The diverse microstructures and strength levels of AHSS products present some challenges for stamping operations. Cutting and punching clearances are greater for AHSS, and as a general rule, should be increased with increasing sheet material strength. The clearances range from about 6% of the thickness for Mild steel but grow to between 10% and 16% for strength levels of 1 GPa or more.
Two-hole punching studies1 were conducted with Mild steel and AHSS. The first measured tool wear (captured in Figure 1), while the second studied burr height formation (Figure 2).
Wear testing was performed with four 1.0 mm thick sheet steels: Mild 140/270, DP 350/600, DP 500/800, and MS 1150/1400. Tool steels were W.Nr. A2 with a hardness of 61 HRC and a 6% clearance for Mild steel tests. PM tools with a hardness range of 60-62 HRC were used for all AHSS testing. For the DP 350/600, the punch was coated with CVD (TiC) and the clearance was set at 6%. Tool clearances for DP 500/800 were 10%, and for MS 1150/1400 were set at 14%.
The studies showed that wear rates for AHSS DP steels punched with surface treated high quality (PM) tool steels were comparable to punching Mild steel with conventional tools. Wear rates for MS were more than twice that of the DP steels. Increasing burr height is often the reason for sharpening trim steels and punches, as burrs can reduce metal formability. For Mild steels the burr height increases with increasing ductility and tool wear.
Figure 2 shows a burr height plateau for AHSS; both materials initially have a burr height related to the material ductility and the sharpness of the tools. AHSS fractures at a maximum possible height that is reached when the maximum local elongation is obtained during punching, after which the burr height does not increase. The Mild steel, which is more formable, will continue to generate higher burr height with increased tool wear.
The burr height increased with tool wear and increasing die clearance when punching Mild steel. AHSS may require a higher-grade tool steel or surface treatment to avoid tool wear, but tool regrinding because of burrs should be less of a problem. If the tool has been surface-treated, grinding the tool will remove the surface treatment, so if possible, the tool must be retreated. If burr height is the criterion, high quality tool steels will result in greater intervals between sharpening when punching AHSS, since the burr height does not increase as quickly with tool wear as when punching Mild steel with conventional tool steels.
As there are many different tool steels, tool steel treatments and tool steel coatings, shops are encouraged to identify the dominant mode of tool failure in order to select the tool steel with the properties to counter that failure mode. There are five main types of cold work failure modes involving tool steels: wear, plastic deformation, chipping cracking, and galling. Figure 3 shows examples of these five failure modes.
The following case study illustrates the importance of clearly identifying the mode(s) of failure on the part, as well as the mode of failure on the tooling, to improve the selection of counter-measures. A dash reinforcement had been in production for several years, stamped with a 280 MPa yield strength HSLA steel. To improve side impact ratings, the part transitioned to DP 600. Immediately after implementation, stamping scrap rates increased significantly. The failures were all determined to be local formability edge fractures; investigation revealed that the blank for this part was configured and that the blanked edge at the edge fracture was part of the final product edge. Figure 4 shows one of the blanked cut-outs which is then drawn and flanged. The edge condition had burrs and a poor burnish to fracture zone. See Figure 5 for a photo of the edge fracture.
The blank die material was examined and determined to be the same D2 tool steel as was when the steel was HSLA. Further examination determined that no clearance adjustments were made for the higher strength steel, maintained at 10% of metal thickness instead of an optimal setting of 15%. Additionally, the inserts used to make the u-shaped cut-outs were wearing and failing at an alarming rate.
The failures were so frequent that a series of back-up inserts had been constructed so the worn/damaged inserts could be quickly replaced. Figure 6 shows worn and broken inserts. It became clear that the failure modes involved both wear and fractures. An alternative, more durable tool steel (trade name caldie) with increased clearances was inserted in the blanking die. This change enabled the inserts to operate with over 90,000 hits and virtually no insert maintenance required (other than routine cleaning). This change significantly reduced the scrap rate, but sporadic edge fractures were still being experienced. As a result, breakdown panels of the draw, trim and flange operations were examined. It was found that the draw die at the location in question was not forming the part to the final length of line. As DP steels have a very high work hardening rate, stretch flanging a blanked edge significantly increases the potential for edge fracture. To compensate, a small metal gainer was added to the draw die to ensure that the flanging operation deformed the steel via bending and straightening, not additional stretch (see also Figure 5). After these two process changes, scrap rates for edge fractures dropped to virtually zero.
Selecting the proper tool steels for specific grades of AHSS is critically important and will reduce long term maintenance, repair and scrap/rework costs. Some of the more elaborate tool steels, are significantly more expensive than those used with mild steels. As a result, some automakers and steel processors don’t use the more durable (and expensive) tool steels on the entire working surface of the die. They strategically identify high wear and difficult to maintain areas and install tool steels as inserts in those locations. Figures 7 and 8 show caldie inserts installed on blank dies in difficult to maintain locations.
1 B. Carlsson, “Choice of Tool Materials for Punching and Forming of Extra- and Ultra High Strength Steel Sheet,” 3rd International Conference and Exhibition on Design and Production of Dies and Molds and 7th International Symposium on Advances in Abrasive Technology, Bursa, Turkey (June 17-19, 2004).
2 Courtesy of Peter Mooney Peter Mooney, 3S-Superior Stamping Solutions, LLC