Development of a Nonlinear Forming Limit Curve of an Advanced High-Strength Steel (DP440) with Applicability to Multi-Step Forming Processes

The modern automotive industry has highlighted a significant trend towards lighter automobiles. They frequently use a conventional forming limit curve (FLC) to assess the formability of widely used DP440 sheet steel. However, when the steel sheet undergoes multiple production steps, the curve becomes less effective in estimating the onset of necking, likely due to the twisting of the strain trajectory the sheet experiences. Therefore, engineers need an improved formability prediction tool to better handle such manufacturing elaboration and develop top-quality dies. This work proposes a phenomenological approach capable of detecting the onset moments of localized necking in DP440 sheets, which undergo progressive deformation under varying strain ratios. The approach begins with the establishment of an experimental bilinear failure strain database. Therein, six Marciniak tests are first carried out to pre-stretch rectangular sheet specimens by three different strain ratios, each with two different deformation degrees, to cover the whole deformation range encountered regularly in metal forming processes. To complete the bilinear failure strain database, all the specimens that have already been stretched are stretched again until breakage, following the Nakajima standard procedure. This produces six FLCs out of DP440 sheets previously pre-strained with six different strain values. Four displacement functions can then be derived from such a database, and a nonlinear FLC expressed in the form of a 3D forming limit surface is eventually composed. A nonlinear strain path up to failure, used to verify the developed FLC, is crafted by conducting a hole expansion test on prepared square specimens followed by a cup draw test on the same specimens both experimentally and simulatively. The nonlinear FLC reliably captures the onset of local necking of the test DP440 sheet deformed under a complex strain path, whereas the conventional FLC does not.