Steel Targets EV Structures, Battery Boxes

The industry fires back at aluminum’s growing‘ gigacasting ’threat.

This excerpt from an ArcelorMittal presentation shows the claimed benefits of its products that now compete with aluminum gigacastings. ArcelorMittal engineers believe steel-intensive ‘multipart solutions ’are superior overall to large aluminum die castings in performance and cost. A recent analysis showed this steel approach offers significant reductions in operators in assembly, the body shop area, the number of robots, hours per vehicle, part count, the number of spot welds, and improved material utilization.(ArcelorMittal)

The steel and aluminum industries are waging a new materials war that could determine which of the long-time competitors dominates EV structures in the next design cycle. Feeling the heat from new aluminum applications, steel companies are aggressively moving to defend their reign as the material of choice in vehicle body structures. They also aim to conquer the EV battery enclosures market, currently dominated by aluminum extrusions. (See AE March 2023 feature) Steel executives are bullish on their new technologies and their approaches to winning on both fronts.

GDIS 2024 keynoter Brad Davey, executive VP of corporate business optimization at ArcelorMittal. (Lindsay Brooke)

“Today, many automakers are talking about high-pressure die-cast aluminum, especially for BEVs,” Brad Davey, executive VP at steel giant ArcelorMittal said. Speaking with SAE Media following his keynote address at the recent 2024 Great Designs in Steel (GDIS) conference in Novi, Michigan, Davey acknowledged that giant HPDC aluminum ‘gigacastings lend themselves to ‘skateboard ’EV platforms due to their potential to consolidate parts and simplify manufacturing. The end benefit, he said,“ is capital-expense assembly line savings,” particularly for EV companies.

“If you can reduce floor space and the number of welding robots in assembly, by using one large casting, it can be an attractive solution – compared with traditional processes, that is,” he said.

Pioneered by Tesla on the Model Y, gigacastings are designed to ultimately form a battery-electric vehicle’s entire underbody in a single aluminum component. The Model Y uses two castings, front and rear, connected by a steel/aluminum battery box. See AE June 2020 feature) The potential benefits have sparked industry-wide interest, especially in tooling suppliers like IDRA, LK Group, Buhler, and Yizumi. In late 2023, GM acquired Livonia, Michigan-based Tooling & Equipment International (TEI), a leader in foundry mold technology. GM engineers used TEI’s expertise to prototype and validate the large underbody castings for the Cadillac Celestiq due later this year.

Despite recent reports that Tesla scaled back its effort to produce a low-cost Model 2 using an industry-first single underbody casting, steelmakers are also concerned about Chinese OEMs using aluminum gigacasting to help drive radical new EV architectures. S&P Global has reported development in China of enormous 12,000- to 16,000-metric-ton HPDC machines.

Gigacasting downsides

Not so fast, the steel stalwarts say. ArcelorMittal’s Davey highlighted gigacasting’s downsides. “The part costs are higher. If you’re not offsetting the part costs with manufacturing savings, you’re going to end up with a higher cost design or no cost improvement,” he said. “There are many issues on yields, defects related to high tooling cost and trying to extend tooling life, and also cracking [of the castings]. He also noted casting’s crash-energy absorption characteristics and questioned OEMs ’ability to refresh/change exterior body styling as quickly and efficiently as on traditional structures.

“Typically, in the past, aluminum solutions have been lower weight, but these are not compared to the optimum steel solution,” Davey said. Optimizing mold flow through the large die-cast parts has, in many cases, required increased wall thickness and larger/heavier castings overall, he said. He also claimed that large die castings are difficult to repair after road accidents, leading to higher auto insurance costs.

Davey prompted chuckles from the GDIS audience when he reported that some automakers are discussing the need to redesign load paths in their vehicles to avoid damage to the large, expensive die castings. “If you didn’t have enough challenges keeping the occupants and batteries in BEVs safe, now you’ve got to protect the die castings as well,” he quipped.

Steel’s multi-part solutions

The “optimal steel solutions” that Davey and other steel execs believe will parry aluminum’s assault are rooted in the industry’s innovation and R&D awakening in the 1980s, when steel fought back against the plastic body panel challenge. New lighter and stronger high-strength, advanced high-strength, and high-strength low-alloy products, combined with new manufacturing processes, co-design services and cost advantages halted the advance of aluminum-intensive bodies.

Even after losing Ford’s F-Series to the light metal in 2012, steel today still makes up over 50% of an average vehicle’s mass. At GDIS, American Iron and Steel Institute CEO Kevin Dempsey noted that 75% of modern steel alloys have been developed in the last 20 years; more than 3,500 steel grades are available today.

An analysis conducted with S&P Global looked at traditional single-part manufacturing and the improvements offered by new steel multi-part solutions. The reference single-part design was compared quantitatively to the steel multipart and then to the aluminum die casting, which was found to be heavier and more costly. (ArcelorMittal)

Steelmaker engineers who “sit at the design table” with vehicle OEMs are using that portfolio, along with tailor-welded blanks, new fabrication techniques, and what Davey calls “multi-part solutions” to expand steel applications in EV body structures, conquest the battery enclosures business, and improve the EV cost structure overall.

Steel industry analyses reveal that nine of the top 10 vehicles with the highest utilization of high-strength steel are EVs, Davey said. He added that the adoption rate of laser-welded, press-hardened steels is running 50% higher in EVs than in hybrid and ICE vehicles.

“We’re winning back the battery boxes,” he said. “Nearly every automaker is in discussion with us and others about how to utilize steel in battery structures. One North American OEM has told us their future for battery boxes is 100% steel, or highly steel-intensive.”

Bring on the competition, Davey said, echoing his industry colleagues at GDIS. “The competition is good for all of us.” His keynote concluded with a warning: “China is adopting these steel solutions at about two times the rate of the Western world.”