Atom-probe tomographic and dilatometric studies of phase-transformations after inter-critical annealing of a low-carbon dual-phase steel

Abstract

Phase-transformations and microstructural evolution during and after inter-critical annealing of a low-alloy, dual-phase steel are studied utilizing dilatometric measurements, and microstructural characterization methods, including scanning electron microscopy, synchrotron X-ray diffraction, and atom-probe tomography. Dilatometric measurements reveal that the sample contains ~72% austenite, after annealing for 5-min at 871 °C, which agrees with the water-quenched microstructure, and ~90% after further annealing to 1 h, which agrees with thermodynamic predictions. A continuous-cooling transformation (CCT) diagram is constructed, within the cooling-rate range between 1 °C/s and 70 °C/s. The incubation time for the ferrite-transformation is <1 s, after inter-critical annealing, and the secondary-phases are mainly pearlite for cooling-rates of <5 °C/s, and martensite for cooling-rates >10 °C/s, which is confirmed utilizing scanning electron-microscope observations. The microhardness increases as the cooling rate increases, and an abrupt slope-change is explained by the major secondary-phase changing from martensite to pearlite between 5 °C/s and 10 °C/s. Atom-probe tomography demonstrates the coexistence of martensite and pearlite as secondary-phases for 30 °C/s and a switch in austenite-ferrite transformation mechanisms from para-equilibrium (70 °C/s) to partitioning local-equilibrium (30 °C/s). A microstructural map is constructed to reveal the austenite decomposition products in the cooling-rate range between 1 °C/s and 70 °C/s. Based on composition measurements of the matrix, utilizing APT, the effects of cooling-rates on solid-solution-strengthening are estimated, and the results demonstrate that solid-solution strengthening, especially carbon-redistribution, accounts for the majority of the strength variations due to different cooling-rates.