Stretch-Flangeability of High Mn TWIP steel

The mechanical properties of austenitic high Mn Twinning Inducted Plasticity (TWIP) steel provide an excellent combination of strength and ductility when tested in uni-axial tension. The performance of TWIP steel during some critical formability tests such as deep drawing, bulge test and cutting edge stretching has not yet been studied extensively. In this contribution, the stretch-flangeability of Fe18Mn0.6C1.5Al TWIP steel and Ti Interstitial-Free (IF) steel were studied by means of hole expansion test. In-situ strain analysis and Infra-red (IR) thermography were carried out during the test. It was found that TWIP steel, despite having a higher uniform elongation in uniaxial tension, had poorer hole expansion properties than Ti IF steel. Strain distribution analysis revealed that the hole edge deformed in a deep drawing mode which was similar to a tensile deformation. Away from the hole edge, the deformation mode changed gradually from deep drawing to stretch forming mode. The IR-thermography of TWIP steel revealed a high degree of adiabatic heating which was absent in the case of IF steel. The crack associated with the edge fracture revealed a local temperature increase at the crack tip of up to 92 degrees C. Two types of hole edge preparations were studied. A high quality hole edge finish resulted in a better hole expansion performance. The fractography of the crack plane surface of TWIP steel and Ti IF were also studied by SEM, and revealed a plastic failure mode in both cases.