AUSTENITE FORMATION IN MANGANESE-PARTITIONING DUAL-PHASE STEEL

Manganese-containing ferritic-pearlitic steels have been studied after intercritical annealing for various times at temperatures of 700 and 725 degree C, during which austenite formation occurred at ferrite grain boundaries. Light and electron optical microscopy, and scanning transmission electron microanalysis and microdiffraction were employed for the study, in which both microstructural development and manganese redistribution were examined in detail. It was observed that the formation of austenite is associated initially with a migration of ferrite grain boundaries. It is proposed that this boundary migration is induced by manganese diffusion along the boundaries, and that this in turn provides the mechanism for rapidly transporting manganese to the growing austenite. On this basis, we find that austenite formation in these steels is associated with manganese-rich migrated ferrite boundaries, and is not dependent on the presence of cementite particles for nucleation.