WorldAutoSteel has a 30-year legacy of steel demonstration all the way back to the Ultra-Light Steel Auto Body (ULSAB), whose engineering report is still being downloaded from our worldautosteel.org site today. The one you may remember best is the FutureSteelVehicle (FSV), results of which we launched in 2011. FSV demonstrated steel innovation for not only Battery Electric vehicles (BEV) but also Fuel Cell vehicles (FCV). Steel E-Motive is the sixth of our global steel industry programs.
So Why Mobility as a Service?
The Automotive sector is undergoing the most rapid change in 40 years. This transformation shifts our thinking – from the movement of vehicles to the efficient movement of people and goods. Over the past eight years, we have conducted extensive research into global trends such as urbanization, transport emissions reduction, as well as the waning interest in vehicle ownership among the young and old. This is especially prevalent in megacities characterized by pollution, congestion, limited parking and enormous ownership costs. Our research concluded that mobility as a service (MaaS) will grow exponentially in high population areas and would place a significant challenge on vehicle design and manufacturing. Therefore, we needed to make sure we as an industry were active and visible in providing STEEL solutions in this new market place.
Steel E-Motive will demonstrate the benefits of steel, linking the properties of the material to the required architectures and attributes for MaaS vehicles.
This program will demonstrate the benefits of steel, linking the properties of the material to the required architectures and attributes for MaaS vehicles. It connects us with original equipment manufacturers (OEMs) and future mobility providers (FMPs), reinforcing steel’s advantages in strength, durability, sustainability and affordability.
An autonomous BEV structure aligns perfectly with steel’s best attributes, however most new concepts trial alternative materials. The global steel industry is investing significantly in product and fabrication development to continually prepare for the next challenge. High Strength and Advanced High-Strength Steel (AHSS) portfolios have grown from the 11 highlighted in the ULSAB program, to more than 60 grades available for use in designing and optimizing Steel E-Motive’s autonomous BEV architecture. Third Generation AHSS (3rd Gen AHSS) will have a prominent role in Steel E-Motive’s body-in-white, taking strength levels ever higher while improving manufacturability. And our industry continues to evolve Press Hardened Steels (PHS) with strength levels upwards of 2000 MPa.
Finally, efficient fabrication processes such as roll stamping, press hardening, and hydroforming use less steel and therefore contribute lower vehicle production emissions. These are the details being highlighted in Steel E-Motive, where we hope to demonstrate that only Steel can make it Real.
Steel E-Motive: A game changing, world first?
Many OEM’s and mobility service providers follow the typical vehicle development process where they adapt an existing vehicle structure to the new vehicle requirements. We don’t have that in Steel E-Motive We believe Steel E-Motive is one of the world’s firsts.
- The first for a Level 5 autonomous vehicle that is compliant with global high-speed crash requirements.
- The first autonomous vehicle to be a conventional high-volume stamped steel body construction, creating an affordable platform for the mobility service provider.
- First to offer a competitive, robust, and sustainable MaaS solution.
For engineers, being first is very exciting but a little nerve wracking – there are no benchmarks out there. There is less to “hang on to.” We’re on our own. Target setting is more challenging; we are the benchmark. Time will tell if we make it to the automotive hall of fame.
We are producing concepts for two BEVs based on a single modular platform. SEM1 (Figure 1) is a front-wheel drive short wheelbase urban version for inter-city travel for four passengers. It has a compact design and vehicle footprint, comparable in footprint to a European B/C segment size. SEM2 (Figure 2) is an all-wheel drive, long wheelbase extra urban version designed to carry up to six passengers. It has an adaptable interior volume that can result in additional luggage capacity compared to SEM1.
