Every industry has its own jargon. In certain settings, these words might be necessary – you wouldn’t want a cardiologist talking to a gastroenterologist about boo-boos and upset tummies. But when these professionals talk with their patients, it’s sometimes necessary for them to use much simpler words. That is, assuming the goal is to actually communicate the issues and concerns.
The steel industry is no different. We use words that have precise meanings in our daily discussions, and we forget that many people we work with don’t have exposure to the terminology that we are accustomed to using. What follows is a brief tour of the words and phrases you are likely to hear when speaking with your metallurgical representative.
Let’s start with the most common word: steel. Simply, steel is just an alloy of iron with up to about 2% carbon. Of course, other elements are in the composition. These fall into two categories: those intentionally added to improve one or more properties (called alloying elements), and those remaining from the steelmaking process that are too costly to remove relative to the benefit the removal would provide (called residual elements). High residuals are usually bad, typically because they lower ductility. But remember high is a relative term. The value may be higher than the standard to which you ordered (which is a cause for rejection), or just higher than what you’ve received in the past. If they are within the tolerance allowed within the standard, the product should still meet your strength and ductility requirements.
I’ve worked with metal formers who believe “steel is steel” and that all grades should behave the same way. According to the World Steel Association, there are more than 3,500 different grades of steel, each with unique properties and characteristics,75% of which were developed in the past 20 years. Certainly, not all of these are sheet steels, but even within this category, there are sizable numbers. When it comes to just advanced high-strength sheet steels, more than 60 unique grades are available today.
The most common sheet steel grade is routinely called mild steel. Mild steels are low-carbon steels with no alloying elements added for substantial strengthening, and for that reason, they are characterized by relatively lower yield strength. However, there is no single grade or chemistry that meets this definition. Grade definitions require the steelmaker to meet certain chemistry or property limits. These grades are ordered to a standard usually written by the steel producer, a pertinent industry society (like ASTM, Euronorm, or JFS), or the end-user OEM. What is generally thought of as mild steel has chemistry, strength, and ductility overlapping many defined grades. Steel users should order to standards that define and constrain important properties like strength and ductility.
If you hang out with enough metallurgists, you are bound to hear passionate discussions about the iron-carbon phase diagram. (Why you are hanging around metallurgists is another topic entirely.) Before explaining the purpose of a phase diagram, it’s important to understand that a phase is a region of a material that is physically distinct, chemically uniform, and can be seen as different from the rest of the material. Ice and water are two phases that exist in my beverage. You’ll find a chocolate chip phase in my vanilla ice cream. And you’ll find ferrite in my steel – tasty!The properties of each of these change if you increase temperature (converting H2O from a solid to a liquid and eventually a gas) or if you add more alloying elements (chocolate chips or carbon). If you add a lot of that alloying element, you can get something entirely different like ripple or pearlite.
A phase diagram is a graphical representation of composition on the horizontal axis and temperature on the vertical axis. Two important phase diagrams are shown below. The far-left side of each represents 100% vanilla or 100% iron. Different phases exist as the temperature increases, or as the product is alloyed with increasing amounts of either chocolate or carbon.
Atoms arrange themselves in three-dimensional patterns called lattices. Think about billiard balls in multiple layers. The balls can be one layer directly above the prior one, or they can be shifted and rest in the crevice formed by adjacent balls in the layer below. The balls are all the same material, but the gap size changes with different arrangements. This is what happens with steel. At lower temperatures, only up to 0.02% carbon fits in the gap. This orientation is called ferrite. At higher temperatures, a different atomic orientation is stable, which we call austenite. Up to 2% carbon can fit into this arrangement of atoms. For low-carbon steels under normal conditions, austenite cannot exist at room temperature – when the steel is slowly cooled, it changes from austenite to a combination of ferrite and a mixture of phases called pearlite. However, heating a certain chemistry to the austenitic zone followed by rapidlycooling just right bypasses the natural conversion to ferrite and pearlite, and creates a structure that contains austenite at room temperature. This leads to the term retained austenite, which is the phase that gives TRIP and 3rd Generation Steels excellent ductility. More on these later.
100% iron is very soft. As a matter of fact, 100% of any element is very soft. As an example, think about gold. 24-carat gold is pure gold. You might think that a wedding ring, as a symbol of long-lasting love and devotion, should be made from 100% gold. In reality, many gold bands are made from 12-carat gold, which is half gold and half impurities. (Showing your love by giving something 50% impure perhaps is not the best marketing approach.) Adding alloying elements to gold is done to improve certain characteristics, like strength, making the alloy appropriate for the applications it serves.
When we talk about ferrite at room temperature, that’s iron with no more than 80 parts per million carbon. That’s really close to pure iron, so when we hear the term ferrite, we should think of something that is really soft, low-strength, and very ductile.
