Effect of boron and titanium on hot ductility of a low carbon niobium microalloyed steel

Hot tensile and compression tests were performed to evaluate the effect of B and Ti on the hot ductility of a low-C Nb-microalloyed steel. Specimens were solution-treated at 1350 °C and cooled at 0.83 °C/s to test temperatures between 750 and 1000 °C. The Nb-microalloyed steel exhibited a wide trough, extending from 950 to 750 °C. The addition of 0.0025 wt.% B improved hot ductility, increasing the reduction of area by more than 10% across the entire temperature range and shifting the trough to 925–800 °C. In the austenitic region, this improvement was attributed to increased grain boundary cohesion caused by B in solid solution and the formation of coarse, non-detrimental BN precipitates, while B also influenced the austenite-to-ferrite transformation kinetics. The simultaneous addition of Ti and B further enhanced hot ductility, narrowing the trough to 850–800 °C and raising the reduction of area above 60%. Detailed characterization using field emission gun scanning electron microscopy and transmission electron microscopy, combined with thermodynamic simulations, revealed that Ti combined with N to form TiN, which increased B in solid solution by reducing BN formation. Additionally, Ti modified Nb precipitation patterns, promoting fewer, coarser (Nb, Ti)(C, N) precipitates at 900 °C. However, at 800 °C, increasing Ti promoted a higher density of fine particles, which deteriorated hot ductility. These results are supported by kinetic simulations. Therefore, the optimum Ti additions correspond to hypo- or stoichiometric ratios with N, where B is protected in solid solution while precipitation effects are minimized.