J-30
Citations:
J-30. JFE New Products & Technologies, “High Strength Steel Sheets with Excellent Formability Series, JEFORMATM,” JFE Technical Report, No. 24 (Mar. 2019); https://www.jfe-steel.co.jp/en/research/report/024/pdf/024-15.pdf
Tensile Testing: Engineering Stress-Strain Curves vs. True Stress-Strain Curves
Tensile testing is one of the most basic formability characterization methods available. Results from tensile testing are a key input into metal forming simulations, but if the right information isn’t included, the simulation will not accurately reflect material behavior.
Metal forming simulation is particularly beneficial on the value-added parts made from advanced high strength steels, since accurate simulations allow for optimal processing with minimal recuts … at least when the right information is used as inputs.
Tensile Testing
During tensile testing, a standard sample shape called a dogbone is pulled in tension. Load and displacement are recorded, and which are then converted to a stress-strain curve. Strength is defined as load divided by cross-sectional area. Exactly when the cross-sectional area is measured during the test influences the results.
Before starting the pull, it’s easiest to measure the width and thickness of the test sample.
Engineering Stress-Strain Curve
At any load, the engineering stress is the load divided by this initial cross-sectional area. Engineering stress reaches a maximum at the Tensile Strength, which occurs at an engineering strain equal to Uniform Elongation. After that point, engineering stress decreases with increasing strain, progressing until the sample fractures.
However, metals get stronger with deformation through a process known as strain hardening or work hardening. As a tensile test progresses, additional load must be applied to achieve further deformation, even after the “ultimate” tensile strength is reached. Understanding true stress and true strain helps to address the need for additional load after the peak strength is reached.
During the tensile test, the width and thickness shrink as the length of the test sample increases. Although these dimensional changes are not considered when determining the engineering stress, they are of primary importance when determining true stress. At any load, the true stress is the load divided by the cross-sectional area at that instant.
True Stress-Strain Curve
The true stress – true strain curve gives an accurate view of the stress-strain relationship, one where the stress is not dropping after exceeding the tensile strength stress level.
True stress is determined by dividing the tensile load by the instantaneous area.
True stress-strain curves obtained from tensile bars are valid only through uniform elongation due to the effects of necking and the associated strain state on the calculations. Inaccuracies are introduced if the true stress-true strain curve is extrapolated beyond uniform strain, and as such a different test is needed. Biaxial bulge testing has been used to determine stress-strain curves beyond uniform elongation. Optical measuring systems based on the principles of Digital Image Correlation (DIC) are used to measure strains. The method by which this test is performed is covered in ISO 16808.
Stress-strain curves and associated parameters historically were based on engineering units, since starting dimensions are easily measured and incorporated into the calculations. These are the values you see on certified metal properties, also called metal cert sheets that you get with your steel shipments.
True stress and true strain provide a much better representation of how the material behaves as it is being deformed, which explains its use in computer forming and crash simulations.
It’s much more challenging to get accurate dimensional measurements once the test has started unless there are multiple loops of the operator stopping the test, remeasuring, then restarting the pull. This is not a practical approach.
Fortunately, there are equations that relate engineering units to true units. Conventional stress-strain curves generated in engineering units can be converted to true units for inclusion in simulation software packages.
As the industry moves to more value-added stampings, metal forming simulation is done on nearly every part. The value-added nature of parts made from advanced high strength steels requires best practices be used throughout – otherwise the results from simulation drift further away from matching reality, leading to longer development times and costly recuts.
Danny Schaeffler is the Metallurgy and Forming Technical Editor of the AHSS Applications Guidelines available from WorldAutoSteel. He is founder and President of Engineering Quality Solutions (EQS). Danny wrote the monthly “Science of Forming” and “Metal Matters” column for Metalforming Magazine, and provides seminars on sheet metal formability for Auto/Steel Partnership and the Precision Metalforming Association. He has written for Stamping Journal and The Fabricator, and has lectured at FabTech. Danny is passionate about training new and experienced employees at manufacturing companies about how sheet metal properties impact their forming success.
