Formability analysis of hot-rolled dual-phase steel during the multistage stamping process of wheel disc

For reduction of vehicle weight and higher fatigue property, the hot-rolled dual-phase steel 700DP was developed to produce wheel disc with multistage stamping process. The wheel disc is one critical component of wheel system whose forming precision is extensively concerned. The critical area of the part characterizes nonlinear strain path during the multistage stamping process, which makes it difficult to accurately evaluate the formability through standard forming limit curve (FLC) method. The concept of equivalent plastic strain was reported to be strain path independent. Hence, in this paper, the formability of 700DP steel was studied through tensile test, Nakazima test, and two-step nonlinear formability test. It was certified that the modeling using the equivalent plastic strain-based FLC (ep-FLC) had better agreement with the two-step nonlinear test results. Then based on current die shape, the feasibility of 700DP steel for wheel disc with three-stage stamping process was studied by finite element method (FEM) with both traditional FLC and ep-FLC. Furthermore, the corresponding experiments were conducted for validation. The results show that the location and critical condition for the occurrence of fracture can be more exactly predicted by the ep-FLC compared with the traditional FLC. The process parameters such as the radius of die corner were then optimized by the modeling analysis. Finally, the wheel disc was successfully manufactured with high precision and sound mechanical property.