On the lüder's strain of static strain aged dual phase steel

Dual-phase steel is characterized by an interesting combination of high strength, good ductility, continuous yielding, high initial work hardening rates, and a low yield stress to tensile strength ratio. The changes in the yield stress and appearance of prolonged lüder's strain in the stress strain curves of static strain aged dual phase steel motivate studying the parameters affecting the formation and width of the lüder's strain as function of aging conditions. Static strain aging of fine grained (5 μm) CMn-CrNb dual-phase (DP700) ferrite-martensite steel was investigated. A 3.2 mm sheet thickness of dual phase steel of martensite volume fraction of 0.306 was used to examine the aging behavior at the temperature range of 100 to 500°C for aging times 5 to 180 minutes. Quasi-static tensile tests and optical microstructure investigation were performed on the aged and the as-deformed dual phase steel. The stress strain curves were determined and analyzed to detect the condition of onset of the lüder's strain. A constant aging time of 30 minutes at different temperatures has showed the onset Lüder's strain to be at 190°C while, a 60 minutes aging at 170°C was needed for the onset of the Lüder's strain. The extent of Lüder's strain was observed to be time dependant for the various applied aging temperatures. Lüder's plateau was characterized by a serrated flow within the Lüder's strain and has been correlated to dynamic micro-mechanisms of hardening and softening.