Local ductility – key parameter for predicting formability of AHSS

Abstract

In order to be able to make targeted use of the advantages of AHSS grades in forming processes it is necessary to determine local material properties. Hence, in addition to edge crack sensitivity measures, such as hole expansion ratio according to ISO 16630 (HERISO), it is currently intensively discussed whether measures, that assess the local ductility of the material, should also be specified in material specifications. One representative is given by the parameter true fracture strain (TFS) that is based on the measurement of the fracture area of a cracked flat tensile specimen. By means of 16 different cold- and hot-rolled steel batches, the relation between the parameters TFS and HERISO was investigated. The outcome is that these parameters do not correlate to a satisfying degree when considering all materials and consequently are regarded as representatives of different material properties. Thus, the issue of the reliability of the measures is considered separately. For the regarded steel grades, the overall scattering of TFS, i.e. neglecting the sample orientation, is found to be 13 – 44 % and the scattering of HERISO is within 5 – 35 %. The high scattering of TFS is mainly caused by the anisotropy of the materials. In case of parallel measurements of the same batch the relative standard deviations of TFS and HERISO are on the same level for cold-rolled strips, whereas in case of hot-rolled strips the relative standard deviation of HERISO is significantly higher. By investigating the influence of the tensile specimen shape for a bainitic hot-rolled strip numerically, it could be shown that the width-to-thickness ratio, that takes the hole fracture area into account, influences the TFS value directly.