Control of interphase-cluster evolution and its contribution to strength and ductility in complex microalloyed HSLA steel

Interphase precipitate (IP) strengthening has been identified as an effective mechanism for enhancing the mechanical properties of advanced steels. Recent breakthroughs in characterization have revealed the unusual strengthening effect of precipitates in their embryonic stage, referred to as clusters, which indicate additional strengthening mechanisms for material strengthening and further opportunities for composition design in IP-strengthened steels. This study investigates the impact of IP on the mechanical and formability properties of complex microalloyed high-strength low-alloy (HSLA) steel. Two types of HSLA steel with a single ferrite microstructure were engineered via the thermo-mechanical control process, differing in that one exhibits only fully-developed IPs, while the other exhibits both the clusters of interphase and IPs. These microstructures were achieved through controlled coiling at 620 and 650 °C in Ti-Nb microalloyed steel. Increasing undercooling below the γ → α transformation temperature intensifies the driving force for phase transformation, leading to a decrease in both intersheet spacing and the size of particles, while the number density of interphase particles increases, promoting the formation of clusters of interphase. These clusters significantly influence dislocation behavior, facilitating dislocation multiplication. Compared to the fully-developed IPs in samples coiled at 650 °C, the presence of the clusters of interphase results in a desirable enhancement in mechanical properties, including a 100 MPa increase in ultimate tensile strength without compromising ductility or stretch-flangeability. These findings highlight the critical role of the clusters of interphase in simultaneously enhancing both strength and plasticity in HSLA steel.