No abstract

Dual-phase steels offer very attractive combinations of strength and ductility owing to the coexistence of different microstructures components and their interactions. These steels are suitable to the automotive industry due to their improved impact resistance increasing the passenger safety along with the vehicle weight reduction. The properties of the dual-phase steels are attributed to the chemical composition, type, size, amount and spatial distribution of different phases that can be obtained during thermomechanical treatments, namely, ferrite and martensite. In this work, the microstructure of as-received DP600 cold rolled steel sheet with 1.2 mm nominal thickness was firstly characterized by means of scanning electron microscopy technique. Then, a representative volume element was obtained from the DP600 microstructure and a micromechanical finite element model is proposed considering the steel chemical composition, average ferrite grain size, martensite volume fraction and mechanical properties of both ferrite and martensite phases. The uniaxial tension loading was simulated by assuming either plane-stress and plane-strain conditions. The numerical predictions corresponding to the plane-strain model are in good agreement with the experimental true stress-strain curve determined along the sheet rolling direction. The proposed finite element micromechanical approach based on the real microstructure proved to be an important tool to evaluate both local and overall behaviors of DP600 steel grade.

The properties of the dual-phase steels are attributed to the chemical composition, type, size, amount, and spatial distribution of different phases that can be obtained during thermomechanical treatments. In this way, modeling of the mechanical behavior of the dual-phase steel constituents, namely, ferrite and martensite, is crucial to the numerical simulation of sheet metal forming processes mainly to forecast the residual stresses per phase. In this work, the microstructure of as-received DP600 and DP800 cold rolled steel sheets with 1.2 mm nominal thickness were firstly characterized by means of scanning electron microscopy technique. The grain sizes and volume fractions of ferrite and martensite phases were obtained by means of digital image analysis. The Mori-Tanaka homogenization scheme was implemented in the finite element code ABAQUS assuming linear isotropic elasticity and isotropic work-hardening behavior for both ferrite (matrix) and martensite (inclusion) phases. The numerical predictions obtained with the Mori-Tanaka homogenization scheme for the macroscopic uniaxial tensile behavior are in good agreement with the experimental curves of both dual-phase steels.

The limit strains of dual-phase steels DP600 and 800 were evaluated in this work with a localization model formulated in plane-stress conditions using elasto-plastic constitutive equations. In this model, a geometrical imperfection parameter is defined from the sheet nominal thickness, initial ferrite grain size and average surface roughness. The proposed identification procedure provided a more physically meaning for this parameter and at best more conservative predictions in the drawing Forming Limit Curve (FLC) range of both investigated dual-phase steels. Nevertheless, the corresponding limit strains in the biaxial stretching region are underestimated with the present theoretical model. Thus, more detailed anisotropic yield function and hardening descriptions must be implemented to improve the accuracy of the FLC prediction of advanced high strength steels.