We are excited to share with you a summary of an article, written for our AHSS Application Guidelines database, by Eren Billur, Billur Makine and Billur Metal Form, Turkey. It is one of the most detailed, comprehensive, and interesting reads on the progression of Press Hardened Steels in vehicle applications that we have seen to date. The big problem was, how to summarize the original 15-page article for this blog! With Dr. Billur’s help, we provide a snapshot here. But please accept this as just a taste of what you can expect to see when our new online AHSS Guidelines is released later this year.
The Beginnings of PHS Use
Press hardening, as we know it today, was developed in Luleå, Sweden, by Norrbottens Järnverks AB (abbreviated as NJA, translated as Norrbotten Iron Works). The first patent application was completed in 1973 and awarded in 1977.N-23 The technology was first commercialized in agriculture components, where the high strength of Press Hardened Steels (PHS) was favored for wear resistance.B-45
In 1984, automotive applications of PHS started with the Saab 9000 side impact door beams, as seen in Figure 1. A total of 4 parts were used in this car.A-66 The uncoated blanks were almost half the thickness of a cold stamped beam.T-26
Figure 1: Door beams of the Saab 9000 (1984-1998): (a) A see-through car in Saab MuseumS-82, (b) the hot stamped part.L-42
The majority of the PHS parts were door beams through the mid-1990s, with approximately 6 million beams produced in 1996. By this time, the demand for bumper beams was also increasing.F-31 By the end of 1996, the European New Car Assessment Program (EuroNCAP) was formed, which increased the pressure on the OEMs for improved crashworthiness.T-26 In 1998, both the new Volvo S80L-44 and Ford Focus5 were equipped with Press Hardened bumper beams.
The year 1998 saw the development of one of the most important breakthroughs in Press Hardening technology. French steel maker Usinor developed an aluminum-silicon (AlSi) pre-coated steel, commercialized as Usibor 1500 (indicating the typical tensile strength, 1500 MPa.C-24, L-39 In 2000, BMW rolled out its new 3 series convertible. In this vehicle, the A-pillar is made from 3 mm thick uncoated, PHS sheet. This was BMW’s first PHS application, and one of the first PHS A-pillar reinforcement.S-83, S-84 Accra started delivering roll formed PHS components for the Volvo V70, initially an optional 3rd row seating support. Approximately 10,000 parts/year were supplied.G-28
AlSi coated steel was first hot stamped at a French Tier 1 supplier, Sofedit.V-15 This grade was first used in the front bumper beam of the 2nd Generation Renault Laguna (2000-2007). Laguna 2 was the first car to receive a 5-star safety rating from Euro NCAP.V-10 AlSi coated blanks were also used in PSA Group’s Citroën C5 (1st Gen: 2001-2007) in the front bumper beam, and the A-pillars. These three parts weighed a total of 4.5 kg, approximately 1% of the total BIW weight, Figure 2a. About one month later, PSA Group started production of the compact hatchback Peugeot 307, which had five hot stamped components (A- and B-pillars and rear bumper beam). Unlike the Citroën C5, these parts were uncoated. The total weight was 12 kg, corresponding to 3.4% of the BIW weight.R-17, P-27
Volvo started producing the XC90 SUV in 2002. The body-in-white with doors and closures weighed 531 kg.B-44 A total of 10 parts, weighing 37 kg are either roll formed or direct stamped PHS. This accounts for approximately 7% of the BIW weight.L-43 During its time, this was the highest use of PHS in car bodies. In Figure 2b, the Press Hardened components other than the 2nd row seat frame, which is a load bearing body part, are shown.
Accelerated Use and Globalization
The use of press hardened parts increased rapidly after the introduction of the VW Passat in 2005. This car had approximately 19% of its BIW (by weight) made from press hardened steels, Figure 2c. Some parts in this car saw the first use of varnish coated blanks in a two-step hybrid process. Three parts were produced using either an indirect or hybrid process, including the transmission tunnel.H-50
Following are a few highlights of PHS use in vehicle applications during this time period :
In 2006, the Dodge CaliberK-37 and BMW X5P-28 were among the first cars to have tailor-rolled and Press Hardened components in their bodies (Figure 3).
