Steels_ Metallurgy and Applications, Third Edition

30 Steels: Metallurgy and Applications

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Strain hardening exponent n

rasing shoot

~rtess

Figure 1.29 Relationships between the plane strain intercept on a forming limit diagram
and the strain hardening exponent as a function of sheet thickness

Springback

Springback occurs on forming whenever the component is withdrawn from the
tool set. The forces used to make the pressing are then no longer applied and the
pressing relaxes in an elastic way to minimize its residual internal stresses. In
general, the plastic strains introduced into many pressings are not uniform over
the pressing. The result is that the elastic relaxation is not uniform and the shape
of the pressing will not be exactly the same as the shape of the punch that has
been used in its manufacture. The change in shape is known as springback and it
clearly varies widely for different pressings. The most important material factor
affecting springback is the yield stress, but gauge and work-hardening coefficient
also have an influence. It is clearly important that all examples of the same
pressing should have a uniform shape in order that they fit together easily with
other pressings when they are assembled into complete structures. It is important,
therefore, that each sheet of the material used should have closely the same yield
stress, gauge and work-hardening coefficient. Press conditions may, however, be
adjusted to minimize the effect of any property variation.

Buckling, distortion and wrinkling

Sometimes, when there are large strain gradients across a pressing or when there
is some other critical condition, the transfer of stress across a pressing may cause
buckling, or the relaxation of the elastic stresses introduced during forming may
cause distortion. The extent of these effects is very dependent on the nature of
the pressing.
Wrinkling consists of corrugations produced mainly near the flanges of a
pressing when the material is subject to in-plane compression. Wrinkling would
make a pressing unsuitable for any exposed applications, but the existence of
wrinkling on internal flanges makes the process of spot welding difficult. Wrin-
kling may be controlled by adjusting press conditions but the main material
parameters that affect the tendency are yield stress and gauge. The tendency for
wrinlding increases with increasing yield stress and decreasing gauge.