Stamping and die designers are interested in the forming capabilities of the steels they specify. Complex stampings are created from several different basic forming modes, which are sensitive to different mechanical properties. For this reason, steel formability, especially for AHSS grades, cannot be characterized by a single number, but instead must be done for each basic forming mode.
Forming modes and key points include:
- Stretching is the sheet metal forming process where the punch which creates the part shape forces the sheet metal to thin since lock beads prevent metal flow inward from the flange area.
- The steel property that improves stretching is the strain hardening exponent, or n-value.
- Dome testing characterizes stretchability.
- Higher n-values flatten strain gradients, reduce localized thinning that leads to early failure, and allows the forming of more complex parts.
- Tensile testing cannot be used to determine bendability, since these are different failure modes. Different bend tests characterize bendability.
- The failure strain in the bend is related to the total elongation of conventional steel, but are limited by other issues in AHSS grades.
- For equal strengths, most AHSS grades have higher total elongations than conventional HSLA steels. However, several AHSS grades have limited local formability based on their microstructure, and may be at risk for cracking during edge expansion. AHSS grades with multiphase microstructures such as DP and TRIP experience shear fracture that severely reduces the bendability before failure occurs.
- Drawing is the sheet metal forming process where the punch that creates the part shape forces the sheet metal to pull in from the flange area.
- The steel property that improves cup drawing or radial drawing is the normal anisotropy or rm value.
- The Limiting Draw Ratio (LDR) characterizes cup drawability.
- Higher r-value increases the LDR, but the absolute value of the LDR also depends on the lubrication, blank holder load, die radius and other system inputs.