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

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Structural materials : properties, microstructure and processing, 2011-04, Vol.528 (10), p.3556-3561
• Main Authors: Cho, Kyung Chul, Mun, Dong Jun, Koo, Yang Mo, Lee, Jae Sang
• Format: Article
• Language: English
• Subjects: Applied sciences; Boron nitride; Continuous cast slab; Cross-disciplinary physics: materials science; rheology; Ductility; Elasticity. Plasticity; Exact sciences and technology; Grain boundaries; Hot ductility; Materials science; Mechanical properties and methods of testing. Rheology. Fracture mechanics. Tribology; Metals. Metallurgy; Niobium; Phase diagrams and microstructures developed by solidification and solid-solid phase transformations; Physics; Precipitates; Precipitation; Structural steels; Surface crack; Titanium; Titanium-nitride
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2011.01.097

► Addition of only Nb without Ti has little influence in the hot ductility of B steel. ► Hot ductility loss of B–Nb steel is due to grain boundary precipitation of BN. ► Adding a small amount of Ti improve the hot ductility of B–Nb steel. ► In B–Nb–Ti steel, hot ductility improvement is related to presence of TiN particle. ► Presence of TiN particles makes the BN precipitates’ distribution more homogeneous. 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.