EFFECT OF PRE-STRAINING AND BAKE HARDENING ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF CMnSi TRIP STEELS

The effects of pre-straining and bake hardening on the mechanical behaviour and microstructural changes werestudied in two CMnSi TRansformation-Induced Plasticity (TRIP) steels with different microstructures afterintercritical annealing. The TRIP steels before and after pre-straining and bake hardening were characterisedby X-ray diffraction, optical microscopy, transmission electron microscopy, three dimensional atom probeand tensile tests. Both steels exhibited discontinuous yielding behaviour and a significant strengthincrease with some reduction in ductility after pre-straining and bake hardening treatment. Thefollowing main microstructural changes are responsible for the observed mechanical behaviours: a decreasein the volume fraction of retained austenite, an increase in the dislocation density and the formation ofcell substructure in the polygonal ferrite, higher localized dislocation density in the polygonal ferriteregions adjacent to martensite or retained austenite, and the precipitation of fine iron carbides in bainite andmartensite. The mechanism for the observed yield point phenomenon in both steels after treatment wasanalysed