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.
Figure 1: Cross section of a punched hole of DP780 showing the four main zones of a typical sheared edge.2
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.
Figure 2: Excessive burr on blank (top) and a global formability split on the formed liftgate (bottom) due to excessive work hardening. Dull trim steels were the cause of this condition. 3
Figure 3: Ideal sheared edge with a distinct burnish zone and a smooth fracture zone.
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.
Figure 4: Sheared edge with the trim steel clearance too small (This edge shows secondary shear due to the tight clearance and increases the probability for edge fractures.
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
Here at WorldAutoSteel, we have been studying the changes in the automotive industry for several years, focusing particularly on ride sharing, autonomous, electric vehicles and steel’s role in that marketplace. George Coates, Technical Director for WorldAutoSteel and The Phoenix Group, has been leading that effort and today contributes an article on the disruption of future mobility to the industry and the great opportunities we see for steel in meeting the challenges providers will face. We hope you enjoy the read, and we welcome your thoughts and comments. What changes and impacts do you envision for vehicle manufacture? How do you feel about the world of autonomy?
Renault’s Future Mobility concept, the E-Z Go
We’re approaching a critical milestone in automotive history when what we know as normal is about to change significantly. Future Mobility describes the revolution that’s already begun. We’re rethinking transportation from the movement of a vehicle to a more efficient concept for moving people and things. We’re about to discover the social advantages of connected, autonomous, shared and electric vehicles. And we’re completely changing the way we view transportation.
By 2030, electric vehicles (EVs) will be mainstream—not just within the premium segment, as they are today. EV’s will be popular and available across all vehicle variants and prolific in the commercial vehicle industry and in public transportation. Owners and fleet providers will experience the lower costs of electricity, lower maintenance costs, and the lower overall total cost of ownership (TCO). Fully autonomous or self-driving vehicles will introduce design freedom never experienced before, with the removal of the steering wheel, foot pedals and conventional dashboard. Communication and comfort will be re-imagined, with a vehicle that’s no longer designed around the driver but designed to serve the needs and comfort of the occupants, who are now users instead of owners.
With the rise of mobility services such as Uber, Didi, and a host of others, vehicle ownership is fast becoming an option. In a very short time, especially in urban areas such as China’s mega-cities, it is becoming cost-efficient to subscribe to a monthly ride share service for all of your transportation needs.
Bill Russo, CEO, Automobility LTD
Bill Russo, CEO of China-based Automobility, in a December 2018 article, Competing in the Digital Internet of Mobility, notes that the digital connectivity of these vehicles will open up profit opportunities well beyond the vehicle hardware. He says “An expanded understanding of mobility use cases and tailoring of the mobility hardware ‘form factor’ to the particular mobility need will be a way to create a value proposition that is rooted in the unique riding experience. In the user-centric world where users are passengers, the focus shifts from traditional driver-centric design to a user-centric productivity space. Instead of traveling in the cockpit, we will move in business class or economy class, depending on our preferences and budget.” Cities will be re-imagined in new social opportunities associated with autonomy, as these vehicles will serve the under-served, and infrastructures will shift in purpose to move people, as opposed to moving vehicles.
Where does steel fit?
The steel industry plans to be right in the middle of this revolutionary change. Fleet owners who provide ride hailing and ride sharing services need to manage the total cost of ownership, while maximizing the user experience for added revenue. To be profitable, they’ll want durable, lasting structures that are affordable to own, provide the user motion as well as emotional comfort, while being efficient to operate, and environmentally friendly – and steel is the only material that meets all these requirements.
On Camera Now: George Coates, Technical Director, WorldAutoSteel and Phoenix Group from worldautosteel on Vimeo.
As always, steel is needed for the crash safety structures, and now add battery protection. Our market intelligence shows that due to the high cost to municipalities and regional governments, autonomous-only vehicles will be limited to dedicated areas for a long time to come. Meanwhile, vehicle-to-vehicle and vehicle-to-infrastructure connectivity will result in dramatic improvements in accident avoidance and reduced fatalities.
