Dynamic strain aging of dual phase steels in forming applications

Bayramin, Berkay
Sheet metals are subjected to certain temperatures (25-200oC) and strain rates (1-10 s-1) during forming applications. These thermomechanical conditions may cause dynamic strain aging (DSA) in various grades of steels including dual phase (DP) steels. DSA represent itself as serrations in the stress-strain curves. Two grades of DP steels (DP590 and DP800) were studied systematically at different strain rates (10-3s-1-1s-1) and temperatures (25oC-400oC) to investigate the DSA effects on the mechanical properties and formability. Test results show that DSA controls the deformation behavior between 200oC - 300°C through serrations in the stress-strain curves of both grades. Increasing strain rates between these temperatures result in severe drops of uniform and total strain. Also with increasing negative strain rate sensitivity (NSRS) these drops in the ductility state weak formability. Besides the tests to identify DSA, a simple microstructural model on dislocation motion is developed to explain the responsible mechanisms for DSA and its effects on the jerky-serrative flow behavior.