Low-carbon strip steels 31Hole expansion effects
Many pressings are made with holes to accommodate screws for fixing and the
forming operation often involves an increase in the size of the punched hole
after it is made. An example would be a wheel centre for a motor vehicle which
usually has four holes for securing the wheel in position. It is found that different
steels have a tendency to split as the hole is expanded. They are said, therefore, to
have different edge ductility. A hole expansion index has, therefore, been devised
to give a measure of the quality of the steel in this respect and represents the
percentage increase in the size of a hole at the moment that a crack forms.
It is found that the hole expansion index is often influenced by the presence of
elongated sulphide inclusions, but the index may easily be improved by reducing
the sulphur content and/or by making additions to the steel to modify the sulphide
shape. Steels with certain types of microstructure are, however, more prone to
cracking at cut edges than steels with other types of microstructure. Thus, for
a given strength, a ferrite- martensite structure gives a lower hole expansion
value than a ferrite - bainite structure.
Bending
Many components are made by a bending operation, for example simple brackets
or tubes and the minimum bend radius that may be produced without cracking
on the outside of the bend are often regarded as measures of the bendability of
the steel. Low-strength steels with high n values are clearly more bendable than
high-strength steels. It is found, however, that for a given strength, the minimum
bend radius is again influenced by the presence of elongated sulphide inclusions.
Recently, new applications have required that the bendability of ultra-high-
strength steel is optimized for tube-making processes. It was found 51 for three
steels that the elongation as measured in a tensile test increased, as expected, with
decreasing gauge length but that the steel with the lowest elongation, measured
over a gauge length of 50 mm, had the highest elongation measured over a
gauge length of 2 mm, as illustrated in Figure 1.30. The important observation
was that the minimum bend radius correlated well with the 2 mm elongation (the
local elongation) rather than with the 50 mm elongation. Other work showed
that a homogeneity index, defined as the standard deviation of hardness over a
specimen, correlated well with the local elongation (Figure 1.31) and confirmed
that the minimum bend radius correlated with the local elongation (Figure 1.32).
It was clear, therefore, that the best bendability is obtained from steels with the
most homogeneous microstructure with the least variation in hardness throughout
the steel. For the steels studied, there was no correlation between either the n
value or the total elongation measured using a 50 mm gauge length and the
minimum bend radius that would just avoid splitting.
Delayed fracture
Another phenomenon which may be important for certain applications is known
as delayed fracture. 53 This is caused by hydrogen absorbed during corrosion
and increases when the tensile strength is above 1170 N/mm 2. Delayed fracture