Why e-mobility needs Smart Steels

Jérôme Favero, Head of Automotive Steel Solutions for ArcelorMittal Global R&D:

Why e-mobility needs Smart Steels.

As automakers are challenged to continuously improve safety and fuel economy, they search for new materials which meet higher standards. For more than 20 years advanced high-strength steels (AHSS) have been helping engineers meet evolving requirements for safety, efficiency, emissions, manufacturability, durability, and quality at a low cost. The range of AHSS available has been constantly evolving for more than two decades. Thanks to the additional research and development undertaken by steelmakers such as ArcelorMittal, automakers are now using the latest grades in more and more applications. To ensure a successful transition to electric vehicles (EVs), carmakers must increase their efforts to extend range, reduce cost, and enhance safety. In this article, Jérôme Favero, head of automotive steel solutions for ArcelorMittal Global R&D, tells us about the new generation of smart AHSS steels which are helping automakers achieve these goals.

To start with, can you define AHSS?

Advanced high strength steels (AHSS) are grades which provide extremely high strength and other advantageous properties while maintaining the high formability required for manufacturing. They offer an extremely diverse range of mechanical properties and can meet the technical requirements of different parts of a vehicle including the body-in-white (BIW) and closures.

Intermediate results of our on-going S-in motion® study into the development of a body in white (BIW) for a mid-size electric sports utility vehicle (SUV) which complies with North American crash standards. The concept limits mass penalties compared to an equivalent ICE vehicle.

AHSS have been developed to enable car manufacturers to increase the safety performance of their vehicles while continuously reducing the weight of the structure. Some are able to absorb or resist strong crash loads without compromising their structural integrity. The development of AHSS has also been driven by the need to light weight vehicles to meet stricter emission regulations. A better performing steel allows the OEM to reduce a part’s thickness without compromising safety.

The first AHSS grades were introduced in the 1990s and had an ultimate tensile strength (UTS) of 600 megapascals (MPa). The latest AHSS grades launched by ArcelorMittal, now reach almost 2000 MPa. By comparison, drawing grades – which are commonly used to stamp deep parts such as door inners – have a UTS of just 300 MPa.

It’s important to note that there is not one AHSS grade, but different AHSS products – or families. Each family has different metallurgies to meet the varied technical requirements of the automotive market. The oldest are the dual phase (DP) grades which offer quite good stamping properties. Next are the complex phase (CP) family which offer good local formability. This allows OEMs to create parts with more complex local geometry or better cut-edge formability to help the assembly process.

Martensitic grades, marketed as the MartINsite® product series by ArcelorMittal, are another family of AHSS. They offer very high mechanical properties (up to 1700 MPa) but with a limited elongation. Thanks to a dedicated roll forming process, this family can be used to develop very cost-effective solutions which are well adapted to anti-intrusion applications such as battery pack protection in electric vehicles (EVs).

Generally, the higher the steel’s tensile strength, the lower its formability. This means that conventional AHSS (such as DP, CP, and martensitic grades) have stamping limitations when their mechanical properties increase. ArcelorMittal has developed two other AHSS families to overcome this difficulty.

The battery of an EV is often located in the floor to maximise available space for the cells

The first is press hardenable steels (PHS) which include the Usibor® and Ductibor® ranges. While Usibor® is designed to provide the best anti-intrusion properties thanks to very high level of mechanical properties, Ductibor® is designed to maximize energy absorption by combining high mechanical properties with excellent ductility (fracture strain). This means that parts made from Usibor® can withstand intrusion, while parts made from Ductibor® can absorb energy by deforming without fracture.

The great advantage of PHS over conventional AHSS is their ability to form very complex shapes with outstanding precision thanks to the hot stamping process. The steel is stamped at high temperature (700°C) when it is very formable and then quenched in the stamping tool to give the part its final mechanical properties. The part holds its shape as there is no springback.

