Meeting the Challenges of Autonomous Vehicles: A Steel E-Motive Student Engagement to Adapt SEM2 from People Mover to Commercial Delivery Vehicle

Meeting the Challenges of Autonomous Vehicles: A Steel E-Motive Student Engagement to Adapt SEM2 from People Mover to Commercial Delivery Vehicle

WorldAutoSteel is focused on advancing steel’s advantages in the automotive, autonomous vehicle, and future mobility industries. To encourage careers in engineering, we are committed to engaging with future engineers at post-secondary education organizations around the globe. Our most recent engineering project, Steel E-Motive, was created to help the industry meet the challenges of future mobility and Level-5 autonomous vehicles and eventually reach net zero emissions targets. 

We engaged Ricardo plc to collaborate with our technical directors to develop a Level 5 Autonomous Vehicle for the Steel E-Motive project. The project uncovered a few challenges that were solved by student engineering teams through Senior Capstone Projects. Here we summarize the Side Door and Door Hinges project, created at Michigan Technological University by the engineering students and faculty members listed herein.

Michigan Tech University – Senior Capstone Project #2: Adaptation of SEM2 from People Mover to Commercial Delivery Vehicle
MTU Senior Capstone Team: Kyle Davis, Nick Palatka, Evan Larson, Logan Pietila, Blake Pietila, Tej Bergin

Introduction and Background

This Senior Capstone Design Team was sponsored by WorldAutoSteel and Ricardo Engineering (UK engineering and consultancy firm) to develop a solution for expanding the serviceability within a 24-hour period for Steel E-Motive 2 (SEM2), their extra-urban electric autonomous vehicle concept.

The SEM2 vehicle is a stretched 6-passenger commuter targeting longer journeys with expanded occupancy or additional luggage capacity. In non-commuting hours, the Mobility Fleet Operator would like to continue revenue generation by quickly adapting the vehicle for commercial delivery services. Currently, occupant packaging contrasts with the storage requirements of a package delivery vehicle; thus, the SEM2 vehicles can only be utilized to either transport people or goods. Our project aims to develop interior seating that enables quick removal and adaptation to an optimal delivery van.

Project Details and Results

From the requirements outlined by WorldAutoSteel, the team focused further research on interior vehicle and seat design. Modern delivery methods, delivery vehicle layout, passenger vehicle seat safety requirements, and seat folding or locking mechanisms were sub-categories of research that hold value within the project’s scope. The main takeaways from our research include different pin and slot mechanisms that are incorporated into a preliminary design for a quick-release system. We also benchmarked a vertical folding seat based on International Harvester designs, and we have modified these for application to the SEM2 vehicle’s specific needs.

The quick-release system will benefit MSP technicians responsible for performing the conversion of multiple SEM2 vehicles in its fleet at their depot during off-commuting hours. In under 30 minutes, the fleet operators (MSP) must be able to convert the vehicle from a vehicle stressing passenger comfort to an autonomous delivery van and vice versa, using common tools and techniques while meeting all necessary safety standards and regulations. The fleet operators that provide the robotaxi service are not expected to see any major disruption in ride services; however, they may observe improved utilization and profitability if they use the vehicles for package delivery.

Our current engineering requirements include:

  • Maximum total payload of 675 kg (6 passengers and 6 seats)
  • Individual seat weight of 30kg (6 seats)
  • Total volumetric storage space requirement of 1 cubic meter
  • Total seating width equal to or less than 1220 mm
  • Changeover time less than 30 minutes and
  • Minimal number of changeover movements (Fewer than 50 for the complete conversion from passenger to cargo transportation)

Concept Solutions

A graphical rendering of our selected system-level autonomous vehicle concept is provided in Figure 1. This design showcases two pairs of T-shaped rails placed in the fore-aft direction of the vehicle. These T-Rails are compatible with a slider system connecting to each seat leg’s bottom.

A sliced view of the rails and slider system are seen in Figure 2. On each slider, in the port-starboard direction, a circular slot approximately 20 mm in diameter (dependent on pin material and size) is cut-out to allow for the insertion of a spring-loaded steel pin. This pin engages both the slider and an equivalent slot cut into the rail, to allow the seat to be locked into a specified position along the rail (Figure 3). The rails run the full length of the vehicle’s interior, allowing 3 seating modules to be placed and locked into a position. Inserting and removing the slider on the rails will be possible through narrow sections where the slider can be vertically lifted or placed on the rail system.

In order to “drop the seats” onto the rails without manual lifting, the team has designed an accompanying “pallet jack accessory” that will be able to hold, transport, and lower the seating modules onto the rail system through the use of an industry-standard pallet jack with a lifting range of 6 inches. The pallet jack accessory can be seen in Figure 1, item C.

Figure 1 – System level concept – rapid adaptation of SEM2 to autonomous delivery services

The slider mechanism, seen in Figure 2 below, will house a ball roller bearing that allows for the translational motion along the rail to slide the seats into position.

Figure 2 – Slider mechanism in Steel E-T-rail system

The roller bearing bolts into the slider mechanism allowing for fast and easy replacement. The T-Rails are then bolted to the structural members of the vehicle, where engineers from WorldAutoSteel have confidence the design can withstand any and all static and dynamic loading scenarios. The rails will feature a narrow section near the center, allowing the seating module to be removed. This narrow section is tapered to allow the slider to be “homed” and slid into its final, fixed position.

The arms of the pallet jack system reach out to allow a set of seats to be placed on the rail at one time. The pallet jack will drive through the open slots on the ground, lift the seats to the vehicle, align itself using markings on the vehicle, and drop the seats into the rails, where they can then be manually moved to their correct position. The system is designed to remove and insert these seats as fast as possible while exerting minimum effort that might stress the MSP technician. With ease of use and safety being the critical elements of every project, we’ve removed manual lifting from the equation and ensured a factor of safety of 2 is kept for all required crash loads under our current design.

