Low formability and reduction of area in twinning-induced plasticity steels despite their excellent tensile elongation

Abstract

The mechanism for low formability and reduction of area (RA) in twinning-induced plasticity (TWIP) steels despite their excellent tensile elongation has been investigated in an attempt to understand and increase their formability. In metals, RA is strongly dependent on the post-necking elongation (PE) rather than uniform elongation (UE); in this study, UE and PE had 9% and 91% contributions to RA, respectively. Between the two representative ductility indicators in tensile tests, the RA is typically higher than the total elongation (TE) in conventional metals. However, the RA of TWIP steels was lower than the TE because TWIP steels had a higher UE and lower PE. This was attributed to the higher strain hardening exponent (n) and lower strain rate sensitivity (m) of TWIP steels. This indicates that TWIP steels have a higher resistance to fracture by the strain gradient, but a lower resistance to fracture by the strain rate gradient. In this study, the small and/or negative m value of TWIP steels reduced the RA value and formability in spite of their higher elongation. The negative m value led to a strain concentration at the necking region, leading to localized necking without diffuse necking followed by shear cracks, resulting in the lower RA and formability. Consequently, it is necessary to increase the m value of TWIP steels in order to increase the formability.