Emergence of Generation 3 Steel

Advances in Gen3 steel and battery technology ‘fuel’ BEV development.

Emergence, according to Google, is the process of coming into being or of becoming important or prominent. The arrival of Generation 3 (Gen3) steels in the marketplace and on commercial vehicles meets this definition.

It is agreed by many experts that these steels will play a prominent role in the automotive industry due to their high strength and high elongation. These properties allow complex parts to be stamped easily and provide efficient load paths that absorb energy in crash situations.

At a recent conference, Ahmed Ayoub from Electra Meccanica introduced the subject of emergent behavior. Wikipedia defines it this way: “In science, emergence occurs when an entity is observed to have properties its parts do not have on their own. These properties emerge only when the parts interact in a wider whole.”

Mr. Ayoub gave the example of combining sand and a funnel to create a method to measure time. I think of how Apple combined a cellphone and a MP3 player, revolutionizing personal electronics and communication with the iPhone.

Emergent behavior is also a good way to describe how advances in Gen3 steels are combining with advances in battery technology to fuel the wave of battery electric vehicles (BEVs) that are currently being developed. I will discuss the advances that are being seen, first in Gen3 steels and secondly in batteries, specifically lithium-ion (Li-ion) batteries and how these innovations create synergies in the BEV market.

Gen3 steel can be described in reference to other steel products. Figure 1 shows the typical “banana diagram,” which depicts the traditional trade-off between higher strength resulting in lower elongation.

STRENGTH/DUCTILITY WITHOUT COST/ JOINING PROBLEMS
Gen3 steels present an opportunity to achieve both high strength and high ductility without the cost and joining problems of alternative materials like press hardened steel (PHS), aluminum or carbon fiber reinforced polymer composites (CFRP).

A Gen3 steel generally is recognized as having a minimum tensile strength and elongation product of 20GPa%. Gen3 steel integration into the Body-in-White (BIW) will reduce weight, reduce cost and lower the total carbon footprint of the structure while taking advantage of the last 100 years of manufacturing knowledge in the auto industry.

To see these improvements, BIW engineers can use the better ductility to design more efficient geometries—while using the increased strength to reduce the material thickness. This two-step process yields vehicle structures that weigh less and have improved crash worthiness.

Typical BIW applications for Gen3 steels are the structural safety components, such as front and rear rails and the entire “safety cage,” which is made up of the A-pillar, roof rail, Bpillar, sill and front body hinge pillar.

GEN3 IN TODAY’S MODELS
Many OEMs are designing with Gen3 steels now for launch in future models. The Nissan QX50 recently launched using a Gen3 steel in the rear rail structure. Earlier this year, FCA released the Jeep Gladiator with its first application of U.S. Steel’s 980 XG3TM steel in a large reinforcement integral to the cab structure. These are good examples of components that can benefit from both the high strength and high ductility of this material to achieve an efficient lightweight design.

A major benefit of the Gen3 steels is that massive manufacturing changes are not required. Using PHS requires the additional capital investments for long roller hearth ovens and new hydraulic presses to replace traditional mechanical presses. Aluminum requires complicated scrap-segregating strategies in the press shop, an overhaul of the entire body shop to implement riveted joints and an expansion of the paint shop.

Gen3 three steels are a relative “drop in” to replace traditional steel designs and can be cold stamped in traditional mechanical presses. Figure 3 shows the stress-strain plots for 780 XG3TM steel and 980 XG3TM steel.

For 780 XG3 steel, we see that it has similar elongation of a traditional high-strength low-alloy (HSLA) 440 material. While the 980 XG3 steel has similar elongation to a dual phase (DP) 590 material. In both cases, parts can be produced with material of nearly double the strength without any forming limitations.

Improved die design techniques need to be implemented to better control the trim-edge quality and springback, but these procedures are well known. Weldability is also maintained by using a lean chemistry, minimizing the heat into the weld by utilizing robust weld cap geometries and minimizing external stresses during the welding process.