Because it will take many years before all vehicles on the road have these technologies in play, the need for passive safety will remain for the foreseeable future. Developing a structural design for the passenger compartment becomes challenging, since there’s a now a need to strike a balance between occupant safety and the occupant freedom. This is enabled by removing the driver and controls from the interior. Steel will be needed to provide the unique properties of both crash energy absorption and deflection, while also managing the loads associated with passengers in multiple and diverse seating configurations. Steel has the ability to provide needed strength while keeping the material thin, which lends more room in the passenger cabin for new seating arrangements and more seats. And battery housings made from steel will provide structural integrity for crash management, while also preventing battery pack damage and leakage.
Lightweighting will continue to be important in an effort to balance smaller battery sizes with maximum range. The steel industry has been and will continue to develop products, such as the ever-growing family of Advanced High-Strength Steels (AHSS), to meet both the mass reduction and the safety targets, affordably. With content innovation and the amazing flexibility of the Iron (Fe) element, researchers still have vast development possibilities for new steels that are stronger, more formable and cost effective.
||George Coates is the Technical Director, WorldAutoSteel as well as The Phoenix Group. Since 1991, George has been providing engineering and consulting services for industry leaders in the steel, automotive, and manufacturing industries. George’s areas of expertise include: management and strategic consulting, project management, automotive stamping throughput improvement, supplier metal conversion, metal formability and reference panel systems, and new vehicle launch manufacturing support. George is an active contributor to WorldAutoSteel technical programs, including project manager / instructor for AHSS Application Guidelines. George earned a B.S. in Metallurgical Engineering, University of Cincinnati, and his MBA at Miami University (Ohio).
Product benchmarking is the process of measuring and analyzing the performance of competitive products. Data from a benchmarking analysis is used at the early stages of product development where performance targets are being set for a new vehicle. As an example of benchmarking, consider setting the mass target for the body structure of a new vehicle program. We want to set a target that is light weight, but also one that is possible to achieve. We benchmark two competitive body structures to help us set the target, Figure 1.
Figure 1: Mass data for two benchmarked vehicles.
From this limited data, it appears a sufficient target for the new program would be 300 kg, the lighter of the two. But there are questions to be resolved: Are these two structures representative of efficient light weighting? Also, if the vehicle under design is of a slightly different size than these two vehicles, how will this affect the applicability of benchmark comparison?
A means to begin to address these concerns is simply to look at more benchmark vehicles. The tear-down database at A2Mac1 Automotive Benchmarking contains mass data for several hundred vehicles. From this database, structure mass for 280 steel sedans is plotted in Figure 2. This expanded data allows us to see a more complete picture of the range of mass exhibited in the market place. Vehicle A and B considered before no longer stand out as exceptional. While this additional data provides an understanding of the average and range of body structure mass, there are concerns with interpreting this chart. Do the lighter structures represent efficient designs or are they just the structures of smaller vehicles?
Figure 2: Body structure mass data for 280 benchmarked vehicles.
We can answer this question by investigating how structure mass varies with mass drivers. Two mass drivers for body structure are vehicle size as measured by plan view area, and structural loading taken to depend on the Gross Vehicle Mass. In Figure 3 we use the same vehicles shown in Figure 2, but now plot structure mass versus each mass driver.
Figure 3: Structure mass vs. vehicle plan view area (left), and gross vehicle mass (right).
The correlation of structure mass for each of these mass drivers is very clearly demonstrated by the trend lines shown: Body structures are heavier for larger cars (left graph), and heavier when they must support greater vehicle mass (right graph). We can quantify these correlations with an equation determined by statistical regression, Equation 1. This equation represents the mass of an average or typical body structure, given its GVM and Area.
mSTRUCT=Mass of body structure (kg)
GVM =Gross vehicle mass (kg)
Area =Plan view area (Length x Width) (m2)
Now for each of the vehicles in our original data set we can calculate the expected structure mass using the vehicle’s GVM and Area. Figure 4 plots the actual measured structure mass vs. the mass expected for that vehicle using Equation 1. The diagonal line indicates those vehicles where the body mass is average or typical. For those structures above the line, body mass is heavier than expected given the area and GVM of the vehicle. And for those below the line, body mass is lighter than expected. This group below the line are the mass efficient body structures that are of interest for fuller analysis.