If additional strength is needed, then more alloying elements must be used in addition to carbon. The next most cost-effective alloying element is manganese which produces higher-strength steels called carbon-manganese steels. These are substitutional solid solutions strengthened, where the atoms of manganese swap into where atoms of iron would otherwise go. These grades have limited ductility, especially at higher carbon and manganese contents, so they are used in structural applications that do not need a lot of formability and are therefore also called structural steels (SS). In the ASTM standard specification covering many sheet steels, ASTM A1008/A1008M, these grades are grouped in the SS category.
Around 1980, steelmakers rolled out a new approach to getting higher strength levels while minimizing the loss of elongation usually seen with higher strengths. They do this by strengthening the ferrite through the addition of very small quantities of titanium, niobium, and vanadium to form carbide and nitride precipitates. These microalloying additionsare used in precipitation hardening of the ferrite to create High Strength Low Alloy (HSLA) steels.
Switching gears a bit to discuss something unrelated to sheet steel but a process with which we might be familiar: forged gears. We want forged gears to be hard and high strength. Typical production of gears involves heating up a steel alloy of certain chemistry, followed by rapid cooling (quenching) them faster than a critical cooling rate. The structure that’s produced is calledmartensite. If the quench rate is only a little too slow, a different phase called bainite can be produced. While martensite is the highest strength phase, it has limited elongation. Bainite is a little lower in strength but has higher elongation and toughness compared with martensite. Bainite shines in applications needing cut-edge ductility during stretch flanging.
Martensite wasn’t commonly foundas a microstructural component during most of the history of automotive sheet steels due to the limited number of companies having an annealing line with appropriate quenching capabilities. This started to change around the turn of the millennium when newer annealing lines were installed with the ability to hold at a specific temperaturewhich may be lower than the annealing temperature followed by quenching to another much lower temperature. This led to greater production of the first generation of Advanced High-Strength Steels (AHSS), including grades that have a microstructure of only martensite.
Dual Phase steels are the most common AHSS. As you might guess, there are two phases in Dual-Phase steels. Ferrite and martensite are the two phases: ferrite is super-soft and comprises the majority of the microstructure, while martensite is super-hard and takes up 10% (590DP) to 40% (980DP) of the microstructure. The more martensite, the stronger the steel. Elongation is the ductility measured in a tensile test, and since most of the structure is ferrite, these steels have exceptional elongation for the strength level. However, there is a large hardness difference between ferrite and martensite, leading to crack initiation sites and resulting in poor cut-edge ductility during stretch flanging.
[A brief digression on testing. Tensile testing takes a standard sample shape, typically looking like a bone you might give a dog to chew on, and pulls it in tension from the edges. The test results include yield strength, tensile strength, and total elongation, commonly called the YTEs or TYEs based on the initials. More information comes out of the tensile test, covered elsewhere. However, the tensile test is usually not used to measure cut-edge ductility. Cut edge ductility is typically characterized by the hole expansion test, where a punched hole is expanded with a conical punch until a through-thickness crack forms.]
Ferrite-bainite steels have a combination of decent elongation (from the ferrite) and excellent cut-edge ductility (from the bainite). Yes, your assumption is correct that there are only two phases in these steels, with ferrite being about 85% of the microstructure. Due to the way these are produced, ferrite-bainite steels are available as hot-rolled products only. That’s in contrast with Complex Phase (CP) steels, which can be found either at hot-rolled or cold-rolled thicknesses.
Soft ferrite is the primary microstructural component in DP steels and the soon-to-be-discussed TRIP steels, which results in low yield strength and relatively high elongation. On the other hand, the primary microstructural components of complex phase steels are bainite and precipitation-strengthened ferrite, with martensite and retained austenite also present in lower amounts. Lacking soft ferrite, these steels have relatively high yield strength and low elongation as measured in a tensile test, but the bainite leads to exceptional cut-edge ductility as measured in a hole expansion test. Multi-phase steels are a related product. Some OEMs group CP and MP steels in the same category, while others say that CP steels are engineered to favor improved bendability and cut edge extension over tensile elongation at the same tensile strength and that MP steels target balancing the fracture resistance needed for better bendability and hole expansion with the necking resistance found with higher uniform elongation and n-value.
TRIP steels contain mostly ferrite surrounding islands of martensite, as well as some bainite and retained austenite. The amount of bainite is pretty low, so it doesn’t add much to the cut-edge ductility. But the magic is in the retained austenite. Austenite is a very ductile phase. What makes this a special phase is that as austenite-containing steels deform, the atoms rearrange and the austenite transforms into martensite, giving the steel enhanced ductility. (Jargon alert: Another word for ductility used by professionals is plasticity.) A quick review: this enhanced ductility comes from austenite transforming to martensite. In other words, these steels have Transformation Induced Plasticity (TRIP).
Wouldn’t it be great to have an alloy that was just austenite? We’d have a high-strength, high-ductility product. There are two types of steels that are in this category. First are austenitic stainless steels in the 3XX family, like SS304 and SS316. In these alloys, austenite is stable at room temperature, but these require approximately 18% chromium and 8% nickel. Next are TWIP steels. These may look like TRIP steels from how they are written, but these steels get their plasticity differently. TWIP steels deform by a mechanism known as twinning, so they are described as Twinning Induced Plasticity Steels (TWIP). Of course, there are no free lunches. To get fantastic formability properties, a lot of alloying is necessary. This drives up the steelmaking complexity and cost. The alloying elements also make welding much more challenging. TWIP steels are called second-generation advanced high-strength steels.
