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An undesirable factor that affects the dimensional precision and final shape of metallic parts produced in cold forming processes is springback phenomenon. An analytical model is introduced to predict springback in U-shaped bending process of DP780 dual phase steel sheet. This analytical model is based on the Hill48 yield criterion and plane strain condition. In this model, the effect of forming history, sheet thinning and the motion of the neutral surface on the springback of U-shaped bending process is taken into account. The anisotropic nonlinear kinematic hardening model is used to consider the impact of complex deformation, including stretching, bending and reverse bending. This model is able to investigate the complex hardening behavior of material such as Bauschinger effect, transient behavior and permanent softening. The effect of the sheet holder force, the coefficient of friction, thickness, material anisotropy and hardening parameters on the sheet springback is studied. It can be seen that analytical model which presented in this paper has good accuracy in the springback prediction in comparison with FEM method and results are close to experimental data. The results shows that the sheet holder force, the coefficient of friction, thickness and material anisotropy have great influences on the springback prediction. Since during the forming process the material experience reverse loading, the hardening parameters of material has a significant influence on the springback prediction. It can be seen that the Bauschinger effect has more influence on the springback prediction than the permanent softening and transient behavior.

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In the present study, using a new method, dual-phase (DP) steel with high strength and good ductility was produced from plain carbon steel with 0.16% carbon. The DP steel with ferrite-martensite structure was obtained using austenitizing, quenching, asymmetric cold rolling, and intercritical annealing at temperatures of 770 and 800 °C and short holding times of 1 and 5 min. Due to the application of uniform shear strain through asymmetric cold rolling, a uniform distribution of the martensite phase was observed in the RD-TD and RD-ND planes. By increasing the holding time, the volume fraction of martensite increased from 8% to 12% at 770 °C and from 10% to 33% at 800 °C for the holding times of 1 and 5 min, respectively. Hardness and strength improved with increasing temperature and time of intercritical annealing. The sample produced at a temperature of 800 °C and a time of 5 minutes showed excellent mechanical properties such as 244 HV hardness and 1020 MPa strength and 12.5% ​​ductility. In addition, due to the high volume fraction of martensite and the consequent reduction of its carbon content, the hardness of this phase decreased and as a result, it showed significant plastic deformation and high strain hardening. The fracture surface of all produced DP steels mainly included dimples, which indicates ductile fracture behavior.

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