Resistance Spot Welding with Advanced High-Strength Steels: Cold Stamped and Hot Formed
The discussions relative to cold stamping are applicable to any forming operation occurring at room temperature such as roll forming, hydroforming, or conventional stamping. Similarly, hot stamping refers to any set of operations using Press Hardening Steels (or Press Quenched Steels), including those that are roll formed or fluid formed.
Automakers contemplating whether a part is cold stamped or hot formed must consider numerous factors. This blog covers some important considerations related to welding these materials for automotive applications. Most important is the discussion on Resistance Spot Welding (RSW) as it is the dominating process in automotive manufacturing.
Setting Correct Welding Parameters for Resistance Spot Welding
Specific welding parameters need to be developed for each combination of material type and thickness. In general, the Hot Press (HP) steels require more demanding process conditions. One important factor is electrode force which should be higher for the HP steel than for cold press type steel of the same thickness. The actual recommended force will depend on the strength level, and the thickness of the steel. Of course, this will affect the welding machine/welding gun force capability requirement.
Another important variable is the welding current level and even more important is the current range at which acceptable welds can be made. The current range is weldability measurement, and the best indicator of the welding process robustness in the manufacturing environment and sometime called proceed window. Note the relative range of current for different steel types. A smaller process window may require more frequent weld quality evaluation such as for weld size.
Relative Current Range (process windows) for Different Steel Types
The Effect of Coating Type on Weldability
In all cases of resistance spot welding coated steels, it is imperative to move the coating away from the weld area during and in the beginning of the weld cycle to allow a steel-to-steel weld to occur. The combination of welding current, weld time and electrode force are responsible for this coating displacement.
For all the coated steels, the ability of the coating to flow is a function of the coating type and properties, such as electrical resistivity and melting point, as well as the coating thickness.
An example of cross sectioned spot welds made on Hot Press Steel with Aluminum -Silicon coating is shown below. It shows two coating thicknesses and the displaced coating at the periphery of weld. This figure also shows the difference in current range for the different coating thickness. The thicker coating shows a smaller current range. In addition, the Al-Si coating has a much higher melting point than the zinc coatings on the cold stamped steels, making it more difficult to displace from the weld area.
Hot Press Steel with Aluminum -Silicon
Liquid Metal Embrittlement and Resistance Spot Welding
Cold-formable, coated, Advance High Strength Steels such as the 3rd Generation Advanced High Strength Steels are being widely used in automotive applications. One welding issue these materials encounter is the increased hardness in the weld area, that sometime results in brittle fracture of the weld.
Another issue is their sensitivity to Liquid Metal Embrittlement (LME) cracking. These two issues are discussed in detail on the WorldAutoSteel AHSS Guidelines website and our recently released Phase 2 Report on LME.
Resistance Spot Welding Using Current Pulsation
The most effective solution for the issues described above is using current pulsation during the welding cycle. A schematic description is shown below.
The pulsation allows much better control of the heat generation and the weld nugget development. The pulsation variables include the number of pulses (typically 2-4), the current level and time for each pulse, and the cool time between the pulses.
In summery, pulsation (and sometime current upslope) in Resistance Spot Welding proved to be beneficial for the following applications:
- PHS steels
- Coated Cold Stamped steels
- Cold stamped Advance High Strength Steels
- Multi materials stack-ups – As described in our articles here on 3T/4T and 5T Stack-Ups
Thanks is given to Menachem Kimchi, Associate Professor-Practice, Dept of Materials Science, Ohio State University and Technical Editor – Joining, AHSS Application Guidelines, for this article.
Current Vehicle Examples
Manufacturers embrace Advanced High Strength Steels as a cost-effective way to satisfy functional and regulatory requirements. The following are just a few examples where automakers have attributed improved performance and lightweighting due to the use of these advanced steels.
