Microstructure and dynamic material performance of high strength and ultra high strength strip steels

Abstract

The combined need for weight reduction and increased safety in modern vehicles has led automobile manufacturers to utilise the unique properties of high strength steels in their vehicles. Standard quasi-static tensile tests allow the static mechanical properties to be evaluated for any given steel grade. However, automobile designers require more advanced data, including high strain rate tensile data to fully understand the performance of a steel grade during deformation. Knowledge of the role of strain rate on a given strip steel grades deformation characteristics is essential to fully predict the behaviour of the steel during a crash event to help ensure the safety of the vehicle passengers. In the present study, three commercial strip steel grades were subjected to room temperature deformation at variable strain rates, ranging from quasi-static to dynamic loading conditions, where their obtained mechanical properties were measured. The steel grades studied were a HSLA grade (XF450), a carbon manganese grade (CMn800) and an advanced high strength steel grade (DP1400), each with very different microstructures. The present study showed that the XF450 with a ferrite (97%)-pearlite microstructure exhibited superior elongation and was the most strain rate sensitive grade. This was the only grade where energy absorption until 10% elongation could be measured. The DP1400 with a mainly martensitic microstructure showed very little change in strength or ductility properties over the tested strain rates.