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

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Structural materials : properties, microstructure and processing, 2013-04, Vol.568, p.88-95
• Main Authors: Shrestha, Sachin L., Xie, Kelvin Y., Zhu, Chen, Ringer, Simon P., Killmore, Chris, Carpenter, Kristin, Kaul, Harold, Williams, James G., Cairney, Julie M.
• Format: Article
• Language: English
• Subjects: Applied sciences; Atom probe tomography (APT); CASTRIP; Cluster strengthening; Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; Equations of state, phase equilibria, and phase transitions; Exact sciences and technology; General studies of phase transitions; Hardness; High strength low alloy (HSLA) steel; Materials science; Mechanical properties and methods of testing. Rheology. Fracture mechanics. Tribology; Metals. Metallurgy; Nb-microalloying; Nucleation; Other heat and thermomechanical treatments; Physics; Solid solution, precipitation, and dispersion hardening; aging; Transmission electron microscopy (TEM); Treatment of materials and its effects on microstructure and properties
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2013.01.021

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 1–2nm and a volume fraction of 0.027±0.004%.