Mechanism of bake hardening in ultralow carbon steel containing niobium and titanium additions

Abstract

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 first 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 fine 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.