Metallurgy and processing of ultralow carbon bake hardening steels

The development of bake hardening steels is reviewed. Classical strain aging theories still have relevance to modern ultralow carbon chemistries, although the drastically reduced carbon contents of these steels can influence the kinetics observed during bake hardening. High solute carbon levels increase the bake hardening response but reduce room temperature aging resistance, hence excess carbon levels are kept within the range 15 - 25 ppm. The strength increase appears to be enhanced by a fine ferrite grain size; the published evidence is contradictory and further research is considered necessary. Bake hardening steels can be produced from interstitial free chemistries either by adjusting the chemistry during steelmaking to leave carbon in solution, or by annealing at high temperature to take carbides formed during processing into solid solution. Care must be taken during coiling, annealing, and temper rolling to ensure that suitable amounts of carbon remain in solution to produce a bake hardening product. Further research into the influence of processing conditions on the metallurgy of these steels is required if they are to be successfully produced in industrial conditions.