Dual phase versus TRIP strip steels: Microstructural changes as a consequence of quasi-static and dynamic tensile testing

The development of present day and future vehicles is being driven by the need to simultaneously reduce mass and increase passenger and pedestrian safety. For this reason, the steel industry has developed strip steel grades with suitable properties, as required for meeting the demands placed on the automotive manufacturers. Two of these strip steel grades are the Dual Phase (DP) and the Transformation Induced Plasticity (TRIP) steels, which are thought to offer solutions for critical crash component criteria. Limited published information is available on the changes in microstructure of these novel strip steel grades at different rates of deformation. This paper examines the change in microstructure of a range of both commercial and experimental DP and TRIP strip steel grades, which were tensile tested at low (0.001 s(-1)) and very high strain rates (200 s-1). The DP and TRIP microstructures were characterised in terms of ferrite grain size, ferrite grain elongation and volume fraction of constituent phases. The specimens were examined following deformation and compared to the as-received condition to assess microstructural changes. This paper concentrates only on microstructural changes through dynamic tensile testing of DP and TRIP grades at low and high strain rates. The full crash performance data from the dynamic tensile tests and crushing of box sections is presented in a separate publication. [S. Oliver, G. Fourlaris and T.B. Jones, 'Dual Phase versus TPIP strip steels: a comparison of dynamic properties for automotive crash performance', Materials Science and Technology, 2006, (submitted for publication)]. (c) 2006 Elsevier Inc. All rights reserved.