Refinement of Cu-M2C precipitates and improvement of strength and toughness by Ti microalloying in a Cu-bearing steel

Abstract

The present work aims to unravel the effect of Ti microalloying on the refinement of co-precipitated nanoscale Cu and M2C carbides, and thereby the improvement of both strength and toughness of an ultrahigh-strength Fe-0.05C-1.3Cu steel. The co-precipitated Cu and M2C carbides in Ti-free and Ti-containing steels was characterized after quenching and aging treatment via atom probe tomography (APT), and their contribution to the yield strength was quantified. APT results show that after being aged at 550 °C for 1 h, nanoscale Cu particles and M2C carbides co-precipitated. And, the density of finer Cu was found to be higher in the Ti-containing steel than that in the Ti-free steel which can produce more nucleation sites for M2C carbides precipitation. After being aged at 550 °C for 1 h, the Ti-containing steel had a yield strength as high as 1055 MPa, and an impact toughness of 132 J at −40 °C, which was an improvement of 61 MPa and 27 J, respectively, in comparison to the Ti-free steel. The higher-density M2C carbides were found to contribute to precipitation strengthening enhancement. The co-precipitation strengthening contribution from the Cu-M2C precipitates of the Ti-containing steel was calculated to be 69 MPa higher than that of the Ti-free steel. This is similar to the difference of 61 MPa from the experiment. The high toughness of Ti-containing steel is attributed to a higher density of high angle grain boundaries and a smaller prior austenite grain size by deflecting crack propagation.