Many integral components are COTS parts and can be bought in bulk to use for mass production as well as reserved parts, helping maintain low cost of ownership for the MSP. The components that are not COTS items, such as the rails and sliders, can be manufactured using high-volume, low-cost fabrication techniques such as stamping. Assembly of the system will be just as easy as all components are connected together using industry-standard fastening and welding techniques.

Conclusions

Validation – To confirm the ease of changeover and our objectives, a simulation was conducted to estimate the time and difficulty of changing from delivery service to people transporter. In a warehouse setting, a location was established as the “vehicle maintenance spot.” We developed a “seat module storage area” approximately 30 meters away. In this simulation, the following steps were conducted:

  • A pallet jack was pre-staged near the vehicle, signaling the beginning of the conversion and timer
  • A technician walked 30 m to the seating storage area and picked up the seats via accessory
  • They carried seats back to the vehicle, aligned the pallet jack with the door, lowered seats onto the rail system, and removed the pallet jack accessory from the vehicle
  • They slid the seats into the correct position and inserted the locking pin into its slot

This simulation was repeated twice to replicate the insertion of all six seats. To remove the seats, the steps would be reversed. After five runs of this simulation, ensuring adequate time to perform each simulated step, the average time to complete the simulation was 5 minutes 17 seconds with a total of 24 required movements.

These values were well within the 30-minute and 50-movement objectives. For further validation, we’ll repeat these simulations in the opposite direction, ie, removing the seats to transform into the delivery van. Finally, in Phase 2 of this project, we’ll continue to evaluate seat frame/track componentry to ensure robustness and durability in the proposed solutions.

More Info About Steel E-Motive

We are grateful to our student teams, their supportive leaders, and the universities providing automotive engineering education to our future industry leaders. Their contributions to Steel EMotive have been invaluable.

Interested in learning more about Steel E-Motive and the infinite tunability of steel for Future Mobility? Download the full engineering report here: Steel E-Motive Engineering Report 

Meeting the Engineering Challenges of Autonomous Vehicles: A Steel E-Motive Student Engagement – Side Door Functionality and Door Hinge Assessment

Meeting the Engineering Challenges of Autonomous Vehicles: A Steel E-Motive Student Engagement – Side Door Functionality and Door Hinge Assessment

As an organization focused on advancing the advantages of steel to the automotive and future mobility industries, WorldAutoSteel is committed to engaging with future engineers at universities and colleges around the globe.

Our most recent engineering project, Steel E-Motive, was designed to unveil and meet the challenges of future autonomous vehicles that will help the automotive industry reach net zero emissions targets. 

For the Steel E-Motive project, we engaged Ricardo plc to collaborate with our technical directors to develop a Level 5 Autonomous Vehicle. The project uncovered a few challenges that were solved by student engineering teams through Senior Capstone Projects.

Here we summarize the Side Door and Door Hinges project, created at Michigan Technological University by the students and faculty members listed herein.

Steel E-Motive Side Door Functionality and Door Hinge Assessment

The MTU Senior Capstone Team members were Gavin Sheffer, Leander Daavettila, Rob Oestreich, Steven Turnbull, Andrew Mitteer, and Jesse Ebenhoeh.

Introduction and Background 

The MTU Senior Capstone Design Team, sponsored by WorldAutoSteel and the Auto/Steel Partnership, was challenged to design a new door hinge for the Steel E-Motive side closure mechanism. The current hinge design was referenced, but a few operational issues were identified for this team’s assessment and engineering study. 

The door had unconstrained degrees of freedom, allowing it to swing freely about one axis. Thus, the project included a review of the kinematics of the door opening and hinge design for attachment to the A and C pillars.

The previous hinge design interferes with the all-wheel steering, meaning in emergency situations, passengers could get trapped in the vehicle. For SEM1, it was also observed that when the wheels are turned, they would block the doors from opening fully, constraining the passenger exit (Figure 1 below).

An emergency release mechanism was needed to allow users to escape if a crash or electrical failure prevented the doors from opening.

Power requirements and electric motor sizing for the hinge mechanism needed to be defined. 

Steel E-Motive Side Door Opening

Figure 1: Side door opening constrained by present hinge mechanism and wheel position.

 

Project Details and Results

MTU’s design solution uses a four-bar linkage hinge design to keep the door parallel to the vehicle’s body to avoid damage to either the door or the body. The team used a 4:1 gear ratio for the drive motor to open the door. Finally, one of the pins in the secondary arm linkage is accessible by passengers and removable, allowing users to manually push the door open in the event of an emergency (see Figure 2).

Their solution includes pressing a button to open and close the door. The stepper motor receives the input from the button and rotates the gears; the gears then rotate the primary arm, which drives the door. The primary arm is mounted to the door in two locations with bolts. The secondary arm is added to the mechanism to create a four-bar linkage, which helps maintain door orientation during operation. An emergency release was designed and added to the secondary arm to release the four-bar linkage. This allows the door to swivel around a turned wheel in the event of an emergency.

Steel E-Motive Door Mechanism

Figure 2: 3D model of the final design less the gear cover for clarity.

 

The emergency release was designed for safety and manufacturability. The emergency release pin was simplified to utilize off-the-shelf pins. A relay is suggested to cut power to the motor and allow the door to be manually pushed open. A gear cover was added for safety to protect the occupants from getting pinched by the gears. To maintain and service the design, each assembly part was designed to be attached with threaded fasteners. A gear drive was created so we could use a more cost-efficient motor. The gear ratio decreased the amount of torque required. The material was chosen based on strength and sustainability.

