6. Aging of thin sheet metal

The widespread use of continuous casting, Al-quenched steels or steels microalloyed with nitrogen-fixing elements, has eliminated the risk of ageing after work-hardening in many products; this is particularly true of thin sheet and heavy plate, at least for the base metal.

Nevertheless, we consider the case of thin sheets in detail for the following three reasons:

• These products are delivered by the factories with a skin pass, which plays an important role in processing;

• the range of grades has been greatly extended with the introduction of continuous annealing;

• there is a growing need for compromises that appeal to users: low yield strength for forming and bake hardening.

The peculiar point that disturbs the observations...