Mechanical anisotropy of ultra strong-and-ductile lamellar dual-phase steel

Abstract

Generally, high-strength metallic materials with large ductility are of technological importance in various engineering fields, including aerospace, fuel efficiency, and emission reduction. This work developed a new type of steel with exceptional yield strength (1.5∼2.0 GPa) and high total elongation (11∼26%) through a simple three-step process involving cold-rolling, warm-rolling and annealing treatments. Microstructural analysis revealed that the steel is composed of nano platelets made up of austenite and martensite. These platelets aggregate within elongated prior austenite grains, forming hierarchical lamellar structures. The study thoroughly investigated the anisotropic mechanical properties of the steel, particularly its strength and yield point behavior in rolling and transverse directions. The primary sources of strength were mainly originated from nano lamellae and high density of dislocations, while the anisotropic strengthening and yield point behavior was attributed to the textures and lamellar boundaries. With increasing warm-rolling reduction, the anisotropy level of dislocation strengthening gradually decreases, while the anisotropy level of boundary strengthening significantly increases. The discrepancy in yield point behavior along the rolling and transverse directions is ascribed to the differing number of boundaries per unit area of cross-section perpendicular to the tensile direction and the shear stress required for dislocation slip. This study provides a deep insight into understanding the anisotropy of strengthening and yield point behavior in nanolamellar steel so as to contribute to its future applications in engineering structural components.