The second new AHSS family is Fortiform®, commonly referred to as the 3rd Generation AHSS or HS/HF steels. Fortiform® grades have higher elongation and offer outstanding formability compared to other cold stamping AHSS with a similar level of mechanical strength.

Using AHSS, ArcelorMittal has developed revolutionary solutions for carmakers who want to build lighter, safer, and more environmentally friendly vehicles. Can you tell us about these?

ArcelorMittal has developed a set of steel solutions known as S-in motion®. We have already completed nine S-in motion® studies into different vehicle types, for example, C-segment vehicles, battery electric vehicles (BEVs), or pick-up trucks, and eight studies of specific vehicle components (for example, battery packs or front seats). More are under development including our new S-in motion® SUV Battery Electric Vehicle project. Each study defines a catalogue of generic steel solutions which highlights the benefits of ArcelorMittal’s offer.

The research is led by our Global R&D center in Montataire (France), but it also involves our worldwide ArcelorMittal R&D teams and automotive suppliers such as engineering offices and die makers. We also work in close cooperation with carmakers in the early design stage of new vehicle platforms to adapt our S-in motion® solutions to match their goals and architecture. We do this through our development centers in Europe and North America and our teams located in Asia.

The main idea behind S-in motion® is to show how our steels can be combined with good design and innovative processes to help OEMs meet safety and environmental regulations at the lowest possible cost. It is a priority for us to develop realistic solutions which are ready for implementation in the customer’s design. We carefully identify all the requirements for the vehicle and the market.

Thanks to ArcelorMittal’s knowledge of the properties of our products, and many previous automotive engineering studies, we can identify the best compromises to lightweight, reduce costs, or improve the performance of the vehicle or component. Our customers like the idea of having material innovations, but also appreciate having an early idea of the technical and cost performance that can be achieved.

Intermediate results of our on-going S-in motion® study into the development of a body in white (BIW) for a mid-size electric sports utility vehicle (SUV) which complies with North American crash standards. The concept limits mass penalties compared to an equivalent ICE vehicle.

Mobility is evolving at a greater pace than ever before. New powertrains are challenging the dominance of the internal combustion engine (ICE) and creating almost unlimited opportunities to transform vehicle design. Electrification is an incredible opportunity for us to demonstrate once again that steel is by far the best solution. The various AHSS grades available from ArcelorMittal offer an optimal balance between strength, performance, and mass reduction and have the lightest impact on the environment.

Development of a practical solid-state battery has generated considerable interest amongst automotive manufacturers. Despite decades of development, many technology challenges remain unsolved and so, at least in the short term, lithium-ion (Li-on) battery systems will be the mainstream solution. What technical points were considered in the design of ArcelorMittal’s S-in motion® Battery Pack solution?

 

Like the S-in motion® study into the BIW of conventional ICE vehicles, there are three very important criteria we consider: safety, weight, and cost.

The design of a BEV is challenging for two main reasons. The first is that the weight of the vehicle is increased by several hundred kilograms due to the battery pack. That means there is much more energy to absorb in the event of a crash. The second is that the battery pack must be preserved as a whole in a crash to avoid fire or chemical hazards for the vehicle’s occupants. We must rethink the design of the BIW, and the battery pack, to meet these safety requirements.

Range is still one of the most important criteria for a BEV. In order to optimize the range of their vehicles, BEV manufacturers are maximizing the size of battery pack to accommodate as many battery cells as a possible. The result is a very large battery pack which must be completely protected in a crash event. But the space available to absorb the energy of the crash (via deformation of the structure) is very limited. This is particularly important for side-pole crash loads. It’s an excellent opportunity to prove that AHSS, and more particularly in this case – martensitic grades such as MartINsite® 1500 – are an excellent way to achieve this level of anti-intrusion protection.

The battery pack itself is most the challenging structure to design as it has its own load cases. It is also one of the most vital parts of an EV. As well as protecting the battery and its equipment from external elements, it must keep the vehicle and its passengers safe from battery leakage, fumes, fire, and electromagnetic fields.

