Advanced Hight-Strength Steels (AHSS) exhibit high work hardening rates, resulting in improved forming capabilities compared to conventional HSLA. However, the same high work hardening creates higher strength and hardness in sheared or punched edges, creating susceptibility to localized strain conditions. In addition, laser cutting samples will also lead to highly localized strength and hardness increases in the cut edge. In general, AHSS can be more sensitive to edge condition because of their higher strength. Therefore, it is important to obtain a good quality edge during the cutting operation. With a good edge, both sheared and laser cut processes can be used to provide adequate formability.
To avoid unexpected problems during a program launch, production intent tooling should be used as early in the development as possible. For example, switching to a sheared edge from a laser-cut edge may lead to problems if the lower ductility, usually associated with a sheared edge, is not accounted for during development.
Trim Blade Design & Blanking Clearances
Cut, sheared, punched or trimmed sheet metal edges have reduced stretchability due to localized work hardening. This work hardened zone can extend one-half metal thickness from the cut edge, therefore, the allowable edge stretchability is less than that predicted by the various forming limit curves. The DP and TRIP steels have islands of martensite located throughout the ferritic microstructure, including the shear zones. These hard particles act as crack initiators and further reduce the allowable edge stretch. These problems are minimized by using laser, EDM or water jet cutting devices that minimize the work hardening and loss of n-value.
Steel company research centers are conducting studies to improve the cutting process by modifying the cutting tool. One program 1 evaluated the design of the punch. Instead of the traditional flat bottom punch, a bevelled design was used. Their conclusion stated the optimized bevel angle was between 3 and 6 degrees, the shear direction was parallel to the rolling direction of the coil and a bevel clearance of 17% was used. With these parameters, the maximum shearing force was significantly reduced, and the hole expansion ratio increased by 60% when compared to a conventional flat punching process.
Multiple studies have examined the trimmed edge quality based on various cutting conditions. These conditions included different clearances, shear angles, and rake angles on mechanical shearing operations, as well as clearances on slitting operations. Laser cut, water jet and milled edges were also examined. A typical mechanically sheared steel edge has four main zones – rollover, burnish, fracture and burr (see Figure 1). Laser cutting, water jet cutting, EDM and milling are a different story as cold working is not the issue with these processes.
Conventional mild and HSLA steels have historically used burr height as the main measure of edge quality. The typical practice was to maintain burr height below 10% of metal thickness as burrs are stress risers that can lead to edge splitting. As an example, Figure 2 following shows the burr in a BH210 steel blank in the window cut-out area; the subsequent image shows edge splitting in the draw die in this localized area.
Due to their progressively higher yield and tensile strengths, AHSS grades experience less rollover and smaller burrs (Refer to our previous blog on burr heights). They tend to fracture with very little rollover or burr. As such, detailed examination of the actual edge condition under various cutting conditions becomes more significant with AHSS as opposed to measuring burr height alone to determine edge quality. Multiple studies have found that local formability edge fractures for AHSS are less likely to occur when there is a clearly defined burnish zone with a uniform transition to the fracture zone. The fracture zone should also be smooth with no voids, secondary shear or edge damage (see Figure 3 for photos of an optimal edge condition). If clearances are too small, secondary shear can occur and the potential for voids due to the multiphase microstructure increases (see Figure 4 for edge with secondary shear due to small trim steel clearance).
Clearances that are too large create additional problems that include excessive burrs and voids. A non-uniform transition from the burnish zone to the fracture zone is also undesirable. These non-ideal conditions create propagation sites for edge fractures. HET results and 2D tension test results show a strong correlation between edge condition, HER and percent elongation results. Microstructural analysis of blanked edges, trimmed edges and slit edges should be conducted on a routine basis to assess the edge condition, particularly after die sharpening, tooling modifications, die repair and set-up.
There are multiple causes for a poor sheared edge condition. They include die clearances that are too large or too small, a cutting angle that is too small, worn, chipped, or damaged tooling, improperly ground or sharpened tooling, improper die material, improperly heat treated die material, improper (or no) coating on the tooling, misaligned die sections, worn wear plates and out-of-level presses or slitting equipment. The higher loads required to shear AHSS also creates additional deflection of dies and processing equipment. Clearances measured under a static condition may change once the die, press or slitting equipment is put under load due to this deflection. As a large percentage of presses, levellers, straighteners, blankers and slitting equipment were designed years ago, the significantly higher loads required to process today’s AHSS may exceed equipment design limits and alter their performance.
1 Hua-Chu Shih, Constantin Chirac, and Ming Shi, “The Effects of AHSS Shear Edge Conditions on Edge Fracture,” Proceedings of the 2010 International Conference on Manufacturing Science and Engineering, MSEC2010-34062
2 Courtesy of P. Mooney, 3S – Superior Stamping Solutions, LLC
3 P. Mooney, “Stamping Technology Seminar” – 3S – Superior Stamping Solutions, LLC training seminar