Experimental investigation and theoretical prediction of forming limit diagram of DP600 steel sheets

Dual phase steels are the ideal substitutes for conventional steels to reduce the weight of vehicles and decrease fuel consumption, and their mechanical properties have attracted much attention. Therefore, this work established strain-based forming limit diagrams (FLDs) for DP600 steel sheets by means of experimental investigation and theoretical prediction. To obtain the analytical limit strains in the biaxial and uniaxial tensile case, the Swift’s diffuse instability model and Hill’s localized instability model were adopted, respectively. The results showed that compared with the theoretical forming limit curve (FLC) predicted by Keeler equation, the theoretical FLCs predicted by the Hill'79 yield criterion or the Logan-Hosford yield criterion were more suitable for predicting the forming limit of DP600 steel sheets. The experimental results showed that the limit major strain of DP600 material is 41% under uniaxial tensile strain path, 43% under biaxial tensile strain path and 18% under plane strain path. Solutions from the analytical methods were compared with experimental FLD established based on combining uniaxial tensile tests and hydraulic bulging tests, only with an error of 3%, proving the rationality and accuracy of the theoretical prediction.