Figure 3: (a) Tailor Rolling ProcessZ-5, (b) B-pillar of BMW X5 (2nd Gen: 2006-2013)P-28
BMW 7 Series (5th Gen: 2008-2015) became the first car to have Zn-coated Press Hardened components in its body-in-white. The car also contained uncoated parts, as shown in Figure 4 (next page). The total PHS usage in this car was approximately 16%.P-20
Figure 4: PHS usage in BMW 7 Series (5th Gen: 2008-2015) (re-created using P-20).
Press hardening also allowed car makers to create unconventional cars. In 2011, Hyundai rolled out the 1st generation Veloster, a 3-door coupé (also known as 2+1, with one door on the driver side and 2 doors on the passenger side), and as such containing axisymmetric front doors. Thus, the car could not have a full B-ring, as illustrated in Figure 5a.B-14, R-19 Another unconventional design was the Ford B-Max subcompact MPV sold in Europe between 2012 and 2017. The car had conventional swing doors in the front and two sliding rear doors. A PHS B-pillar was integrated in the doors, providing ease of ingress. Its PHS components (integrated B-pillar in front and rear doors, door beams and cantrail) are shown with blue color in Figure 5b.B-14, L-45
Figure 5: Unconventional car designs with PHS: (a) Hyundai Veloster, asymmetric 2+1 doors coupé (re-created after Citation R-19), and (b) Ford B-Max, sub-compact MPV with integrated B-pillars in the doors.L-45
In 2013, the Acura MDX (3rd Gen: 2013-2020) became the first car to have a Hot Stamped door ring. The part was a tailor welded blank comprised of two sub-blanks, as shown in Figure 6a. The design saved about 6.2 kg weight per car and had high material utilization ratio thanks to sub-blank nesting optimization.A-67, M-46 One of the most recent PHS applications was in 2017 Chrysler Pacifica with 5 sub-blanks, as shown in Figure 6b. This car also has a PQS550 sub-blank at the lower B-pillar region.D-28
Figure 6: Hot stamped door rings: (a) First application in 2013 Acura MDX had 2 sub-blanks, (b) a more recent application in 2017 Chrysler Pacifica has 5 sub-blanks with PQS550 at the lower B-pillar (re-created after Citations B-14, A-67, D-28).
Tubular hardened steels have been long used in car bodies, with minimal forming. Since 2013, a special 3-D hot bending and quenching (3DQ) process has been employed. One of the earliest uses of this technology was Mazda Premacy (known as Mazda 5 in some markets). The same process was also used in making the A-pillars of the Acura NSX (Honda NSX in some markets, 2016-present), as seen in Figure 7a.H-29 Since 2018, tubular parts formed with internal pressure, called form blow hardened parts, are used in the Ford Focus (4th Generation) (Figure 7b) and Jeep Wrangler (4th Generation).B-16, B-17
Figure 7: Tubular hardened steel usage in A-pillars of: (a) 2015 Acura NSXH-29, (b) 2018 Ford Focus.B-16
PHS Use in xEVs: Hybrid Electric, Battery Electric,
Plug-in Hybrid Electric & Fuel Cell Electric Vehicles
The first commercially available Hybrid Electric Vehicle (HEV) was the Toyota Prius (1st Gen: 1997-2003). The second-generation Prius (2003-2009) had very few Press Hardened components, as shown with red color in Figure 8a. This was the first time Toyota used hot stamped components.M-47 The third generation Prius (2009-2015) had approximately 3% of its BIW Press Hardened. In the 4th generation Prius released in 2015, the share of >980 MPa steels has risen to 19%.U-10 Figure 8b shows the Press Hardened parts in this latest Prius.K-38
Figure 8: PHS usage in Toyota Prius: (a) 2nd generation (2003-2009) and (b) 4th generation (2015-present) (re-created after Citations M-47 and K-38)
The 2012 Tesla Model S and Model X launched using aluminium bodies, with PHS reinforcements in the pillars and the bumpers. Model S is known to have a roll-formed PHS bumper beam. High volume Model 3 and Model Y have a significant amount of press hardened components in their bodies.T-35
In 2011, General Motors started production of its first Plug-in Hybrid Electric Vehicle (PHEV), the Chevrolet Volt (known as Opel Ampera in EU and Vauxhall Ampera in the UK). This car had six Hot Stamped components, including A and B pillars, accounting for slightly over 5% of the BIW mass.P-29
The smaller BEV Chevrolet Bolt, launched in 2017, had aluminum closures, but a steel-intensive BIW that is 80% steel, 44% of which is Advanced High-Strength Steels including 11.8% PHS. Figure 9.A-69
Figure 9: Chevrolet Bolt Body Structure and Steel Content.A-69
In December 2020, Hyundai announced their new electric platform, E-GMP. The platform will utilize Press Hardened steel components to secure the batteries.H-52
Automakers have turned to PHS to manage the extra load of Fuel Cell powertrains as well. The first-generation Toyota Mirai had only Press Hardened B-pillars, cantrails and lateral floor members.T-38 The second generation has a number of parts with PHS in its under body as well.T-39
In 2018, Hyundai Nexo became the first fuel-cell car to be tested by EuroNCAP, achieving a 5-Star rating. The car has PHS A- and B-pillars, rocker reinforcements, and several under body components, as seen in Figure 10.H-53
Figure 10: Press hardened steel usage in Hyundai Nexo Fuel Cell vehicle: (a) side view and (b) top view (re-created after Citation H-53).
