Commercial dual phase steel (DP 800) supplied by Swedish
Steel AB (SSAB) in the form of rolled sheet with a thickness
of 2mm from industry regular production has been investigated.
The chemical composition of the material studied is
listed in Table 1. The microstructure consists of ferrite (F)
grains and martensite (M) islands together with some percent
of retained austenite (A), as shown in Fig. 1. The volume fraction
of M is roughly 0.33. In this study, deformation to 3.5% at
room temperature at a strain rate of 10−4 s−1 and subsequent
heat treatment at 180 ◦C, 30min in air were purposely used to
simulate pressing of plates followed by the paint-bake cycle
involved in the manufacturing process of automobile body
structures. Prestraining to 3.5% was used since it corresponds
to a strain level where approximately half of the hardening
capacity up to the instability point has been exhausted and
also may represent a situation where the paint-baking effect
could be favorably used. Symbols AR, PS and BH are used here
to represent as-received, pre-strained, as well as pre-strained
and bake hardened conditions respectively