Figure 4: Actual measured structure mass compared to that expected using equation 1.
Note that looking only at structure mass, as in Figure 2, does not lead to understanding which structures are efficient. For example, Vehicle A in Figure 1 is the lighter of the two structures, 300 kg vs. 325 kg. However, after accounting for the two vehicle’s area and GVM, it can be seen from Figure 4 that Vehicle A is above the diagonal line, indicating a heavier than expected structure, while Vehicle B is on the line indicating it has a typical structure mass.
As a further example, consider the WorldAutoSteel FutureSteelVehicle (FSV). The FSV project, completed in 2011, investigated the weight reduction potential enabled with the use of Advanced High-Strength Steels (AHSS), advanced manufacturing processes, and the use of computer optimization. The resulting material use and body structure mass are shown in Figure 5.
Figure 5: FSV material application and resulting body structure mass.
We can now graph the actual FSV structure mass with expected mass, Figure 6. The data point is well below the diagonal line quantifying the exceptional mass reduction enabled through extensive AHSS use.
Figure 6: FSV body structure compared with 280 normalized benchmarked structures.
Finally, statistical benchmarking reveals which current products would benefit most from lightweighting. Looking again at the plot of actual vs. expected body structure mass for a fixed expected mass, in this case 300 kg, Figure 7. For this set of similarly sized vehicles, there is a wide range of variability in actual mass, indicated by the arrow. For the several vehicles above the diagonal, these body structures are heavier than expected and have significant potential for lightweighting.
Figure 7: Variability in structure mass for similar size vehicles.
For more information on the statistical benchmarking method, see the studies referenced in No. 2 and 3 below. Dr. Malen’s statistical benchmarking methodology also is documented in SAE Paper No. 2015-01-0574
1. A2Mac1.com, Automotive benchmarking.
2. Malen, D., Nagaraj, B., Automotive Mass Benchmarking 2017 study
3. Hughes, J. & Malen, D., Statistical Benchmarking of Automotive Closures, Great Designs in Steel, 2015,
4. FutureSteelVehicle Overview Report, April 2011,
Dr. Donald E. Malen University of Michigan
Dr. Donald E. Malen is an adjunct faculty member at the University of Michigan where he teaches graduate level courses in Automobile Body Structure and Product Design. Prior to this, he was an engineer with General Motors Corporation for 35 years. His background at GM was in automotive body structure design and analysis, and systems engineering. While at GM, he worked on many new vehicle programs and has brought this experience to his teaching and writing. Dr. Malen consults and conducts international seminars on Body Engineering, Innovation, Lead Time Reduction, and Decision Making During Preliminary Design. He holds several patents related to automobile body structure and vibration. His education includes a Ph.D. in Mechanical and Industrial Engineering from the University of Michigan, an MS from Massachusetts Institute of Technology, and a BSME from General Motors Institute (Kettering University).
Advanced Steel Processing Technologies For
Reduced Cost, Reduced Mass and Improved Functional Performance
Laser (Tailor) Welded Blanks
A laser welded blank is two or more sheets of steel seam-welded together into a single blank which is then stamped into a part. Laser welded blank technology allows for the placement of various steel grades and thicknesses within a specific part, placing steel’s attributes where they are most needed for part function, and removing weight that does not contribute to part performance. For example, Figure 1 shows a laser welded body side aperture (outer) with multiple grades and thicknesses. This technology allows for a reduction in panel thickness in non-critical areas, thus contributing to an overall mass reduction of the part.
Figure 1: Body-side outer with exposed laser welds and multi-piece construction.