There will be more on the history and detailed information about some of the technical terms used in this article (such as form blow hardening or 3DQ) in the AHSS Guidelines which will be available by mid-2021. PHS technology, both in terms of steel making and automotive use, is still evolving and is expected to have a higher share in the near future.
Author: Dr. Eren Billur, Technical Manager Billur Makine and Billur Metal Form
Dr. Billur, is a mechanical engineer, automotive enthusiast, and a second-generation business owner on metal forming, located in Ankara, Turkey. He has completed his doctoral studies on press hardening technology, with Prof. Taylan Altan at The Ohio State University in 2013. Dr. Billur is continuing to work on advanced material characterization, new generation press hardening steels, 3rd generation AHSS and servo-press technologies. He has authored/co-authored more than 20 scientific papers (including proceedings) and contributed to four books, including “Hot Stamping of Ultra High Strength Steels” published in late 2018. Since 2015, he is acting as technical manager of Billur Makine and Billur Metal Form companies, both located in Ankara, Turkey.
A-66. J. Aspacher, “Hot Stamping Market Overview & Process Review,” Presented at Schuler Hot Stamping Workshop, May 14, Dearborn, MI, USA, 2013.
B-16. J. Büchner, T. Oberlander and B. Benneker, “Ford Focus,” Presented at EuroCarBody 2018, October 16-18, Bad Nauheim, Germany, 2018.
B-17. A. Bagley and T. Rybicki, “Jeep Wrangler,” Presented at EuroCarBody 2018, October 16-18, Bad Nauheim, Germany, 2018.
B-44. J. Bernquist, “Safety cage design in the VOLVO XC90,” Great Design in Steel – 2004, Sponsored by American Iron and Steel Institute.
B-45. E. Billur, G. Berglund and T. Gustafsson, “History and Future Outlook of Hot Stamping,” Chapter 3 in Hot stamping of ultra high-strength steels: From a Technological and Business Perspective, Edited by E. Billur, Springer, 2019, pages 31-44.
M-46. R. Zum Mallen and J. Riggsby, “Development of a Global First SUV Body Construction,” Presented at Automotive Engineering Expo, June 4-6, Nürnberg, Germany, 2013.
M-47. N. Matsuyama and S. Morimatsu, “The New Toyota PRIUS Car Body,” Presented at EuroCarBody 2005, October 25-27, Bad Nauheim, Germany, 2005.
N-23. Norrbottens Jaernverk, A. B., “Manufacturing a hardened steel article,” Great Britain Patent, GB1490535, 1977.
P-20. M. Pfestorf, “Multimaterial lightweight design for the body in white of the new BMW 7 series,” in International Conference of Innovative Developments for Lightweight Vehicle Structures, May 26-27, Wolfsburg, Germany, 2009.
P-28. M. Pfestorf and J. Rensburg, “Functional Properties of High Strength Steel in Body in White,” Great Design in Steel – 2006, Sponsored by American Iron and Steel Institute.
P-29. W. Parsons and A. DeZess, “The New Opel Ampera,” Presented at Strategies in Car Body Engineering 2012, March 21-22, Bad Nauheim, Germany, 2012.
R-17. A. Reinhardt, “Development of hot stamped Ultra High Strength Steel parts on the Peugeot 307 and the Citroën C5,” Presented at EuroCarBody 2001 – 3rd Global Car Body Benchmarking Conference, Bad Nauheim, Germany (2001).
