Time-dependent springback of advanced high strength steels

Aluminum alloys are known to undergo time-dependent springback following forming while typical autobody steels of the 1990s do not. This behavior has been attributed to larger residual stresses relative to the yield stress in aluminum and to faster creep rates. In order to determine whether “advanced high strength steels” (AHSS), with high ultimate tensile strength/Young’s modulus ratios, also exhibit time dependence, draw-bend springback tests were performed using three dual phase (DP) steels (DP 600, DP 800, and DP 980), a transformation-induced plasticity (TRIP) steel (TRIP 780), and three traditional steels (DQSK, AKDQ, and HSLA). The AHSS alloys showed time-dependent springback at room temperature. Similar to aluminum alloys, the early shape change was proportional to log time for the first few days to weeks, after which the rate of change was lower. The final time-dependent shape change of AHSS was approximately 1/3 of that observed for aluminum alloys under similar conditions. Finite element simulations of the draw-bending, subsequent springback, and post-springback shape change based on creep modeling for DP 600 were in good agreement with the experiments. Effective Young’s moduli for tensile unloading were measured and they decreased progressively for unloading from larger pre-strains. Springback simulations showed that Young’s moduli affect both initial springback, as well as time-dependent springback significantly. Deformation-induced heating is another complication for AHSS, with springback changing by up to 8% by its influence.