Steels_ Metallurgy and Applications, Third Edition

Low-carbon strip steels 29

Major

strain

deformation

...... I I , .I., ;" i

Compressive

9 ,,, , , | ,|, ,|,l i|

m

~safe

~strain

l interc~:)t

-. , ,, , +_, ,Minor

0 Strain

,~ Tensile

Plane

strain

Figure 1.28 Forming limit diagram

there would be the risk that some pressings would be unsatisfactory. It would
then be necessary to adjust the press, modify the lubrication, modify the blank
shape, clean or modify the tools or use a more formable steel, to ensure that the
pressings would become consistently satisfactory. Clearly modifying the tools
would only be an option during the development and die try-out stages of a
pressing before the design itself has become fixed or if the overall effect does
not change panel shape.
The forming limit diagram itself may be established by subjecting a series
of samples of the material to be used to a range of forming tests to give failure
involving different ratios of major and minor strains, including negative ones. The
strains close to the failure points are measured using the grid marking technique
and plotted on the diagram in the normal way to establish the forming limit line
itself.
It is useful to note, as illustrated in Figure 1.29, that the plane strain intercept
on the major strain axis of a forming limit diagram is a function of the sheet
thickness and the work-hardening coefficient n. Thus higher-strength steels with
lower n values have a lower forming limit curve than lower-strength steels. It has
also been found that the presence of macro inclusions reduces the forming limit. 49
In addition, fine elongated sulphide inclusions may also reduce the forming limit,
but a chemical addition to modify the inclusion shape to make them globular may
restore the forming limit to a higher level. 5~

Other forming effects

The above remarks have concentrated primarily on the factors that influence the

possibility of local necking or splitting during the formation of any pressing. It

should be remembered, however, that there are other factors that may lead to

a pressing being unsatisfactory. These include wrinkling, hole expansion effects

and constancy of shape, sometimes known as shape fixability which is related to

springback.