Steels_ Metallurgy and Applications, Third Edition

Low-carbon strip steels 117

Figure 1.116 Costs of different lightweight construction methods lsl

The micro-alloyed steels mentioned earlier were suitable for use for relatively
simple structural parts, but their reduced formability and increased springback
compared with lower-strength mild steel prevented their use for the large body
panels, which in general involved difficult forming operations. In addition, the
formability of any steel decreases with decreasing thickness as indicated by
the lower plane strain intercept on the forming limit diagram. Any decrease
in thickness, therefore, only exacerbates the loss in formability that arises from
the increase in strength.
The reaction of the automobile industry world-wide was to request steels with
a relatively small increase in strength over mild steel, with the loss in formability
reduced to a minimum while still retaining high r values. The first of these steels
were the rephosphorized steels which, as indicated previously, retained higher
r values and higher elongation values than would have been obtained from any
micro-alloyed steel of similar strength. Steels with minimum proof stresses of
180, 220, 260 and 300 N/mm 2 have been available in Europe, though the grades
with the lower strength are the ones which have been more commonly used. The
early rephosphorized steels were batch annealed and based on an aluminium-
killed composition, but a more recent trend has been to anneal different types of
steel continuously, either on a continuous annealing line or on a hot dip galva-
nizing line. The hot dip-coated steels, usually with a galvanneal coating for the
motor industry, tend to be based on an IF-type composition to be more compat-
ible with a continuous annealing cycle. These steels, as mentioned previously,
may also be solid solution strengthened with manganese, boron or silicon as
well as a small phosphorus addition to reduce problems that would otherwise be
associated with high phosphorus levels.
The solid solution-strengthened steels made by alloying either an aluminium-
killed or an IF base composition are sufficiently formable to enable difficult body
panels to be produced without local necking or splitting, but the pressing has still
been subject to greater springback than lower-strength mild steel. The trend has,
therefore, been to use bake-hardening steels. These steels usually have a minimum
bake-hardening index of 40 N/mm 2. Very approximately, therefore, it is possible
with a bake-hardening steel to achieve the same formability and spfingback as