The first stage of ArcelorMittal’s S-in motion® Battery pack study was to identify the load cases and regulations which apply. However, these are not always the same in different regions. As BEV technology is not yet mature, there are some things which are not standardized.

Underfloor intrusion requirements are a good example. Today there are no regulations concerning underfloor intrusion, but it is logical that deformation or contact with the battery cells must be avoided. This means that special attention must be given to the shield under the battery pack.

We ask questions like: what happens if an automatic bollard lifts a car, or what happens if you need to change a tire and the jack penetrates the battery pack? When there are no regulations, we start discussions with relevant industry bodies that study real-life accidents to develop safe designs.

One other difficulty we have is that the regulations which do exist are sometimes only applied in a specific region. To design a solution which is compliant worldwide, we consider the most severe crash standards. Some examples include:

  • Crush test (China GBT31467.3). This regulation specifies that there should be no fire or explosion if a load of 100 kilonewtons (kN) is applied to the structure using a pole.
  • Underfloor intrusion: no fire or explosion with a load of 35 kN.
  • Shock test: 25 g of acceleration applied to the battery pack without plastic deformation occurring.
  • Drop test (Korea KMVSS Ver 01). No risk of explosion when the battery pack is dropped from five meters.

The S-in motion® Battery Pack concept has been validated against these severe standards and achieved an attractive cost/weight performance. The high mechanical properties of ArcelorMittal’s MartINsite® 1500 grade showed excellent results in these tests when combined with an appropriate design. The MartINsite® grade was able to resist high load-deformation due to its very good yield strength of more than 1200 MPa. The three main sub-modules (upper cross member, frame, and lower shield) were found to benefit from the use of this steel grade. MartINsite® 1500 is widely used in the concept due to the geometrical requirements of the part. These geometries, which are mainly profiles, are compatible with the roll-forming process which makes the solution very cost efficient.

The S-in motion® Battery Pack study includes several best-in-class and customizable battery pack solutions with different architectures (for example, with a tray or different material strategies). This provides answers to various technical requirements which are specific to different OEMs. Each showcases the ability of AHSS to meet structural requirements while providing attractive weight and cost benefits.

The feasibility of the concepts has been checked by ArcelorMittal’s automotive teams. Formability was confirmed through numerical simulations while the manufacturing and assembly aspects were assessed by a well-known Tier-1 supplier who could industrialize this type of solution.

A cost assessment was performed by an external company which specializes in the automotive business. That assessment confirmed that our design for a C-segment battery pack is cost-effective and can achieve savings of between €80 and €100 compared to an equivalent aluminum version. The steel solution also achieved a very similar weight performance. In the economics of battery electric vehicles, this level of saving is of major importance.

Sustainability is also a key driver of vehicle electrification and is also covered in ArcelorMittal’s S-in motion® studies. ArcelorMittal’s battery pack solution provides a 36 percent reduction in CO2-equivalent emissions compared to a similar solution made from aluminum. The scope of the life cycle assessment (LCA) performed includes production and recycling of the structure (using average worldwide values).

For a C-segment vehicle battery pack made of steel, cost savings of between €80 and €100 are expected compared to aluminum

ArcelorMittal’s steel battery pack for a C-segment vehicle offers a 36 percent reduction in lifecycle CO2-equivalent emissions compared to an aluminum solution

Can you describe the performance testing that was carried out on the S-in motion® solution?

As mentioned earlier, the most severe load cases were considered. ArcelorMittal has applied all of them in our S-in motion® Battery Pack study. Let’s start with the crush test applied in China.

Chinese crush test (GBT31467.3)

The crush test is the load case which defines the frame design. The objective is to reach 100 kN of pressure without contact between the structure and the battery cell modules. during the test, the battery pack is placed on a flat floor against a wall. A quasi-static load is applied at the weakest zone using a rigid pole with a diameter of 150 mm.