R-19. B. Ramirez, “Media Launch: 2013 Veloster Turbo,” Hyundai Motor North America Newsroom, 2012.
Greetings from the virtual offices of WorldAutoSteel! I thought it would be a good idea, as AHSS Insights’ Editor, to let you know what is happening here at WorldAutoSteel at the beginning of 2021.
Steel E-Motive – a new industry demonstration
No doubt you have received some emails about our Steel E-Motive program. If you are familiar with WorldAutoSteel’s history, you know our roots are in the UltraLight Family of Research and FutureSteelVehicle, geared to demonstrate the latest steel solutions for vehicle structures of their times.
Our member companies believe that the best way to show steel capabilities is to demonstrate them, and we have done that with body structures, closures and suspensions in the past. Now we are embarking on a whole new adventure, demonstrating the future of steel products and manufacturing technologies in the development of body structures for Level 5 autonomous, electric, ride sharing/hailing vehicles for Mobility as a Service (MaaS). The challenges and opportunities in these vehicles are creating a lot of excitement here as we work together with Ricardo, our world-class engineering partner, and our global steel members, to discover viable steel solutions for this new market.
For me personally, as my sixth experience with steel industry demonstration programs, it has been an exciting ride to see us once again tackle a major program like this, not to mention doing so in a year when we cannot meet and communicate face to face.
Our usual practice is to meet somewhere in the world around a common table several times a year, break up into working groups tackling various topics, brainstorming together with our engineering partner. Until we can once again freely move around the world, we are doing the same thing everyone else is—meeting virtually, with sometimes 40+ people on a call, with often 15 different native languages represented, gathered around a virtual white board. I’m sure many of you have felt the same struggle of making sure everyone has a voice in that setting.
In September 2020, we had 17 web conferences, including whole team meetings, sub-team working groups with subject matter experts, and even OEM advisory groups as we closed in on the end of our Phase 0 decision making. Despite the complications, we’ve been able to get the tasks done, and done well.
But what really excites me the most is that in a year of such uncertainty amid a global pandemic, the steel industry chose to move forward with its plans to invest in the future with new vehicle applications and steel innovation. I cannot be prouder of that bold decision. And we cannot wait to start showing you what we are doing. Unlike our other programs where we made a big announcement at the beginning of a program and then were silent until the end, Steel E-Motive is continuously communicating all the way along our development timeline until we finish at the end of 2022.
Get ready to don your Oculus goggles – we will be releasing VR eventually to show you our designs. Sign up for updates at www.steelemotive.world so you are sure to receive our news.
AHSS Application Guidelines DATABASE
Yes, that says database! Many of you are familiar with our 400+ page tome on the metallurgy, forming and joining of Advanced High-Strength Steels. Like any big reference book, it is daunting, yet it is the most popular document on our website, with people from all over the world still downloading the last volume released in 2017. We have had a three-year cadence of updates stretching all the way back to the late 1990s, authored at that time by Dr. Stuart Keeler. Now it is time to get it off the shelves, and onto the web where it can be more easily searched, and even better, more consistently updated with rapidly evolving technology and best practice experience.
We are also, wherever possible, linking to all referenced technical papers that are available through scientific journals and other sites. Our clickable citations thus far number over 500 with probably two-thirds linked to the source papers. We are not just creating an online volume; we’re building a community of resources on Advanced High-Strength Steels to which you will want access if you have anything to do with vehicle engineering and material application.
We’re expecting to launch the new database in May 2021. If you received an email for this blog, then you’ll get notified when the database will become available too. If someone forwarded you this article, make sure you Subscribe. The AHSS Insights blog will be integrated with the whole database, and ahssinsights.org will be its combined home. Spread the word!
We think we have a nice lineup of blogs for the remainder of this year. Next month, we will release a summary article on press-hardened steel usage, written for the new Guidelines, that we think you’ll find interesting, as well as a bit of an appetizer of things to come.
Thank you, loyal readers. From all the staff at WorldAutoSteel, we send you best wishes to stay safe, stay healthy and make every day count.
