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 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
