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.