Conclusions

The CAD geometry and the quarter-scale prototype met all of our engineering requirements and objectives. The CAD model defines the mass and emergency release mechanism, and the kinematics are verified by the quarter-scale prototype. The FEA simulation verified set and sag under normal and abusive loading conditions. 

In identifying specifications required for the full model, the projected production cost for one mechanism is $471. This includes stamped and cast components, off-the-shelf components such as the stepper motor and fasteners, and the assembly cost. 

The team compared using AHSS for the components as opposed to an aluminum alloy. Steel components are stronger, half the price of aluminum, and produce 1/3 of the carbon emissions compared to the same amount of aluminum.

The prototype was 3D printed from PLA plastic at a quarter scale. This serves as a model to be shown by Auto/Steel Partnership for future presentations. It was also used to verify the kinematics of the door motion.

Learn more about Steel E-Motive and download the FREE Engineering Report here: Steel E-Motive 

 

Resistance Spot Welding: 5T Dissimilar Steel Stack-ups for Automotive Applications

Resistance Spot Welding: 5T Dissimilar Steel Stack-ups for Automotive Applications

Urbanization and waning interest in vehicle ownership point to new transport opportunities in megacities around the world. Mobility as a Service (MaaS) – characterized by autonomous, ride-sharing-friendly EVs – can be the comfortable, economical, sustainable transport solution of choice thanks to the benefits that today’s steel offers.

The WorldAutoSteel organization is working on the Steel E-Motive program, which delivers autonomous ride-sharing vehicle concepts enabled by Advanced High-Strength Steel (AHSS) products and technologies.

The Body structure design for this vehicle is shown in Figure 1. It also indicates the specific joint configuration of 5 layers AHSS sheet stack-up as shown in Table 1. Resistance spot welding parameters were developed to allow this joint to be made by a single weld. (The previous solution for this welded joint is to create one spot weld with the bottom 3 sheets indicated in the table and a second weld to join the top 2 sheets, combining the two-layer groups to 5T stack-up.)

NOTE: Click this link to read a previous AHSS Insights blog that summarizes development work and recommendations for resistance spot welding 3T and 4T AHSS stack-ups: https://bit.ly/42Alib8

 

 

Table 1. Provided materials organized in stack-up formation showing part number, name, grade, gauge in mm, and coating type. Total thickness = 6.8 mm

 

The same approach of utilizing multiple current pulses with short cool time in between the pulses was shown to be most effective in this case of 5T stack-up. It is important to note that in some cases, the application of a secondary force was shown to be beneficial, however, it was not used in this example.

To establish initial welding parameters simulations were conducted using the Simufact software by Hexagon. As shown in Figure 2, the final setup included a set of welding electrodes that clamped the 5-layer AHSS stack-up. Several simulations were created with a designated set of welding parameters of current, time, number of pulses, and electrode force.

Figure 2. Example of simulation and experimental results showing acceptable 5T resistance spot weld (Meets AWS Automotive specifications)

 

 

Thanks is given to Menachem Kimchi, Associate Professor-Practice, Dept of Materials Science, Ohio State University and Technical Editor – Joining, AHSS Application Guidelines, for this article.

 

Stronger AHSS Knowledge Required for Metal Stampers

Stronger AHSS Knowledge Required for Metal Stampers

This month’s blog was contributed by Peter Ulintz, Precision Metalforming Association. This content originally appeared in the September 2023 issue of MetalForming Magazine under the title Stronger AHSS Knowledge Required for Metal Stampers” and has been reproduced with the permission of MetalForming Magazine.

Metal stampers and die shops experienced with mild and HSLA steels often have problems making parts from AHSS grades. The higher initial yield strengths and increased work hardening of these steels can require as much as four times the working loads of mild steel. Some AHSS grades also have hardness levels approaching the dies used to form them.

Dies Get Tougher

Metal stampers and die shops experienced with mild and HSLA steels often have problems making parts from AHSS grades. The higher initial yield strengths and increased work hardening of these steels can require as much as four times the working loads of mild steel. Some AHSS grades also have hardness levels approaching the dies used to form them.

The higher stresses required to penetrate higher-strength materials require increased punch-to-die clearances compared to mild steels and HSLA grades. Why? This clearance acts as leverage to bend and break the slug out of the sheet metal. Stronger materials need longer levers to bend the slug. The required clearance is a function of the steel grade and tensile strength, and sheet thickness.

Increasing cutting clearance can result in punch cracking and head breakage due to higher snapthrough loads and reverse-unloading forces within the die. Adding shear angles to the punch face helps reduce punch forces and reverse unloading.

Tight-cutting clearances increase the tendency for die galling and chipping. The severity of galling depends on the surface finish and microstructure of both the tool steel and work material. Chipping can occur when process stresses are high enough to cause low-cycle fatigue of the tooling material, indicating that the material lacks toughness.

Stamping Tool Failure Modes (Citations T-20 and U-7)

 

Tempering of tools and dies represents a critical heat-treatment step and serves more than one purpose, but of primary concern is the need to relieve residual stresses and impart toughness. Dies placed in service without proper tempering likely will experience early failure.

Dies made from the higher-alloy tool-steel grades (D, M or T grades) require more than one tempering step. These grades contain large amounts of retained austenite and untempered martensite after the first tempering step and require at least one more temper to relieve internal stresses, and sometimes a third temper for even greater toughness.

Unfortunately, heat treatment remains a “black-box” process for most die shops and manufacturing companies, which send soft die details to the local heat treat facility, with hardened details returned. A cursory Rockwell hardness test may be conducted at the die shop when the parts return. If they meet hardness requirements, the parts usually are accepted, regardless of how they may have been processed—a problem, as hardness alone does not adequately measure impact toughness.

