No abstract

The need to simultaneously reduce vehicle emissions and increase the safety of passengers is encouraging the automotive industry to incorporate new technologies and materials into today's vehicles. To remain competitive, the steel industry has developed steel grades with increased energy absorbing properties allowing down gauging of body in white components to address the competition from alternative materials such as aluminium alloys and composites. Two of the more important developments are the introduction of dual phase (DP) and transformation induced plasticity (TRIP) grades for the automotive industry. These grades offer superior strength/ formability and work hardening properties compared to conventional high strength grades of similar tensile strength. Utilising thinner gauge components with increased energy absorbing properties would permit addressing the mass/safety issues by the automotive industry. This paper relates the crash performance of a range of both commercial and experimental DP and TRIP grades. Dynamic tensile testing was performed at low and very high strain rates within the range of 0 . 001-200 s 21 , to allow an extensive analysis of the effect of strain rate on the material properties. Crash testing was also performed on closed top hat sections at low, medium and high strain rates and the results compared to the dynamically tested tensile specimens. This study helped clarify the enhanced performance offered by high strength DP and TRIP strip steel grades during dynamic tensile testing and impact loading conditions. This advantageous behaviour is attributed to the favourable microconstituents present in these novel grades and their deformation characteristics. This paper concentrates only on the crash properties measured from dynamic tensile tests. The microstructural analysis is presented in a separate publication. 1

As a result of their unique combination of strength and ductility dual phase steels play an important role in reducing weight in automobile components and improving crashworthiness. The purpose of this paper is to quantify the crash performance of dual phase steels, as defined by the influence of low and high strain deformation rates (0 . 001 s 21 and 100 s 21 respectively), on the tensile and work hardening properties of a range of commercial dual phase products. The objective is to establish whether dual phase steels maintain their desirable mechanical property characteristics of low yield strength, high tensile strength and high work hardening rates during plastic deformation under the application of a high strain rate loading. The results confirmed that the yield/proof strength and tensile strength increased with increasing volume fraction of second phase constituents and increasing strain rate. In particular, a dual phase steel with a microstructure consisting of a significant volume fraction (.10-15%) of additional second phase material (bainite) is shown to display superior energy absorption properties. However, this is accompanied by poor ductility and work hardening characteristics.

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.