Unveiling Precipitation Behavior and Strengthening Mechanisms in Ti-Nb-Mo Steels

In this work, the effects of Nb and Mo additions on the precipitation behavior and strengthening mechanisms of three ultra-low carbon Ti-Mo-Nb steels with a predominantly ferritic microstructure were investigated under two different thermo-mechanical processing (TMP) routes. A water-quenching step after hot rolling followed by furnace cooling was found to refine the average precipitate size and increase their volume fraction, leading to a significant strength improvement. Specifically, this process increased the yield strength by approximately 110~180 MPa, reaching levels above 750 MPa, with the 22Mo-Nb steel achieving a peak ultimate tensile strength of ~790 MPa. The precipitates exhibited dispersed, interphase, and grain boundary morphologies, none of which correlated directly with the TMP route or steel composition. While variations in Mo content showed little influence on precipitate characteristics, the addition of Nb markedly promoted precipitation. The strength of these Ti-Mo-Nb ferritic steels is primarily determined by precipitation strengthening. Through optimized TMP parameters and microalloying additions, the overall precipitation strengthening contribution was elevated to the 300~400 MPa range.