Deformation Behavior in High-Strength Dual-Phase Steel Sheets during Bend Forming

Abstract

The individual deformation behavior of ferrite and martensite was investigated microscopically by using nanoindentation technique to clarify bendability of dual-phase steel sheets. The cold-rolled steel sheets with composition of Fe-0.16mass%C-1.0mass%Si-1.5mass%Mn were annealed at 775°C for 600 s, water-quenched and tempered at 350°C for 300 s. V-shape bending tests with bending angle of 90° were performed for annealed specimens with the thickness of 1.0 mm. Nanoindentation experiments were performed with several peak loads to measure the distribution of the hardness in sub-micron areas, using a cube corner type indenter. The microstructure of the specimen obtained by the heat treatment was ferrite-martensite dual-phase. Ferrite was the dominant microstructure with a volume fraction of approximately 65 %. The nanohardness for ferrite and martensite was 3.2 GPa and 5.9 GPa in the annealed specimen, respectively. Both ferrite and martensite were hardened in the specimen with a bending radius of 1.0 mm. Especially, ferrite was hardened locally in the specimen. The nanohardness for ferrite in the markedly hardened area was 4.0 GPa and it was 3.6 GPa in another area. A distinct plastic flow such as a shear band was observed near the markedly hardened area. Voids nucleated on the ferrite side in the area. It was concluded that the development of shear bands during bending deformation with a small radius caused the local and remarkable work-hardening and that a larger partition of strain into ferrite than martensite domain resulted in the nucleation of micro voids in ferrite.