Design and Development of Lightly Alloyed Ferritic Fire-Resistant Structural Steels

To improve safety in case of building fires, stricter building codes have been proposed requiring structural steels to maintain two-thirds of their room-temperature yield strength after exposure to 873 K (600 °C) for longer than 20 minutes. To address this need, we have designed lightly alloyed structural steels, employing computational thermodynamics in combination with fundamental principles of precipitation strengthening and its temperature dependence, precipitate stability, characterization by optical microscopy and atom probe tomography (APT), and mechanical testing at room and elevated temperatures. The design process resulted in low-carbon ferritic steels with small alloying additions of V, Nb, and Mo that maintain over 80 pct of room-temperature yield strength in compression, and nearly 70 pct in tension, after 2 hours of exposure at 873 K (600 °C). APT demonstrates the formation of nanoscale MX and M₂X (where M = V + Nb + Mo and X = C + N) precipitates after exposure to 873 K (600 °C). The favorable high-temperature mechanical properties are discussed with a model of precipitation strengthening by detachment-stress-mediated dislocation pinning at nanoscale semi-coherent MX precipitates.