Multi-phase steels are complex to cut and form, requiring specific tooling materials. The tooling alloys which have been used for decades, such as D2, A2 or S7, are reaching their load limits and often result in unacceptable tool life. The mechanical properties of the sheet steels achieve tensile strengths of up to 1800 MPa with elongations of up to 40%. Additionally, the tooling alloys are stressed by the work hardening of the material during processing.
The challenge to process AHSS quickly and economically makes it necessary for suppliers to manufacture tooling with an optimal tool steel selection. The following case study illustrates the tooling challenges caused by AHSS and the importance of proper tool steel selection.
A manufacturer of control arms changed production material from a conventional steel to an Advanced High-Strength Steel (AHSS), HR440Y580T-FB, a Ferrite-Bainite grade with a minimum yield strength of 440 MPa and a minimum tensile strength of 580 MPa. However, the tool steels were not also changed to address the increased demands of AHSS, resulting in unacceptable tool life and down time.
According to the certified metal properties, the 4 mm thick FB 600 material introduced into production had a 525 MPa yield strength, 605 MPa tensile strength, and a 20% total elongation. These mechanical properties did not appear to be a significant challenge for the tool steels specified in the existing die standards. But the problems encountered in production revealed serious tool life problems.
To form the FB 600 the manufacturer used D2 steel. D2 was successful for decades in forming applications. This cold work tool steel is used in a wide variety of applications due to its simple heat treatment and its easily adjustable hardness values. In this case, D2 was used at a hardness of RC 58/60.
While tools manufactured from D2 can withstand up to 50,000 load cycles when forming conventional steels, these particular D2 tools failed after only 5,000 – 7,000 cycles during the forming of FB 600. The first problems were detected on a curl station where mechanical overload caused the D2 tools to break catastrophically, as seen in Figure 1. Since the breakage was sudden and unforeseeable, each failure of the tools resulted in long changeover times and thus machine downtime.
Figure 1: Breakage seen in control arm curl tool made from D2, leading to premature failure. Conversion to a PM tool steel having higher impact resistance led to 10x increase in tool life.
Since the cause of failure was a mechanical breakage of the tools, a tougher alternative was consequently sought. These alternatives, which included A2 and DC53 were tested at RC 58-60 and unfortunately showed similar tool life and failures.
Metallurgical analysis indicated that the failure resulted from insufficient impact strength of the tool steel. This was caused by the increased cross-cut that the work-hardened AHSS exerted on the curl. As an alternative material, a cold work steel with a hardness of 58-60, a tensile strength of about 2200 – 2400 MPa and high toughness was sought. These properties could not be achieved with conventional tool steels. The toolmaker used a special particle metallurgy (PM) tool steel to obtain an optimum combination of impact strength, hardness and wear resistance.
Particle metallurgy (PM) tool steels, due to their unique manufacturing process, represent improvements in alloy composition beyond the capabilities of conventional tool steels. Materials with a high alloy content of carbide formers such as chromium, vanadium, molybdenum and tungsten are readily available. The PM melting process ensures that the carbides are especially fine in particle size and evenly distributed (reference Table 1). This process results in a far tougher tool steel compared to conventional melting practices.
Table 1: Elemental Composition of Chosen Tool Steel
The manufacturer selected Z-Tuff PM® to be used at a hardness of RC 58-60. Employing the identical hardness as the conventional cold work steel D2, a significant increase in impact strength (nearly 10X increase as measured by un-notched Charpy impact values) was realized due to the homogeneous microstructure and the more evenly distributed precipitates. This positive effect of the PM material led to a significant increase in tool life. By switching to the PM tool steel, the service life is again at the usual 40000 – 50000 load cycles. By using a steel with an optimal combination of properties, the manufacturer eliminated the tool breakage without introducing new problems such as deformation, galling, or premature wear.
AHSS creates tooling demands that challenge the mechanical properties of conventional tool steels. Existing die standards may not be sufficient to achieve consistent and reliable performance for forming, trimming and piercing AHSS. Proper tool steel grade selection is critical to ensuring consistent and reliable tooling performance in AHSS applications. Powder metallurgical tool steels offer a solution for the challenges of AHSS.
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 bend 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. Figure 1 provides an example of a roll forming line.
Figure 1: Example Roll Forming Line1
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.2
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.2 Figure 2 shows example automotive applications that are ideal for the roll forming process.
Figure 2: 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 3).
