The recent development of steels with carbide-free acicular microstructures containing retained austenite

Abstract

After a lengthy period of improvement in carbon steel ferrite/pearlite microstructures by microalloying and
controlled processing, some attention has focused back more recently upon acicular forms of microstructure.
In particular, an interesting advance in this area has been the development, primarily by alloying with Si, of
so-called ‘carbide-free’ bainitic steels, where carbon-stabilized retained austenite is substituted for cementite.
Transformation induced plasticity (TRIP) sheet steel with enhanced properties, principally targeted for
automotive use, and future potential ‘nanobainite’ steel, are two noteworthy examples. Even more recently, this
concept of developing steel microstructures containing carbon-enriched retained austenite has been extended
further by introducing novel heat treatment procedures to replace bainite with martensite. This nonequilibrium
‘quenching and partitioning’ process route, as it is known, offers possibilities of immediate
advantage: enhanced strength from a martensitic structure protected from the more damaging effects of
carbon, along with the promise of new properties from the retained austenite phase which could potentially
contain a very high controlled concentration of carbon.