Paint Bake Strengthening in Water Quenched Continuously Annealed Steels

The paint bake strengthening, i.e., the increase in yield strength due to strain aging during the paint curing cycle, can be used to achieve additional strengthening in formed steel parts. Many parameters affect this strengthening in water quenched continuously annealed steels. This paper describes the effects of quenching and overaging temperatures, the mode and direction of deformation, manganese level, and the nature of microstructures on the paint bake strengthening in continuously annealed steels. The most important parameter affecting the paint bake strengthening is the overaging temperature. It is shown that for temper rolled steels overaged below 274°C (525°F), introduction of a certain minimum strain during stamping is not necessary to take advantage of the paint bake strengthening. Also, paint bake strengthening in partially recrystallized and fully recrystallized dual phase steels are found to be comparable. It is suggested that when good formability is necessary, specifying the minimum yield strength in the formed part after paint bake aging may be more desirable than specifying it in the as-shipped condition.