Figure 1: SEM1 Vehicle Specifications (© WorldAutoSteel 2022)
Figure 2: SEM2 Vehicle Specifications (© WorldAutoSteel 2022)
Body in White Steel Usage
Steel E-Motive benefits from a broad portfolio of steel grades and fabrication process, as identified by our member steel experts. The design is nearly finalized, and material selections are being evaluated against various performance targets with the representative structure shown in Figure 3 with high PHS usage at this stage in the design (as of May 2022). This is mainly driven by the safety requirements. Steel E-Motive BIW steel and steel technologies include:
- Right steel grade in the right place
- Significant proportion of >1500MPa grades, primarily for occupant and battery intrusion zones
- Mixture of stamped, roll formed, roll stamped, press hardened steel and hydroformed parts
- Spotweld, laser weld and structural adhesive
Figure 3: Steel E-Motive’s Body-in-White Steel usage as of May 2022. (© WorldAutoSteel 2022)
At the Core of the Steel E-Motive Concept Is an Innovative Battery Design
Figure 4 shows Steel E-Motive’s battery frame design’s construction:
- Battery modules and cooling plates are mounted to an AHSS carrier frame (off-line).
- The carrier frame is mounted to the body structure (in general assembly).
- The BIW floor acts as the top cover and provides sealing.
- The AHSS bottom cover plate provides impact protection.
This design provides significant cost and weight savings, as well as improved NVH. This extremely efficient package does not compromise safety and enables a flat floor with a lower step-in height.
Figure 4: Steel E-Motive Battery package assembly. (© WorldAutoSteel 2022)
Competitive Body Stiffness with an Open B-Pillarless Body Structure
With clean sheet design, and generally less package constraints in a Level 5 vehicle, our design teams have had more freedom to engineer and optimize the crash and stiffness structural loadpaths. We used topology, optimization, and Virtual Reality tools to determine the most efficient structural loadpaths (Figure 5). The results informed the joint designs and enabled optimization of the joining and structural adhesives. These steps and the advantage of steel’s high modulus resulted in impressive performance.
Figure 5: Topology Load Path Optimization. (© WorldAutoSteel 2022)
The approach for achieving body stiffness was as follows. Results are shown in Figure 6 following.
- Topology load path optimization
- Appropriate section size, profiles, part integration and flange / joint design
- Strut towers integrated with key body members, such as A-pillars, vertical dash brace
- Contribution from structural battery frame and battery cover closing, roof structure trusses
- Rigidly connected front and rear subframes
- Optimized joining and use of structural adhesives
- Capitalizing on the Inherent high modulus of steel
Figure 6: SEM Torsional Rigidity animation. (© WorldAutoSteel 2022)
Static torsional stiffness 38,000Nm/deg
Global trimmed BIW modes >28Hz
Local attachment static stiffness ten times bushing stiffness
Front Crash Structure Engineered to Balance the Requirements of 56kph USNCAP FFB, IIHS ODB, IIHS SORB and EuroNCAP MPDB Load cases
One of the most challenging aspects of the Steel E-Motive program has been achieving the front crash performance that minimizes occupant injury. The challenge has been compounded by the overall compact size of the vehicle and the short front overhang dimensions, meaning less space to manage and balance the required crush energy with intrusion resistance.
For the IIHS 25% Small Overlap test, we worked from the outset to achieve a barrier “glance off.” The goal is to deflect the vehicle off the barrier by the time the barrier reaches the hinge pillar. This results in a reduced amount of vehicle kinetic energy converted to crush energy. The vehicle continues after the impact with some onward velocity and kinetic energy. This strategy results in reduced intrusion to the passenger compartment and a much lower vehicle pulse (below 20g), which translates into lower occupant injury. We are very excited by this outcome, as in our benchmarking we have not seen many (if any) vehicles of this size managing to achieve a glance off for this test. Figures 7 and the bullets following provide a look at the results.
Figure 7: IIHS 25% Small Overlap test. (© WorldAutoSteel 2022)
- IIHS “good” rating achieved (based on predicted intrusions).
- Our strategy for IIHS Small Overlap test was to achieve a “glance off” the barrier, which is a significant challenge given the vehicle’s short front overhang.
- Front suspension engineered to detach on impact. This is important for achieving glance off.
- Glance off results in some continued onward vehicle velocity after the impact.
- This results in reduced crush energy, lower vehicle pulse and intrusions = enhanced occupant protection
Figure 8 points out features of the front crash structure. Most of the crush energy in FFB and ODB is absorbed by conventional longitudinal mid-rails, which are made of cold stamped, tailor welded blank Dual Phase steels. The plan view angle of the longitudinals has been optimized to provide load reaction early in the SORB event while remaining largely inside of the SORB barrier.
