Proportional and non-proportional hardening behavior of dual-phase steels

The elastic–plastic response of sheet materials during non-proportional paths is seldom incorporated in constitutive equations used for routine sheet forming simulation, but can have a significant effect on formability and springback. Monotonic tension and compression, coaxial tension–compression (T–C), coaxial compression–tension (C–T), and two-stage/non-coaxial tensile tests were performed for three grades of dual phase steels: DP590, DP780, and DP980. The reverse flow curves have three characteristics: reduced yield stress (Bauschinger effect), rapid transient strain hardening over a few percent strain, and long-term or “permanent” softening. The departure of the reverse hardening curves from monotonic ones is larger than with other typical sheet forming alloys, presumably because of the effects of large second-phase martensite particles in dual-phase steels.