Microstructure-property relationship in cold rolled complex phase steel

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In complex phase (CP) steels, partially bainitic microstructure smooths the property gradient between soft ferrite and hard martensite phases, meaning that CP steels exhibit better local formability and hole expansion tolerance. In this study, in order to investigate the influence of microstructure constituents (ferrite, bainite, and martensite) on local plasticity behavior, microstructure characterization and micromechanical characterization were performed and coupled on a cold rolled steel CP800. First, different microscopic techniques (light optical microscopy (LOM), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and transmission electron microscopy (TEM)) were used to identify the microstructure constituents. A criterion for segmenting microstructure components, based on electron backscatter diffraction (EBSD) and EPMA measurements, was next proposed. Second, we implemented a novel method employing SEM-based microscopic digital image correlation (μDIC), together with ex-situ mechanical testing. This method enables the joint high-resolution mapping of deformation-induced micro-strain evolution. Furthermore, the prior austenite grain (PAG) was reconstructed from CP800, in order to achieve understanding of slip-based deformation mechanisms within bainite phase. The final digitized microstructure constituent map was correlated with a micro-strain analysis. The strain partitioning behavior between microstructure constituents, as well as between different interfaces, in complex phase CP800 during deformation was intensively examined and precisely quantified.