Effect of niobium and titanium addition on the hot ductility of boron containing steel

Hot ductility of boron containing steel (B steel) with adding Nb (0.03 wt.%) (B–Nb steel) and B–Nb steel with adding Ti (0.0079 wt.%) (B–Nb–Ti steel) was quantified using hot tensile tests. The specimens were solution-treated at 1350 °C and cooled at 20 °C s−1 to tensile test temperature (T) in the range of 750 ≤ T ≤ 1050 °C. After that, they were strained to failure at a strain rate of 2.5 × 10−3 s−1. For the B–Nb steel, severe hot ductility loss was observed at 850 ≤ T ≤ 950 °C, which covered the low temperature in which austenite (γ) single-phase exists, and the high temperature at which γ and ferrite (α) coexist. Ductility loss in the B–Nb steel was caused by the presence of a network of BN precipitates, rather than by Nb(C, N) precipitates at the γ grain boundaries. In contrast, hot ductility of the B–Nb–Ti steel was remarkably improved at 850 ≤ T ≤ 950 °C. In the B–Nb–Ti steel, BN precipitates preferentially on TiN particles, resulting in increased BN precipitation in the γ grain interior and a decrease in the network of BN precipitates at the γ grain boundaries. These changes reduce strain localization at the γ grain boundaries and therefore increase the hot ductility of the steel.