Predicting the effect of ultrafine ferrite on the deformation behavior of DP-steels

The development of ultrafine ferrite grain size has become an attractive way to improve the behavior of dual phase (DP) steels. It is expensive to experimentally investigate the effect of the various governing variables on the deformation behavior for optimum combination of properties in DP-steels. In this work, the effect of the ultrafine ferrite phase on the deformation behavior of DP-steels is investigated for volume fractions of martensite (Vm) of 6.8%, 13.7%, 18.5%, 27.8% and 31.4% and at ferrite grain sizes of 3, 5, 10, 15, 20, 25, 30, 40, 50 and 60 μm using micromechanical modeling of cells. The influence of the ultrafine ferrite grain size is investigated through relating the deformation fields in the material constituents to the mechanical properties that are complemented with them. This investigation is carried out through employing the different influential governing factors, which have significant influence on the material behavior in the model. These include ferrite grain size, martensite volume fraction, martensite softening by carbon dilution and particle size distribution. The effect of grain refinement on the strain hardening rate is observed at small nominal strains, but at large nominal strains the grain refinement effect diminishes in agreement with experimental observations. The deterioration in ductility is revealed to be less at intermediate and high Vm. The model also reveals that at small grain size, the effect of Vm on the strain hardening rate is less, compared to large grained matrix. The crucial influence of ferrite grain refinement is shown to be triggered at the onset of plastic deformation of martensite. It is shown that the finer the ferrite grain size, the more restriction of deformation in the ferrite phase and the earlier martensite plastically deforms.