An optimized frame design and the high mechanical properties of MartINsite®1500 ensure the anti-intrusion performance of the S-in motion® Battery Pack. The results showed the battery pack could resist more than 100 kN of pressure with a short, but not fatal, intrusion.

Underfloor intrusion test

Underfloor intrusion test

The underfloor intrusion load case represents a significant accident from underneath the vehicle. To pass, the battery pack must be able to resist up to 35 kN of pressure without damage to the battery cells. This quasi-static load case is performed using a rigid impactor with a diameter of 20 mm. The pressure is applied to the weakest area underneath the battery pack. Flat sheets of MartINsite® 1500 can resist this pressure without affecting the battery cells.

Shock test (International SAE J2464, EU ECE R100)

The shock test is the most severe load case and assesses the performance of the battery pack structure in driving conditions. Two load cases must be validated:

  1. A vertical 25 g z-acceleration force is applied to the battery pack while it is fixed by its brackets. Structural rupture and contact with the battery cell modules are not allowed.
  2. Proof shocks. This is a test with lower severity. The entire structure is submitted to different accelerations (2 g x-direction; 2 g y-direction; 4 g z-direction). No plastic deformation of any structural parts of the battery pack can occur.

The shock test includes two separate load cases

Drop test

Several drop load tests exist to assess the structure of the battery pack. The Korean drop test (KMVSS Ver 01) is the most severe. It requires the battery pack to be dropped from a height of five meters and at an angle of 15 degrees. To pass this test, the battery pack must resist structural rupture and there can be no localized contact with cell modules. This drop test highlights the important contribution of the AHSS longitudinal cross-member and demonstrates again why these steels are needed.

During the Korean drop test, the battery pack is dropped at a 15-degree angle from a height of five meters

Vibration and fatigue validation (criteria: first mode > 35 Hz)

Vibration and fatigue simulation were carried out on the S-in motion® Battery Pack. The goal of these tests was to ensure that the battery pack architecture is not sensitive to vibration or fatigue during the use phase of the vehicle’s life. This ensures the durability of the S-in motion® solution.

The results showed that the battery pack was not sensitive to vibrations of up to 35 hertz (Hz). This confirms that the concept meets international vibration requirements. The results are strongly affected by the weight of the battery cells, but other factors such as the assembly, fixing points, and the thickness of the parts also have an impact.

Vibration testing

The fatigue performance of the battery pack was assessed in line with international standard ISO 12405-2. During the test, the battery pack design is subjected to a power spectral density (PSD) simulation to assess failure risks in assembly areas. The design team performed a complete concept study and used numerical simulations to check that the main technical requirements of the standard were met by ArcelorMittal’s AHSS steels.

I understand that the battery pack is a key element of ArcelorMittal’s electrification projects. But what other projects is ArcelorMittal working on?

Major changes are occurring in the BIW due to electrification. The chassis has to be redesigned to successfully bear the added mass of the battery pack and to ensure compatibility with the new powertrain. We are currently completing a design solution for the BIW and chassis of BEVs. Some cost-effective solutions with excellent performance have already been identified. The final results are expected before the end of 2019.

I’ve mentioned AHSS a lot. These steels are the best solution for the BIW and other parts of EVs. But we can’t forget the electric components of the vehicle and its powertrain. ArcelorMittal has developed a catalogue of electrical steel grades under the iCARe® brand. They provide solutions which cover all the requirements of traction motors in hybrid and full electric powertrains.

The iCARe® family was initially launched with three members: iCARe® Save for the highest motor efficiency; iCARe® Torque for the highest motor performance; and iCARe® Speed which allows engine designers to develop compact and light machines. Now there’s a new member of the iCARe® family: iCARe® 420 Save. This new group of electrical steels combines low losses with improved and guaranteed yield strength. This results in increased power density and improves the energy efficiency of electric traction motors. You can find more information about these developments on our dedicated automotive website: https://automotive.arcelormittal.com/

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