On Formability Limitations in Stamping Involving Sheared Edge Stretching

The use of advanced high strength steels (AHSS) such as dual phase (DP), transformation induced plasticity (TRIP) and stretch flanging (SF) steels of the tensile strength of 600 MPa range are well established in automotive components production. This is due to their superior crash energy absorption ability and vehicle weight reduction potential. Recent trends show rapid growth in applications of even higher strength grades such as 800 MPa and 1000 MPa tensile strength and above. They are mostly used for fabrication of crash sensitive components to meet much higher safety requirements in side impact and roll-over accidents. One of the few concerns during the fabrication of AHSS components is the formability limit in flanging and hole expansion operations. Questions have been raised about the applicability of existing manufacturing experience with conventional high strength low alloy steels (HSLA) to new generations of AHSS. In this paper, sheared edge failure modes are presented for these steels in various loading modes. The influence of blanking clearance and various edge morphology parameters on the formability of a variety of AHSS is discussed. Differences between AHSS and HSLA failure mechanisms are illustrated. The applicability of the conventional approach to formability limits in the sheared edge stretching conditions is critically evaluated. Based on experimental data, practical recommendations regarding edge preparation and clearance in blanking or hole punching operations are also provided for numerous AHSS.