Change in Concentration of Boron Segregated on Austenite Grain Boundary after Hot Deformation and during Recrystallization in Low Carbon Steels

The effect of prior austenite (γ) recrystallization and BN precipitation on B segregation behavior at prior austenite grain boundaries (PAGBs) was investigated by electron backscatter diffraction (EBSD) and time of flight secondary ion mass spectroscopy (TOF-SIMS). We measured changes in the BO2− signal intensity (SI) at PAGBs of 0.09C steel during isothermal holding after deforming at 850°C. The γ recrystallization started within 10 seconds of isothermal holding, and SI at recrystallized PAGBs 10 seconds after deforming was almost the same as prior to the deformation, whereas SI at non-recrystallized PAGBs showed lower values. When subjected to isothermal holding for more than 10 seconds, BN precipitation increased and SI at PAGBs decreased. This decrease in SI was attributable to the decrease in the amount of soluble B caused by BN precipitation. We also confirmed that SI at PAGBs of 70% Ni steel decreased immediately after deforming at 700°C, to levels lower than prior to the deformation. These findings indicate that the amount of segregated B at PAGBs decreases with deformation and the subsequent recovery behavior of SI depends on the recrystallization state. In addition, we measured the change in Vickers hardness (ΔHv) of 0.09C steel associated with B addition to investigate the effect of the amount of segregated B at PAGBs on the hardenability. ΔHv expressed in a linear function and increased as SI at PAGBs increased. These results suggest that the hardenability of B added steel is strongly related to segregated B at PAGBs.