Role of interphase precipitation in microalloyed hot strip steels

Transmission electron microscopy and microhardness measurements of individual ferrite grains were employed to investigate the state of interphase precipitation in commercial microalloyed steels. Interphase precipitation sheets tended to align parallel to the rolling plane, leading to frequent observation of particle row formation in longitudinal and transverse sample sections. As a rule, interphase precipitation occurred in some but not in all of the ferrite grains, thus limiting its contribution to the overall strength in comparison with carbonitride precipitation in austenite and dislocation hardening. In the particular case of a highly microalloyed Nb + Ti steel, interphase precipitation was present in about 50% of the ferrite grains. Interphase precipitation was also found to be associated with the loss of strength of as rolled steels due to normalising. In addition, arguments are presented to relate interphase precipitation to the effect of coiling temperature on as rolled strength.