DROP-IN QUALITIES OF 980 XG3
Hyundai and U.S. Steel showcased this drop-in quality of 980 XG3 steel by replacing the PHS inner and outer front body hinge pillar of a current model sedan. The 980 XG3 parts were produced at the same gauge and geometry of the PHS components and welded into the vehicle via the production assembly line. The vehicle was then subjected to the IIHS Small Overlap Front (SOF) impact test.

The test vehicle performed better than the production vehicle, measuring less intrusion into the occupant compartment. The results proved that Gen3 steels can offer a weight-neutral, cost-saving alternative to PHS while yielding comparable or better crash performance. The entire project is documented in SAE publication 2018-01-0117.1

Lightweighting with Gen3 materials is very economical, typically costing less than $2/kg saved when upgrading stamped components from an advanced high-strength steel (AHSS) to Gen3. This is 10 to 20 percent less expensive than PHS.

Meanwhile, lightweighting with a low-density material like aluminum typically costs $6/kg saved, and CFRP can cost more than $20/kg saved.2 This low cost of lightweighting using Gen3 steels will be discussed in more detail later to show the material selection strategy for BEV structures that is developing.

BEV GROWTH
BEVs are a growing trend on their own and are also part of the larger movement to autonomous, connected, electric and shared (ACES) vehicles. This growth started in 2011 when there were three BEVs available. In 2018, that number grew to 18. By 2020, that number is expected to grow to 50-plus unique models.

The decreasing cost of BEVs is the driver behind this explosive growth. A Bloomberg study predicts that by 2024, a BEV will have a cost equivalent to that of a traditional internal combustion engine (ICE) (Figure 4).3

This reduction in cost comes primarily from advances in the lithium-ion batteries. In 2010, Li-ion batteries cost approximately $1,200/kWh. According to Bloomberg (Figure 5), those prices dropped to $176/kWh in 2018— an 85-percent decrease.4

Experts are predicting the costs to continue to drop in the coming years. GM and VW have set targets at less than $100/kWh. Tesla projects approaching $80/kWh from investments made in its Gigafactories.

New technologies are also on the horizon. Nad Karim, the founder of KeraCel Inc., is aiming at $85/kWh utilizing 3-D printing and solid-state battery technology. He predicts that these battery-efficiency gains combined with low-cost solar panel development will change our economy from being measured in barrels of oil to a new “electric economy.”

TOTAL COST OF OWNERSHIP
In the same sense, as the automotive ownership model continues to move toward shared vehicles with mobility service providers, the key characteristic of future vehicles will be a measurement of the total cost of ownership. When it comes to specifying with which materials to produce these new vehicles, more than ever before, the total cost will be the major factor.

As a former BIW structural engineer, I am convinced that we can produce a vehicle out of any material and meet the vehicle function objectives. However, the question of the best material is more than an engineering calculation. It comes down to how best to meet the vehicle economic requirements for variable cost, weight and total investment while meeting the vehicle function objectives.

For the BIW engineer, this means picking the material that will give an efficient lightweight design at an acceptable cost. For electric vehicles, one clear customer functional objective is to have a total range equivalent to one tank of gas in an ICE vehicle.

Extending the range comes down to determining the best place to spend variable cost—additional lightweighting or additional battery capacity. To determine which is optimal, we will plot the cost of lightweighting in $/kg saved versus the battery cost in $/kWh.

Using powertrain simulation models, we can plot the “Equal cost for unit of range increase” line (Figure 6). Lightweighting always leads to increased fuel efficiency. For BEVs, typically a 10-percent reduction in weight will lead to a 6-percent increase in vehicle range.

Figure 6 now gives us the trade-off of when it is less expensive to add battery capacity and when we should spend for lightweighting.

For example, at a battery cost of $450/kWh, lightweighting at a cost lower than $10/kg saved will be a better option. If higher than $10/kg saved, increasing the battery size is more cost effective. Therefore, in 2014, a business case could be made to lightweight with aluminum rather than increasing the battery size. In 2018, at a battery price of $176/kWh, the equal price trade-off is at $3 to $4/kg saved. Now, lightweighting with advanced high-strength steel and the newly available Gen3 steels is a much more economically attractive option.