Figure 3: Achievable minimum r/t values for bending and roll forming for different strength and types of steel.3
As seen in Figure 3, roll forming allows smaller radii than a bending process. Figure 4 compares CR1150/1400-MS formed with air-bending and roll forming. Bending requires a minimum 3T radius, but roll forming can produce 1T bends.4
Figure 4: CR1150/1400-MS (2 mm thick) has a minimum bend radius of 3T, but can be roll formed to a 1T radius.4
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 5).
Figure 5: Angular change increases with increasing tensile strength and bend radii.5
Figure 6 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 6: 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.3
The Auto/Steel Partnership, “Steel Bumper Systems for Passenger Cars and Light Trucks,” (Sixth Edition)6 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 steels6:
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, 860MPa 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 7). Reduced press forces are another factor that can influence whether a company roll forms rather than stamps truck beds.
Figure 7: Roll Forming can replace stamping in certain applications.7
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.
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.
Authored by Dr. Daniel Schaeffler, President and Chief Executive Officer, Engineering Quality Solutions, Inc., www.EQSgroup.com
A New Software Application for Thin Wall Section Analysis
Advanced High-Strength Steel (AHSS) grades offer increased performance in yield and tensile strength. However, to fully utilize this increased strength, automotive beam sections must be designed carefully to avoid buckling of the plate elements in the section. A new software application, Geometric Analysis of Sections—GAS2.0, available through the American Iron and Steel Institute, is a tool to aid in this design effort.
Plate Buckling in Automotive Sections
To understand how plate buckling affects the strength of a thin walled beam consider Figure 1. A square beam is made of four identical plates connected at their edges. Under an axial compressive load each plate may buckle. Considering just one of the plates, the stress that will cause buckling depends on the ratio of plate width and thickness (b/t). Thinner wider plates with large b/t ratio will buckle at a lower stress than thicker narrower plates.
Figure 1: Plate Buckling Behavior.
Now consider a plate of mild steel (200 MPa yield stress) which has been designed to buckle just as yield stress is reached, Point A in Figure 2. The plate would have a b/t ratio of approximately 60. This design is taking full advantage of the yield strength of the material.
Now consider the same plate but substituting an AHSS grade (600 MPa yield stress) as shown in Figure 2. The plate will buckle at the same 200 MPa before reaching the material’s potential, Point B in the figure. To take advantage of this materials yield strength, the proportions of the plate will need to be changed, Point C. This illustration demonstrates the need to consider plate buckling particularly in the application of AHSS grades.
Figure 2: AHSS Substitution in a Plate.
Moving from a single plate to the more complex case of a beam section of several plates, consider Figure 3. On the left is the beam made of four plates with a compressive load causing the plates to just begin to buckle. However, this condition does not represent the maximum load carrying ability of the beam. The load can be increased until the stress at the corners of the buckled plates are at the material yield stress, center in Figure 3. Note that in this condition the stress distribution across the plate is nonlinear with lower stress in the center of each plate. One means to model this complex state is by using an imaginary Effective Section. Here the center portion is visualized as being removed and the remainder of the section is stressed uniformly at yield. The amount of plate width to be removed is determined by theory.1, 2, 3, 4 The effective section is a convenient way to visualize the efficiency of a section design given the material grade and provides an estimate of the maximum load carrying ability of the beam.
Figure 3: Concept of Effective Section.
Geometric Analysis of Sections – GAS2.0
Geometrical Analysis of Sections software determines the effective section for complex automotive sections. Figure 4 illustrates the GAS2.0 user interface. The user has the ability to construct sections or to import section data from a CAD system. Material properties for 63 steel grades are preloaded with the ability to also add user-defined steel grades. Two types of analysis are available. Nominal analysis, which provides classical area properties of the section, and Effective analysis which determines the effective section at material yield. Figure 5 summarizes both the tabular results and graphical results for each type of analysis.
Figure 4: GAS2.0 User Interface.
Figure 5. GAS2.0 Analysis Results.
Figure 6 illustrates an example of an Effective Analysis for a rocker section. In the graphical screen, the effective section is shown in green. Ideally, the whole section would be effective to fully use the materials yield capability. Also shown in the graphical screen are the section centroid, orientation of the principle coordinates, and stress distribution. In the right text box are tabular results. At the bottom of the tabular results is the axial load that causes this stress state and represents the ultimate load carrying ability of this section.
