Local formability of medium-Mn steel

Abstract

• Local formability of medium-Mn steel is described by forming limits at fracture. • Ductile fracture occurs prior to plastic localization in MMnS. • Large uniform elongation indicates superior global formability of MMnS. • Local formability of MMnS is not enhanced by pronounced strain hardening. Global formability and local formability are critical in different metal forming processes. Edge cracking, controlled by the local formability, is a dominant factor limiting the application of advanced high strength steels (AHSS) in automotive industries. The local formability of a medium-Mn steel (MMnS), a promising candidate of the third generation of AHSS, is evaluated based on forming limit curves at fracture and compared with a dual-phase DP1000 steel using the damage mechanics approach. The superior tensile properties of the investigated MMnS, high tensile strength, pronounced strain hardening, large uniform and total elongation, lead to a very good global formability, which is an indicator of necking resistance. However, the local formability of the investigated MMnS, which is an indicator of fracture resistance and quantified by the plastic strain at fracture under different stress states, is worse than the DP1000 steel. By comparing the local and global formability of the two AHSS, it is confirmed that ductile fracture is the dominant failure mode in the MMnS and the onset of localized necking occurs prior to ductile fracture in the DP1000 steel. To achieve an accurate determination of the local formability, the effects of stress states need to be considered, which cannot be derived explicitly from uniaxial tensile tests. In addition to tensile properties, more attention should be paid to the local formability of new AHSS to assess their potential application in automotive industries.