Electric vehicles came to market demonstrating new technologies and new challenges for OEMs and their suppliers. A significant focus of current research and development has been related to the cells’ chemistry, which is improving at a fast pace. In addition to this, new requirements regarding safety and performance are demanded which causes the battery tray to constantly evolve. In this direction, Gestamp defines an innovative steel battery box solution, which can be applied in a wide range of electric vehicle segments.
Four basic requirements have been established for this revolutionary system: High energy capacity, stampings with high formability, simplified assembly process and high safety performance. These four pillars were used to develop the first cell-to-pack concept. The cross members were deleted, allowing more space for additional cells. The enclosures were redesigned taking advantage of extra deep drawing steel alloys. The joining technologies were selected taking into account the process energy input to decrease final distortion and improve final assembly quality. Stamping Structural Parts
The new Gestamp steel battery box design achieved an energy storage increase of 15% using the same exterior packaging. Components have simple geometries with a high degree of manufacturability. The body and battery housing work as a holistic system resulting in a high crash performance, lighter weight solution.
With the rapid growth of electric vehicle (EV) market, overall battery safety becomes more and more important. One of the most challenging tasks is to minimize the intrusion to battery enclosure during the crash under side impact load. Rocker and rocker reinforcement are critical for the strength of the local periphery area to absorb lateral load. In this study, a vertical stacked-up steel tube design (CVST) is proposed for the rocker reinforcement, using steel grades of 1500 and 1200 MPa. Its performance is evaluated against a comparable aluminum extrusion design. With mass parity, the CVST design achieves better performance in peak force in three points bending analysis. Its advantages in cost of the simple steel tube design as well as the greenhouse gas emissions versus extruded aluminum solutions are assessed.
Ideal proportional loading conditions with linear strain paths are rarely encountered in automotive forming and fracture applications. Despite this, the majority of forming and fracture models such as forming limit curves and phenomenological fracture surfaces have been proposed under the assumption of linear strain paths. In the present study, the influence of nonlinear strain paths on the fracture behavior of DP1180 automotive steel was experimentally investigated. The DP1180 was subjected to bilinear strain histories with the first path being proportional in-plane stretching in uniaxial, plane strain and equal-biaxial conditions. Fracture coupons were then extracted and tested for the second loading stage from shear to biaxial tension. The experimental data was then used to evaluate popular phenomenological fracture modes used in industry and academia such as the Johnson-Cook and GISSMO damage models. It is shown that phenomenological fracture models that employ strain-based damage indicators can result in significant errors in non-linear strain paths. An alternate modelling approach is proposed by defining a stress-based “fracture potential” that can be calibrated with proportional test data and does require a phenomenological damage model for non-linear loading. Finally, recommendations and best-practices are discussed to minimize the testing required to characterize the constitutive and fracture behavior for crash applications.
OEMs are rapidly moving toward electrification of their vehicle fleets. This powertrain transition from internal-combustion engine (ICE) to battery electric vehicles (BEV) present a significant opportunity for steel, particularly with battery enclosures. While many OEMs actively consider steel battery enclosures for smaller mass-market vehicles, they have tended to use aluminum enclosures for larger platforms. The preference for aluminum may be attributed to the need for lightweighting, non-availability of efficient steel designs for benchmarking, short program times for detailed design, and lack of lightweight designs being developed and promoted by tier suppliers. ArcelorMittal has developed a family of steel battery enclosure solutions to address the specific and varied needs of our automotive customers. These solutions combine innovative design along with the judicious use of the steel grades available from ArcelorMittal. Some options include:
1. Module cooling via an external steel cooling structure attached to the bottom of the tray. 2. Cold stamped and roll formed concepts that take advantage of 1300Y1500T steel grades to maximize weight reduction. 3. A concept with a single piece, press hardenable steel (PHS) inner structure to minimize part count.
All solutions are engineered to meet commonly specified OEM requirements for side crush and natural frequency. Manufacturing and assembly feasibility assessments are undertaken for all designs to ensure the viability of the solutions.
Automotive manufacturers are moving rapidly towards producing battery electric vehicles (BEV) in an effort towards a clean and sustainable future. Electrification introduces flexibility into vehicle architecture by adapting to an updated powertrain while reimagining the vehicle structure.
OEMs are moving towards high volume production of BEVs. Alternative materials like aluminum, magnesium and composites are good choices for low volumes but are not ideal for high production volumes. For managing economics of scale, cost optimization is critical. To optimize investment, OEMs are simplifying assembly by reducing assembly steps using so-called mega structures. Our steel laser welded Multi Part Integration™ (MPI) concepts are excellent solutions to meet these challenges.
Laser welded blank (LWB) technology is a proven solution enabling performance improvement, part consolidation, and weight optimization in vehicles. Additionally, cost improvement, modularity, and sustainable steel use make LWBs an ideal solution to battery electric vehicles (BEV) architectural challenges.