The 3rd Generation Advanced High-Strength Steels (3rd Gen AHSS or 3rd Gen)are made possible by another advance in annealing technology, allowing steelmakers to produce a refined microstructure. Nearly all 3rd Gen steels have retained austenite in the microstructure and therefore benefit from a high strength, high ductility combination. The latest annealing lines used to make these steels come equipped to not just hold and quench to defined temperatures but have reheating capability followed by another hold and quench to different temperature targets. This allows for the creation of an engineered balance and distribution of ferrite, bainite, martensite, and austenite in the microstructure.
The resultant tensile property ranges from 3rd Gen steels produced at different companies may be similar, but their methods of getting those properties are a function of chemistry and the capabilities and characteristics of the equipment used to produce them. A different chemistry approach may result in different weldability, for example, so users are encouraged to perform thorough due diligence before switching suppliers. The days of steel being simply a commodity are in the past as it relates to these highly engineered higher strength steels.
Final thought 1: What’s an MPa?
This note may have a global readership, but this answer is focused on the countries that haven’t embraced the metric system. Megapascals, abbreviated MPa, is a measure of strength, just like pounds per square inch (psi) or force per area. Like Celsius and Fahrenheit or inches and millimeters, we can convert between them easily enough. There are 1000 psi in a ksi, with k being the abbreviation for kilopounds. And there are 6.895 ksi in an MPa. Make your life easier and focus on a 7:1 difference. 100 ksi is about 700 MPa.
Final thought 2: What about Press Hardening Steels?
Press hardening steel for hot stamping is a separate topic with a lot of nuances. One of the biggest differences is how the properties develop. For cold stamping operations, the stamping company is responsible for creating the formed part from sheet metal supplied to the necessary strength. With press hardening steels, the stamping company creates both the shape and the strength. Different grades come from a combination of different chemistries from the steelmaker and different heating and cooling profiles at the stamping location. The chosen corrosion protection approach impacts the various options. Learn more at the Press Hardening Primer on this site.
Final thought 3: Don’t hesitate to ask questions.
If your metallurgical representative says something that you don’t understand, ask for clarification. Your suppliers want to be your valued partner for more than just a simple transaction. Quite likely, your met rep is passionate about their offerings and would love to talk about them. If you get a deeper understanding of what makes one product different from another, then you’ll be in a better position to weigh the benefits against the inevitable constraints, leading to an optimized material selection. Remember, communication is the key to success for all parties.
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.
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
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
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 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
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.
In addition, increasing the number of passes has been shown to be an effective technique to lower residual stresses and therefore improve dimensional accuracy. Multiple bending sequences, especially in the transverse direction also improve dimensional accuracy, providing the steel has sufficient inherent formability to accommodate the additional bends.X-4
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
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.
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
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.
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 550, 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.
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.
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.
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 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: 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: 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 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 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-58This 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 18: Some Roll Stamping Automotive Applications.K-58
Thanks 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.
As more companies aim to reduce their product’s time to market, research and design engineers have begun integrating predictive modeling into their process. These models, whether finite element based or artificial intelligence based, all rely on quality mechanical testing results. Companies within the automotive industry have seen that accurately predicting large scale tests such as crashworthiness trials can greatly expedite the time it takes to get their products to market. One of the more important details in predicting these expensive and time-consuming tests is to understand how the materials within the design are affected by the higher rates of deformation or their strain rates.
Traditional standardized tensile tests have been used for over a century – ASTM E8 was first approved in 1924. Testing laboratories using standards like ASTM E8, ISO 6892-1, and JIS Z-2241 produce repeatable and reproducible mechanical properties for metals undergoing tensile deformation, but each of these standards requires the test to be run at a speed orders of magnitude lower than those occurring during events like sheet metal forming and automotive crash. A tensile test run according to ASTM E8 to obtain the yield properties of a metal is run at a strain rate of 0.00035 strain per second (0.00035/s). For comparison, a stamping process has strain rates on the order of 1 to 10 strain per second, and an automotive crash can have strain rates up to 1,000 strain per second (Figure 1).
Figure 1. Strain rates of different events.
Historically, no guidelines have been available as to the testing method to obtain high strain rate mechanical properties. Decisions on specimen dimensions, measurement devices, and other important issues which are critical to the quality of testing results were made within each individual laboratory. As a result, data from different laboratories were often not directly comparable. A WorldAutoSteel committee evaluated various procedures, conducted several round-robins, and developed a recommended procedure, which evolved into what are now the first two parts of ISO 26203, linked below.
Published standards addressing tensile testing at high strain rates include:
Steel alloys typically possess positive strain rate sensitivity, or m-value when tested at ambient temperature, meaning that strength increases with strain rate. This has benefits related to improved crash energy absorption.