Figure 8: Front crash structure engineered to balance the requirements of 56kph USNCAP FFB, IIHS ODB, IIHS SORB and EuroNCAP MPDB load cases. (© WorldAutoSteel 2022)
Following in Figures 9 and 10 are animations of the FFB results:
MaaS vehicles will need to accommodate quick ingress and egress as well as provide comfort and safety for the occupants. Consequently, we have flipped the front occupant around to a rear facing configuration and provided a B-Pillarless wide door aperture to enable comfortable and quick access for passengers. This changes the approach required for occupant protection in a front crash. Effectively we are dealing with a high-speed rear impact situation for the occupant. Current rear impact tests cover lower speed rear end shunts. Figure 11 notes the key points and challenges that Steel E-Motive is designed to meet.
Figure 11: Different approach and considerations are required for the protection of rear facing front occupants. We are effectively addressing a high-speed rear impact event. (© WorldAutoSteel 2022)
Side Crash Structure Consists of Absorption and Intrusion Prevention Zones, Compensating for Large Body Aperture
The side structure includes roll-stamped martensitic door waist rail beams and a one-piece Tailor Welded Blank, Press Hardened Steel door ring outer. A- and C-pillars in line with occupants provide good side impact protection. (You can learn more about the door design in our May blog).
In the section AA schematic in Figure 12 the TRIP690 hydroformed tube interlocking door B-pillar is shown (wrapped over the rocker and cantrail). The load travels through the side impact crush “hex” beam, which is a two-piece roll formed DP590 component.
Figure 12: Side crash structure consists of absorption and intrusion prevention zones, compensating for large body aperture. (© WorldAutoSteel 2022)
Steel E-Motive Design Demonstrates Good Side Crashworthiness and Good Levels of Occupant and Battery Protection
In addition to occupant protection tests, additional side impact load cases have been simulated to ensure optimal battery protection. The design maintains a less than 30 mm clearance to the battery.
In reviewing the design according to IIHS standards and based on the predicted intrusions, we are confident this vehicle would achieve an IIHS “good” rating. See Figures 13 and 14 following:
Figure 13: USNCAP 32kph side pole (battery protection). (© WorldAutoSteel 2022)
In addition to occupant protection test, additional side pole load cases to ensure battery protection
>30mm clearance to battery maintained
Figure 14: IIHS 60kph side barrier II (occupant protection). (© WorldAutoSteel 2022)
IIHS “good” rating (based on predicted intrusions).
Total Cost of Ownership: Vehicle and Body Is Designed for Conventional Fabrication and Assembly Processes
The Steel E-Motive body has been designed with low cost in mind to provide the foundation for a lower total cost of ownership for fleet owners. The steel body design is optimized to maximize material utilization and minimize scrap rate. Steel E-Motive is suitable for >250,000 units/year production and is compatible with existing global automotive manufacturing facilities using conventional press and fabrication tools. We are also using Life Cycle Assessment as an integral part of the engineering process to ensure that Steel E-Motive is responsible for the lowest possible emissions throughout its entire life cycle. We will report on environmental performance and sustainability as a part of our final results.
Steel E-Motive Key Outcomes
The Steel E-Motive program is delivering an exciting futuristic vehicle, optimized from the ground up for autonomous MaaS application. We are addressing key challenges through careful design, application of simulation tools and efficient use of the latest Advanced High-Strength Steels and fabrication processes. Steel’s inherent characteristics of low production emissions, lightweighting capabilities for mass efficiency, infinite recyclability and product durability underscores its suitability as an integral part of stakeholder strategies to offer sustainable mobility solutions, today and in the future.
Be sure to follow us on our journey as we enter our final months of design, engineering and reporting by subscribing at the Steel E-Motive website. We welcome your questions about this program using the Comment box below.
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We at WorldAutoSteel, side-by-side with our engineering partner Ricardo, are rounding the corner on two years in the design process for the Steel E-Motive’s SEM1 and SEM2 autonomous, electric, mobility as a service (MaaS) vehicles. The excitement is really building around our global virtual office as we are getting closer and closer to finalizing the concept designs. We’re finishing the material and manufacturing process optimization now, and soon it will be pencils down. We’ll be making the full engineering reports freely available to the world’s engineers, as we have always done through the years, beginning with our UltraLight Family of Vehicles. We cannot wait to release our results for the world to see!
