Hierarchical strain distribution analysis formed in DP steel using a combination of metallographic image and digital image correlation method

For a better understanding the relation between the heterogeneous microscopic strain caused by metallographic microstructure and the macroscopic strain concentration responsible for ductile fracture, digital image correlation (DIC) method was applied to the digital images obtained by a scanning electron microscope (SEM) to analyze a strain distribution in a low carbon dual-phase steel in multiple scales. As well as grain boundaries, a substructure in martensite grains and a corrosion shade in ferrite matrix were clearly observed in SEM image and they enhance the random nature of SEM digital image. Therefore, the combination of SEM image and DIC method (SEM-DIC method) is able to evaluate a heterogeneous strain distribution in metal in multiple scales. The development of strain distribution looks different depending on the magnification of SEM observation due to the difference in pixel-dependent spatial resolution; micro-scale strain map indicated that strain is localized in ferrite matrix, while macro-scale strain map revealed that strain seems to exist over a width of tensile specimen. This result suggests that macroscopic strain concentration is accompanied by a percolation of microscopic localized strains. In addition, the strain in martensite grains is increased considerably as the hardness of martensite is reduced by tempering treatment, which leads to the reduction in the strain gap between ferrite and martensite. As a result, the developments of not only microscopic localized strain but also macroscopic strain concentration are retarded, leading to the significant recover of ductility in DP steel.