Experimental analysis and evaluation of forming limit diagram for interstitial free steels

In this work, the suitability of Interstitial Free (IF) steel sheets of thickness 0.6 mm, 0.9 mm, 1.2 mm and 1.6 mm for press forming operations were examined by obtaining the forming limit diagrams. The microstructural aspects, tensile properties and formability parameters were studied. Forming Limit Diagrams were evaluated for the above sheet metals of four different thicknesses and they were compared. The tensile properties and formability parameters were correlated with the FLD. The fracture surfaces of the formed samples were observed using scanning electron microscope (SEM). Using the SEM fractography, the fracture behaviour and formability were analyzed. An attempt has been made to relate the strain ratio, average depth of cup and ratio of maximum depth of cup formed to minimum depth of cup formed with various parameters (involving n-value and r-value) namely n, r, nr, n + r, n2 + r, n2 + r2n + r2, n2r, nr2, n2 and r2. An attempt has been made to find out void sizes in the fracture surface and relate it to the shear strain developed in the sheet metal at fracture, depth of cup formed, limited minor strain and strain ratio. The formability of all sheets have been compared and the sheet with 1.6 mm thickness has been found to be superior.