The specific response as a function of strain rate is grade dependent. Some grades get stronger and more ductile as the strain rate increases (left image in Figure 2), while other grades see primarily a strength increase (right image in Figure 2). Increases are not linear or consistent with strain rate, so simply scaling the response from conventional quasi-static testing does not work well. Strain hardening (n-value) also changes with speed in some grades, as suggested by the different slopes in the right image of Figure 2. Accurate crash models must also consider how strain rate sensitivity impacts bake hardenability and the magnitude of the TRIP effect, both of which are further complicated by the strain levels in the part from stamping.
Figure 2: Two steels with different strength/ductility response to increasing strain rate.A-7
Importance of Proper Testing Equipment
Knowing that the strain rate directly affects mechanical properties, many research test laboratories have run tensile tests using the same specimen geometry and machine as standardized but have increased the speed the machine runs during the test. This typically allows for tests to be performed at strain rates of up to 0.1 strain per second. From this data, an extrapolated curve can be fit to approximate the properties of the materials at higher strain rates. One model used to predict the increase in strength of a material deformed at a higher strain rate is the Cowper-Symonds model:
Equation 1
where σd is the strength of the material at a strain rate έ, σs is the strength of the material at a theoretical strain rate of zero strain per second, and C and p are model parameters.
Figure 3 shows two best fit curves for this Cowper-Symonds model to the tensile strengths of a cold rolled steel across a range of strain rates. The first considers only data that can be obtained using traditional tensile testing machines (strain rates less than 0.1 strain per second) while the second uses data up to 2,000 strain per second. Both models have minimal errors at strain rates below 0.1 strain per second, but as the limited model begins to extrapolate data beyond this strain rate regime, the associated error begins to grow exponentially causing large errors at the strain rates typical in crashes.
Figure 3. Example of extrapolation of tensile strength vs strain rate data using a Cowper-Symonds model. The dashed curve is an extrapolation based only on data acquired using traditional tensile testing machines, where strain rates are less than 0.1 strain per second. The solid curve is the extrapolation when considering data from equipment capable of achieving 2,000 strain per second.
Testing Methods and Equipment
The biggest obstacle to measuring how a material responds to different strain rates is that it requires several different types of equipment. This is due to the need to run tests at up to six or seven different orders of magnitude to fully characterize the material. Figure 4 shows which mechanical testing equipment is most used to perform tensile tests based on the strain rate of the test. A broader range of testing methods for more strain rates can be found in the ASM Handbook, Volume 8: Mechanical Testing and Evaluation.A-88 The limits for the strain rate range that each type of test equipment can achieve varies based on the design and attributes of each specific machine as well as the specimen geometry used in testing. In tensile and compression testing, going from a longer specimen to a shorter specimen allows for a specific machine to increase its upper limit on strain rates.
Figure 4. Testing equipment most used to test materials at strain rates between 0.0001 strain per second and 10,000 strain per second in uniaxial tension.
Modified Servo-Hydraulic Machines
Modified servo-hydraulic testing machines are specifically engineered to characterize the dynamic mechanical properties of materials at high strain rates, often reaching strain rates up to 500 strain per second. Unlike conventional servo-hydraulic machines, which used closed-loop control for precise lower-speed testing, these modified systems incorporate design features that overcome the limitations of standard hydraulic controls at higher speeds. Many of these machines utilize extreme high flow valves along with slack adapters. The higher flow valves allow for higher accelerations while the slack adapters decouple the actuator from the specimen while the actuator accelerates to a desired test speed.
Split Hopkinson Pressure Bars
A split Hopkinson pressure bar (sometimes referred to as Kolsky bar or simply SHPB) is an impact-based device that is designed to characterize the dynamic mechanical properties of materials at strain rates above 100/s. The SHPB system uses a striker rod to generate a stress wave which induces plastic deformation in a specimen placed between two elastic bars. The stress wave generated by the striker bar in the first elastic bar (called the incident bar) is measured by a strain gauge fixed at the midpoint of the incident bar. When the stress wave reaches the specimen, part of the stress wave is transmitted through the specimen into the second elastic bar (called the transmitted bar) where it is measured by a second strain gauge. The rest of the stress wave is reflected off the specimen and returns down the incident bar to be re-measured by the strain gauge. Figure 5 shows how the stress waves propagate through a SHPB system during a compression test. There are various options to modify the compression SHPB setup to run a tensile test, but the most common is to replace the striker bar with a tube that slides on the incident bar where it impacts a flange on the end of the incident bar causing a tensile stress wave to propagate towards the specimen instead of a compression wave.
Figure 5. Animation showing how a stress wave propagates through a
split Hopkinson pressure bar system during a high strain rate compression test.
