shows engineering tensile curves at different strain
rates and temperatures for the studied DP steel. Although the
as-received variant (AR) exhibits continuous yielding behavior,
pre-straining (PS) and superimposed bake hardening (BH)
result in reappearance of a distinct yield point. Rather flat
flow curves are then obtained, indicating a relatively small
but approximately linear strain hardening beyond the yielding
point. Increased flow stresses are observed at increased
strain rates and/or decreased temperatures. This is consistent
with general knowledge on the role ofthermal activation upon
straining. The only exception is the deformation at 100 ◦C. At
this temperature there is a cross point of the two curves with
strain rates 10−4 s−1 and 10−1 s−1 respectively, indicating a
gradual change from positive to negative strain rate sensitivity
with increasing strain. Clearly, an enhanced work hardening
rate is obtained for low strain rates at this temperature. It is
also interesting to notice the serrated stress–strain curve atthe
initial stage of plastic deformation, which is an indication of
dynamic strain aging under this condition, as shown in Fig. 2.
The effect of the PS and PS + BH treatment on the yield
and tensile strengths under different testing conditions is displayed
in Fig. 3. Compared to the as-received material, PS and
PS + BH treatment increase both Rm and Rp0.2 at ambient temperature,
+20 ◦C. This is also true for the temperatures −60 ◦C
and 100 ◦C. The yield strength is generally much more affected
than is the ultimate tensile strength. Table 2 shows the strain
hardening ratio Rm/Rp0.2, often used as a single parameter to
describe strain hardening or formability in engineering terms.
The data indicate that strain hardening capacity is strongly
reduced by preforming and bake hardening, similarly for all
temperatures and strain rates investigated.
The flow stress increase due to the PS + BH treatment is
often named bake hardening arising from an aging process in
the deformed material. Both Cottrell locking of dislocation