Effect of Wire Electrical Discharge Machining on Hole Expansion Ratio of 1 GPa Low Carbon Low Alloyed Steel

The stretch flangeability of ferritic steel grade with tensile strength ≥1 GPa in hole expansion tests can be significantly improved by using the wire electrical discharge machining (W-EDM) process for hole-making instead of conventional punching tools. This improvement is attributed to the notably enhanced cut edge quality of the machined holes. In this study, the average hole expansion ratio (HER) of a novel 0.1C-0.3V-0.25Mo-0.08Ti-0.08Nb steel increased from 24% to 91% when W-EDM was used in hole preparation. A comparison between the fractured surfaces of punched and W-EDM-machined holes after HER testing revealed different failure mechanisms in the steel sheet. At the onset of cracking, fractures in the W-EDM specimens exhibited ductile behavior, whereas quasi-cleavage fracture was observed in the punched specimens. Based on texture measurements and metallographic investigations, it was concluded that reducing the intensity of the adverse shear texture component {112} near the steel sheet surface and eliminating microstructural constituent variations improved the stretch flangeability of the Ti-Mo-V-Nb steel in both hole-making processes.