Excessive damage increase in dual phase steels under high strain rates and temperatures

Damage formation in dual phase steels is a complex process and it may be sensitive to the deformation conditions and mechanisms. In this study, the damage parameter is measured and compared under quasi-static and industrial forming conditions (temperatures: 25 vs 200, 300°C and strain rates: 10 ⁻³ vs 10 s ⁻¹ ) for DP590 and DP800 steels. Resonance frequency and ultrasonic sound velocity techniques are utilized for the measurements to test the effectiveness and validity of each technique. At a given strain, the damage values can be up to 700% higher at industrial forming conditions, under which dynamic strain aging (DSA) controls the deformation behavior. DSA results in lower formability and is the likely mechanism responsible from the abnormal damage evolution. Measured damage parameters are also confirmed with the void fraction characterization by microscopy, which also provided details on the void shape and distribution with respect to the deformation conditions.