High Strain Rate Testing Challenges
Many other challenges complicate testing materials at higher strain rates. Three of the major challenges are
Challenges of measuring strain at high speeds
Challenges of accounting for inertial effects
Challenges of accounting for adiabatic heating
Strain Measurements in High Strain Rate Testing
During standardized mechanical testing, clip-on extensometers and deflectometers provide excellent extension measurements. As the speed of the test increases, the mass of the extensometer inhibits its use due to slippage of the contact points or interference with the specimen, both of which lead to erroneous test results and potential damage to the device. During high strain rate tests, the simplest means of measuring specimen displacements is to derive them based on the stress waves from the test. A detailed derivation of this method can be found here. This method works well for compression testing, but during tensile testing, events such as slippage of the specimen within the grips or deformation of the radius section of the specimen add to the displacements of the test. These additional displacements overshoot the tensile strain of a specimen during the test. This has led to the nearly universal adoption of optical strain measurements for high strain rate tension tests.
The most common optical method is digital image correlation (DIC). DIC correlates a series of images taken during the deformation of a specimen and calculates the corresponding strains of the specimen. It does this by tracking a black-and-white speckle pattern painted on the specimen’s surface which creates a series of high-contrast features as shown in Figure 6.
Figure 6. Two examples of 2D digital image correlation (DIC) showing the true equivalent strain fields during tensile tests performed at 0.1 strain per second and 1000 strain per second.
When testing round or more complex specimens, two cameras are required to track the surface of the specimen in three-dimensional space. This is referred to as 3D DIC, and it often requires more rigorous calibration for use in testing due to its multi-camera complexity. Alternatively, there are a series of one-dimensional options for strain measurements. The simplest utilizes a high-speed line scan camera to measure the displacement of a specimen along the test direction. While the two- and three-dimensional approaches bring in more data, the one-dimensional approach has been shown to provide excellent resolution at a lower adoption cost.Z-16
Accounting for Inertia in High Strain Rate Testing
At strain rates around one to ten strain per second, inertial effects can begin to complicate the interpretation of test results. These effects can be broken into two varieties: inertia of the specimen being tested and inertia of the equipment being used. Both directly affect the load values measured during a given test. In a modified servo-hydraulic load frame, large grips can create a large difference in stiffness (known more specifically as mechanical impedance) between each grip and the specimen. This difference causes more of any generated stress wave to be reflected at the interface as opposed to transmitting through; thus, as the wave travels back-and-forth through the specimen, the load it experiences “rings up”. This is sometimes referred to as “load ringing”. During this transient period of ring-up, the wave is amplifying or changing shape each time it reaches an end of the specimen. From this, the specimen experiences a load gradient across its gauge section depending on where the wave is at that point. After a certain number of reflections have occurred, the stress wave within the specimen becomes uniform and the specimen is determined to be in stress equilibrium. The number of oscillations it takes for this to occur is greatly dependent on the maximum frequency of the stress wave that enters the specimen and the ratio of mechanical impedances between the specimen and its grips. The lower the difference in impedances, the lower the energy that is reflected back into the specimen. The overall time of this period is not affected by the strain rate of the test being performed. This is because the phenomenon is based more so on the number of times the wave traverses the specimen which is solely dependent on the wave speed and length of the specimen. The wave speed of a material (assuming one—dimensional wave propagation) is calculated by:
Equation 2
Where c is the wave speed of the material, E is the Young’s modulus of the material, and ρ is its density. When viewing test results, the load ringing can be seen as a decaying sinusoidal that begins when the specimen is first loaded. Figure 7 shows an example of how this could look across various strain rates. Each data set shares the same decay time constant, frequency, and magnitude of oscillations. Holding these three as constant simulates running tensile tests using the same specimen geometry, machine, and grips but varying the strain rate of the test. In the data set at lower strain rates, the effect of load ringing is minor with no notable difference between the actual response and the measured response at 10 strain per second. At 30 and 100 strain per second, the yield portions of the stress-strain curves are noisy, but the overall hardening profile is still clean. At 300 strain per second, the oscillations affect most of the hardening potion of the stress-strain curve. At 1000 strain per second, the overall profile of the stress-strain curve can be made out including strain to failure, but no details regarding hardening, uniform elongation, or tensile strength can be stated without large error bands. At 3000 strain per second, very little can be discerned from the data.
Figure 7. Illustration of theoretical frequency responses of dynamic tensile tests performed at various strain rates. All tests share the same decaying time constant, frequency, and magnitude of oscillations.
Some laboratories have adopted an inverse method that takes data with excessive load ringing and derives a stress-strain model. This is done by simulating the equipment used to perform the test along with the specimen tested in a finite element model. Then, repeated iterations of the stress-strain profile are sequentially optimized until the simulation best fits the data read from the test which includes the load oscillations. While this method has shown great potential, it is often too time intensive and expensive to justify in most industrial applications.
