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 in¯uence 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 ® ne 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 in¯uence of processing conditions on the metallurgy of these steels is required if they are to be successfully produced in industrial conditions.MST/5160

The strain aging behaviour of an ultralow carbon steel partially stabilised by additions of titanium and niobium was studied as a function of prestrain, aging temperature, and aging time. The increase in strength owing to bake hardening occurred in two stages. The ® rst was independent of prestrain and reached a maximum of 30 MPa after 100 min aging at 100°C. This increase is attributed to the pinning of dislocations by solute carbon atoms. The second hardening stage reached a maximum of 40 MPa after aging at 200°C for 100 min. A dependence on prestrain was observed in this stage, with maximum values being obtained in material prestrained by 1%. It thus appears that the second stage is caused by the formation of ® ne precipitates on dislocations. Evaluation of the information gained shows that the same bake hardening increment can be achieved under less severe paint baking conditions than those normally encountered. With the increasing emphasis on environmental issues, this will allow automobile manufacturers to reduce energy consumption without compromising part performance.MST/5082

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