Low-carbon strip steels 67The minimum levels of bake-hardening index that are usually considered to
provide a useful benefit are 30 or 40 N/ram 2. An increase in yield stress of
about 40 N/mm 2, for example, would compensate for a reduction in gauge of
about 0.1 mm to give the same dent resistance, ll3 The maximum levels of bake
hardening that are commonly used are 50 or 60 N/ram 2. Higher indices than
these would lead to excessive room temperature strain ageing with the result that
pressings would be subject to stretcher strain defects on forming. Recently, it
has been indicated, ll4 however, that a sufficiently low and satisfactory degree
of strain ageing is obtained for exposed parts when the bake-hardening index
is below 50 N/mm 2. Bake-hardening indices above 50 N/mm 2 for exposed parts
were, therefore, regarded as suitable only for just-in-time delivery before forming.
Figure 1.73 gives an example of the return of yield point elongation for steel with
different bake-hardening indices after an accelerated strain-ageing test for one
hour at 100*C. A yield point elongation of just 0.2% was regarded as satisfactory
if stretcher strain markings are to be avoided, ll3
The earliest type of bake-hardening steel available was rimming steel which
acquired its bake-hardening tendency as a result of free nitrogen in solution.
These steels were, however, subject to high levels of room temperature strain
ageing with the result that forming without stretcher strain markings could only
be guaranteed for a short period. With the widespread use of continuous casting,
nitrogen is usually combined as stable aluminium nitride. The bake hardening
must, therefore, be obtained from solute carbon. Consequently, the chemistry and
processing of the steel must be manipulated to leave sufficient carbon in solution
after annealing. It is usually accepted that the ideal level of solute of carbon is
in the range 5-20 ppm, but certain workers regard a figure low in this range as
essential whereas others indicate an optimum figure high in 'the range. The ideal
figure depends on the type of steel and, in particular, its grain size since it is
well known that bake hardening depends on grain size as well as on the solute
carbon or nitrogen content. ~5
Bake-hardening steels may be processed by batch annealing or by continuous
annealing including continuous annealing on galvanizing or aluminizing lines.
4 ....... , o
o~- 0 o 0
._~ 2-
., foO
'j.o
~ 0. , .! , I .. , t
0 2 4 6 8 10 12
BH (kgf/mm 2)Figure 1.73 Relationship between bake hardenability and YP-EI caused by strain ageing
(After Takechi, H. l l3)