Adiabatic Heating in High Strain Rate Testing
The final challenge when testing materials at high strain rates comes from a by-product of plastic deformation of materials: adiabatic heating. Quasi-static tests allow for iso-thermal testing where the rate of heat being generated from plastically deforming the specimen is exceeded by the rate that heat is lost to the surrounding environment. As the strain rate of the test is increased, so too does the rate of plastic work and heat generation within the specimen. This internal heating is also compounded with the need for high intensity lighting to illuminate any high-speed optical methods for measuring strain of specimens. Because of these heat sources without equivalent cooling, the material response as measured by a high strain rate test is jointly affected by the higher strain rate as well as the elevated temperature of the specimen. These two effects have been shown experimentally to not be independent of one another, further complicating the analysis and interpretation of these tests. More complex multi-physics material models are often employed to account for these coupled effects in finite element model simulations.
Thanks are given to Trey Leonard, for his contributions to this page. Trey is the founder and CEO of Standard Mechanics, LLC, where he delivers advanced mechanical testing services and solutions across a wide range of applications, from automotive design to consumer electronics. His expertise spans formability, fatigue, and strain rate sensitivity testing. Beyond conducting tests, he partners with customers to ensure the development of high-quality, calibrated material cards and models for accurate finite element simulations. Dr. Leonard earned his Ph.D. in Mechanical Engineering from Mississippi State University, where he pioneered and licensed technologies in dynamic material testing and characterization. Building on this foundation, he continues to develop innovative testing methods and technologies that advance the field of dynamic mechanical testing. He also contributes to the broader engineering community through his work with ASTM International, where he serves on the E28 Committee on Mechanical Testing, regularly reviewing and improving industry standards.
Key materials characteristics for formed parts include strength, thickness, and corrosion protection. Tailored products provide opportunities to place these attributes where they are most needed for part function, and remove weight that does not contribute to part performance.
Figure 1 highlights some of the areas within the body structure where companies have considered transitioning to welded tailored blanks. Other tailored products may be suitable in other areas.
Figure 1: Applications suited for welded tailored blanks.A-31
Tailored products offer numerous advantages over the conventional approach involving the stamping and assembly of individual monolithic blanks which have a single grade, thickness, and coating, including:
Improved materials utilization
Certain parts, like door rings, window frames, and door inner panels, have large cutout areas contributing to engineered scrap. Converting these to welded tailored blanks allows for optimized nesting of the individual components. Figure 2 presents an example of optimized nesting associated with body side aperture designs using a tailor welded blank. Reduced blank width requirements may allow for additional suppliers or use of master coils yielding slit mults. In the other extreme, blank dimensions larger than rolling mill capabilities are now feasible.
Elimination of reinforcement parts and reduced manufacturing infrastructure requirements
In areas needing additional thickness for stiffness or crash performance, conventional approaches require stamping both the primary part and an additional smaller reinforcement and then spot welding the two parts together. The tailored product directly incorporates the required strength and thickness. Compared with a tailored product, the conventional approach requires twice the stamping time and dunnage, creates inventory, and adds the spot welding operation. Tolerance and fit-up issues appear when joining two formed parts, since their individual springback characteristics must be accommodated.
Part consolidation
Similar to the benefits of eliminating reinforcements, tailored products may combine the function of what would otherwise be multiple distinct parts which would need to be joined.
Weight savings
Conventional approaches to body-in-white construction requires individual parts to have flat weld flanges to facilitate spot welding. Combining multiple parts into a tailored product removes the need for weld flanges, and their associated weight.
Improved NVH, safety, and build quality
Joining formed parts is more challenging than joining flat blanks first and then stamping. Tailored products have better dimensional integrity. Elimination of spot welds leads to a reduction in Noise, Vibration, and Harshness (NVH). A continuous weld line in tailored products means a more efficient load path.
Enhanced engineering flexibility
Using tailored products provides the ability to add sectional strength in precise locations to optimize body structure performance.
Easily integrated with advanced manufacturing technologies for additional savings
Tailored products incorporated into hot stamping or hydroforming applications magnify the advantages described here, and open up additional benefits.
TEASER!: A future post will highlight how these tailored products are applied to press hardening steels to create a single component having strength levels tuned to the needs of each segment of the body structure. Stay tuned!
WorldAutoSteel has a 30-year legacy of steel demonstration all the way back to the Ultra-Light Steel Auto Body (ULSAB), whose engineering report is still being downloaded from our worldautosteel.org site today. The one you may remember best is the FutureSteelVehicle (FSV), results of which we launched in 2011. FSV demonstrated steel innovation for not only Battery Electric vehicles (BEV) but also Fuel Cell vehicles (FCV). Steel E-Motive is the sixth of our global steel industry programs.
So Why Mobility as a Service?
The Automotive sector is undergoing the most rapid change in 40 years. This transformation shifts our thinking – from the movement of vehicles to the efficient movement of people and goods. Over the past eight years, we have conducted extensive research into global trends such as urbanization, transport emissions reduction, as well as the waning interest in vehicle ownership among the young and old. This is especially prevalent in megacities characterized by pollution, congestion, limited parking and enormous ownership costs. Our research concluded that mobility as a service (MaaS) will grow exponentially in high population areas and would place a significant challenge on vehicle design and manufacturing. Therefore, we needed to make sure we as an industry were active and visible in providing STEEL solutions in this new market place.
