Transformation plasticity in boron-bearing low carbon steel

The transformation plasticity (TP), which indicates that permanent strain remains after solid-solid phase transformation, even under much smaller stress than the yield stress, has been described by a vacancy diffusion mechanism in the migrating interface during diffusional phase transformation. In this study, the influence of boron (B) addition on the TP of low carbon high strength steel was investigated through the observation of the B segregation in the phase interface between primary austenite phase and ferrite phase using secondary ion mass spectroscopy. The B segregation at the austenite-ferrite phase interface was confirmed to cause drastic decrease of the TP strain by comparison of the dilatation behavior of B-bearing and B-free steels under a tensile force during slow cooling, where the diffusional phase transformation occurs in B-bearing steel. Furthermore, it was also confirmed that the velocity of B diffusion is larger than the migration velocity of interface at the given temperature through a calculation based on Fick’s law.