THE INFLUENCE OF METALLURGICAL DEFECTS ON FORMABILITY OF SHEET STEELS

Experiments have been performed to analyze the initiation of strain localization and its relationship to the microstructure in medium- and high-strength grades of sheet steel. Out-of-plane stretching tests were used to determine the forming limit curves between the strain states of uniaxial tension and equibiaxial tension for four steels. Failure under negative minor strains correlated with a constant thickness strain criterion. Localized necking under positive minor strains resulted from the development of plane-strain conditions due to geometrical and frictional constraints. Localized necking and fracture occurred predominantly along the rolling direction for these steels. Comparison of these results with both deformation and flow theories of localized necking suggest that flow theory is a more appropriate model of formability.; In-plane equibiaxial stretching tests were performed for two medium-strength steels with and without artificially-induced defects; these tests were used to examine the M-K theory of localized necking for a sheet containing defects. Geometrical "soft" spots were formed by selective electropolishing of the desired area. These long, narrow grooves reduced the forming limit in accordance with the M-K theory. Metallurgical "hard" spots were formed by laser heating and rapid cooling of the selected area. During stretching a localized neck formed in a ring around the "hard" defect although fracture occurred predominantly along the rolling direction. Both "hard" and "soft" defects were found to reduce the forming limit although failure occurred outside the "hard" spot. These experimental results along with finite-element calculations suggest that "soft" and "hard" defects may cooperate to cause localized necking under in-plane equibiaxial stretching.