Effect of austenitic state before ferrite transformation on the mechanical behavior at an elevated temperature for seismic-resistant and fire-resistant constructional steel

In this study, steels with different volume fraction of bainite and ferrite were obtained by tuning the rolling process to change the austenitic state before ferrite transformation. The volume fraction of bainite in the steel when the austenite was undeformed was ~83%, and in the steel when the austenite was deformed was ~36%. The yield strength at room temperature of the experimental steel with undeformed austenite was ~63.5 MPa higher than the experimental steel with deformed austenite, while the yield strength at 600 °C of the former was ~104.8 MPa higher than the latter. The main strengthening methods that led to difference in yield strength at elevated temperature are precipitation strengthening, dislocation strengthening and grain boundary strengthening. The experimental steel with higher volume fraction of bainite was characterized by higher Young's modulus and dislocation density at 600 °C, while the volume fraction of precipitates was relatively lower. The higher Young's modulus was mainly because of higher volume fraction of alloyed cementite precipitated from steel with higher volume fraction of bainite at 600 °C. Furthermore, the boundary densities of packet boundaries, block boundaries and sub-block boundaries had an important influence on the yield strength of steel at 600 °C.