KIA EV9
The Kia EV9, Kia’s first three-row electric flagship SUV, is based on the Electric Global Modular Platform (E-GMP).K-59 Kia EV9 won the 2024 North American Utility Vehicle of the Year™ (NACTOY) AwardK-60, and was named a 2024 Top Safety Pick by IIHS, the Insurance Institute for Highway Safety.K-61 Kia deployed hot stamped parts in the passenger safety cage for enhanced passenger protection and crash energy management.K-62
Figure 1: Hot stamped parts increase the average tensile strength in the 2024 Kia EV9K-62
Tesla Cybertruck and Model Y
Much press has been given to the “ultra-hard stainless steel” used on the Cybertruck skin panelsT-46, but there are several high strength and advanced high strength steel parts on the vehicle as well. According to the Cybertruck Collision Repair Manual,T-47 Tesla defines mild steel as having a tensile strength less than 270 MPa. The tensile strength of high strength steels ranges from 300 MPa to 700 MPa. Ultra high strength steels are those with a tensile strength greater than 800 MPa. Figure 2 presents a breakdown of materials used in the body structure.
Figure 2: Cybertruck Body Materials. Dark blue = mild steel; yellow = high strength steel; red = ultra high strength steel; orange = stainless steel.T-47
A video by Munro Live with Lars Moravy, Tesla’s Head of Vehicle Engineering, shows that the Cybertruck body side inner is formed from a laser welded press hardened steel.M-65-2 An interview with Thomas Ausmann, former global advanced manufacturing technical advisor at Tesla, confirms that Tesla hot stamps the double-door rings, which represents the first hot-stamped part that Tesla had ever produced internally at any of its plants.B-78 Figure 3 shows a Cybertruck hot-stamped body side inner.
Figure 3: The Cybertruck double door ring made from a laser welded blank is the first hot stamped part that Tesla ever produced internally at any of its plants M-65-2 B-78
The Model Y Collision Repair Procedures ManualT-48 highlights that there are several ultra high strength steel parts in the body structure. Another video from Munro LiveM-70 confirms that the ultra high strength steel in the body side aperture is press hardened, hot stamped steel. Simwon NA is the likely supplier of these hot stamped parts.Y-15
Figure 4: Press Hardened Hot Stamped Steel in the Model Y Body Side Outers and Inners. Dark blue = mild steel; yellow = high strength steel; red = ultra high strength steelT-48
Li Auto L8
The Li L8 is a luxury range-extended battery electric SUV equipped with an autonomous driving system produced by Chinese manufacturer Li Auto. Hot-formed steel is used in safety-critical areas such as the A-pillar, B-pillar, C-pillar, door sills, and door intrusion beams, accounting for 28.9% of the entire body-in-white, with high strength steels accounting for over 75% of the body structure. Hot-formed steel parts are shown in red in Figure 5, with ultra high strength steel shown in yellow, high strength steel shown in dark gray, and mild steel parts colored in blue.X-2
Figure 5: Nearly 30% of the Li Auto L8 body-in-white is made from Hot Stamped Press Hardened Steels.X-2
Honda Civic
The 2025 Honda Civic Hybrid, based on the 11th Generation Honda Civic platform launched for the 2022 model year, uses high strength and advanced high strength steel throughout their Next-Generation Advanced Compatibility Engineering™ (ACE™) body structure. Honda defines high-strength steel (HSS) as any steel with a tensile strength of 340 MPa or higher. Ultra-high-strength steels (UHSS) are those with a tensile strength of 980 MPa or higher.H-68
Figure 6: The body construction of the 2025 Honda Civic uses high-strength steel and advanced high-strength steel for enhanced passenger protection.H-67
Nissan Rogue
The 3rd Generation Nissan Rogue, launched for the 2021 Model Year, makes extensive use of advanced high strength steels, including 3rd Gen AHSS.
Nissan deploys AHSS grades for 35% of the body structure, an increase of more than 10% compared to the prior version.L-67 Hot stamped press hardened steels, not used in the prior model, helps this Nissan Rogue achieve improved safety, fuel efficiency, and customer satisfaction. Figure 7 shows how various steel grades are deployed in the body structure.
Figure 7: Nissan Rogue Body-in-White Uses Press Hardened Steels and 3rd Generation Advanced High Strength Steel Grades.L-67
The Rogue’s B-pillar is cold stamped from a tailor welded blank of super high formable 980 (SHF 980) and super high formable 1180 (SHF 1180) steel, allowing Nissan to realize the same benefits of hot stamping at a much higher productivity, as highlighted in Figure 8 L-67. Both of these super high formable grades can be considered 3rd Generation Advanced High Strength Steels. (See the information on the 2018 Infinity QX50 SUV here) .
