Roles of transformation interfaces in the design of advanced high strength steels

Abstract

Alloy partitioning and segregation or precipitation at transformation interfaces have been important in designing the mechanical properties of final products. Some representative examples in high-strength low-alloy steels are briefly discussed, with an emphasis on the transformation interface. For achieving better strengthening-ductility balance in TRIP-aided low alloy steels, it is essential to control carbon enrichment in austenite during ferrite and bainite transformations and the stability of retained austenite. Recent studies on alloy partitioning including carbon enrichment into austenite, provides a deeper insight into transformation mechanisms. We show that energy dissipation during interface migration and strain energy accumulation are important factors to control carbon enrichment into untransformed austenite during the ferrite/bainite transformation. As a counterpart of TRIP steels, interphase precipitation of alloy carbides during ferrite transformation leads to high yield strength and good elongation in low-carbon ferritic steels. A key for further strengthening is clarification of the conditions for refining the carbide dispersion. It is shown that incoherent ferrite/austenite interfaces are superior nucleation sites for carbides.