It is an essential requirement process for any sheet or sheet metal part which is to be fabricated or the end use because, when material is leveled, it is compressed or stretched about its neutral. As the fibers of the steel pass between two alternating, opposing rolls, they are bent in opposite directions in direct proportion to their distance from the neutral centerline.
The center of the material is in an elastic condition and wants to remain in its original condition, while the surface of the material has been stressed beyond its initial yield point. Because the reverse-bending actions have stressed the surface, the material is unable to return to its initial near-flat shape, and opposing forces now are present in the material.
This results in material that is flat but contains large amounts of internal stress. These stresses are released when the sheet is processed into parts using mechanical or thermal processes.