Figure 8: The Nissan Rogue uses a laser welded blank formed from two 3rd Generation Advanced High Strength Steels. L-67
A critical enabling technology in the use of SHF 980 and SHF 1180 is the development of design guidelines for welding stacks that include those materials. These guidelines use weld gun control and panel positioning to prevent unneeded additional tensile stress in the weld stack.L-66 Minimizing the tensile stress in the weld stack helps address the risk of liquid metal embrittlement as does extending the hold time portion of the spot weld cycle in order to lower the temperature prior to releasing the electrodes.L-67
Chevrolet Blazer EV
The Chevrolet Blazer EV is built on the same architecture as the Chevrolet Equinox EV, Cadillac Lyriq, Honda Prologue, Acura ZDX EV, among others.E-14
Fifteen percent of the body structure are ultra high strength steels, including multiphase, martensitic, and 3rd Generation Steels having a tensile strength of at least 980 MPa. An additional 11% are stamped from press hardened steels. The breakdown of the Blazer EV body structure is shown in Figure 9.
Regarding the battery pack, part of the General Motors battery management system known as the Rechargeable Energy Storage System (RESS), 43% of the all-steel construction is made from grades with tensile strength of at least 980 MPa. (Figure 10).
Instead of using press hardening steels for the B-pillar, General Motors stated that there was a cost savings in addition to a mass savings by using 3rd Generation AHSS in this application. This required development of a material grade specification capable of use globally, along with forming and welding practices for robust production. (Figure 11).
Figure 9: 35% of the Chevrolet Blazer EV body structure is made from Advanced High Strength Steels with a tensile strength of at least 590 MPa. E-14
Figure 10: 43% of the Chevrolet Blazer EV Rechargeable Energy Storage System structure is made from Advanced High Strength Steels with a tensile strength of at least 980 MPa. E-14
Figure 11: Use of 3rd Generation Advanced High Strength Steels in the B-Pillar of the Chevrolet Blazer EV led to cost savings and mass savings while maintaining crash and safety performance. E-14
Using Life Cycle Assessment to Determine Steel E-Motive Concept Vehicle Emissions
The transportation industry’s contribution to greenhouse gas (GHG) emissions and global warming is well documented and understood. Vehicle OEMs, fleet operators, and transport users all have responsibilities to reduce environmental impacts on the planet and contribute to meeting global emissions regulations.
Mobility as a Service (MaaS) solutions like WorldAutoSteel’s flaghip Steel E-Motive (SEM) program have the potential to contribute to a reduction in GHG emissions, helping to achieve these global targets and specific policy objectives. The Steel E-Motive engineering report, released in 2023, addresses the impact of emissions reduction using Life Cycle Assessment, with key results summarized in this article.
Introduction to Life Cycle Assessment
Life Cycle Assessment (LCA) is a methodology that evaluates the environmental impact of a product across its entire lifecycle. By understanding the impact across the entire vehicle life cycle, vehicle manufacturers evaluate trade-offs and assess the net impact of the product they’re using.
Cradle-to-grave assessments utilize a boundary that includes impacts from the production phase (including raw material extraction and vehicle production), the use phase (including fuel or electricity as well as consumables like tires and fluids) and the end-of-life phase, which could include disposal and/or recyling of the product, as shown in Figure 1. We applied LCA throughout the development of the SEM concept.
Figure 1. SEQ Figure \* ARABIC 1 Life Cycle Assessment, considering the entire life of the vehicle, from raw material extraction to end of life
LCA can cover a range of environmental impacts; however, for the SEM program, we focused on GHG emissions through the GWP-100 indicator and total energy consumption using Cumulative/Primary Energy Demand and Fossil Energy Consumption indicators.
Reference Taxi (Baseline) Vehicle
A key consideration in LCA calculations is establishing an appropriate reference vehicle. For this program, the following criteria was used:
- Present day (~2020) battery electric vehicle (BEV) operating in taxi mode with a driver and one occupant with vehicle/battery lifetime assumptions of 300,000km, and use of 100 percent conventional steel/aluminum.
- Vehicle end-of-life methodology using the Avoided Burden Approach, where recycled metals are assumed to displace equivalent quantities of their virgin counterparts and assigned corresponding emission and energy demand credits.
