A new strategy for developing a Nb microalloyed fire-resistant steel: Effects of boron and cooling rate

This study investigates the correlation among cooling rates, boron addition, and their subsequent effects on the microstructure and mechanical properties of niobium microalloyed steels, specifically with an intent to develop new fire-resistant steels. Slower cooling rates enhanced the ferrite transformation and (Nb,Ti)C precipitation, promoting a shift toward an equilibrium-phase microstructure. A boron addition of 30 ppm was sufficient to increase the fire resistance of niobium microalloyed steels when slowly cooled after thermocontrolled rolling. At rapid cooling rate boron induced the formation of upper bainite. Boron is mainly segregated at the interface of (Nb,Ti)C precipitates, possibly hindering the coarsening kinetics of nanosized carbides and contributing to the maintenance of a high yield strength during fire exposure simulations. The boron addition promoted a ratio between yield strength at 600 °C and room temperature greater than 66%., boron also shifted the strain-temperature curve to higher temperatures in transient tensile tests. These findings contribute to the potential use of boron additions in advancing cost-effective, high-performance, fire-resistant steel development, highlighting the central role of microstructural control and optimized thermomechanical treatment.