ArcelorMittal Tailored Blanks (AMTB) will showcase our next generation of MPI battery pack concept in steel that enable re-designing of the vehicle architecture surrounding the passenger and battery space. Our design concept enables cell-to-body integration, which can make the battery pack modular with a reduced part count by potentially eliminating the floor and provide additional rigidity to the cabin space. The concept has been developed to provide an answer to the sealing challenges that OEMs face when it comes to a modular battery pack. An upper and lower LWB clam shell design helps seal the battery modules while managing crash loads using press hardened steel.
Our presentation intends to make a strong case for steel-based architectures using MPI designs which are key enablers in weight reduction, cost improvement, performance optimization and reducing assembly complexity, cost and time while improving sustainability of future BEV designs.
As the automotive sector moves further into electrification, overall vehicle mass increases due to battery content and long-range expectations. The IIHS 2.0 Side Impact event is of particular interest, as the impact barrier’s mass and speed has increased. The body structure must manage higher levels of energy, ensuring vehicle crash worthiness, occupant safety and battery integrity.
Gestamp utilizes extreme-size stamping strategies to address higher impact energy levels. Benefits include part count reduction, reduced vehicle build time/complexity, green steel attributes and recyclability. Gestamp uses a holistic approach to vehicle energy management, addressing the impact event in stages with the body structure. Although all impact events are critical, the side impact is one Gestamp has a great deal of experience with, providing many innovative solutions. Gestamp’s focus today is how to best use the steel door Ring and sill/rocker structure to manage the IIHS 2.0 Side Impact event, while protecting the occupant and battery enclosure.
Gestamp’s OLPB Door Ring and Wave Rocker innovations offer a high strength low-cost solution to the energy management challenge. The OLPB Door Ring, comprised of spot-welded blanks prior to hot stamping, offers (~7-10%) cost savings over traditional architectures while reducing mass, part count and plant complexity. The Steel Wave Rocker is made from either Press Hard or Hydroformed Steel, and absorbs maximum energy in a compact environment to prevent intrusion into the battery enclosure.
Design and manufacturing of modern electric vehicle (EV) battery trays currently don’t follow a uniform standard and features a variety of different requirements, materials and joining technologies. While one design is based on extruded aluminum profiles, other models use higher-strength steel alloys. Some battery boxes can be replaced frequently for recharging; others remain in the vehicle for the long term. In any case, the connection between the battery tray and the car body is not only exposed to mechanical stresses with every movement, but must also be corrosion-resistant over the long term and waterproof even under cyclic loads. This presentation illustrates the requirements for fasteners, particularly in the application of battery trays, and how their performance can be proven and verified. The relevant simulation, testing and measurement methods are explained and presented with practical examples of different designs made from extruded profiles and higher-strength steels.
Sumitomo Heavy Industries developed a new press forming technology Steel Tube Air Forming (STAF) for forming Body-in-White (BIW) parts such as A-pillar reinf., bumper reinf., side frame and so on. The concept of STAF is concentrated on maximum weight reduction and reduction of manufacturing cost with a single process. In the STAF process, a steel tube is processed through “a single step” in the tooling of press machinery. A steel tube is jouel heated (high-speed), air-formed and hardened.
STAF-formed parts have characteristic appearance with optimally designed flanges, TS over 1500 MPa, and continuously varied closed cross-section structure. First of all, STAF-formed parts can significantly improve basic performance against conventional hot-stamped parts due to its closed and flanged geometry. Sumitomo can expect weight reduction by around 30%. More than anything, the most unique part of the process is forming various flanges, which can integrate surrounding parts into STAF, improve joining and enhance performance. STAF’s flanges dramatically reduces part count, thereby reducing manufacturing costs and tooling investments. Furthermore, we puts a compact jouel heating device into practical use, replacing the conventional large heating furnace. The heating process will bring not only super power-saving but significantly reduces CO2 emissions from equipment.
As described above, STAF is the latest technology that can drastically improve performance and reduce weight and manufacturing costs.
Urbanization and Net Zero Emissions policy ambitions are leading contributors to the transportation shift to mobility on demand. Significant growth in Mobility as a Service (MaaS) (ride sharing transportation) is anticipated, and these vehicles will emphasize autonomous vehicle technologies and electrification. This presentation details the development of a new body structure design for a Level 5 fully autonomous vehicle, using the latest Advanced High-Strength Steel (AHSS) grades and fabrication processes.
