Influence of prior microstructure on normalising response of C–Mn–Al–Nb steels

The microstructural response to normalising of commercial C–Mn–Al–Nb steels produced using various manufacturing routes and rolling schedules has been investigated over the range of normalising temperatures 840–940°C and the temperature required for optimum mechanical properties was identified. The morphological development of austenite was studied using reheat quench experiments by which it was shown that the morphology of coarse bainitic microstructures when present in the as rolled steels, was inherited by the austenite on normalising. The transformation behaviour on cooling from normalising temperatures close to the Ac₃ and the evolution of the grain refined ferrite–pearlite microstructure were studied using both dilatometry and electron microscopy. The variations in mechanical properties observed are explained in terms of microstructural features, such as ferrite grain size and volume fraction and distribution of pearlite, and there was found to be a strong relationship between the ferrite grain size before and after normalising. It can be inferred from the microstructural observations that the mode of austenite transformation (as determined by the as rolled ferrite–pearlite microstructure) confers a limited memory to these steels which is particularly evident at low normalising temperatures. From observations of the etching response of austenite grain boundaries at low normalising temperatures, it can be suggested that the memory may be a result of segregation of impurities to austenite/ferrite interfaces either during cooling after rolling or reheating during normalising.

MST/673