Machines Get Stronger

The increased forces needed to form, cut and trim higher-strength steels create significant challenges for pressroom equipment and tooling. These include excessive tooling deflections, damaging tipping-moments, and amplified vibrations and snapthrough forces that can shock and break dies—and sometimes presses. Stamping AHSS materials can affect the size, strength, power and overall configuration of every major piece of the press line, including material-handling equipment, coil straighteners, feed systems and presses. 

Here is what every stamper should know about higher-strength materials:

  • Because higher-strength steels require more stress to deform, additional servo motor power and torque capability may be needed to pull the coil material through the straightener. Additional back tension between the coil feed and straightening equipment also may be required due to the higher yield strength of the material in the loop as the material tries to push back against the straightener and feed system. 
  • Higher-strength materials, due to their greater yield strengths, have a greater tendency to retain coil set. This requires greater horsepower to straighten the material to an acceptable level of flatness. Straightening higher-strength coils requires larger-diameter rolls and wider roll spacing in order to work the stronger material more effectively. But increasing roll diameter and center distances on straighteners to accommodate higher-strength steels limits the range of materials that can effectively be straightened. A straightener capable of processing 600-mm-wide coils to 10 mm thick in mild steel may still straighten 1.5-mm-thick material successfully. But a straightener sized to run the same width and thickness of DP steel might only be capable of straightening 2.5 mm or 3.0-mm thick mild steel. This limitation is primarily due to the larger rolls and broadly spaced centers necessary to run AHSS materials. The larger rolls, journals and broader center distances safeguard the straightener from potential damage caused by the higher stresses. 
  • Because higher-strength materials require greater stress to blank and punch as compared to HSLA or mild steel, they generate proportionally increased snapthrough and reverse-unloading forces. High-tensile snapthrough forces introduce large downward accelerations to the upper die half. These forces work to separate the upper die from the bottom of the ram on every stroke. Insufficient die-clamping force could cause the upper-die half to separate from the bottom of the ram on each stroke, causing fatigue to the upper-die mounting fasteners. 
  • Because energy is expended with each stroke of the press—and this energy must be replaced—critical attention must focus on the size (horsepower) of the main drive motor and the rotational speed of the flywheel in higher-strength-steel applications. The main motor, with its electrical connection, provides the only source of energy for the press and it must generate sufficient power to meet the demands of the stamping operation. The motor must be properly sized to replace the increased energy expended during each press stroke. For these reasons, some stampers consider the benefits of servo-driven presses for these applications.

As steels becomes stronger, a corresponding increase in process knowledge is required in terms of die design, construction and maintenance, and equipment selection.

You can read more about these topics at these links:

Tooling and Die Wear
Coil Processing Straightening and Leveling
Press Requirements

Peter Ulintz

Thanks go to Peter Ulintz, of the Precision Metalforming Association (PMA) for authoring this article. Ulintz was employed in the metal stamping and tool & die industries for 38 years before joining Precision Metalforming Association (PMA) in 2015. He provides industry-related training and seminars in Stamping Press Operation and Setup; Designing and Building Metal Stamping Dies; Die Maintenance and Troubleshooting; Metal Stamping Design for Manufacturability; Deep Draw Tooling and Process Technology; Stamping Higher Strength Steels; and Problem Solving in the Press Shop. Peter is a contributor to ASM Handbook, Volume 14B, Metalworking: Sheet Forming (2006) and writes the monthly column, Tooling by Design, for PMA’s monthly publication, MetalForming Magazine.

 
Die-Free Blanking of Class A Quality & Structural Parts

Die-Free Blanking of Class A Quality & Structural Parts

You’ll find this content as part of our page on Laser Blanking, but this month, we want to highlight it in our AHSS Insights blog.  We thank Schuler North America for contributing this insightful case study.

Production of Class A quality and structural parts without a blanking die is possible, even for high-volume serial production. Laser blanking enables flexible, cost-effective, and sustainable manufacturing and is capable of reaching 45 parts per minute.  DynamicFlow Technology (DFT) from Schuler provides highly productive, die-free blanking with lasers—directly from a continuously running steel coil. DFT combines the advantages of flexible laser cutting with the speed of conventional blanking.

 

Laser Blanking Lines at a Glance

Figure 1 – Laser blanking lines offer additional flexibility over conventional blanking approaches.

Laser blanking technology addresses market challenges such as frequent die changes, the need to increase capacity, and improving plant floor utilization, material utilization, and downstream processes.

 

LASER BLANKING ELIMINATES FREQUENT DIE CHANGES 

It is important to remember that there are no dies with laser blanking technology, and no dies mean no die changes. Overall Equipment Effectiveness (OEE) of up to 80% can be achieved with laser blanking technology. In fact, 4 to 6 million parts per year of various materials are produced with the help of DFT—including mild steel, high-strength steel, and advanced high-strength steel. Even processing press-hardening steels with an aluminum-silicon coating is possible with laser blanking.  Surface and cutting quality can be maintained over this spectrum of steel grades.  Laser blanking technology can even achieve effective small batch production of Class A outer body panels and structural parts typically up to 3mm thick.

 

LASER BLANKING INCREASES PLANT OUTPUT

Competitive high-speed and high-output results can be achieved in multiple ways with laser blanking technology. The above-ground coil-fed line, optimized for short setup time, can handle coils with material widths up to 2,150 mm, weighing up to 30 tons. The material transport is smooth and controlled, simplifying setup and leading to uninterrupted processing within the laser cell.

There are three highly dynamic and simultaneously moving laser cutting heads within the laser cell of these lines. These laser cutting heads cut the programmed blank contour from a continuously moving material coil. Cutting speeds can exceed 100 meters per minute. The material is protected against any process contamination throughout the cutting process by custom-designed cutting clearance and material transport. 

Figure 2 reveals the high-speed and high-output results for outer body parts. Each part is measured by improved output per minute and hour to achieve an OEE of 80%. Laser blanking lines can achieve up to 45 parts per minute and reduce costs per blank.

