Preliminary study of the inhomogeneous precipitate distributions in Nb-microalloyed plate steels

Three as-thermomechanically controlled rolled (TMCR) microalloyed steels containing Nb levels between 0.023 and 0.057 wt% have been characterised in terms of phase balance and ferrite grain size distribution (optical microscopy and image analysis). In addition, transmission electron microscopy (TEM) has been carried out on carbon extraction replicas from sub-surface and mid-thickness positions to determine fine precipitate size distribution and areal number density in ferrite and pearlite. Precipitate distributions were also determined after simulated re-heating schedules. The ferrite grain size distributions have been related to precipitate distributions and rolling schedules, whilst the development of precipitate distributions has been considered based on solidification, reheating and deformation behaviour.