- Assumption of 50 percent pyrometallurgical recycling for the battery packs.
- Estimated reference taxi vehicle curb weight using the statistical reference data study (Figure 2), resulting in an estimated curb weight of 1,949kg.
- Material utilization based on data from a similar vehicle specification, as shown in Figure 3.
- Vehicle occupancy rate assumptions of 1.4, based on a combination of both “empty” and passenger-carrying journeys.
Figure 2. Vehicle curb weight versus box volume comparison. Reference vehicle data; source www.a2mac1.com
Steel E-Motive “Default” Vehicle
SEM vehicle life cycle calculations assume a hypothetical 2030 manufacture and start-of-operation date of 2030 to 2035. We updated the electricity grid supply mix to include the average of the International Energy Agency (IEA) scenario estimates for 2030 and 2040.
- We applied the nominal SEM1 vehicle curb weight of 1,512kg in the LCA model, and updated the vehicle Bill of Materials.
- As with the reference vehicle, we adopted the Avoided Burden Approach as the default for end-of-life calculation.
Life Cycle Assessment Results
Figure 3 below highlights absolute calculated life cycle GHG emissions, in units of kgCO2e/ passengerꞏkilometer studied, with the individual contributions of vehicle manufacturing, vehicle use, and end-of-life phase presented.
The analysis evaluated two reference/baseline conditions and nine SEM sensitivity studies, see Figure 4. These included alternative assumptions on LCA end-of-life modeling methodology, lifetime vehicle activity (and battery lifetime), alternative operational energy consumption sensitivities, sensitivities on the use of ‘green’ steel, and vehicle occupancy rates.
The accompanying pie chart shows the breakdown and contributions to the vehicle manufacture GHG for the baseline SEM scenario (2).
Figure 3. SEQ Figure \* ARABIC 2 life cycle assessment GHG results
Figure 4. Reference/baseline conditions and SEM sensitivity studies
Life Cycle Assessment Conclusions
Based on the parameters outlined, applying LCA to SEM concept demonstrated the designs’ potential to reduce lifecycle greenhouse gas emissions by up to 86 percent compared to a present-day battery electric vehicle operating as a taxi.
This potential can be realized by adopting the following measures:
- Reducing vehicle production and manufacturing embedded emissions by utilizing 100 percent reduced carbon (“green”) steel
- Improving battery technology and increasing the use of renewable electricity in battery manufacturing; as well as increasing/improving battery recycling
- Ensuring the vehicle weight of autonomous vehicles is managed, and the potential weight reduction benefits realized and implemented. The SEM body structure and battery housing demonstrate good weight efficiency.
- Increasing the overall lifespan of the vehicle and battery. The fatigue and durability properties of AHSS can enable enhanced vehicle lifetime. The SEM battery design allows easy replacement of specific modules, enabling an overall extended battery life.
- Autonomous vehicle control smooths the driving cycle. The vehicle acceleration and deceleration rates can be optimized to match the driving conditions and road topography, reducing energy consumption and subsequent GHG emissions.
- Increasing passenger occupancy rates to at least three per vehicle via MaaS.
The projected net GHG emissions for the SEM vehicle operating with the flexibilities described above already represent a significant reduction when compared to the current baseline.
Achieving net zero emissions would require additional measures like offsetting manufacturing impacts (e.g., through compensatory credits from atmospheric carbon capture and storage) and transitioning to a 100 percent renewable electricity grid.
Moving Toward Net Zero
Taking a Life Cycle Assessment approach to the SEM concept demonstrates the possibilities for engineering future mobility vehicles that continue to move us closer to a net zero future. For more information about the Steel E-Motive program, download the engineering report here: https://bit.ly/SEM_Eng_Report
Thanks go to Russ Balzer for his contribution of this article to the AHSS Insights blog. As.technical director at WorldAutoSteel, he leads technical programs and oversees the organization’s work in research, modeling, and advocacy for Life Cycle Assessment in the automotive sector. An LCA Certified Professional through the American Center for Life Cycle Assessment (ACLCA), he also acts as the WorldAutoSteel liaison to the worldsteel LCA Expert Group.