Control of microstructure and mechanical properties of low-carbon bainitic weathering bridge steel with ultrahigh toughness at ultra-low temperatures

The ultra-toughness of ultra-fine low-carbon bainitic weathering bridge steel at ultra-low temperatures, along with its microstructure and the strengthening and toughening mechanisms under various production processes were investigated in this study. Additionally, a bainite transformation model was established. It was shown that the exclusive transformation mechanism observed is the packet granular bainite transformation, which conforms precisely to a one-dimensional growth model. The exceptional impact performance of the experimental steels was exhibited at ultra-low temperature, where it achieves remarkable impact absorption energy of 91 J at −130 °C and impressive 53 J at −196 °C. Unlike traditional low-temperature steels, the super toughness at low-temperature in this material does not rely on (residual) austenite; instead, it primarily originates from fine grains, a high proportion of high-angle grain boundaries (HAGBs), and packet-boundary granular bainite. Furthermore, the uniform distribution of dislocations due to recrystallization behavior also contributes significantly to the excellent impact toughness. Lower final rolling temperature and cooling temperature provide enhanced nucleation driving forces and nucleation sites, contributing substantially to an average grain size of 3.7 μm. The primary strengthening mechanisms were attributed to grain refinement (52 %), followed by dislocation strengthening (24 %).