There are several advantages to a laser welded blank, compared with conventional blanks made from a single grade and part thickness. They include:
- Superior vehicle strength and rigidity
- Consolidation of parts, where one blank can replace several different parts
- Lower vehicle and part weight
- Reduced steel usage
- Improved safety
- Elimination of reinforcement parts
- Elimination of assembly processes
- Reduction in capital spending for stamping and spot-welding equipment
- Reduced inventory costs
- Improved dimensional integrity (fit and finish)
- Achievement of high-performance objectives with lower total costs
- Reduction in Noise, Vibration, and Harshness (NVH)
- Elevated customer-perceived quality
A laser-welded coil (Figure 2) is a continuous coil of steel comprised of individual, separate coils of steel with varying thickness and grades. The basic process takes separate coils, prepares their edges for contiguous joining, and laser welds these together into one master coil. The new strip is then readied for blanking, or to be used as a continuous feed into a transfer press line.
As in laser-welded blanks, the laser-welded coil allows for similar advantages – targeted strength or stiffness where required, while allowing for overall part weight reduction by incorporate thinner materials where possible.
Figure 2: Laser-welded coil process
Potential use of a laser welded coil in an automotive application, using a pro-die-stamping process, includes the following:
- Roof frames
- Roof bows
- Side members
- Seat cross members
- Exhaust systems
This is a manufacturing process of flexible cold strip rolling by varying the gap between two rolls, allowing for different strip thicknesses in the direction of rolling. Figure 3 illustrates the manufacturing principles. The accurate measuring and controlling technology ensure that strip thickness tolerances are maintained. A tailor-rolled coil can be either used for blanking operations (for stamping or tubular blanks) or can be directly fed into a roll-forming line.
Figure 3: The principle of producing a Tailor Rolled Coil.
A comparison of various mechanical sheared edges with water jet, laser and milled edges showed that laser-cut edges achieved elongations that approached that of the ideal milled edge. As a response to increasing AHSS volumes and strength levels, multiple companies have developed laser blanking lines, where a coil is blanked via a laser or series of lasers. These new lines are capable of cutting blanks on a high-volume basis.
There are several advantages to this approach when processing AHSS. Improved edge conditions are less susceptible to edge fracture, and thus is significant. Additional savings can be achieved through the elimination of expensive blank die construction and tooling maintenance costs (no blank die is needed) and thus no expensive trim steels are required (AHSS usually requires more durable and more expensive tool steels). Less floor space is needed because there are no blank dies to store. With no blank dies to remove and replace, faster line transitions occur, which means greater uptime and increased productivity. There is also the opportunity to optimize material utilization through either blank nesting optimization or blanking two or more different blanks out of the same steel strip.
Figure 4 shows an example of optimized laser blank nesting of three different blanks from the same coil. Blank contours can easily be modified after production launch as well. As many AHSS grades are rolling-direction sensitive with respect to edge and shear fracture, alternative nesting can potentially optimize the blank orientation to minimize these types of local formability failures.
Figure 4: Schematic of 3 different blanks to be laser blanked from the same coil to minimize engineered scrap. Not only is engineered scrap minimized, but the superior edge condition significantly reduces the potential for edge fracture.
The decision for laser blanking should be made during development, in order maximize overall process efficiency and avoid building a blank die or other non-essential tooling. Figure 5 shows a typical laser blanking line specifically designed to process AHSS.
Figure 5: Laser blanking line specifically designed to process AHSS. Note the cartridge-based straightener (far left), specifically designed to ensure blanks are flat after processing.
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%.
Figure 1: Punching up to DP 500/800 with surface treated high quality tool steels can be comparable to Mild steel with conventional tools.1
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.
Figure 2: Burr height comparison for Mild steel and AHSS as a function of the number of hits. Results for DP 500/800 and MS 1150/1400 are identical and shown as the AHSS curve.1
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.
Figure 3: Five main modes of tooling failure.2
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.
|Figure 4: End of a blank on a dash reinforcement where the blanked cut-out becomes part of the product on the finished part. Close examination shows a poor edge condition.2
||Figure 5: Local formability edge fracture emanating from a blanked edge in a stretch flange operation, and location of metal gainer eventually added to the draw die.
||Figure 6: Worn and broken D2 inserts being used on a DP 600 AHSS steel.
