Abstract
Hot stamping components with tailored mechanical properties have excellent safety-related performance in the field of lightweight manufacturing. In this paper, the constitutive relation and damage evolution of bainite, martensite, and mixed bainite/martensite (B/M) phase were studied. Two-dimensional representative volume element (RVE) models were constructed according to microstructure characteristics. The constitutive relations of individual phases were defined based on the dislocation strengthening theory. Results showed that the damage initiation and evolution of martensite and bainite phases can well described by the Lou-Huh damage criterion (DF2015) determined by the hybrid experimental–numerical method. The calibrated damage parameters of each phase were applied to the numerical simulation, followed by the 2D RVE simulations of B/M phase under different stress states. To study the influence of martensite volume fraction (Vm) and distribution of damage evolution, the void nucleation and growth were evaluated by RVEs and verified by scanning electron microscope (SEM). Three types of void nucleation modes under different stress states were experimentally and numerically studied. The results showed that with the increase of Vm and varying martensite distribution, the nucleation location of voids move from bainite to martensite.
Highlights
Since ultra-high strength steel (UHSS) can save fuel consumption by reducing vehicle weight and improving collision performance, it has been widely used in the automobile industry
The results indicate that the void nucleation in bainite is caused by high strain
Constitutive equations of bainite and martensite phase were obtained by the reverse engineering approach
Summary
Since ultra-high strength steel (UHSS) can save fuel consumption by reducing vehicle weight and improving collision performance, it has been widely used in the automobile industry. Due to the increasing demand for high strength body-inwhite components, the hot stamping process has been spread widely [1]. During the hot stamping process, the boron steel is heated to a temperature above the austenitizing temperature, maintained and transferred to a press, where a cold die is used to obtain parts with “customized” mechanical properties. Maikranzvalentin et al [5] proposed a hot stamping strategy using differential heating and cooling combined with thermal–mechanical integration processes for customized microstructure design, so as to obtain complex parts with expected performance. In order to tailor the mechanical properties of components, Liang et al [6] used resistance heaters in the hot stamping tools, where the tools can be classified into heated zone, transition zone, and unheated zone. By controlling the heating and cooling rate in segmented tools, George et al [7] obtained lab-scale B-pillars with customized performance
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