Cluster strengthening of Nb-microalloyed ultra-thin cast strip steels produced by the CASTRIP® process

This study investigated the microstructure and mechanical properties of Nb-microalloyed ultra-thin cast strip (UCS) steels produced by the CASTRIP® process aged at temperatures below and above the Ac1 temperature (525–800 °C). Aging provided a rapid strength increase without loss in ductility. This is significant because, ordinarily, one of these properties would increase at the expense of the other. Atom probe tomography (APT) and transmission electron microscopy (TEM) revealed that the strengthening was associated with the formation of Nb-rich solute atom clusters and nanoscale precipitates within the microstructure, which retard the movement of dislocations during deformation. The changes in strength and hardness were compared to the microstructural evolution across a range of different aging temperatures and explained in terms of the nucleation kinetics. Cluster-finding algorithms were used to analyse atom probe data, providing quantitative information about the dispersion of these clusters and precipitates. Optimum strengthening was observed where the clusters particles had a radius of 1–2 nm and a volume fraction of 0.027±0.004%.