Abstract: The mechanical properties of cold-rolled transformation-induced plasticity steel TRIP780 and bake hardening steel BH180 are compared. By U-shaped part stamping testing, sensitivity of blank holder force and punch radius on springback of the two steels are analyzed. Springback of the U-shaped parts occur during the time of one minute to 12 weeks after it is unloaded are measured. Emphasize is put on the analysis and discussion on the time-dependent springback which meant springback varied with time. X-ray refraction testing is performed to study the trend of retained austenites in TRIP780 steel. Results show that the investigated TRIP780 steel not only combined high strength, preferable ductility and large springback, but also it has prominent time-dependent springback, which is hardly found in BH180 steel. The amount of time-dependent springback of TRIP780 steel occurs in the first day after it is unloaded reaches the climate and then it tends to be stable. Twelve weeks after unloading, the max percentage of time-dependent springback to the initial springback could reach 20%. X-ray refraction indicates that there exists an inherent relationship between the time-dependent springback and the transformation-induced plasticity effect. TRIP780 steel is more sensitive to the blank holder force, which can be used in the springback control.

Key words: time-dependent springback;transformation-induced plasticity(TRIP);bake hardening(BH);blank holder force