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

Abstract

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 degrees C and cooled at 20 degrees C s(-1) to tensile test temperature (T) in the range of 750 <= T <= 1050 degrees C. After that, they were strained to failure at a strain rate of 2.5 x 10(-3) s(-1). For the B-Nb steel, severe hot ductility loss was observed at 850 <= T <= 950 degrees C, which covered the low temperature in which austenite (gamma) single-phase exists, and the high temperature at which gamma and ferrite (alpha) 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 gamma grain boundaries. In contrast, hot ductility of the B-Nb-Ti steel was remarkably improved at 850 <= T <= 950 degrees C. In the B-Nb-Ti steel, BN precipitates preferentially on TiN particles, resulting in increased BN precipitation in the gamma grain interior and a decrease in the network of BN precipitates at the gamma grain boundaries. These changes reduce strain localization at the gamma grain boundaries and therefore increase the hot ductility of the steel. (C) 2011 Elsevier B.V. All rights reserved.