Effect of grain size on the static strain aging of a ULC-bake hardening steel

Abstract

Bake hardenability in ultra low carbon bake hardening (ULC-BH) steels is primarily influenced by the (i) solute carbon content in the matrix, (ii) dislocation density and (iii) grain boundaries. Grain boundaries in fine-grained steels significantly change the dislocation distribution and carbon-concentration profiles from grain interior to the grain boundary and correspondingly, the bake hardening response may change. The present study examines the effect of different grain sizes (66 – 15 lm) on the bake hardening behavior of a ULC-BH steel (total carbon content 24 wt.ppm) as a function of tensile strain (2 – 5 %) and aging temperatures (50 – 170 °C). The carbon concentration profiles across the grains of different sizes have been evaluated both through modeling calculations and experimental measurements. Results showed that the bake hardening decreases with decreasing grain size in general, but there is a critical fine grain size corresponding to a specific amount of tensile strain, at which the bake hardening increases significantly due to the contribution of grain boundary carbon and increased dislocation density adjacent to the grain boundaries.