New BIW options for heavier EV batteries: HR1500 martensitic in 2 to 4 mm gauge

As EV batteries continue to grow – both in range and weight – automotive designers are looking for stronger materials for both battery enclosures and related body-in-white (BIW) safety systems. Since cold-rolled martensitic steel is limited to 2.1 mm thickness, hot-rolled 1500 MPa martensitic – available now in 2 to 4 mm thicknesses – and conventional roll forming are ready to meet these heavier EV needs.

EV distance range dominates buyer decision making

For most EV shoppers, distance range is the primary consideration. Period. Consequently, until a new, lightweight battery technology becomes available, EV designers are faced with accommodating ever-increasing battery weights. Increased battery and vehicle weight poses new challenges for crash protection – for both the battery pack and the vehicle occupants.

As car designers run their material optimization loops for these heavier EV batteries, the maximum thickness of 2.1 mm for cold-rolled martensitic steel is causing them to consider new alternatives. 

Anticipating heavier EV batteries: HR1500M in 2 to 4 mm gauges

Anticipating this situation, SSAB has spent years developing its hot-rolled 1500 MPa tensile strength Docol® martensitic steel.

“Automotive designers needing stronger battery enclosures and BIW safety systems – due to heavier EV batteries – are now readily possible,” says Robert Ström, Design Specialist Automotive, SSAB Knowledge Service Center. “That Docol® HR1500 martensitic also enables lightweighting and total production cost efficiencies – that is, no hot stamping – are great side benefits. But the key focus here is managing crash forces with a combination of effective designs and very strong materials.”

Robert Ström, Design Specialist Automotive, SSAB Knowledge Service Center

Robert Ström, Design Specialist Automotive, SSAB Knowledge Service Center. 

Making the most efficient shapes for crash loads

Traditionally, many production engineers have hesitated to cold form gigapascal (1000+ MPa) steels. But many are now considering the high performance of roll-formed parts made from martensitic steels. Roll forming is a highly effective means for shaping profiles that can be used in both battery enclosures and vehicle reinforcements. 

“Crash forces, by nature, are not usually complicated: they want to go the shortest way from point A to B,” notes Ström. “So the most effective reinforcements are relatively simple looking. SSAB studies show that the most efficient shapes for several load cases are closed profiles that actually require roll forming.”

“But,” continues Ström, “it is important to design for roll forming from the beginning. And, our EV Design Concept, when used from beginning of the car design, can inspire EV designers in new ways to deliver exceptionally strong battery and occupant protection for heavy EVs.”

The Docol® EV Design Concept

The Docol® EV Design Concept as applied to an EV battery enclosure. The car’s sill beam (in red) absorbs the side pole impact – protecting the battery cells – while the crash load path travels through the (red) floor cross members. Both sill and cross member are roll formed in Docol® martensitic steel.

Excellent mechanical “cuttability"

Despite a tensile strength of 1500 MPa, Docol® HR1500 martensitic steel has excellent mechanical “cuttability” when following SSAB’s recommended cutting parameters.

Cut edge surface shape

Analyzing the mechanically cut edge surface shape of a 3.5mm thick sheet of Docol® HR1500 martensitic. Note how the cut is smooth, uniform, and crack-free.

“In addition to roll-forming, HR1500 martensitic can also be cold stamped,” says Ström. “Japanese car OEMs and their Tier 1s are now cold stamping some BIW parts made from 1.5 GPa steels. Companies in other parts of the world are running cold-stamping trials for 1500 MPa and 1700 MPa side impact beams and bumper reinforcements. We anticipate companies will soon be both roll-forming and cold-stamping Docol® HR1500 martensitic in 2 to 4 mm thicknesses.”

Updates on the Docol® EV Design Concept

“Our Docol® EV Design Concept generates a lot of interest from EV OEMs and Tier 1 suppliers of all sizes,” says Ström. “The EV Concept includes ideas for battery enclosures, sill beams, and cross members. The Concept is just that: a design theory, not full designs for ready-to-make components.”

“The EV Concept design theory has actually been widely used and the results are very positive. Customers are adapting our fundamental ideas to their specific needs – just as we intended. Customers are using different parts of the EV Concept for the rocker structure, the floor cross beams, or members in the battery pack itself for protecting battery cells.”

“Most of the steel in the EV-concept is still cold rolled,” explains Ström. “But Docol® hot-rolled 1500M has let us apply the EV-concept theories on even heavier BEVs.”

“Auto designers are coming to us with a highly diverse set of design philosophies for protecting the battery cells. The EV Concept ideas are adaptable to these different approaches.”

“We always keep the Concept in the back of our mind as the challenges of heavier EV batteries grow,” says Ström. “Working, early on with the EV OEM, we have a real opportunity to develop the best possible design – particularly now, with the new opportunities offered by 2 to 4 mm thick Docol® HR1500 martensitic.”

EV Concept

Another view, after side pole impact, of the EV Concept at work. The full potential of the EV Concept for battery enclosures uses 3D roll-formed “mesh pattern” (not shown) for space-efficient battery protection.

3D roll forming martensite: who will be first?

One part of the EV Concept remains pure potential: 3D roll forming of martensite steel for battery enclosure profiles that are one half the height of other designs.

“To our knowledge, no automaker is using 3D roll-forming technology in serial production,” notes Ström. “It’s a ‘chicken-and-the-egg’ problem: nobody else has done it, so automakers are reluctant to be the first one.”

“The EV Concept’s battery enclosure idea features a cross pattern mesh of 3D roll-formed beams that reduce the mesh’s height by a factor of two. The Concept is there. The technology is there. We think it’s just a matter of time until an EV OEM, or Tier 1 provider of battery enclosures, does it.”

Interested in getting a trial sample of Docol® hot-rolled 1500 MPa martensite? Contact us for samples, EV Design Concept suggestions, forming questions, or any other technical support matters. Your heavy new EV battery will thank you. 

 

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