Deep drawing and bulging forming limit of dual-phase steel under different mechanical properties

Due to the different chemistries and manufacturing processes applied, the mechanical property of DP780 steel produced by different production lines may be inconsistent. In order to investigate the effect of mechanical property on the forming limit, two representative stamping processes, the round-cup drawing test and the dome test, are conducted separately on two DP780 specimens with different mechanical properties. Finite element models for the round-cup drawing test and the dome test are built to analyze the stress and strain evolutions during the forming process. Based on the results of experiment and the finite element analysis, the deep drawing and bulging forming limit of DP780 steel under different mechanical properties are predicted and compared with the Cockcroft-Latham ductile fracture criterion. The limit stroke and limit strain at fracture are obtained under variations of strength coefficient K, strain hardening exponent n, Lankford coefficient r, and thickness t in a specified range. It is found that an increase in K results in a significant drop in the limit stroke and limit strain for both the drawing and bulging forming processes. The deep drawing forming limit increases while the bulging forming limit decreases as r increases. The variation of n has a positive but not drastic influence on the deep drawing and bulging forming limit. The variation of t affects the limit drawing stroke but has almost no influence on the bulging forming. Due to these considerable effects of mechanical properties, the criteria for evaluating the performance of DP steel should be revised to improve the stamping manufacturability of DP steel.