is to heat the steel to a temperature that will restore formability to the stiff, elongated iron crystals, or grains, formed during cold-reduction.
Steel that has been deformed at room temperature, or cold-worked, retains a large amount of potential energy in the form of internal stress between the individual atoms. Further forming of the steel will require much greater forces than was required prior to cold reduction, and is more likely to cross the threshold at which the atomic bonds are broken and the steel cracks. By heating the steel, the atoms are given enough energy to migrate within the crystal structure, with higher temperatures hastening the process. Specifying the temperature to which the steel is heated controls the configuration of the crystals, and consequently the forming properties of the final product.
Metallurgists refer to three different stages in the annealing process: stress-relief, recrystallization, and grain growth.
During stress-relief, the first stage reached at around 900 degrees Fahrenheit, the atoms move only small distances, pushed and pulled by surrounding atoms into a configuration in which the internal stresses are reduced but the boundaries between crystals remain unchanged. Halting the heat treatment after this stage retains most of the strength of the Full Hard product, but allows for greater formability. This strategy is called a stress-relief anneal or sunshine anneal, the latter a facetious suggestion that the relatively low temperatures might be achieved by setting the coil outside on a hot summer day.
Depending to some extent on the grade of steel and amount of cold-work performed on it, at around 1000 degrees Fahrenheit new crystals begin to form at the boundaries of the original, rolled grains. These crystals grow roughly into spheres, realigning atoms from the cold-worked grains until their boundaries meet up with those of other newly formed grains. Once the cold-worked grains have been consumed, the steel is fully recrystallized. At the completion of this phase the grains are very small, little more than a ten-thousandth of an inch across, and make for steel that is still stiffer than many applications call for.
After the steel has recrystallized, it continues to soften as the grains consume other newly formed crystals in the stage known as grain growth. The strategy for annealing the vast majority cold-rolled sheet involves growing grains to a certain size: crystals that are in the neighborhood of a thousandth of an inch across provides the most popular combination of strength and formability. A common strategy for Drawing Quality steel involves manipulating impurities to allow the crystals to grow in only two directions, resulting in a coarse, ‘pancake’ grain structure that draws uniformly without stretching or tearing.