Effects of Phase Transformation and Hot Rolling on the Microstructure and Mechanical Properties of a Novel High-Performance Bridge Steel

The novel 690 MPa grade high-performance bridge steel was subjected to dynamic and static continuous cooling transformation and rolling experiments at different final cooling temperatures. The effects of different cooling rates on microstructure transformation under deformation and undeformed conditions were studied using OM and TEM. Under the undeformed condition, high-density interphase precipitated particles were formed when austenite transforms into ferrite at lower cooling rates. Under deformation conditions, the microstructure was mainly bainite when the cooling rate reaches 5 °C/s. With the increase in cooling rate, the microstructure of bainite changed from needle like to lath like. As the final cooling temperature decreased, the overall hardness, strength and yield ratio showed an upward trend, while the plasticity showed a decreasing trend. The experimental steel exhibited outstanding mechanical properties. When the final cooling temperature was 417 °C, the impact energy at − 20, − 40 and − 60 °C was 168, 125 and 92 J, respectively. With the decrease in final cooling temperature, the microstructure changed from ferrite and a small amount of granular bainite to granular bainite and a small amount of ferrite, and then to lath bainite. When the final cooling temperature was 417 °C, the proportion of large-angle grain boundaries reached a maximum of 64%, significantly improving the low-temperature toughness of the experimental steel.