In the meantime, we have been releasing as much information as we can along the road. This month’s blog (on the Steel E-Motive site) concentrates on the unique closure design developed for Steel E-Motive. A B-Pillar integrated configuration (red component on the right side door in the animation below), which specifies Advanced High-Strength Steels in the A- and B-Pillars as well as for the door ring, affords a wide-open entry for passengers to enter and exit safely and comfortably. And when not in use for transporting people, the wide aperture can accommodate the loading and unloading of goods.
Steel E-Motive is achieving excellent side pole and side impact results with this closure design. You can read a little about Ricardo’s approach on designing for crash in our April blog here.
Here’s a little animation of the side closure design to hopefully encourage you to click over to the Steel E-Motive blog where we tell the whole story. Keep this window open on your browser and come back here with any questions or comments you may have. You can add them below in the Comments area of this page. We’d very much appreciate hearing your thoughts.
Steel E-Motive’s AHSS A- and C-Pillar placement provide good occupant protection in side pole crash scenarios. The A- and C-Pillar combined with AHSS door ring and door reinforcements compensate for the removed B-Pillar.
Since June 2020, we’ve been working with our engineering partners, Ricardo, on Steel E-Motive, a vehicle engineering program which is developing virtual concepts for two fully autonomous and connected electric vehicles designed for mobility as a service (MaaS) applications. We are using Advanced High-Strength Steel (AHSS) technologies and products to design autonomous vehicle concepts to enable MaaS solutions which are safe, affordable, accessible and environmentally conscious.
Steel E-Motive engineering and validation will be completed in 2022, at which time we will prepare communication and delivery of the final results. But right now, we are in the midst of crash simulation and material optimization to solve the new challenges posed by these vehicles.
Steel E-Motive is designed to operate in a mixed traffic mode operation (both driver-operated and autonomous vehicles); hence the vehicle should meet the current and future global high-speed crash test standards. The compact vehicle dimensions and proportions result in relatively short front and rear overhangs. To enhance user experience and take advantage of full autonomy, the front occupants are positioned in a rear facing configuration presenting a more significant challenge for crashworthiness in frontal impact. This requires a revised front crash strategy and load management approach for Steel E-Motive.
Neil McGregor, Steel E-Motive’s Chief Engineer, has written a blog about it, and you can find it posted on our Steel E-Motive website (web spiders prohibit us from duplicating the article here). Take a moment to read it there, but feel free to ask questions or comment about it here!
Our colleagues at JFE Steel recently provided us with a new case study based on laboratory evaluations they conducted in Japan. The article is part of our Martensite article, but we this month, we want to highlight it in our AHSS Insights blog.
Martensitic steel grades provide a cold formed alternative to hot formed press hardening steels. Not all product shapes can be cold formed. For those shapes where forming at ambient temperatures is possible, design and process strategies must address the springback which comes with the high strength levels, as well as eliminate the risk of delayed fracture. The potential benefits associated with cold forming include lower energy costs, reduced carbon footprint, and improved cycle times compared with hot forming processes.
Highlighting product forms achievable in cold stamping, an automotive steel Product Applications Laboratory formed a Roof Center Reinforcement from 1.4 mm CR1200Y1470T-MS using conventional cold stamping rather than roll forming, Figure 6. Using cold stamping allows for the flexibility of considering different strategies when die processing which may result in reduced springback or incorporating part features not achievable with roll forming.
Figure 6: Roof Center Reinforcement cold stamped from CR1200Y1470T-MS martensitic steel.U-1
Cold stamping of martensitic steels is not limited to simpler shapes with gentle curvature. Shown in Figure 7 is a Center Pillar Outer, cold stamped using a tailor welded blank containing CR1200Y1470T-MS and CR320Y590T-DP as the upper and lower portion steels.U-1
Figure 7: Center Pillar Outer stamped at ambient temperature from a tailor welded blank containing 1470 MPa tensile strength martensitic steel.U-1
Another characteristic of martensitic steels is their high yield strength, which is associated with improved crash performance. In a laboratory environment, crash behavior is assessed with 3-point bending moments. A studyS-8 determined there was a correlation between sheet steel yield strength and the 3-point bending deformation of hat shaped parts. Based on a comparison of yield strength, Figure 8 shows that CR1200Y1470T-MS has similar performance to hot stamped PHS-CR1800T-MB and PHS-CR1900T-MB at the same thickness and exceeds the frequently used PHS-CR1500T-MB. For this reason, there may be the potential to reduce costs and even weight with a cold stamping approach, providing appropriate press, process, and die designs are used.