High productivity achieved with laser blanking

Figure 2: High productivity achieved with laser blanking  

 

LASER BLANKING IMPROVES MATERIAL UTILIZATION

Up to 90% of blank costs are determined by the material price. The most significant leverage would be to reduce scrap and save on materials. Schuler conducted research based on the production of 300,000 cars per year, at 350 kg per car and $1,000 USD per ton of steel to provide a realistic inside look at how much cost savings can be achieved with laser blanking. The result was $1 Million USD saved with just 1% of material savings. This is extremely significant as material costs keep increasing.

Laser blanking is the digital way to cut blanks. All that’s needed to create a blanking program is a drawing to be loaded and a material to be selected. The part-specific program can be created offline and modified at any time. It is designed to create optimal combinations of material utilization and output—resulting in a high level of flexibility that significantly reduces development time for optimal blanks while also allowing for need-based production. This makes production planning easier, and it also opens the door to continuous contour optimizations for the forming process. Additionally, laser cutting does not require any gaps between individual parts due to smart nesting capabilities that cannot be achieved in comparison to die nesting or flatbed laser nesting. The combined smart, flexible nesting functions unlock new potential for material savings. Manufacturers can optimize individual blanks and eliminate the separating strip or connection bridges. Scrap savings in the forming process can also be achieved as there are no geometric restrictions due to cutting dies, and manufacturers can continuously optimize or adapt parts.

Figure 3 showcases the comparison of die nesting (the two graphics on the left) versus a laser-optimized blank contour and material savings via smart, laser blanking line nesting (the two images on the right). 

Die nesting compared with laser-optimized blank contours highlighting potential

Figure 3: Die nesting (left) compared with laser-optimized blank contours highlighting potential material savings (right)

Overall, laser blanking lines can have an equivalent throughput to conventional blanking lines, but laser blanking lines can achieve up to 10% greater material utilization.

 

You can read the full Case Study, including how laser blanking reduces infrastructure costs and improves downstream processes here: Laser Blanking Case Study

 

Schuler will present laser blanking technology, along with a variety of digital tools that create the “Press Shop of the Future” at FABTECH Chicago 2023 (booth # D41306). Tiago Vasconcellos, Sales Director at Schuler North America, will present “How Smart is Your Press Shop?” during FABTECH’s Educational Conference. The presentation will use The Smart Press Shop, a newly formed joint venture between Porsche and Schuler, as an exemplary case study for smart manufacturing standards. Attendees will discover innovative and practical ways to incorporate digitalization into production and become a state-of-the-art stamping facility directly from Schuler. 

About Schuler Group

Schuler offers customized cutting-edge technology in all areas of forming—from the networked press to press shop planning. In addition to presses, Schuler’s products include automation, dies, process know-how, and service for the entire metalworking industry. Schuler’s Digital Suite brings together solutions for networking forming technology and is continuously being developed to further improve line productivity and availability. Schuler customers include automotive manufacturers and suppliers, as well as companies in the forging, household appliance, and electrical industries. Schuler presses are minting coins for more than 180 countries. Founded in 1839 at the Göppingen, Germany headquarters, Schuler has approximately 5,000 employees at production sites in Europe, China and the Americas, as well as service companies in more than 40 countries. The company is part of the international technology group ANDRITZ.

Schuler’s global portfolio of world-renowned brands include BCN (Bliss Clearing Niagara) Technical Services, Müller Weingarten, Beutler, Umformtechnik Erfurt, SMG Pressen, Hydrap Pressen, Wilkins & Mitchell, Bêché, Spiertz Presses, Farina Presse, Liebergeld, Peltzer & Ehlers, Schleicher, and Sovema Group.

About Schuler North America

Schuler North America (Schuler), headquartered in Canton, Michigan, is the North American subsidiary of Schuler Group. Schuler provides new equipment, spare parts, and a portfolio of lifecycle services for all press systems—including preventative maintenance, press shop design and optimization, turnkey installations, retrofits for existing systems, and localized production and service. Schuler’s best-in-class position in the metalworking and materials industry serves automotive manufacturers and tier suppliers, as well as home appliance, electronics, forging, and other industries.

 

Roll Forming

Roll Forming

Roll Forming takes a flat sheet or strip and feeds it longitudinally through a mill containing several successive paired roller dies, each of which incrementally bends the strip into the desired final shape. The incremental approach can minimize strain localization and compensate for springback. Therefore, roll forming is well suited for generating many complex shapes from Advanced High-Strength Steels, especially from those grades with low total elongation, such as martensitic steel. The following video, kindly provided by Shape Corp.S-104, highlights the process that can produce either open or closed (tubular) sections.

The number of pairs of rolls depends on the sheet metal grade, finished part complexity, and the design of the roll-forming mill. A roll-forming mill used for bumpers may have as many as 30 pairs of roller dies mounted on individually driven horizontal shafts.A-32

Roll forming is one of the few sheet metal forming processes requiring only one primary mode of deformation. Unlike most forming operations, which have various combinations of forming modes, the roll-forming process is nothing more than a carefully engineered series of bends. In roll forming, metal thickness does not change appreciably except for a slight thinning at the bend radii.

Roll forming is appropriate for applications requiring high-volume production of long lengths of complex sections held to tight dimensional tolerances. The continuous process involves coil feeding, roll forming, and cutting to length. Notching, slotting, punching, embossing, and curving combine with contour roll forming to produce finished parts off the exit end of the roll-forming mill. In fact, companies directly roll-form automotive door beam impact bars to the appropriate sweep and only need to weld on mounting brackets prior to shipment to the vehicle assembly line.A-32 Figure 1 shows an example of automotive applications that are ideal for the roll-forming process.

Figure 1: Body components that are ideally suited for roll-forming.

Figure 1: Body components that are ideally suited for roll-forming.

