Abstract
The role of microstructural damage in controlling the edge stretchability of Complex-Phase (CP) and Dual-Phase (DP) steels was evaluated using hole tension experiments. The experiments considered a tensile specimen with a hole at the center of specimen that is either sheared (sheared edge condition) or drilled and then reamed (reamed edge condition). The damage mechanism and accumulation in the CP and DP steels were systematically characterized by interrupting the hole tension tests at different strain levels using scanning electron microscope (SEM) analysis and optical microscopy. Martensite cracking and decohesion of ferrite-martensite interfaces are the dominant nucleation mechanisms in the DP780. The primary source of void nucleation in the CP800 is nucleation at TiN particles, with secondary void formation at martensite/bainite interfaces near the failure strain. The rate of damage evolution is considerably higher for the sheared edge in contrast with the reamed edge since the shearing process alters the microstructure in the shear affected zone (SAZ) by introducing work-hardening and initial damage behind the sheared edge. The CP microstructures were shown to be less prone to shear-induced damage than the DP materials resulting in much higher sheared edge formability. Microstructural damage in the CP and DP steels was characterized to understand the interaction between microstructure, damage evolution and edge formability during edge stretching. An analytical model for void evolution and coalescence was developed and applied to predict the damage rate in these rather diverse microstructures.
Highlights
Advanced high strength steels (AHSS), in particular dual-phase (DP) steels, have seen widespread adoption for automotive body-in-white and chassis applications due to their high strength and good formability
DP steels derive their strength and ductility from a multiphase microstructure which comprises a high-strength martensitic phase within a soft ferritic matrix [1]. These steels exhibit good in-plane formability, as characterized by a forming limit diagram approach, for example. They are susceptible to premature failure during industrial stretch-flanging operations and often exhibit failure strains at the sheared edge that are below the forming limit strain [2,3]
plastic limit-load (PLL) model have been proposed is dueloading to necking failure of the inter-void ligament, which occurs transverse to the[34,50,51], principaland loading have been shown to give excellent agreement with unit cell computations
Summary
Advanced high strength steels (AHSS), in particular dual-phase (DP) steels, have seen widespread adoption for automotive body-in-white and chassis applications due to their high strength and good formability. Kumar et al [15] and Sudo et al [16] reported higher stretch-flangeability for ferritic-bainitic DP steels due to the lower relative hardness between the ferrite and bainite phases This difference in microstructural properties leads to variation in failure mechanism and alters the edge stretchability of a material. This correlation between damage parameters and sheared edge formability is important in order to quantify the relationship between microstructure and sheared edge stretching performance as well as to understand the interaction between residual shear strain field and damage evolution An investigation into these interrelationships is needed to contribute to future work to develop a suitable fracture model for finite element simulations of edge stretching
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