Transformation during the isothermal deformation of low-carbon Nb-B steels
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In modern low-carbon and ultralow-carbon bainitic steels, boron is added to suppress the formation of polygonal ferrite, thereby increasing the hardenability of the steel.[1–7] In order to enhance the hardening effect of boron, some strong carbide and nitride formers, such as Nb and Ti, are also added; these tie up the C and N, allowing free boron to segregate to the grain boundaries. In recent studies, borocarbides were not observed in low-carbon (0.03 wt pct) and low-boron (0.0011 wt pct) steels treated with Nb or Ti; by contrast, they were seen in microalloyed steels[8,9] containing higher amounts of carbon (0.057 wt pct) and boron (0.0016 wt pct). This suggests that, for a given steel composition, the question of whether boron will segregate or precipitate depends not only on the carbon content, but also on the amount of boron in solution. In other words, the state of the boron is determined by the ‘‘solubility product’’ of boron and carbon.[10] A small amount of boron addition generally lowers the transformation temperature. The addition of boron in combination with Nb or Ti can lower the transformation temperatures still further. Recently, Bai et al.[11,12] investigated the effects of chemical composition and of the processing parameters on the transformation temperatures and resulting microstructures in ultralow-carbon Nb-B treated steels being deformed while undergoing continuous cooling. Their results confirm that the concentrations of Nb and boron in solution have a strong effect on the transformation temperature. Their work also showed that the amount of deformation in the no-recrystallization temperature range, as well D.Q. BAI, Postdoctoral Fellow, S. YUE, Associate Professor, and J.J. JONAS, Professor, are with the Department of Metallurgical Engineering, McGill University, PQ, Canada H3A 2B2. T.M. MACCAGNO, formerly with the Department of Metallurgical Engineering, McGill University, is with the Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, Canada T6G 2G6. Manuscript submitted August 14, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
as the cooling rate, significantly affect the transformation temperature and final microstructure, especially in high-boron steels. The addition of Mo to their Nb-B steels, however, was observed to stabilize the transformation behavior, thereby minimizing the range of observed transformation temperatures. Large intergranular and intragranular Fe23(C,B)6 particles were found in the Nb-containing steels with higher boron contents (48 and 64 ppm), and these were considered to promote the formation of polygonal ferrite. The present study was conducted with the intent of gaining a better understanding of the influence of strain and of the addition of Nb and Mo on the transformation behavior of ultralow-carbon B grades under isothermal conditions. For this purpose, a Mo-Nb-B, a Mo-B, and two Nb-B steels were selected for investigation. The transformation kinetics were determined by analyzing the resulting flow curves, while
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