Roll forming can produce AHSS parts with:

  • Steels of all levels of mechanical properties and different microstructures.
  • Small radii depending on the thickness and mechanical properties of the steel.
  • Reduced number of forming stations compared with lower strength steel.

However, the high sheet-steel strength means that forces on the rollers and frames in the roll-forming mill are higher. A rule of thumb says that the force is proportional to the strength and thickness squared. Therefore, structural strength ratings of the roll forming equipment must be checked to avoid bending of the shafts. The value of minimum internal radius of a roll formed component depends primarily on the thickness and the tensile strength of the steel (Figure 2).

Figure 2: Achievable minimum r/t values for bending and roll forming for different strength and types of steel.S-5

Figure 2: Achievable minimum r/t values for bending and roll forming for different strength and types of steel.S-5

As seen in Figure 2, roll forming allows smaller radii than a bending process. Figure 3 compares CR1150/1400-MS formed with air-bending and roll forming. Bending requires a minimum 3T radius, but roll forming can produce 1T bends.S-30

Figure 3: CR1150/1400-MS (2 mm thick) has a minimum bend radius of 3T, but can be roll formed to a 1T radius.S-30

Figure 3: CR1150/1400-MS (2 mm thick) has a minimum bend radius of 3T, but can be roll formed to a 1T radius.S-30

The main parameters having an influence on the springback are the radius of the component, the sheet thickness, and the strength of the steel. As expected, angular change increases for increased tensile strength and bend radius (Figure 4).

Figure 4: Angular change increases with increasing tensile strength and bend radii.A-4

Figure 4: Angular change increases with increasing tensile strength and bend radii.A-4

Figure 5 shows a profile made with the same tool setup for three steels at the same thickness having tensile strength ranging from 1000 MPa to 1400 MPa. Even with the large difference in strength, the springback is almost the same.

Figure 5: Roll formed profile made with the same tool setup for three different steels. Bottom to Top: CR700/1000-DP, CR950/1200-MS, CR1150/1400-MS.S-5

Figure 5: Roll formed profile made with the same tool setup for three different steels. Bottom to Top: CR700/1000-DP, CR950/1200-MS, CR1150/1400-MS.S-5

Citation A-33 provides guidelines for roll forming High-Strength Steels:

  • Select the appropriate number of roll stands for the material being formed. Remember the higher the steel strength, the greater the number of stands required on the roll former.
  • Use the minimum allowable bend radius for the material in order to minimize springback.
  • Position holes away from the bend radius to help achieve desired tolerances.
  • Establish mechanical and dimensional tolerances for successful part production.
  • Use appropriate lubrication.
  • Use a suitable maintenance schedule for the roll forming line.
  • Anticipate end flare (a form of springback). End flare is caused by stresses that build up during the roll forming process.
  • Recognize that as a part is being swept (or reformed after roll forming), the compression of metal can cause sidewall buckling, which leads to fit-up problems.
  • Do not roll form with worn tooling, as the use of worn tools increases the severity of buckling.
  • Do not expect steels of similar yield strength from different steel sources to behave similarly.
  • Do not over-specify tolerances.

Guidelines specifically for the highest strength steelsA-33:

  • Depending on the grade, the minimum bend radius should be three to four times the thickness of the steel to avoid fracture.
  • Springback magnitude can range from ten degrees for 120X steel (120 ksi or 830 MPa minimum yield strength, 860 MPa minimum tensile strength) to 30 degrees for M220HT (CR1200/1500-MS) steel, as compared to one to three degrees for mild steel. Springback should be accounted for when designing the roll forming process.
  • Due to the higher springback, it is difficult to achieve reasonable tolerances on sections with large radii (radii greater than 20 times the thickness of the steel).
  • Rolls should be designed with a constant radius and an evenly distributed overbend from pass to pass.
  • About 50 percent more passes (compared to mild steel) are required when roll forming ultra high-strength steel. The number of passes required is affected by the number of profile bends, mechanical properties of the steel, section depth-to-steel thickness ratio, tolerance requirements, pre-punched holes and notches.
  • Due to the higher number of passes and higher material strength, the horsepower requirement for forming is increased.
  • Due to the higher material strength, the forming pressure is also higher. Larger shaft diameters should be considered. Thin, slender rolls should be avoided.
  • During roll forming, avoid undue permanent elongation of portions of the cross section that will be compressed during the sweeping process.

Roll forming is applicable to shapes other than long, narrow parts. For example, an automaker roll forms their pickup truck beds allowing them to minimize thinning and improve durability (Figure 6). Reduced press forces are another factor that can influence whether a company roll forms rather than stamps truck beds.

Figure 6: Roll Forming can replace stamping in certain applications.G-9

Figure 6: Roll Forming can replace stamping in certain applications.G-9

Traditional two-dimensional roll forming uses sequential roll stands to incrementally change flat sheets into the targeted shape having a consistent profile down the length. Advanced dynamic roll forming incorporates computer-controlled roll stands with multiple degrees of freedom that allow the finished profile to vary along its length, creating a three-dimensional profile. The same set of tools create different profiles by changing the position and movements of individual roll stands. In-line 3D profiling expands the number of applications where roll forming is a viable parts production option.

One such example are the 3D roll formed tubes made from 1700 MPa martensitic steel for A-pillar / roof rail applications in the 2020 Ford Explorer and 2020 Ford Escape (Figure 7).  Using this approach instead of hydroforming created smaller profiles resulting in improved driver visibility, more interior space, and better packaging of airbags. The strength-to-weight ratio improved by more than 50 percent, which led to an overall mass reduction of 2.8 to 4.5 kg per vehicle.S-104

Figure 7: 3D Roll Formed Profiles in 2020 Ford Vehicles using 1700 MPa martensitic steel.S-104

Figure 7: 3D Roll Formed Profiles in 2020 Ford Vehicles using 1700 MPa martensitic steel.S-104

Roll forming is no longer limited to producing simple circular, oval, or rectangular profiles. Advanced cross sections such
as those shown in Figure 8 provided by Shape Corporation highlight some profile designs aiding in body structure
stiffness and packaging space reductions.