Figure 8: Effect of Yield Strength on Bending Moment. The right image shows the typical yield strength range of CR1030Y1300T-MS and CR1200Y1470T-MS as well as typical yield strength values of several Press Hardened Steels.S-8
To read more about Martensitic steels, including its practical applications, visit the Steel Grades page here.
Many thanks to Toshiaki Urabe, Principal Researcher, JFE Steel, and Dr. Daniel Schaeffler, President, Engineering Quality Solutions, Inc., for providing this case study.
One of the tasks we took great care in completing during the update of the AHSS Application Guidelines was to provide a definition for what constitutes a third generation (3rd Gen) Advanced High-Strength Steel. We had been asked this question many times, and often in addressing the question among our technical editors, we would get various responses. Consequently, we made it a part of a discussion with our AHSS Guidelines Working Group, made up of steel subject matter experts from around the world at our member companies. The following article reflects the outcome of that discussion and is the definition adopted by WorldAutoSteel.
First Generation Advanced High-Strength Steels (AHSS) are based on a ferrite matrix for baseline ductility, with varying amounts of other microstructural components like martensite, bainite, and retained austenite providing strength and additional ductility. These grades have enhanced global formability compared with conventional high strength steels at the same strength level. However, local formability challenges may arise in some applications due to wide hardness differences between the microstructural components.
The Second Generation AHSS grades have essentially a fully austenitic microstructure and rely on a twinning deformation mechanism for strength and ductility. Austenitic stainless steels have similar characteristics, so they are sometimes grouped in this category as well. 2nd Gen AHSS grades are typically higher-cost grades due to the complex mill processing to produce them as well as being highly alloyed, the latter of which leads to welding challenges.
Third Generation (or 3rd Gen) AHSS are multi-phase steels engineered to develop enhanced formability as measured in tensile, sheared edge, and/or bending tests. Typically, these steels rely on retained austenite in a bainite or martensite matrix and potentially some amount of ferrite and/or precipitates, all in specific proportions and distributions, to develop these enhanced properties.
Individual automakers may have proprietary definitions of 3rd Gen AHSS grades containing minimum levels of strength and ductility, or specific balances of microstructural components. However, such globally accepted standards do not exist. Prior to 2010, one steelmaker had limited production runs of a product reaching 18% elongation at 1000 MPa tensile strength. Starting around 2010, several international consortia formed with the hopes of achieving the next-level properties associated with 3rd Gen steels in a production environment. One effortU-11, S-95 targeted the development of two products: a high strength grade having 25% elongation and 1500 MPa tensile strength and a high ductility grade targeting 30% elongation at 1200 MPa tensile strength. The “exceptional-strength/high-ductility” steel achieved 1538 MPa tensile strength and 19% elongation with a 3% manganese steel processed with a QP cycle. The 1200 MPa target of the “exceptional-ductility/high-strength” was met with a 10% Mn alloy, and exceeded the ductility target by achieving 37% elongation. Another effort based in EuropeR-22 produced many alloys with the QP process, including one which reached 1943 MPa tensile strength with 8% elongation. Higher ductility was possible, at the expense of lower strength.
3rd Gen steels have improved ductility in cold forming operations compared with other steels at the same strength level. As such, they may offer a cold forming alternative to press hardening steels in some applications. Also, while 3rd Gen steels are intended for cold forming, some are appropriate for the hot stamping process.
