Microstructures and Flow Behavior of Ductibor 500-AS Steel for a Range of As-Quenched Conditions

Ductibor 500-AS steel is used in hot-stamping applications within tailor-welded blanks to introduce more ductile, energy-absorbing regions within ultra-high strength structural components. This study aimed to explore the sensitivity of the microstructure and mechanical properties of Ductibor 500-AS to changes in cooling rate. For this purpose, the microstructures and hardening behavior of Ductibor 500-AS steel sheet were examined for a range of quench conditions, including austenitization followed by air, die, or water quenching. Optical and scanning electron microscopy investigations revealed a wide range of microstructures, comprising either mostly ferritic, mixed ferritic-martensitic, or mostly martensitic with some tempered martensite/bainite. Tensile and shear tests conducted on the as-quenched samples exhibited a high degree of sensitivity to cooling rate as well as strong mechanical anisotropy. The flow curves of the different material conditions obtained using a shear-conversion technique and fit to a modified Hockett-Sherby model were validated using the finite-element method (FEM) simulations of the uniaxial-tension tests in LS-DYNA, considering isotropic von Mises and/or anisotropic Barlat Yld2000 yield criteria. The predicted load-displacement response and strain localization patterns using the isotropic and anisotropic models were shown to agree with the measured results. However, only the anisotropic model was able to predict the local strain histories consistent with the experiments. Consequently, it is important to account for both of the quench-rate sensitivity and plastic anisotropy of Ductibor 500-AS in crash-safety simulations of structural components utilizing this steel.