Characterization and Modeling of Deformation, Springback, and Failure in Advanced High Strength Steels (AHSSs)

New generations of advanced high strength steels (AHSSs), expected with tension strength exceeding 1000 MPa, sufficient cold-stamping ductility, and low-alloyed microstructure, are being highly sought for automotive lightweighting applications without compromising on neither performance nor cost standards. From the standpoint of metallurgy, the most straightforward solution is to realize complex microstructures in the new AHSSs with significant fractions of the strongest ferrous phase, martensite, in combination with the most ductile phase, austenite. Nevertheless, the preliminary-developed new AHSSs exhibit unique and complex deformation, failure, and springback characteristics compared with the conventional steel grades, which thereby bring significant challenges to the current manufacturing infrastructure in automotive sector. To thoroughly investigate this uniqueness and complexity, this research work started from a full set of mechanical experimentation on some target AHSSs to comprehensively understand their deformation, springback, and failure. In the selected AHSSs, in particular, a 980 MPa grade quenched and partitioned (QP980) steel, with tri-phase microstructure of retained austenite, martensite, and ferrite, was primarily investigated, since it would be a baseline material of the developing AHSS grades. Another highlight is that, during the entire experimental work, a stereo digital image correlation (DIC) system was widely employed to not only in-situ accurately measure the full-field material deformation and displacement, but also control the loading directions when necessary. Furthermore, based on the experimental results, a new phenomenological model was proposed to properly characterize not only anisotropy, complex hardening induced by the Bauschinger effect, but also tension-compression asymmetry and the transformation-induced plasticity (TRIP) effect. Then this new model was implemented via user material subroutine in LS-DYNA. Last but not the least, this work eventually wrapped up with a case study of component-level finite element forming and springback predictions compared with the corresponding actual panels, as an implementation and verification of the previous experimentation and modeling