Abstract
Fatigue hardening-softening behavior of a newly developed "TRIP-aided bainitic (TB) steel" which is composed of a bainitic ferrite matrix and retained austenite films was examined. A significant fatigue hardening, similar to "TRIP-aided dual-phase (TDP) steel" and conventional ferrite-martensite dual-phase steel, was exhibited in the TB steel, particularly when austempered at temperatures higher than the martensitestart temperature. This was mainly caused by an increase in the strain-induced martensite content and development of severe cell structure inthe matrix. Compressive internal stress in the matrix resulting from the stable and strain-hardened retained austenite films was expected tomake a relatively small contribution to the fatigue hardening, iffering from thatof the TDP steel.
