On the Factors Influencing the High Stretch–Flangeability of a Low-Density 1180 MPa Fe–Mn–Al–C–Nb δ-QP Steel

Low-density δ-quenching and partitioning (δ-QP) steels with excellent strength and ductility have been recently developed. However, there are still rare reports on the formability of δ-QP steels, which are critical for satisfying the manufacture of structural parts during the application in automotive industry. In the present work, an 1180 MPa Fe–Mn–Al–C–Nb δ-QP steel with a high ductility was adopted for the stretch–flangeability study. The δ-QP steel was developed by separated quenching and partitioning processes. A good hole expansion ratio (HER) of 34.9 ± 0.9% was obtained in the quenched steel, but it has been further increased to 52.2% by the tempering treatment. The improved stretch–flangeability was attributed to the enhanced austenite stability and deformation uniformity. On the one hand, the stability of austenite was increased by carbon partitioning during tempering, which reduced crack possibility via the suppression of the fresh martensite formation. On the other hand, the tempering treatment released the internal stress caused by martensitic transformation and reduced the difference in strength among different phases, resulting in an increase in the resistance to crack initiation and propagation.