Roll Form Designs

Figure 8: Roll forming profile design possibilities. Courtesy of Shape Corporation.

In summary, roll forming can produce AHSS parts with steels of all levels of mechanical properties and different microstructures with a reduced R/T ratio versus conventional bending. All deformation occurs at a radius, so there is no sidewall curl risk and overbending works to control angular springback.

Case Study: How Steel Properties Influence the Roll Forming Process

Many thanks to Brian Oxley, Product Manager, Shape Corporation, and Dr. Daniel Schaeffler, President, Engineering
Quality Solutions, Inc., for providing this case study.

Optimizing the use of roll forming requires understanding how the sheet metal behaves through the process.
Making a bend in a roll formed part occurs only when forming forces exceed the metal’s yield strength, causing plastic
deformation to occur. Higher strength sheet metals increase forming force requirements, leading to the need to have
larger shaft diameters in the roll forming mill. Each pass must have greater overbend to compensate for the increasing
springback associated with the higher strength.

Figure 9 provides a comparison of the loads on each pass of a 10-station roll forming line when forming either AISI 1020
steel (yield strength of 350 MPa, tensile strength of 450 MPa, elongation to fracture of 15%) or CR1220Y1500T-MS, a
martensitic steel with 1220 MPa minimum yield strength and 1500 MPa minimum tensile strength.

 

Graph showing pass loads with maximum force levels. Gold and purple 3D bar chart

Figure 9: Loads on each pass of a roll forming line when forming either AISI 1020 steel (450 MPa tensile
strength) or a martensitic steel with 1500 MPa minimum tensile strength. Courtesy of Roll-Kraft.

 

Although a high-strength material requires greater forming loads, grades with higher yield strength can resist stretching
of the strip edge and prevent longitudinal deformations such as twisting or bow. Flange edge flatness after forming
either AISI 1020 or CR1220Y1500T-MS is presented in Figure 10.

 

Figure 10: Simulation results showing flange edge flatness of a) AISI 1020 and b) CR1220Y1500T-MS.
Assumptions for the simulation: AISI 1020 yield strength = 350 MPa; CR1220Y1500T yield strength = 1220 MPa. Higher yield strength leads to better flatness.

 

Force requirements for piercing operations are a function of the sheet tensile strength. High strains in the part design
exceeding uniform elongation resulting from loads in excess of the tensile strength produces local necking, representing
a structural weak point. However, assuming the design does not produce these high strains, the tensile strength has only
an indirect influence on the roll forming characteristics.

Yield strength and flow stress are the most critical steel characteristics for roll forming dimensional control. Receiving
metal with limited yield strength variability results in consistent part dimensions and stable locations for pre-pierced
features.

Flow stress represents the strength after some amount of deformation, and is therefore directly related to the degree of
work hardening: starting at the same yield strength, a higher work hardening steel will have a higher flow stress at the
same deformation.

Two grades are shown in Figure 11: ZE 550 and CR420Y780T-DP. ZE 500, represented by the red curve, is a recovery
annealed grade made by Bilstein having a yield strength range of 550 to 625 MPa and a minimum tensile strength of 600
MPa, while CR420Y780T-DP, represented by the blue curve, is a conventional dual phase steel with a minimum yield
strength of 420 MPa and a minimum tensile strength of 780 MPa. For the samples tested, ZE 550 has a yield strength of
approximately 565 MPa, where that for CR420Y780T-DP is much lower at about 485 MPa. Due to the higher work
hardening (n-value) of the DP steel, its flow stress at 5% strain is 775 MPa, while the flow stress for the HSLA grade at 5%
strain is 620 MPa.

In conventional stamping operations, this work hardening is beneficial to delay the onset of necking. However, use of
dual-phase steels and other grades with high n-value can lead to dimensional issues in roll-formed parts. Flow stress in a
given area is a function of the local strain. Each roll station induces additional strain on the overall part, and strains vary
within the part and along the edge. This strength variation is responsible for differing springback and edge wave across
a roll-formed part.

Unlike conventional stamping, grades with a high yield/tensile ratio where the yield strength is close to the tensile
strength are better suited to produce straight parts via roll forming.

Line graph of stress/strain curves

Figure 11: Stress-strain curves for CR420Y780T-DP (blue) and ZE 550 (red). See text for description of the grades.

 

Total elongation to fracture is the strain at which the steel breaks during tensile testing, and is a value commonly
reported on certified metal property documents (cert sheets). As observed on the colloquially called “banana diagram”,
elongation generally decreases as the strength of the steel increases.

For lower strength steels, total elongation is a good indicator for a metal’s bendability. Bend severity is described by the
r/t ratio, or the ratio of the inner bend radius to the sheet thickness. The metal’s ability to withstand a given bend can be
approximated by the tensile test elongation, since during a bend, the outermost fibers elongate like a tensile test.

In higher strength steels where the phase balance between martensite, bainite, austenite, and ferrite play a much larger
role in developing the strength and ductility than in other steels, bendability is usually limited by microstructural
uniformity. Dual phase steels, for example, have excellent uniform elongation and resistance to necking coming from
the hardness difference between ferrite and martensite. However, this large hardness difference is also responsible for
relatively poor edge stretchability and bendability. In roll forming applications, those grades with a uniform
microstructure will typically have superior performance. As an example, refer to Figure 11. The dual phase steel shown
in blue can be bent to a 2T radius before cracking, but the recovery annealed ZE 550 grade with noticeably higher yield
strength and lower elongation can be bent to a ½T radius.