Steel E-Motive will demonstrate the benefits of steel, linking the properties of the material to the required architectures and attributes for MaaS vehicles.
This program will demonstrate the benefits of steel, linking the properties of the material to the required architectures and attributes for MaaS vehicles. It connects us with original equipment manufacturers (OEMs) and future mobility providers (FMPs), reinforcing steel’s advantages in strength, durability, sustainability and affordability.
An autonomous BEV structure aligns perfectly with steel’s best attributes, however most new concepts trial alternative materials. The global steel industry is investing significantly in product and fabrication development to continually prepare for the next challenge. High Strength and Advanced High-Strength Steel (AHSS) portfolios have grown from the 11 highlighted in the ULSAB program, to more than 60 grades available for use in designing and optimizing Steel E-Motive’s autonomous BEV architecture. Third Generation AHSS (3rd Gen AHSS) will have a prominent role in Steel E-Motive’s body-in-white, taking strength levels ever higher while improving manufacturability. And our industry continues to evolve Press Hardened Steels (PHS) with strength levels upwards of 2000 MPa.
Finally, efficient fabrication processes such as roll stamping, press hardening, and hydroforming use less steel and therefore contribute lower vehicle production emissions. These are the details being highlighted in Steel E-Motive, where we hope to demonstrate that only Steel can make it Real.
Steel E-Motive: A game changing, world first?
Many OEM’s and mobility service providers follow the typical vehicle development process where they adapt an existing vehicle structure to the new vehicle requirements. We don’t have that in Steel E-Motive We believe Steel E-Motive is one of the world’s firsts.
The first for a Level 5 autonomous vehicle that is compliant with global high-speed crash requirements.
The first autonomous vehicle to be a conventional high-volume stamped steel body construction, creating an affordable platform for the mobility service provider.
First to offer a competitive, robust, and sustainable MaaS solution.
For engineers, being first is very exciting but a little nerve wracking – there are no benchmarks out there. There is less to “hang on to.” We’re on our own. Target setting is more challenging; we are the benchmark. Time will tell if we make it to the automotive hall of fame.
We are producing concepts for two BEVs based on a single modular platform. SEM1 (Figure 1) is a front-wheel drive short wheelbase urban version for inter-city travel for four passengers. It has a compact design and vehicle footprint, comparable in footprint to a European B/C segment size. SEM2 (Figure 2) is an all-wheel drive, long wheelbase extra urban version designed to carry up to six passengers. It has an adaptable interior volume that can result in additional luggage capacity compared to SEM1.
Steel E-Motive benefits from a broad portfolio of steel grades and fabrication process, as identified by our member steel experts. The design is nearly finalized, and material selections are being evaluated against various performance targets with the representative structure shown in Figure 3 with high PHS usage at this stage in the design (as of May 2022). This is mainly driven by the safety requirements. Steel E-Motive BIW steel and steel technologies include:
Right steel grade in the right place
Significant proportion of >1500MPa grades, primarily for occupant and battery intrusion zones
Mixture of stamped, roll formed, roll stamped, press hardened steel and hydroformed parts
Battery modules and cooling plates are mounted to an AHSS carrier frame (off-line).
The carrier frame is mounted to the body structure (in general assembly).
The BIW floor acts as the top cover and provides sealing.
The AHSS bottom cover plate provides impact protection.
This design provides significant cost and weight savings, as well as improved NVH. This extremely efficient package does not compromise safety and enables a flat floor with a lower step-in height.
Competitive Body Stiffness with an Open B-Pillarless Body Structure
With clean sheet design, and generally less package constraints in a Level 5 vehicle, our design teams have had more freedom to engineer and optimize the crash and stiffness structural loadpaths. We used topology, optimization, and Virtual Reality tools to determine the most efficient structural loadpaths (Figure 5). The results informed the joint designs and enabled optimization of the joining and structural adhesives. These steps and the advantage of steel’s high modulus resulted in impressive performance.
Front Crash Structure Engineered to Balance the Requirements of 56kph USNCAP FFB, IIHS ODB, IIHS SORB and EuroNCAP MPDB Load cases
One of the most challenging aspects of the Steel E-Motive program has been achieving the front crash performance that minimizes occupant injury. The challenge has been compounded by the overall compact size of the vehicle and the short front overhang dimensions, meaning less space to manage and balance the required crush energy with intrusion resistance.
For the IIHS 25% Small Overlap test, we worked from the outset to achieve a barrier “glance off.” The goal is to deflect the vehicle off the barrier by the time the barrier reaches the hinge pillar. This results in a reduced amount of vehicle kinetic energy converted to crush energy. The vehicle continues after the impact with some onward velocity and kinetic energy. This strategy results in reduced intrusion to the passenger compartment and a much lower vehicle pulse (below 20g), which translates into lower occupant injury. We are very excited by this outcome, as in our benchmarking we have not seen many (if any) vehicles of this size managing to achieve a glance off for this test. Figures 7 and the bullets following provide a look at the results.
