Roles of pre-formed martensite in deformation behavior and strain partitioning of medium-carbon bainitic steel by quasi-in-situ tensile tests

Microstructural features play a major role in strain distribution and TRIP effect, influencing the plastic defor-mation behavior of multiphase steel. In this work, the bainitic multiphase microstructure with or without pre-formed martensite (PM) was obtained by the below -MS and above -MS austempering process. The evolution of strain partitioning and TRIP effect on plastic deformation were studied by combining micro digital image cor-relation (mu-DIC) analysis with in-situ tensile test in the multiphase microstructure. The changes of microstructure during deformation were systematically investigated by various characterization methods. The results show that the regions of strong strain partitioning in multiphase microstructure are different. Compared with the above -MS austempered sample without PM, the strain partitioning of the below -MS austempered sample containing PM is concentrated in the pre-formed martensite region while not in the blocky retained austenite (RA) region. The strain concentration in the PM region of the below -MS austempered sample is smaller than that in the blocky RA region of the above -MS austempered sample at the same engineering strain. The PM shares the stress load on RA during plastic deformation, thus can coordinate microstructure deformation and delay the early occurrence of the TRIP effect of the below -MS austempered sample. Moreover, the introduction of PM in the below -MS aus-tempered sample increases the mechanical stability of RA, which is beneficial to produce an effective TRIP effect and delay necking to high strain. Consequently, the plasticity of the medium-carbon bainitic multiphase steel containing PM is enhanced by the effective TRIP effect and uniform strain partitioning.