Steels_ Metallurgy and Applications, Third Edition

26 Steels: Metallurgy and Applications

40

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n value

Figure 1.24 A plot of total and uniform elongation versus n value for several cold-rolled,
high-strength steels continuously annealed on an experimental line using a range of
annealing cycles (without temper rolling)

There is a close relationship between the n value of the steel and both the
uniform and the total elongation measured in a tensile test. This relationship is
illustrated for several types of high-strength steel in Figure 1.24. It is useful to
note that if the n value were to be truly constant throughout a tensile test, the n
value would be numerically equal to the true strain at maximum load which is
the uniform elongation.
The most important factors that influence the n value of a steel are its strength
and the strengthening mechanisms used to develop the strength. This is illustrated
in Figure 1.25. It is seen that for each type of steel the n value decreases with
increasing strength and that, for example, solid solution strengthening gives a
lower loss in n value per unit increase in strength than strengthening by precipi-
tation effects and grain refinement. It is clear that the highest possible n value is
obtained when the strength of any type of steel is as low as possible and this is
the reason why low-strength mild steels are always more formable than higher-
strength steels. As mentioned previously, much emphasis has been given during
the development of higher-strength steels to choosing or developing strength-
ening mechanisms that give the highest possible n value (and hence elongation)
for the strength needed.
The n value is important because it is the second of the two main material
parameters that determine the distribution of strain across any pressing. In general,
a high n value for a steel leads to a more uniform strain distribution than a low
n value steel. A higher n value enables the forming strains, therefore, to be
distributed across a pressing with less likelihood of local necking and failure
than for a low n value.
The strain distributions obtained across a simple hemispherical bulge may be
used to illustrate the effects of n value on strain distribution. Figure 1.26 shows