No abstract

Ferrite/martensite dual-phase (DP) steels have been used for automobile body parts because of their excellent strength/ductility balance. It is known that damage forms with plastic deformation in the DP steels, and is associated with their microstructure. It is thus possible to control the crack/void growth by designing the distribution and morphology of ferrite/martensite. In this work two types of DP steels, with almost same fraction but different distribution/morphology of martensite, were prepared by changing heat treatment process. Effects of microstructure on the damage evolution behavior during tensile tests at 20, -100, and -180 ºC were discussed with quantitative method of damage growth and scanning electron microscopy (SEM). It was found that (1) martensite morphology affected the size of crack/void damage and the mechanism of damage nucleation; (2) nucleated damage was arrested to grow by plastic deformation of ferrite in both DP steels; (3) the arrested damage started to grow again in further deformation, resulting in failure; (4) this restarting of damage growth occurred at a smaller strain at low temperatures of -100 and -180 ºC than 20 ºC.