Microstructural characterization and thermodynamic analysis of precipitates in ultra-low-carbon bake hardened steel

•We provide the optimum amounts of Nb in ultra-low-carbon steel.•The density of fine precipitates and yield strength are closely connected.•The bake hardenability was mostly affected by a fine NbC precipitates.•The observed MnS–Cu2S precipitate manifests a core–shell structure.

The effects of niobium (Nb) addition on the morphologies, size distributions, crystallography, and thermodynamics of precipitates in ultra-low-carbon bake hardened steels (ULC-BH) were investigated using energy-dispersive X-ray spectrometry (EDS), electron diffraction and high resolution transmission electron microscopy (TEM). The calculated nucleation rate of NbC precipitates indicate that the observed fine precipitates and saturation of yield strength with Nb 90 ppm steel are closely connected to thermodynamic factors. A TEM analysis was also carried out for other fine precipitates in two nitride modes: hexagonal AlN and fcc TiN. The Cu2S–MnS complex precipitate was found to manifest a core–shell structure; the core part formed Cu2S and the shell part formed MnS. MnS and Cu2S have an orientation relationship between (001)MnS//(001)Cu2S and [001]MnS//[001]Cu2S.