Figure 6: GAS2.0 Graphical Results.
It is clear that much of the material in the section of Figure 6 is not fully effective. GAS2.0 allows the user to conveniently modify the section. For example, in Figure 7 a bead has been added to the left side wall increasing its bulking resistance. Note that the side wall is now largely effective, and the ultimate load at the bottom of the text box has increased substantially.
Figure 7: Improved Design Concept.
Role of GAS in the Design Process
GAS2.0 can play a significant role in early stage design, see Figure 8, by quickly creating initial designs which are more likely to function and to ensure that adequate package space is set aside for structure. This will result in fewer problems to fix later in the design sequence. During the detail design stage, GAS2.0 can supplement Finite Element Analysis by identifying problems earlier, and by screening design concepts for those with the greatest promise prior to more detailed analysis by FEA.
Figure 8: Role of GAS2.0 in Design Process.
GAS2.0 is available for free download at www.autosteel.org, Included in the resources at autosteel.org is an American Iron and Steel Institute introductory webinar conducted by Dr. Don Malen on 16 June 2020, as well as a number of GAS2.0 tutorials and training modules.
You are most likely wondering why WorldAutoSteel is writing a blog about a bicycle. It is because when we talked to Jia-Uei Chan, Regional Business Development at our member company, thyssenkrupp Steel Europe (TKSe), about the journey of inventing the world’s first Advanced High-Strength Steel road bike, we were incredibly inspired. This is more than a story about a steel bicycle. This is the story of steel innovation, conceived in a WorldAutoSteel members workshop to brainstorm ideas on transforming steel’s image to the sophisticated and advanced material it is. Their journey led to new steel applications, patentable processes, and in the steelworks bicycle, ideas that we think can inspire new automotive applications as well. And anyway, who doesn’t like an inspiring story?
Bikes of this genre have some of the same requirements of modern vehicles: lightweight, strength and durability, affordability, and high performance. To achieve these, the thyssenkrupp steelworks team developed what they called inbike® technology, which combines high-strength steel, half-shell technology and automated laser welding.
How it was made
The bike frame is made from DP 330/590 steel, used for its cold forming abilities, stamped as thin as 0.7mm. The steel blanks are pressed into a die to form two half-shells in a deep-drawing process.
A major challenge was to bring these two half shells together in such a way that minimized gaps and achieved a tight fit, enabling automated laser welding (this process requires no gaps over 6 meters of contact length), while ensuring that the frame achieves an elegant, seamless look. Enter innovation.
At the stamping plant, the half-shells were fitted with “dimples,” (See Figure 1) tiny bumps on the welding flanges that create channels at the weld seam for the zinc, preventing vaporized zinc from remaining trapped in the seam during subsequent welding. The half shells were then clamped in a special device and shipped to the laser specialist (See Figure 2).
Figure 1: Tiny bumps prevent vaporized zinc from remaining trapped in the seam during subsequent welding.
Figure 2: Frame half-shells clamped in the device for laser welding.
The particular challenge lay in the reliable processing and fusing of both frame halves by means of automated laser welding in such a way that no damage to the frame would occur, while also ensuring the weld seam lay as close as possible to the bend radius of the frame halves. The complex frame shape is welded by following a sophisticated trajectory in a 3-D space. After countless continuous improvement exercises, the steelworks team was able to achieve a very flat, elegant weld seam design. This translates into a very stable bike, with a frame that has the needed rigidity in the bottom bracket area to enable high biomechanical power transmission, but with high elasticity in the seat tube configuration to make for an unusually comfortable ride. In comparison, aluminium and carbon fiber bikes are very stiff and characteristic of an unpleasant ride experience.
Inventing the possibility
Tackling a project that is such a reach beyond the norm is never easy. The thyssenkrupp steelworks team repeatedly heard from qualified experts that the project was actually not feasible. At the same time, they had partners who were so fascinated by the challenge that they wanted to make it possible. Chan related to WorldAutoSteel that there were many times when giving up was the more attractive option. Endurance won out. And as it turns out, the half-shell technology invented out of necessity for this bike could find an application in the tough requirements of an electric vehicle battery case.
Says Chan, “We genuinely believed that steel is the perfect material for a road bike. And we wanted to break with convention and make the most out of steel with high-tech engineering.” Have a look at steelworks.bike, and you will undoubtedly agree they did just that.