Page 43 - MetalForming June 2019

limit of successful forming in most low- carbon steels can be determined via a shortcut where the lowest point on the FLC, FLC0, can be calculated as a func- tion of only sheet thickness and the strain-hardening exponent, n-value, measured in a tensile test.
Instead of calculating FLC0 based on test properties of the evaluated try- out coil, use the minimum-allowed part thickness and the minimum n- value associated with the chosen grade. This worst-case forming limit repre- sents the lowest strains that result in necking failure on any shipment received within the grade specification. If the measured strains on the part fall below this threshold, stamping success should be independent of tensile- property variability.
Changing the steel vendor may lead to forming differences. Each company will supply a product that meets the
specification, but likely occupies a dif- ferent range within the allowable tol- erance. Use the grade’s minimum n- value and minimum part thickness to reduce the effect of different suppliers on forming conditions.
This approach cannot be used with alloys where the FLC does not pre- dictably depend on mechanical prop- erties measured in a tensile test. Here, use a larger marginal zone to account for the influence that sheet metal prop- erty variation has on forming behavior. Standard guidance does not exist, so work with your material supplier regarding your specific challenges.
Low-End Properties?
It is easy to plug in grade-minimum n-value into the FLC0 formula, but the task becomes more complex when work- ing with forming-simulation programs that require inputs of multiple tensile-
test parameters. Some specifications list minimum-allowed values for yield strength, tensile strength, total elonga- tion, n-value and r-value. Inexperienced simulation analysts may use these min- imum values for convenience to project worst-case conditions. Reality is not that neat. Low yield strength usually can be found on products with high elongation and n-value. Complicating matters, most specifications do not contain maximum allowed values of elongation, n-value and r-value. Choose simulation inputs to reflect realistic combinations of formability parameters.
Want to learn more about different sheet metals and formability? Plan to attend PMA’s Sheetmetal Formability of Steel, Aluminum and Stainless Steels seminar in Grand Rapids, MI, June 11- 12. Visit www.pma.org to register or con- tact Marianne Sichi, msichi@pma.org for information. MF
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MetalForming/June 2019 41
The Science of Forming