Analysis of work hardening behavior in ferrite-martensite dual-phase steels using micro-grid method

To clarify the contribution of martensite increase to work-hardening during transformation-induced plasticity (TRIP), the changes in local strain distribution with tensile deformation were investigated for dualphase steels with different volume fractions of martensite in ferrite using the precise marker method. Particular attention was paid to the changes in strain and stress bearing by martensite and ferrite with tensile deformation. The precise marker method is especially useful for local strain analysis of multiphase materials. Three types of steels with volume fractions of 25%, 50%, and 75% martensite were subjected to local strain analysis at several stages of deformation. The important results are as follows: (1) in steels with a large volume fraction of martensite, the contribution of the plastic deformation of martensite to the overall tensile strain is large from the beginning of the tensile deformation, and (2) the difference in strain bearing by martensite and ferrite increases through tensile deformation in both 25% and 50% martensitic steels. Using the constitutive equations for the stress-strain response, the strain distributions in each phase were translated into stress distributions. Then, the stress-strain response was numerically estimated by applying the general rule of mixtures using the average values of strain and stress, and compared with the experimental results. The relationship between the controlled martensitic transformation and tensile deformation behavior in TRIP steel is also discussed.