and precipitation hardening can contribute to the increase
of the strength in this material. Bake-hardenable steels must
contain a certain minimum amount of solute carbon. During
the heat-treatment at 180 ◦C, interstitial C diffuses to the
dislocations produced during pre-straining, resulting in both
a discontinuous yield point and strengthening. According to
Hundy [10], dissolved C and N contents of about 0.002 wt% are
sufficient to lock the dislocations. The intercritical annealing
used in producing this DP steel leads to considerably higher
carbon content in the ferritic phase. After completion of the
first stage in which the atmosphere locking develops, carbon
clusters or carbides precipitate in ferrite and martensite during
the annealing [7,9,10] which further increase the resistance
to the movement offree dislocations. Even though softening of
martensite by tempering starts at the annealing temperature
used (+180 ◦C), it is counteracted by the carbide formation that
causes secondary hardening. The formation of fine iron carbides
in martensite after bake hardening has been observed
in TEM investigations [11]. In fact, carbide formation is a continuous
process starting from the formation of carbon clusters
as precursors followed by a compositional evolution and size
growth. Therefore, the carbide formation is more sluggish
than carbon diffusion. The yielding phenomenon observed
after prestraining in Fig. 2a can be explained as an aging effect
at ambient temperature which is less effective and caused
mainly by Cottrell atmospheres. It has been reported that a
clear yielding point occurs after aging in air only for 30min in
microalloyed dual phase steels [12]. The presence of fine C-rich
clusters with sizes ranging from 0.7 to 2.5nm was detected by
atom probe tomography in the martensite after 4% prestraining
before bake hardening [