Synergism between B and Nb improves fire resistance in microalloyed steels

TL;DR

This study demonstrates that small additions of boron to Nb-microalloyed steels significantly enhance fire resistance by improving high-temperature mechanical properties through a boron-niobium synergistic effect, supported by experimental and DFT calculations.

Contribution

It reveals that boron addition in Nb-microalloyed steels markedly improves fire resistance, a novel insight into alloy design for high-temperature applications.

Findings

Boron increases the temperature at which 66% yield strength is maintained from 460°C to 574°C.

DFT calculations indicate boron lowers vacancy formation energy, enhancing microstructural stability.

The boron-niobium combination acts as an effective pinning-based strengthening agent.

Abstract

The development of new fire-resistant steels represents a challenge in materials science and engineering of utmost importance. Alloying elements such as Nb and Mo are generally used to improve the strength at both room- and high-temperatures due to, for example, the formation of precipitates and harder microconstituents. In this study we show alternatively that the addition of small amounts of boron in Nb-microalloyed steels may play a crucial role in maintaining the mechanical properties at high temperatures. The 66\,\% yield-strength criteria for fire resistance is achieved at $\approx 574$\,{\deg}C for a boron steel, whereas without boron this value reaches $\approx 460$\,{\deg}C, a remarkable boron-induced mechanical strengthening enhancement. DFT calculations show that boron additions can lower the vacancy formation energy when compared to pure ferrite and, for Nb-B steels, there is a further 24\,\% reduction, suggesting that the boron-niobium combination acts as an effective pinning-based strengthening agent.