Quantitative study of carbonitride precipitation in niobium and titanium microalloyed hot strip steel

A commercial microalloyed steel was examined by optical and transmission electron microscopy in order to determine the origin and the amount of carbonitride precipitation which had formed as the result of industrial processing on a hot strip mill. Almost one-half of the total microalloy addition (0.06Ti and 0.02Nb) had remained in the form of eutectic or undissolved particles after soaking. Intragranular nucleation of fine carbonitride particles was observed to have occurred only in austenite. No additional carbonitride particles had formed in ferrite during coiling and almost one-half of the microalloying elements dissolved during soaking had remained in solution at the end of the industrial rolling process. According to well established structure-property relationships, a precipitation strengthening potential of 60-80 MN m(-2) could be assigned to the carbonitride particles which had nucleated in austenite.