Page 37 - MetalForming November 2011

the useful formability is exhausted and the stamping under- goes local necking and fracture. However, when these steels fail, the failure mode is still ductile, not brittle.
4) “We interstage anneal the stamping to increase tough- ness and reduce brittleness.” The purpose of annealing sheetmetal is to generate the correct microstructure from work-hardened material. The mill cold rolls the steel to attain the correct thickness. The resulting steel grains are small, elongated in the rolling direction and strong. The annealing process changes the cold-worked grains back to lower-strength equiaxial grains of the proper size to achieve the required ductility needed for forming. Toughness and brit- tleness are not typical goals of the annealing process.
Interstage annealing of stampings must be done with good metallurgical knowledge and extreme care. Annealing activates the crystallographic renewal when the energy of work hardening combines with the energy of the heat to reach the critical energy level needed for the transformation. The steel mill knows exactly the amount of cold work in the coil and annealing temperature to create the ideal microstructure.
However, attempts to anneal a formed stamping in the press shop must overcome one major obstacle (see graph). The amount of cold work generally varies from near zero in flat areas to excessive in highly deformed locations. When the stamping reaches the annealing temperature, some areas with low work hardening will not change, while areas with high work hardening will have sufficient energy to duplicate the annealing process at the steel mill. Somewhere in between these two extremes, an area will exist in the stamping with just the minimum amount of work hardening needed to create a new grain structure. At that location a few grains will grow to extreme size. The material surface will be grainy and rough, and the grains will only tolerate a small amount of defor- mation before failure.
Annealing partially formed stampings creates a high risk of failure. In contrast, sometimes heavily worked, very thick stampings are given interstage anneals to reduce the strength of the intermediate stamping. The problem here is not to increase formability but to reduce the press load so that the press will not stall during forming MF
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