IIHS “good” rating achieved (based on predicted intrusions).
Our strategy for IIHS Small Overlap test was to achieve a “glance off” the barrier, which is a significant challenge given the vehicle’s short front overhang.
Front suspension engineered to detach on impact. This is important for achieving glance off.
Glance off results in some continued onward vehicle velocity after the impact.
This results in reduced crush energy, lower vehicle pulse and intrusions = enhanced occupant protection
Figure 8 points out features of the front crash structure. Most of the crush energy in FFB and ODB is absorbed by conventional longitudinal mid-rails, which are made of cold stamped, tailor welded blank Dual Phase steels. The plan view angle of the longitudinals has been optimized to provide load reaction early in the SORB event while remaining largely inside of the SORB barrier.
MaaS vehicles will need to accommodate quick ingress and egress as well as provide comfort and safety for the occupants. Consequently, we have flipped the front occupant around to a rear facing configuration and provided a B-Pillarless wide door aperture to enable comfortable and quick access for passengers. This changes the approach required for occupant protection in a front crash. Effectively we are dealing with a high-speed rear impact situation for the occupant. Current rear impact tests cover lower speed rear end shunts. Figure 11 notes the key points and challenges that Steel E-Motive is designed to meet.
Side Crash Structure Consists of Absorption and Intrusion Prevention Zones, Compensating for Large Body Aperture
The side structure includes roll-stamped martensitic door waist rail beams and a one-piece Tailor Welded Blank, Press Hardened Steel door ring outer. A- and C-pillars in line with occupants provide good side impact protection. (You can learn more about the door design in our May blog).
In the section AA schematic in Figure 12 the TRIP690hydroformed tube interlocking door B-pillar is shown (wrapped over the rocker and cantrail). The load travels through the side impact crush “hex” beam, which is a two-piece roll formed DP590 component.
Steel E-Motive Design Demonstrates Good Side Crashworthiness and Good Levels of Occupant and Battery Protection
In addition to occupant protection tests, additional side impact load cases have been simulated to ensure optimal battery protection. The design maintains a less than 30 mm clearance to the battery.
In reviewing the design according to IIHS standards and based on the predicted intrusions, we are confident this vehicle would achieve an IIHS “good” rating. See Figures 13 and 14 following:
Total Cost of Ownership: Vehicle and Body Is Designed for Conventional Fabrication and Assembly Processes
The Steel E-Motive body has been designed with low cost in mind to provide the foundation for a lower total cost of ownership for fleet owners. The steel body design is optimized to maximize material utilization and minimize scrap rate. Steel E-Motive is suitable for >250,000 units/year production and is compatible with existing global automotive manufacturing facilities using conventional press and fabrication tools. We are also using Life Cycle Assessment as an integral part of the engineering process to ensure that Steel E-Motive is responsible for the lowest possible emissions throughout its entire life cycle. We will report on environmental performance and sustainability as a part of our final results.
Steel E-Motive Key Outcomes
The Steel E-Motive program is delivering an exciting futuristic vehicle, optimized from the ground up for autonomous MaaS application. We are addressing key challenges through careful design, application of simulation tools and efficient use of the latest Advanced High-Strength Steels and fabrication processes. Steel’s inherent characteristics of low production emissions, lightweighting capabilities for mass efficiency, infinite recyclability and product durability underscores its suitability as an integral part of stakeholder strategies to offer sustainable mobility solutions, today and in the future.
Be sure to follow us on our journey as we enter our final months of design, engineering and reporting by subscribing at the Steel E-Motive website. We welcome your questions about this program using the Comment box below.
We at WorldAutoSteel, side-by-side with our engineering partner Ricardo, are rounding the corner on two years in the design process for the Steel E-Motive’s SEM1 and SEM2 autonomous, electric, mobility as a service (MaaS) vehicles. The excitement is really building around our global virtual office as we are getting closer and closer to finalizing the concept designs. We’re finishing the material and manufacturing process optimization now, and soon it will be pencils down. We’ll be making the full engineering reports freely available to the world’s engineers, as we have always done through the years, beginning with our UltraLight Family of Vehicles. We cannot wait to release our results for the world to see!
In the meantime, we have been releasing as much information as we can along the road. This month’s blog (on the Steel E-Motive site) concentrates on the unique closure design developed for Steel E-Motive. A B-Pillar integrated configuration (red component on the right side door in the animation below), which specifies Advanced High-Strength Steels in the A- and B-Pillars as well as for the door ring, affords a wide-open entry for passengers to enter and exit safely and comfortably. And when not in use for transporting people, the wide aperture can accommodate the loading and unloading of goods.
Here’s a little animation of the side closure design to hopefully encourage you to click over to the Steel E-Motive blog where we tell the whole story. Keep this window open on your browser and come back here with any questions or comments you may have. You can add them below in the Comments area of this page. We’d very much appreciate hearing your thoughts.
Thanks are given to
Thanks are given to 
