High Strain Rate Deformation Behavior of Dual Phase Steels: The Role of Martensite Volume Fraction and Distribution

Abstract

Ferrite-martensite dual phase steels (DP steels) are drawing a great demand in the automobile industries for their high strength to weight ratio, excellent formability, and high yield strength to ultimate strength ratio. The presence of hard martensite phase within softer ferrite matrix makes these steels a potential material for microstructural engineering to produce steels with fabulous properties. Moreover, being a potential material in automobile components, dual phase steels are studied extensively in order to evaluate and understand their properties under various loading conditions. The properties of dual phase steels are found to be influenced largely by (a) amount, (b) size and distribution, and (c) morphology of the individual phases. As dual phase steels are extensively used in crash sensitive components, it becomes important to investigate the effect of these microstructural parameters on the deformation behavior of dual phase steels under high strain rate conditions. In the present work, three different annealing routes viz. CAL, CHCL, and CAS cycle were employed to obtain DP microstructure having different martensite volume fraction, morphology, and distribution. CAL cycle was used to simulate the conventional cycle used in industry to obtain the DP590 grade (referred here as DP-1) having martensite phase distributed on the grain boundaries of ferrite (with martensite fraction ~ 20 %). CHCL cycle was used to obtain in-grain martensite distribution in addition to grain boundary martensite with volume fraction of martensite similar to CAL cycle (DP steel obtained was referred as DP-2). CAS cycle was employed to obtain DP microstructure (referred here as DP-3) having higher volume fraction of martensite (~ 67 %) surrounded by ferrite network/channel distribution. The main focus of present research was to analyze the effect of these different DP microstructures on the tensile properties at different strain rates. The tensile tests were done at different strain rates in the range 2.7×10−4 to 650 s-1. In case of DP-1 and DP-2 steels, the UTS and YS increased with increase in strain rate whereas percentage uniform elongation was decreased with increase in strain rate. However, the percentage elongation values of CHCL samples were higher with comparable strength compared to CAL samples for all high strain rate levels. DP-3 showed excellent results at high strain rate conditions. The energy absorption intensity of CAS cycle was highest among the three cycles at high strain rates, CHCL cycle also showed higher energy absorption intensity than the conventional CAL cycle at high strain rate conditions. Modified C-J method was used to analyze the strain hardening behaviour of the three different steels at quasi-static conditions. All the three steels showed three stages of strain hardening. Initial strain hardening rate of the DP-3 was highest (iv) and DP-2 also showed higher initial strain hardening than DP-1 at all strain rate levels. The initial strain hardening rate was increased significantly at higher strain rates for all the three steels. Furthermore, the fracture surface and fracture tips of samples were also characterized in FEG-SEM, to correlate the obtained mechanical properties with deformation mechanisms present in the different DP microstructures used in this study. The higher volume fraction of martensite and core/shell microstructure of DP-3 and in-grain distribution of DP-2 was considered responsible for the improved properties at high strain rates.