Influence of Martensite Morphology on Sheared-Edge Formability of Dual-Phase Steels

The automotive industry is increasingly demanding the availability of advanced high-strength steels (AHSS) with both higher strength and higher ductility. While there have recently been major improvements in the trade-off between ductility and strength, the sheared-edge formability of AHSS remains a critical issue. AHSS sheets exhibit cracking on sheared edges during stamping and forming operations, below the predictions of forming limit diagrams. The current study investigates the effects of microstructure on sheared edge formability measured through hole expansion experiments. Five commercially produced dual-phase (DP) steels with different microstructural configurations, but with equivalent tensile strengths of approximately 1 GPa, were evaluated. Quantitative stereological measurements of martensite size, contiguity, mean free distance, and number of colonies per unit area were made. The results showed the number of martensite particles per unit area, which represents microstructure homogeneity, correlates directly with the hole expansion ratio (HER). The overall trend is that HER increases as the number of martensite colonies per unit area increases; i.e. HER increases with a fine, evenly dispersed distribution of martensite. Additionally, HER decreases as the calculated carbon content in the martensite of these steels increases, which is attributed to the effect of carbon on martensite strength.