The vehicle concept was created within the Steel E-Motive project, a collaboration between WorldAutoSteel and Ricardo, the UK-based engineering and sustainability consultancy. The vehicle has been designed with the new mode of transport in mind, with a strong focus on the user, the fleet operator and the vehicle’s operating environment. A change from driver to fully autonomous operation eliminates the requirement for driver interfaces and controls and enables occupants to be seated in unconventional locations and orientations. Legislative requirements such as driver vision and obscuration are also removed, which opens up further freedoms such as the ability to place structure where glazing previously existed. These freedoms have enabled the creation of a unique and spacious transportation environment, while being compact in size and agile around city center.
Despite compact dimensions with short front and rear overhangs, sophisticated engineering and the use of AHSS tailored to the specific vehicle requirements result in compliance to global high-speed crash and safety requirements. This presentation reveals more details of this 2.5-year concept design development program on the Steel E-Motive vehicle and body structure, the steel grades and technologies used and the performance achieved.
The transition to electric mobility (E-mobility) and the resulting changes in the car body structures place new demands on structural components. Especially prismatic, profile shaped components increase in their application. From an economic point of view, however, these new components can only be produced to a limited using the manufacturing processes that have been predominant in the body construction of internal combustion engine vehicles. Roll forming, in contrast, opens up new possibilities for the efficient production of these new types of structural components. It is – not only for the purpose of crash performance – becoming particularly apparent in the structures for cutting edge electric vehicles that profiles having a multi-chambered cross-section are subject to growing demand. At the same time, the battery masses to be carried increase the need for weight-reducing measures in the body structure, so that high and ultra-high-strength steel (UHSS) alloys with strengths of up to 1700 MPa and more are increasingly being used. As a result of these material requirements or cross-section requirements, numerous manufacturing processes for profile-shaped structures such as extrusion, classic bending processes and deep drawing reach or exceed their capability limits. Roll forming, on the other hand, can meet these requirements and remains as an economical alternative for manufacturing the required multi-chamber profiles from high-strength materials. In this presentation, we will present current strategies for the production of such profiles by roll forming. In addition to the theoretical presentation of the general suitability of this manufacturing process, we will use concrete examples to discuss the manufacturing strategy as well as how to deal with the specific challenges of high strength materials (including changing material properties). As examples we will use manufacturing lines for components of well-known OEMs, which have been realized by Dreistern in the past years.
The importance of true fracture strain was initially highlighted in the representation of local formability in material selections among various advanced high-strength steels (AHSSs) of similar tensile strength. Inspired by the relative studies, a precedent work compared the true fracture strain results measured via either digital image correlation (DIC) or fracture surface laser scanning on different AHSS tensile test samples. That work concluded that the DIC-based testing results comparatively underestimated the fracture strain. As a continued study, the present work further analyzed the DIC-based testing procedure and attributed such an underestimation mainly to the volume constancy assumption. Furthermore, this work pointed out that also because of the same assumption, the fracture surface laser scanning method to some extent overestimated the true fracture strain results. Nevertheless, it was observed that different AHSS grades were affected discrepantly by such two measurement methods. Therefore, scanning electron microscope (SEM) was applied to inspect the morphology of various micro-voids and dimples on different fracture surfaces to explain such a discrepancy. To bypass the volume constancy assumption, this work proposed two alternative methods, including a DIC-based thinning measurement method and a hybrid method, and discussed their pros and cons. In addition, the effects of DIC-recording frame rate and using different yield functions to derive the effective strain were also studied in this work. Last but not the least, by extending the considerations to damage-fracture modeling for forming and crash simulations, the importance of the true fracture strain accuracy was further highlighted.
Coating Free Press Hardened Steel (CFPHS) is a novel steel grade patented by General Motors Company with improves mechanical properties and surface quality over the current market favorite, AlSi coated 22MnB5. CFPHS has increased tensile strength and improved toughness over 22MnB5, enabling light-weighting of vehicles and improved crash protection for vehicle occupants. To enable vehicle architects to design parts with confidence using this material, it is important to develop computer-aided engineering (CAE) material cards that directly correlate to the actual material performance. This presentation reviews two CAE to trial result correlations demonstrating the compliance of the developed material cards with manufactured parts.
This presentation compares results of bend testing performed via CAE simulation to physical results from two components: a door beam and impact beam. The simulations show a close correlation between expectations from CAE to the physical testing results. This demonstrates that the efforts put into developing the materials cards have resulted in reliable simulation results. These CAE material cards can be utilized going forward to design new components for future programs, or to simulate the functionality of CFPHS in a drop-in application.
A family of new ultra high-strength steel (UHSS) multi-phase (MP) grades has been developed by ArcelorMittal to introduce cold-stamping UHSS products with improved elongation, bendability, flangeability, local formability, and possibly fracture limits, to address unique design challenges posed by automotive structures with very high strength requirements.
