On the Strengthening Effects Affecting Tensile and Low Cycle Fatigue Properties of Low-Alloyed Seismic/Fire-Resistant Structural Steels

In the present study, low carbon ferritic and bainitic steels with different contents of Mo, Ti, and Nb were designed for both seismic and fire-resistant applications. The microstructure of steels containing 0.3 wt% Mo–0.02 wt% Nb (‘A’ hereinafter) was mainly composed of bainite. By contrast, the microstructure of steels with 0.2 wt% Mo–0.13 wt% Ti (‘B’ hereinafter) consisted of ferrite with a high density of nano-sized (Ti,Mo)-rich MX precipitates. The results showed that the bainitic microstructure (‘A’ steel) was quite favorable to high-temperature strength and thermal stability. The yield strength of ‘A’ steel at both room and 600 °C temperatures increased consistently with increasing thermal exposure time (600 °C/200–1000 h), since the precipitation of NbC particles occurred while maintaining bainitic ferrite platelets with a high density of dislocations during exposure. In the ‘B’ steel, the formation of nano-sized (Ti,Mo)-rich MX particles was effective to impede dislocation movement, leading to excellent plasticity (lower yield ratio) at room temperature. However, their contribution to precipitation hardening was not so much at 600 °C, as compared to the bainitic strengthening. During low cycle fatigue tests at room temperature, the main different feature between the two steels is that the ‘A’ steel showed cyclic softening while cyclic hardening was evident in the ‘B’ steel. The bainitic microstructure showed a better fatigue life due to increased ductility manifested by cyclic softening, by which dislocation cell was developed. Graphic Abstract