Low.carbon strip steels 19the initial decrease in the (111) components is compensated for by a subse-
quent increase especially in the (111)[112] component during the later stages of
recrystallization. 37 The influence of the dissolved carbon is mainly to increase the
proportion of the minor components with a consequent decrease in the propor-
tion of the important (111) y fibre components that lead to good drawability.
The higher carbon content is also accompanied by a finer grain size. 37 Nitrogen
in solution also leads to low r values a fine grain size. The effect of nitrogen in
solution is, therefore, similar to the effect of carbon in solution. 2~ It is usually
necessary to ensure that the solute carbon and nitrogen is reduced to a low level
in the hot band structure and during recrystallization itself, if high r values are
to be obtained by cold rolling and annealing. The development of high r values
in batch-annealed, aluminium-killed steel, however, as further discussed below,
has a different requirement.
Cold.forming behaviour
Cold formability and strength, as indicated above, represent the two most impor-
tant requirements for low-carbon strip grade steels. For many applications, the
main requirement is to be able to form the part without splitting, necking or
wrinkling. The most suitable steel, therefore, is one with a low strength and high
formability. For structural applications, the strength of the steel is more important
and must be above a given minimum value. It is found, however, that there is
a general tendency for the cold formability of any type of steel to reduce as the
strength increases. The reduced formability of higher-strength steels tends, there-
fore, to limit their use to those applications which do not require the very highest
formability. Many of the developments of higher-strength steels, therefore, have
been specifically aimed at providing higher strength while minimizing the loss
in formability that would otherwise have taken place.
It is now generally accepted that, for many applications, the cold formability
of sheet steel may be resolved into two separate but related components, namely
its drawability and its stretchability. The drawability of a steel is its ability to
be drawn in to make a component without local necking or splitting whereas the
stretchability of a steel is its ability to be stretched to form a component, again
without local necking or splitting. Stretching involves major and minor strains
in the plane of the sheet that are both positive, whereas drawing involves major
and minor strains, one of which is positive and one of which is negative. Many
applications, however, involve both drawing and stretching. Success in forming
for such applications involves, therefore, the factors which influence both drawing
and stretching.
A simple measure of the deep drawability of a steel may be obtained by
forming fiat-bottomed cylindrical cups from circular blanks, as illustrated in
Figure 1.16(a). The maximum ratio between the circular blank diameter and the
punch diameter that may be drawn in a single stage to form a cylindrical cup
without necking or splitting is a measure of the drawability of the steel. This
ratio is called the limiting drawing ratio and may vary up to about 2.5 for very
good deep-drawing steels.