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

Abstract

The effects of pre-straining and bake hardening on the mechanical behaviour and microstructural changes were
studied in two CMnSi TRansformation-Induced Plasticity (TRIP) steels with different microstructures after
intercritical annealing. The TRIP steels before and after pre-straining and bake hardening were characterised
by X-ray diffraction, optical microscopy, transmission electron microscopy, three dimensional atom probe
and tensile tests. Both steels exhibited discontinuous yielding behaviour and a significant strength
increase with some reduction in ductility after pre-straining and bake hardening treatment. The
following main microstructural changes are responsible for the observed mechanical behaviours: a decrease
in the volume fraction of retained austenite, an increase in the dislocation density and the formation of
cell substructure in the polygonal ferrite, higher localized dislocation density in the polygonal ferrite
regions adjacent to martensite or retained austenite, and the precipitation of fine iron carbides in bainite and
martensite. The mechanism for the observed yield point phenomenon in both steels after treatment was
analysed.