Static Strain Aging of Microstructural Constituents in Transformation-Induced-Plasticity Steel

A pronounced difference in static strain aging was observed between unstrained and prestrained low-alloy multiphase transformation-induced-plasticity (TRIP) steel. High bake-hardening (BH) values were obtained for prestrained TRIP steel. This aging behavior was shown to be directly related to the multiphase microstructure, by analyzing static strain aging in the ferrite and bainite separately. The ferrite in TRIP steel exhibits very limited strain aging, due to the Cottrell atmosphere formation. The bainite constituent, which contains retained austenite, has a pronounced static strain-aging response in the 2 pct prestrained condition. This is a result of internal stresses generated by the strain-induced martensite and the low-temperature aging of this phase. In the absence of strain-induced martensite, the BH values of TRIP steel are low and are due only to the Cottrell atmosphere formation. The TRIP steel exhibits an intermediate BH response, due to the composite effect of its ferrite and bainite constituents. The influence of the cross-sectional changes and volume changes during tensile tests carried out to determine the BH values is also reviewed.