Through Process Characterisation of Steel for Hydroformed Body Structure Components

This paper considers the change in material properties from sheet steel coil through to finished hydroform for a range of body-in-white components specified for a collaborative pre-development project between British Steel and Rover Group. The hydroforming process, unlike the majority of traditional stamping operations, incorporates more than one step during which significant work hardening of the input material may occur. It is therefore necessary to recognise the magnitude of such changes and to consider whether this modification in properties is exploitable for the purposes of component design. Additionally, inter-stage heat treatments will modify material properties and, since the through-process strain paths may not be linear (unlike many stamping operations) the usefulness of the traditional Forming Limit Curve, as an indicator of the ductility limit of materials, is called into question. Several examples of material property mapping and strain development are presented and discussed.