Manufacturing guidelines when using ultra high strength steels in automotive applications

In recent years, interest in utilising ultra high strength steels (UHSS) to reduce vehicle weight, whilst maintaining/improving vehicle safety, has increased. However, it is widely known that these products do not behave in the same way as traditional materials and lack of knowledge could inhibit their widespread use as automotive manufacturers will tend to stick with what they know. To address this lack of knowledge, this project will consider the forming capabilities of the materials and the factors affecting springback. The weldability of UHSS needs to be assessed to determine the viability of these products for assembly. Inservice performance, i.e. crash resistance and fatigue performance, must also be investigated. Crash performance is said, in literature, to be enhanced by utilising these steels and the evaluation of the influence of steel chemistry and processing, with regard to optimising performance, will be of great value to the steel/automotive industry. The approach adopted in devising the work programme was to consider the aims from the perspective of the end-user. It was difficult to envisage that this project would answer all questions posed, however, an attempt was made to define objectives with useful information on immediate manufacturing challenges. The overall aim of this project was as the title suggests 'to generate manufacturing guidelines' as to the use of these steels in automotive applications. To this end, the following objectives were set: To investigate the joining parameters for UHSS and establish optimum joining process and conditions. (SSAB/Corus) To determine the forming capabilities of UHSS. (Arcelor/Corus) To determine the factors influencing springback after forming of UHSS and investigate the methods of controlling the said phenomenon. (Arcelor/Corus) To develop design methodologies for UHSS by the use of finite element analysis (FEA). (Corus/SSAB) To establish crash performance for UHSS using a combination of practical testing and FEA. (TKS) To establish fatigue characteristics on base material and optimised joints in UHSS. In particular, to investigate the influence of yield strength and joining technique on the fatigue strength of UHSS. (SSAB) The study was sub-divided into two stages: (i) an initial stage designed to carry out preliminary investigations and (ii) a more in-depth study, for full analysis of behaviour. It is recognised that when designing components, thickness may be reduced to save weight, whilst maintaining performance by taking advantage of increased strength. Therefore, samples were obtained from as wide a range of strength levels and thicknesses as possible.