BAINITIC TRANSFORMATIONS IN EXTREMELY LOW-CARBON STEELS

The models so tar proposed in order to explain the mechanism of bainitic transformations have been reviewed and the problems arising from them have been discussed. The morphology and the crystallography of bainitic structures are quite similar to those of martensite which forms via diffusionless processes. The kinetic properties of bainite, however, exhibit typical of diffusional (reconstructive) natures such as C-curve formations in TTT diagrams. If bainitic transformations occurred in a displacive mechanism where at least the substitutional atoms are assumed to displace diffusionlessly, bainitic reaction would disappear in the interstitial-free steels, the Bs-temperatures, being equal to the Ms-temperatures. It has, however, been confirmed that bainitic reactions really occur in such extremely low carbon steels with their own C-curves. In order to get a comprehensive understanding for these conflicting results, some experiments using extremely low carbon steels have been carried out. It has been demonstrated that the results can be comprehensively explained by the model where the lattice change and the relaxation of elastic stress due to it are assumed to occur via individual atomic jumps and a lattice invariant shear, respectively.