The effect of martensite volume and distribution on shear fracture propagation of 600–1000 MPa dual phase sheet steels in the process of deep drawing

Due to its excellent strength and formability combinations, dual phase (DP) steels offer the potential to improve the vehicle crashworthiness performance without increasing car body weight and have been increasingly used in new vehicles. However, a new type of crack mode termed as shear fracture is accompanied with the stamping application of these high strength DP steel sheets. This paper presents a study of DP shear fracture through macroscopic identification and micro-level metallographical observation. By the cup drawing experiment to identify the limit drawing ratio (LDR) of three DP AHSS with strength level from 600 MPa to 1000 MPa, the study compared and categorized the macroscopic failure mode of these three types of materials. The metallographical observation using scanning electron microscopy (SEM) along the direction of crack was conducted for the DP steels to discover the micro-level propagation mechanism of the fracture and its relation with the martensite volume and distribution. Plasticity analysis and finite element method (FEM) simulation were provided to explain the observed behavior, which was further confirmed by the comparison between DP fracture and high strength low alloy (HSLA) fracture.

► Martensite volume and distribution is identified for DP AHSS from 600 MPa to 1000 MPa.
► DP steel sheets are ductile failures with dimple appearance.
► DP800 sheets crack and propagate along the ferrite–martensite interfaces.
► DP1000 fracture sheets crack and propagate in a mixed propagation mode.
► Plasticity stress analysis explains the fracture mode of DP steel sheets.