The aim of this research is to investigate generation of micro voids affecting stretch flange formability in cold-rolled Dual-Phase (DP) steel sheets having a low volume fraction of martensite. The volume fraction of martensite was the dominant factor of the hole expanding ratio instead of the difference in hardness between ferrite and martensite when the difference in hardness between ferrite and martensite was large. On the other hand, the difference in hardness became the dominant factor of the hole expanding ratio in DP steel sheets having a low difference in hardness. Micro voids around the punched hole and the fracture edge of tensile deformation were observed in order to understand these different results. The void density is able to be associated with the hole expanding ratio under the same strain condition and the void density depends on the martensite spacing.
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