Time-dependent springback of advanced high strength steels

• Published in: International journal of plasticity Vol. 29; pp. 42 - 59
• Main Authors: Lim, H., Lee, M.G., Sung, J.H., Kim, J.H., Wagoner, R.H.
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier Ltd 01.02.2012; Elsevier
• Subjects: A. Creep; A. Stress relaxation; Alloys; Aluminum base alloys; Applied sciences; B. Residual stress; Computer simulation; Condensed matter: structure, mechanical and thermal properties; Creep; Exact sciences and technology; Forming; Fracture mechanics (crack, fatigue, damage...); Fundamental areas of phenomenology (including applications); High strength steels; Inelasticity (thermoplasticity, viscoplasticity...); Mechanical and acoustical properties of condensed matter; Mechanical properties of solids; Metals. Metallurgy; Modulus of elasticity; Other forming methods; Physics; Production techniques; Solid mechanics; Springback; Steels; Structural and continuum mechanics; TRIP steels; A. Stress relaxation; B. Residual stress; A. Creep; Springback; Elastic constant; Geometrical shape; Creep; Deformation modulus; High strength steel; Rupture; Forming; Car body; Ultimate limit; Aluminium alloy; Experimental study; Low alloy steel; Modeling; Tensile strength; Young modulus; Finite element method; Yield strength; Effective medium model; Dual phase steel; Residual stress
• ISSN: 0749-6419; 1879-2154
• DOI: 10.1016/j.ijplas.2011.07.008

► Time-dependent springback occurs in advanced high-strength steels (AHSS), not in traditional steels. ► The time-dependent springback of AHSS is approximately one-third of that for aluminum alloys. ► The principal mechanism of time-dependent springback is creep driven by residual stresses after forming. ► FE simulations of time-dependent springback show good qualitative agreement with draw-bend experiments. ► Time-dependent springback is affected by the modulus effect and deformation-induced heating. 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.