Roll forming is a continuous, lengthwise process which progressively bends sheet metal into a desired profile. The gradual nature of the bending enables the forming of ultra-high strength materials. Roll forming is typically applied in vehicle areas where high strength is required, such as bumper systems and rockers. As battery electric vehicles have become more prevalent the demands on those structures have changed; due to the increased vehicle mass, more energy must be absorbed, and in a smaller space to protect the battery system. The MP1300 and 1500 grades developed by ArcelorMittal are presented here as a potential for these applications. The improved bendability will allow for better energy absorption in crash at similar strength levels due to enhanced fracture resistance. The MP grades also help to alleviate challenges in roll forming such as further reducing the radius (R/t) of the section for better packaging, and improving spring-back and part forming operations post roll forming.
Mill trials have already taken place, on both uncoated and galvanized products. The microstructure concept and the characterization of major attributes, including HER and R/t, from sampled coils will be described. Roll forming trials have been conducted with these grades to characterize the bendability, and application specific parts have been produced to validate the improvement in energy absorption; all those results will be disclosed.
After pre-straining and bake treatment, most automotive steels show hardening behavior. In some cases, as the pre-strain increases, the bake hardening effect also increases. This characteristic can be a strong point of advanced high-strength steel (AHSS), especially for crash parts design. Historically, ASTM Standard A653/A653M has been used to evaluate the Bake Hardening Index (BHI). But on high tensile strength and low elongation materials, the BHI often can not be properly evaluated because failure occurs outside the gauge section due to a lack of remaining elongation. Depending on the part design, large strains (over 5%) distributed after forming. The Auto/Steel Partnership steel testing and harmonization team (STHT) concluded that an improved test procedure is needed for large pre-strain and bake hardening effect evaluation. In 2022 the STHT performed two rounds of bake hardening tests on two materials at two labs (POSCO and General Motors Company). The first round utilized the existing ASTM methodology. The second round utilized a modified methodology based on the pre-strain of a larger specimen. The new test procedure has resulted in more accurate BHI values in large pre-strain conditions as compared to the present ASTM test standard.
As the automotive industry gravitates to higher strength steel applications to aid in vehicle light weighting and better safety performance, additional panel springback is encountered using conventional cold stamping processes. To address this, hybrid beads were investigated as an alternate approach to traditional stake beads as a method to induce plastic deformation in the stamping operation and consequently lessen springback. The advantage of hybrid beads, when compared to stake beads, is an improvement in material utilization. To accomplish this, stamping simulations and physical die trials were performed to design and compare different hybrid bead geometries. In all, five different hybrid bead designs were evaluated in laboratory scale die trials and two of these designs were subsequently used in larger production scale die trials. During these production scale die trials, both the robustness of the hybrid beads and the associated forming forces required to engage them emerged as significant issues. However, the hybrid beads proved effective in reducing panel springback and side wall curl when sufficient forming forces were available. Forming simulations were also conducted and agreed with the physical die trials. Additional work is required to further develop alternate hybrid bead geometries for successful implementation.
Accurate characterization of edge fracture of advanced high-strength steel (AHSS) is important for several needs such as material approval, troubleshooting of stamping failures and development of fracture criteria for finite element analysis (FEA) prediction. While the standard hole expansion test has been used for material approval purposes, its use for the other needs of resolving edge fracture issues is hindered by several issues such as test repeatability, and dependence of results on initial hole diameter. In this study, a half-dome test with a spherical punch was employed to perform edge fracture tests on ArcelorMittal’s Gen 3 AHSS, Fortiform®980. For comparison, hole expansion tests were also conducted. Edge fracture strain, strain distribution and strain path in the edge vicinity areas were determined using Digital Image Correlation (DIC). It was found that strain path in the edge vicinity area for half-dome test was closer to uniaxial tension than that from hole expansion test. Furthermore, the half dome test could determine directional dependence of fracture strains with respect to the rolling direction. This study demonstrates that half-dome test could be a valid candidate for comprehensive and accurate edge fracture characterization.
ArcelorMittal and its hot press tooling technology partner, American Tooling Center (ATC), have successfully commissioned a production intent hot stamping B-pillar tool using 1.6 mm Usibor® 1500 AS. The B-pillar tool was designed to represent a reasonably difficult part using a monolithic blank and same gauge laser welded blank (LWB) combinations. The intent is to study stamping conditions and process windows for robust parts, using various press hardened steel grades (Usibor® 1500, Usibor® 2000, Ductibor® 1000, Ductibor® 500), under real world manufacturing conditions. The scope of this presentation focuses on the commissioning activities using monolithic Usibor® 1500 AS and the value of FEA is demonstrated. A final buy-off run was successfully performed along with physical testing of parts, examining part quality and consistency. For example, part strength, part distortion, and manufacturing performance (thinning and wrinkling) were measured on parts sampled throughout the run. Detailed press traces and process data were also logged during the buy-off trial. Key to successful commissioning of the tools was continual communication and collaboration between ATC and ArcelorMittal.
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