Like all steel products, 3rd Gen properties are a function of the chemistry and mill processing conditions. There is no one unique way to reach the properties associated with 3rd Gen steels – steelmakers use their available production equipment with different characteristics, constraints, and control capabilities. Even when attempting to meet the same OEM specification, steelmakers will take different routes to achieve those requirements. This may lead to each approved supplier having properties which fall into different portions of the allowable range. Manufacturers should use caution when switching between suppliers, since dies and processes tuned for one set of properties may not behave the same when switching to another set, even when both meet the OEM specification.
There are three general types of 3rd Gen steels currently available or under evaluation. All rely on the TRIP effect. Applying the QP process to the other grades below may create additional high-performance grades.
- TRIP-Assisted Bainitic Ferrite (TBF) and Carbide-Free Bainite (CFB)
- TRIP-Assisted Bainitic Ferrite (TBF) and Carbide-Free Bainite (CFB) are descriptions of essentially the same grade. Some organizations group Dual Phase – High Ductility (DP-HD, or DH) in with these. Their production approach leads to an ultra-fine bainitic ferrite grain size, resulting in higher strength. The austenite in the microstructure allows for a transformation induced plasticity effect leading to enhanced ductility.
- Quenched and Partitioned Grades, Q&P or simply QP
- Quenching and Partitioning (Q&P) describes the processing route resulting in a structure containing martensite as well as significant amounts of retained austenite. The quenching temperature helps define the relative percentages of martensite and austenite while the partitioning temperature promotes an increased percentage of austenite stabile room temperature after cooling.
- Medium Manganese Steels, Medium-Mn, or Med-Mn
- Medium Manganese steels have a Mn content of approximately 3% to 12%, along with silicon, aluminum, and microalloying additions. This alloying approach allows for austenite to be stable at room temperature, leading to the TRIP Effect for enhanced ductility during stamping. These grades are not yet widely commercialized.
We have much more information on 3rd Gen steels here in the Guidelines. Take a look at the full 3rd Generation Steels article for much more detail on the three types listed above.
The rule of thumb estimates used in 1989 during my internship with an automotive stamping supplier were simple calculations for the peak load. Tonnage for trim and pierce operations depended on the length of line of trim, material thickness and the shear strength of the material. Tonnage for forming operations depended on the size of the form, material thickness and material tensile strength. These calculations typically over-predicted the tonnage requirement, but due to the relatively low strength compared to AHSS, the overall part size that dictated the required press size became the limiting factor rather than the tonnage requirement.
Applying these same rules of thumb to the advanced steels in use today will likely under-predict the tonnage requirements. To understand why, let us examine the guidelines I used over 30 years ago.
For piercing a hole: Tonnage = d * t * 80 Equation 1
In this equation d is the punch diameter in inches, t is the material thickness in inches, and it calculates tonnage in tons. This was a simple and effective way to estimate the tonnage of all the holes pierced. Equation 1 is a simplification of the proper calculation being the length of line doing the work, in this case the circumference of a circle, multiplied by the sheet thickness and the material’s shear strength (ꚍ). The generic equation for any type of piercing or trimming is Tonnage = P * t * ꚍ where P is the perimeter or length of line of the trim, t is the sheet thickness and ꚍ is the shear strength of the material. A typical estimate for the shear strength (ꚍ) of mild steel is 60% of the tensile strength (T). Therefore, the equation development for a simple hole piercing looks like:
|Generic trim equation:
||Tonnage = P * t * ꚍ
|Specific for a round hole:
||Tonnage = πd * t * 0.6T
||Tonnage = d * t * 0.6Tπ
|Mild steel T = 300 MPa = 43.5 ksi:
||0.6 * 43.5 * 3.14 = 82
|Pierce a round hole:
||Tonnage = d * t * 80
Knowing how the rule of thumb was derived allows us to highlight some possible sources of error. First, the equation assumes trimming of the full thickness. In reality, a typical trim operation for steel consists of 20% to 50% trimming and the remainder is breakage. The press needs to apply load only for the trimming portion. Second, shear strength is not a fixed percentage of tensile strength. The actual shear strength should be measured for each specific grade as the microstructure differences of the AHSS will affect the material strength in shear. Lastly each of these errors are multiplied since today’s AHSS material has a tensile strength of three to five times that of mild steel. To see this, we can consider a simple example of piercing a 1-inch hole in 0.06 inch (1.5 mm) thick mild steel. Mild steel tensile strength typically ranges from 40 ksi to 55 ksi (280 MPa to 380 MPa). Looking at Equation 1 relative to Equation 2 with low- and high -end assumptions:
|Equation 1 estimate
||Tonnage = 1 * 0.06 * 80 = 4.8 tons
|Equation 2 minimum
||Tonnage = 3.14 * 0.06(20%) * 0.6(40) = 0.9 tons
|Equation 2 maximum
||Tonnage = 3.14 * 0.06(50%) * 0.6(55) = 3.1 tons
This simple example shows sources of error that could lead to an estimate ranging from 0.9 to 4.8 tons to pierce a single hole. A similar exercise could apply to a drawing operation. In this situation, most rules of thumb attempt to use the perimeter or surface area of the part, the material thickness and the material tensile strength to predict the tonnage needed. Sources for error in this type of calculation include: 1) Using the perimeter of the draw area, tending to under-predict; 2) Using the surface area of the part, tending to over-predict; and 3) Using the tensile strength of the material, also tending to over-predict as it assumes the material is stretched right to the level of splitting. Correction factors have been developed over time, but it is still easy to see there are many possible sources of error in these types of calculations.
