The forming myth
Why elongation is not a good measure of formability when it comes to advanced high strength steel.
Let’s begin with a question:
Is it possible to form complex components from steel with a tensile strength of 1400 MPa that displays only 3-4 percent elongation?
The correct answer, while maybe surprising to some, is yes – and here is why. Elongation is not equal to formability when it comes to advanced high strength steel (AHSS) and ultra-high strength steel (UHSS).
The misconception that the formability of a steel is measured solely by its elongation value is a belief that, unfortunately, is still held by some manufacturers. The result is many missed opportunities to upgrade automotive components as well as high strength steel that is not being utilized to its full potential. Upon closer inspection, it is easy to see how the myth emerged.
It all began with the elongation test, which is performed by simply pulling apart a sample of steel until it breaks. The deformation that occurs before failure is then measured as a percentage – the elongation value. Originally, this test was developed when traditional, mild steels were most common, before the emergence of high strength steels.
When performing the test on mild steels, it is true that their formability is very closely linked to their elongation, as the deformation occurs consistently over 80 mm. However, due to their complex microstructures, AHSS and UHSS do not behave the same way – and measuring elongation over 80 mm does not give an accurate forming value.
Lars Troive, Forming Expert at SSAB explains:
“Traditional, classic grades of mild steel have more of a global deformation behavior. While an advanced high strength steel grade may display an elongation of only three or four percent, this is measured over 80mm, whereas, much more deformation occurs locally.”
A good way to measure the actual forming limit of AHSS and UHSS is by creating a 2 by 2 mm grid on a tensile test specimen and looking at the area close to the failure area after the test. For example, if we look at a tensile test performed on the advanced high strength steel Docol® 1000DP, we see that the deformation is concentrated to the area where the failure occurred.
“By just looking at a distance of 2 mm and measuring how much the grid box is deformed, it can be seen that it has been elongated by 20%. The elongation over 80 mm, on the other hand, is only 10%. And when forming a detail, almost all deformation occurs locally. Therefore, a conventional tensile test measuring the elongation over a distance of 80 mm does not say anything about how formable a high strength steel is,” says Troive.