The forming characteristics of sheet metal materials are substantially determined with the help of test methods such as the tensile test and the forming limit curve (FLC). These tests evaluate the base material's formability. It is not always possible to make statements on the formability of a sheet edge manufactured by shearing. This is because the shearing process can damage the cut edge and result in a significant reduction in the forming capability, particularly in higher strength materials in the edge area. A material that reacts sensitively to previous damage at the edge is said to be susceptible to edge cracks. In the research environment, there are a number of test methods used to evaluate the susceptibility to edge cracks. The only standardized test method at this time is the hole expansion test as defined by ISO 16630.
In order to meet the challenge of the reduced edge forming capability caused by shearing in the material, Salzgitter Flachstahl has enhanced and systematically optimized its multiphase steels in this regard. This multiphase steel generation has the term xpand® added to its name. The name xpand® is based on the Latin word expandere (= extend, expand) and stands for the steels' guaranteed hole-expansion ratio. A material's hole-expansion ratio λ indicates the material's resistance to crack propagation and is a measure of its edge crack sensitivity and edge formability. This characteristic is determined in accordance with the ISO 16630 standard. In this test procedure, a hole with a punching die diameter of dp = 10 mm and a relative cutting clearance of 12% is sheared into the sheet sample. This hole is then expanded with a conical die. As soon as a crack that runs through the entire thickness of the plate can be seen, the expansion is stopped and the resulting hole diameter Dh is determined. The hole-expansion ratio is then defined as the ratio between the increase in the hole diameter (Dh - D0) and the original hole diameter D0 (Figure 1). To determine the material characteristics, the test is repeated three times and the average hole-expansion ratio is used. The steels in the xpand® family have optimal characteristics for forming sheared plate edges and consequently allow an expanded product range at the customer. This advantage can be seen, for example, in the "collar forming" manufacturing procedure where the collar can be higher than the standard possible value.
You can find the individual xpand® steels from Salzgitter Flachstahl here.