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.
Figure 7: Caldie insert used to address wear and cracking issues on the blank die.2
Figure 8: Caldie insert in a difficult to maintain location on a blank die. 2
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
Die wear is affected by, but not limited to, steel strength and surface friction, contact pressure, sliding velocity, temperature, die surface treatment and lubrication. As the material strength and hardness is increased, an increase in die wear occurs which leads to quality issues in the stamping. In addition, frequent die wear requires more frequent die maintenance or replacement, affecting turnaround times, productivity and cost.
Actions can be taken to prevent excessive wear on die materials when forming Advanced High-Strength Steels. To prevent such wear, new die materials and better coatings have been developed to maintain their hardness without compromising the toughness of the material. Hard material coatings and nitriding have also been used to improve the tribological properties of die surfaces.
Tool and Die Materials
In general, the considerably higher strength levels associated with AHSS grades exerts proportionally increased forces on the die material. AHSS might reach strength levels four to five times higher than mild steels, approaching 2000 MPa. This is based on complex metallurgy that delivers high concentrations of martensite and very high work hardening rates.
The higher forces required to form AHSS require increased attention to tool and die material specifications. Key requirements that need to be specified are the stiffness and toughness of the die substrate, as well as the surface treatments and hardness of the die coating. Life and performance of a draw die is determined by the amount of wear/galling that accumulates during forming, which then defines specific die maintenance intervals. When selecting die materials, important considerations are:
- Sheet metal being processed, characterized by strength, thickness, and surface coating.
- Die construction, machinability, radii sharpness, surface finish, and die hardness, specifically on draw beads and radii.
- Cost per part.
Thickness reduction for weight saving is one primary reason for applications of AHSS. Unfortunately, the reduced thickness of the steel increases the tendency to wrinkle. Higher blankholder loads are required to suppress these wrinkles. Any formation of wrinkles will increase the local load and accelerate the wear effects. Figure 1 shows a draw die with severe die wear due to excessive wrinkling on a DP780 part. It is not uncommon to replace these high wear areas with a more durable tool steel insert to minimize this type of excessive wear condition.
Figure 1: Draw die with significant wear due to excessive wrinkling on a DP780 part.
Tool steel inserts for forming dies must be selected according to the severity of the forming. Surface coatings are recommended for DP 350/600 and higher grades. When coatings are used, it is important that the substrate has sufficient hardness/strength to avoid plastic deformation of the tool surface, even locally. Therefore, a separate surface hardening, such as nitriding, is recommended before the coating is applied. Prior to coating application, it is important to use the die in in pre-production to provide time to adjust forming conditions and establish die performance Surface roughness must be as low as possible before coating. Ra values below 0.2 µm are recommended. Steel inserts with a TiC/TiN coating are recommended for local high-pressure conditions that cause accelerated die wear and zinc flaking.
Table 1, provided by the Auto/Steel Partnership, describes recommended construction materials for stamping dies and components, based on significant research efforts. North American Automotive Metric Standards (NAAMS) are the product of a consortium between Ford, GM, FCA and various North America automotive steel producers.
Table 1 – Recommended Materials for Specific Die Components – NAAMS Standards
Tool steels for cutting, trimming, and punching operations must be similarly selected. For these operations, tensile strength is more important than yield strength. Tool hardness between 58 and 62 HRC is recommended. Coatings may be used to reduce tool wear, but for the highest strength steels (above 1000 MPa tensile strength) coatings may fail due to local deformation of the die material substrate.
To prevent this, hardening of the substrate prior to coating is again strongly recommended. High performance tool steels, such as powder metallurgy (PM) grades, can be expensive but are justified because of their low wear rate and increased life. Ceramic tool inserts have extreme hardness for wear resistance, high heat resistance, and optimum tribological behavior, but have poor machinability and severe brittleness. High costs are offset by reduced maintenance and increased productivity. While not commonly used, the ceramic tool inserts offer a possible solution to high
interface loading and wear.