The Edge Formability of High-Strength Cold-Rolled Steel

Little information is available about the influence of inclusions and edge condition on the edge Formability of cold-rolled steels. For this reason a series of high-strength cold-rolled steels was evaluated through the hole expansion test. The steels examined were a rephosphorized steel microalloyed 50 and 60 ksi steels and recovery annealed 60 and 70 ksi steels.

Edge formability has been found to be greatly influenced by the condition of the punched edge. Samples that were “dressed” (i.e. by removal of the shear burr along with the worked metal at the edge) exhibited a larger percentage hole expansion when compared to those evaluated in the as-blanked condition. The effect of inclusions on crack initiation is more significant in the de-burred condition than in the as-blanked condition because of the lower incidence of defect sites present at the edge. Hole expansion was found to be better in materials having higher total elongation and plastic anisotropy r_m. Likewise an increase in tensile strength was found to be detrimental to hole expansion performance. The percentage hole expansion was not affected by variations in the observable crack length. Therefore this test is insensitive to crack length within the normal limits of the test and thus is not operator-dependent. Microstructural analysis supports the argument that cold-rolled high-strength containing a high density of inclusions will exhibit reduced edge formability.