Characteristics of Box-Annealed Bake-Hardenable Steels

Bake-hardenable steel optimizes the combination of higher formability (low strength and high ductility) needed for part formation and higher final strength needed for dent resistance in the finished part. In outer panel design, it is important to understand the bake-hardening mechanism and its relationship to strain states in the part, bake temperatures and time. Bake-hardening steel exhibits carbon strain-aging that significantly increases the material's strength after part formation. The aging mechanism is created in box-anneal sheet product by the control of annealing parameters and heat composition. Property data from bake-hardenable material currently being used on a door outer application is discussed. Two different aging studies show the relationships between bake hardenability and the variables of strain, baking temperature and baking time. A third study demonsrates that, once strain is induced, strain-age hardening begins to occur at room temperature.