The lightweighting strategy that would make sense given the combination of high quality Gen3 steel and current Li-ion battery costs would be to use steel for the majority of the BIW structure and aluminum or CFRP sparingly when the vehicle needs to “buy” a few more pounds of mass savings. That is exactly what we are seeing in the industry.

BEV EXAMPLES
For three real-life examples, let’s examine the Nissan Leaf, Tesla Model 3 and the next generation of electric vehicles coming from Volkswagen.

In 2011, the Nissan Leaf was launched, made up from a steel BIW with doors designed in aluminum. If we consider lightweighting the door structure with aluminum to cost about $6/kg saved, this would be cost effective if the battery cost is more than $250/kWh—which it was in 2011. For 2018, Nissan redesigned the Leaf and changed to steel doors. As we previously saw, this is the cost-effective solution with battery prices at $176/kWh. As battery cost continue to decrease, the steel cost advantage will continue to grow.

Tesla recently launched the Model 3, a mid-size BEV sedan with a steel BIW and aluminum closures after using primarily aluminum in models S and X.

Tesla recently launched the Model 3, a mid-size BEV sedan with a steel BIW and aluminum closures. The Model 3 comes after the successes of the Model S and Model X, which were both produced primarily from aluminum.

ECONOMICS BEHIND MOVING TO STEEL
Let’s examine the economic fundamentals behind this decision to move to a primarily steel structure. Using the Model S as a baseline and adjusting its mass to represent the smaller volume of the Model 3, we would calculate that the Model 3 BIW would weigh approximately 245 kg if made in aluminum. The actual mass of the steel Model 3 BIW is 320 kg, representing a 75-kg increase.

Looking at the cost, a conservative approach to the calculation shows that the Model 3 BIW cost approximately $650 less in steel than it would in aluminum. In other words, it would have cost Tesla $8.67/kg saved to use aluminum rather than steel. Figure 6 confirms that the appropriate action was to produce a mass-efficient structure from steel and invest the $650 savings into the battery, powertrain and interior features of the vehicle.

Volkswagen recently gave some details on their next generation of EV vehicles, including the platform they will be built upon, the Modular electric-drive matrix (MEB). Contrary to expectations, but in line with the cost-benefit analysis we have been exploring, Volkswagen’s new concept does not use exotic materials to save weight.

Press releases and recent articles about the MEB describe how VW engineers considered carbon fiber, but decided it wasn’t suitable for the high-volume production and moderate-cost goals the company has for these cars. Even aluminum is used only sparingly, with most of the MEB structure built from high-strength steel.

Environmental sustainability must be touched on in this discussion. Gen3 steels have been shown to be a superior balance of strength, ductility and the low-cost option as compared to PHS, aluminum and CFRP. Gen3 material is also the most environmentally efficient. Figure 7 shows the total life-cycle assessment of the greenhouse gas (GHG) associated with producing equivalent amounts of each material.

Even with taking into account the lower density of aluminum and CFRP, the data show that over the life cycle of the vehicle, AHSS and Gen3 steels have a distinct advantage. Even within the high-strength steel category, it can be calculated that parts made in traditional cold stamping operations, using Gen3 steels, will produce about 6 percent fewer GHG emissions than the equivalent parts made from the hot stamping process using PHS.

Automakers using Gen3 steels will see a reduction in their total carbon footprint as they begin to implement these materials where they had previously used PHS, aluminum or CFRP.

IN SUMMARY
There is an emergence of Gen3 steels in the automotive industry. Including the introduction of U.S. Steel’s 980 XG3 steel on the 2019 Jeep Gladiator. Industrywide, there is a shift in vehicle ownership models, and a new electric economy is being created. These factors are working with the emergent behavior of Gen3 steels and lithium-ion batteries to produce low-cost, high-range and safe battery electric vehicles—now and in the foreseeable future.

 

Authored by Michael Davenport

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