Plasticity and structure evolution of ferrite and martensite in DP 1180 during tension and cyclic bending under tension to large strains

Abstract

This paper describes the main results obtained from a detailed experimental investigation into the dislocation and twinned structures that develop in dual phase (DP) steel sheets. In particular, the morphology and defect-structures present in DP 1180 steel having 55% ferrite and 45% martensite, were examined using transmission electron microscopy (TEM). Microstructure of the as-received DP 1180 consists of a twin-free martensite phase in the form of lath and island morphologies and a ferrite phase exhibiting a relatively low dislocation content (3.41 × 108 cm−2). The steel was deformed in simple tension (ST) to fracture and in continuous-bending-under-tension (CBT) to six cycles. As the CBT test facilitates stretching of the sheet considerably beyond the point of necking, strength increases and substantially exceeds that achieved in ST at fracture. As a consequence of plastic deformation, nano-twins are found in the island-shaped martensite, while the lath-shaped martensite remains twin-free. Additionally, some martensite regions develop a needle-like morphology after CBT. As dislocation density in the ferrite matrix increases with plastic strain, dislocation structures and accumulations of dislocations near the ferrite/martensite interface develop in the ferrite. Unlike in the as-received and ST-deformed DP 1180 samples, high density of dislocations (1.47 × 109 cm−2) and dislocation tangles are observed in ferrite regions of the CBT processed sample. We propose that the primary mechanisms enabling the achievement of high strength, while maintaining residual ductility upon CBT are intense plastic slip in the ferrite and profuse nano-twinning in the martensite regions.