AHSS Magnifies Press Tonnage Prediction Challenges
A number of reasons explain why the inherent challenges with old-school rules of thumb are exaggerated with AHSS:
- Strength: The strength of today’s cold stamped steels is quite incredible. Where a mild steel may have a tensile strength of 280 MPa, it is now common to cold stamp dual phase (DP) steels and 3rd Generation steels with up to 1180 MPa. In addition, new materials having a tensile strength of 1500 MPa with enough elongation to allow for cold stamping are starting to enter the market. This five-fold increase in strength acts as a multiplying factor for any errors in traditional predictions.
- Formability: The formability of AHSS has also increased dramatically. Today a DP 590 steel and even a 980 3rd Generation steel can have nearly the same elongation as a high-strength low alloy (HSLA) steel of 30 years ago. This affords the part designers the ability to incorporate more complex forms into a part including using darts and beads to increase a part’s stiffness, tight radii and deeper draws. All of these add to the tonnage used and are generally not part of the old school rule of thumb calculations.
- Springback Corrections: Springback is linearly related to the yield strength of a material. Therefore, stamping AHSS grades require more features to be added to the die process to control springback. These may include draw beads (used to control material flow early in the press stroke), stake beads (used at the bottom of the stroke to minimize springback) and tighter radii (Figure 2). These features are typically off product, in the addendum, and are easily ignored by typical rule of thumb calculations.
Figure 2: Draw and Stake Bead PlacementA-6
- Hardening Curves: The complex microstructure of AHSS offers many advantages to increase formability. All AHSS grades produce microstructural phase transformations during the stamping process. This allows the lower yield strength in the as-rolled material, which aids in formability, to increase during the stamping operation. This yield strength increase can be as much as 100 MPa. Models that estimate these hardening curves of the material are ignored when doing hand calculations.
- Other Considerations: Lastly the typical rule of thumb calculations, as we have discussed, only consider the part characteristics. They generally do not include the other sources that consume energy during the stamping process including off-product feature (beads, pilot holes, etc.), spring stripper pressure, pad pressure from nitrogen springs or air cushions, driven cams and part lifters. Many of these could be ignored 30 years ago with mild steels, but they become more significant with the strength of today’s AHSS.
Accurately predicting press requirements is a decades-long, industry-wide issue. Auto/Steel Partnership (A/SP), a partnership between automotive OEMs, steel mills and affiliate suppliers, teamed up with formability software suppliers to improve press tonnage prediction accuracy. A/SP’s efforts, including this project, looks to bridge the gap between research laboratories and the shop floor.
Stamping companies should keep press tonnage monitors in good working order, and upgrade to systems that can capture full through-stroke force curves. Engaging with organizations like A/SP, OEMs and steel mills, allows efficient information sharing and capturing best-practices. Get the steel mill involved early, even in the die design phase. All steel mills have teams of application engineers to help OEMs and their suppliers transition into using the newest grades of steel – they want stampers to succeed and have the tools and data to help.
Read more about Press Tonnage Prediction in the expanded article.