Abstract
The need for improved safety and reduced weight has increased the interest to apply extra and ultra high strength steels in the automotive industry. In the literature, the steels with tensile strength over 600 MPa are reported to enhance the vehicle crash performance. The use of ultra high strength dual-phase steels is expanding in potential applications like seat frames, bumper reinforcements, side impact beams, cross members and brackets. Dual-phase steels exhibit an excellent combination of strength and ductility. This is due to the microstructure, which consists of soft ferrite matrix with embedded islands of hard martensite. This results in beneficial yielding behaviour with high initial work hardening rate and high total elongation.
Due to the high initial work hardening rate, the mechanical properties of the dual-phase steel become significantly altered during plastic deformation. There seems to be limited data on the mechanical properties and high strain rate behaviour of as-shipped steel sheets and tubular products manufactured thereof. In the present paper, the mechanical properties and high strain rate behaviour of dual-phase steel sheets and tubular products have been compared.
A family of commercially produced and continuously hot dip galvanised dual-phase steels with tensile strengths ranging from 600 to 1000 MPa was used in the experiments. The high strain rate behaviour was determined in the selected case with as-shipped specimens and the case with bake hardened specimens by using a high speed tensile testing machine and a compressional split Hopkinson pressure bar equipment. In addition a range of commercially produced cold formed high frequency welded steel tubes made of cold rolled- and hot dip galvanised dual phase steels has been investigated. The mechanical properties of the tubes and high strain rate behaviour of the selected tube have been shown. The strain hardening and the strain rate behaviour are compared between as-shipped steel samples and tubes made thereof. All tubes investigated are industrially available.
It is important to know the range of strain rate and the material behaviour when analysing mechanical events. As an example a vehicle crash is a complex and highly non-linear mechanical event that involves strain rates from the quasistatic to the high strain rate region. The present results for dual phase steels, including the influence of plastic these steels for structural parts leading to safer and environmentally friendly vehicles.