Remember that each roll forming station only incrementally deforms the sheet, with subsequent stations working on a
different region. Roll formed parts do not need to use grades associated with high total elongation, especially since
these typically have a bigger gap between yield and tensile strength.

 

Coil Shape Imperfections Influencing Roll Forming

Along with the mechanical properties of steel, physical shape attributes of the sheet or coil can influence the roll
forming process. These include center buckle, coil set, cross bow, and camber. Receiving coils with these imperfections
may result in substandard roll formed parts.
Flatness is paramount when it comes to getting good shape on roll formed parts. Individual OEMs or processors may
have company-specific procedures and requirements, while organizations like ASTM offer similar information in the
public domain. ASTM A1030/A1030M is one standard covering the practices for measuring flatness, and specification
ASTM A568/A568M shows methods for characterizing longitudinal waves, buckles, and camber. (link to
https://www.astm.org/a1030_a1030m-21.html and https://www.astm.org/a0568_a0568m-19a.html)

Center buckle (Figure 12), also known as full center, is the term to describe pockets or waves in the center or quarter
line of the strip. The height of pocket varies from 1/6” to 3/4”. Center buckle occurs when the central width portion of
the master coil is longer than the edges. This over-rolling of the center portion might occur when there is excessive
crown in the work roll, build-up from the hot strip mill, a mismatched set of work rolls, improper use of the benders, or
improper rolling procedures. A related issue is edge buckle presenting as wavy edges, originating when the coil edges
are longer than the central width position.

 

Center Buckle

Figure 12: Coil shape imperfection: Center Buckle

 

Coil set (Figure 13a), also known as longitudinal bow, occurs when the top surface of the strip is stretched more than the
bottom surface, causing a bow condition parallel with the rolling direction. Here, the strip exhibits a tendency to curl
rather than laying flat. To some extent, coil set is normal, and easy to address with a leveler. Severe coil set may be
induced by an imbalance in the stresses induced during rolling by the thickness reduction work rolls. Potential causes include different diameters or surface speeds of the two work rolls, or different frictional conditions along the two arcs
of contact.

Crossbow (Figure 13b) is a bow condition perpendicular to the rolling direction, and arcs downward from the high point
in the center position across the width of the sheet. Crossbow may occur if improper coil set correction practices are
employed.

Figure 13: Coil shape imperfections: A) Coil set and B) Crossbow

Figure 13 caption: Coil shape imperfections: A) Coil set and B) Crossbow A-30

Camber (Figure 14) is the deviation of a side edge from a straight edge, and results when one edge of the steel is
elongated more than the other during the rolling process due to a difference in roll diameter or speed. The maximum
allowable camber under certain conditions is contained within specification ASTM A568/A568M, among others.

Figure 14 - Camber

Figure 14: Coil shape imperfection: Camber

 

Coil shape imperfections produce residual stresses in the starting material. These residual stresses combined with the
stresses from forming lead to longitudinal deviations from targeted dimensions after roll forming. Some of the resultant
shapes of roll formed components made from coils having these issues are shown in Figure 15. Leveling the coil prior to
roll forming may address some of these shape concerns, and has the benefit of increasing the yield strength, making a
more uniform product.

 

Figure 15

Figure 15: Shape deviations in roll formed components initiating from incoming coil shape issues: a) camber b) longitudinal bow c) twist d) flare e) center wave (center buckle) f) edge wave. H-66

 

Roll Stamping

Traditional roll forming creates products with essentially uniform cross sections.  A newer technique called Roll Stamping enhances the ability to create shapes and features which are not in the rolling axis.

Using a patented processA-48, R-9, forming rolls with the part shape along the circumferential direction creates the desired form, as shown in Figure 16.

Figure 7: Roll Stamping creates additional shapes and features beyond capabilities of traditional roll forming. (Reference 1)

Figure 16: Roll Stamping creates additional shapes and features beyond capabilities of traditional roll forming. A-48

This approach can be applied to a conventional roll forming line.  In the example of an automotive door impact beam, the W-shaped profile in the central section and the flat section which attaches to the door inner panel are formed at the same time, without the need for brackets or internal spot welds (Figure 17. Sharp corner curvatures are possible due to the incremental bending deformation inherent in the process.

Figure 8: A roll stamped door beam formed on a conventional roll forming line eliminates the need for welding brackets at the edges. (Reference 2)

Figure 17: A roll stamped door part formed on a conventional roll forming line eliminates the need for welding brackets at the edges.R-9

A global automaker used this method to replace a three-piece door impact beam made with a 2.0 mm PHS-CR1500T-MB press hardened steel tube requiring 2 end brackets formed from 1.4 mm CR-500Y780T-DP to attach it to the door frame, shown in Figure 18. The new approach, with a one-piece roll stamped 1.0 mm CR900Y1180T-CP complex phase steel impact beam, resulted in a 10% weight savings and 20% cost savings.K-58 This technique started in mass production on a Korean sedan in 2017, a Korean SUV in 2020, and a European SUV in 2021.K-58

 

Figure 10: Some Roll Stamping Automotive Applications (Citation D)

Figure 18: Some Roll Stamping Automotive Applications.K-58

 

Photo of Brian OxleyThanks are given to Brian Oxley, Product Manager, Shape Corporation, for his contributions to the Roll Forming Case Study and Coil Shape Imperfections section. Brian Oxley is a Product Manager in the Core Engineering team at Shape Corp. Shape Corp. is a global, full-service supplier of lightweight steel, aluminum, plastic, composite and hybrid engineered solutions for the automotive industry. Brian leads a team responsible for developing next generation products and materials in the upper body and closures space that complement Shape’s core competency in roll forming. Brian has a Bachelor of Science degree in Material Science and Engineering from Michigan State University.