Design and modelling of ultra-high strength steels: nanoprecipitation and plasticity

摘要

Understanding the changes occurring in the mechanical properties during martensitetempering is essential in the development of new industrial grades. The aim of thisresearch was to develop new ultra-high strength steels via nanoprecipitation control,which requires an understanding of the processing-microstructure-property relationshipin medium carbon (0.5-0.6 wt.%) steels throughout tempering. Much of the work hasbeen centred in understanding the role of silicon at the precipitation level and in therecovery of martensite.By using an existing spring steel grade, the effect of interrupted ageing (IA) in tempered martensite has been studied. In IA, an intermediate step between quenching and tempering is introduced, where quenched martensite is left to rest at room temperature for a defined period of time. By allowing carbon segregation into dislocation cores, the incorporation of IA resulted in a more stable microstructure and hardness improvement. The effect of silicon in the epsilon to cementite carbide transition has also been studied. The classical nucleation theory was applied in order to model cementite formation under paraequilibrium conditions, thus incorporating silicon during nucleation.Characterisation using high energy X-rays showed the inhibiting effect of silicon in theoverall cementite precipitation. The second effect of silicon was observed in the martensite recovery. A series of experiments were carried out in order to capture the various microstructural changes taking place during tempering: precipitation, grain size and dislocation density evolution. It was observed that the addition of silicon reduces the rate of martensite recovery, owing to the reduced cross-slip in the ferrite lattice.A plasticity model based on irreversible thermodynamics and EBSD characterisationwas applied to identify the effective grain size. The results from these two techniquesrequire further research. Nevertheless, based on the post-failure analysis by TEM, itappears that at relatively early tempering stages, even low angle lath boundaries cancontribute to strengthening, where piled-up dislocations have been observed at lathboundaries.