The push for lower greenhouse gas emissions has resulted in strict standards for vehicular emissions and the need for significant gains in fuel efficiency. These stringent norms have highlighted the importance of vehicle lightweighting, with new lightweight materials like aluminium, carbon fibre composites and plastics competing with conventional materials like steel for a piece of the automotive pie.
weight reduction we can get from steel
is adequate to close the powertrain
gap” – Dr Blake Zuidema
Some vehicle manufacturers are trying out vehicles that feature a significant quantity of aluminium, while others are exploring the possibility of using carbon fibre reinforced plastics in their vehicles. With these materials eating into a vehicle’s share of steel, manufacturers of steel have upped the ante and have come out with new grades of steel that are both strong and light.
The race is on to find out the best and most efficient way to reduce vehicle weight, without compromising on the economics. Steel, through these newer grades of advanced high-strength steels (AHSS), has made a strong comeback, being able to offer weight reduction without the need for major investments in infrastructure.
ArcelorMittal, the world’s largest steel manufacturer, has been at the forefront of technological developments with regard to new types of steel. According to Dr Blake Zuidema, Director Automotive Product Applications at ArcelorMittal, there has been an exponential growth in the number of grades of steel that are being used to achieve weight reduction in cars. From a handful of grades in the 1960s, there are now more than 200, and that number keeps growing. Although the challenges have increased, developments in steel technology have kept pace, he says.
ArcelorMittal, for instance, has put considerable emphasis on understanding the whole body structure design process. The company has CAD studios and FDA models and essentially all of the same design and analysis tools that a car manufacturer has to design body structures. In fact, ArcelorMittal is itself engaged in the design of body structure systems.
“That has given us a very clear understanding of how the properties of our steel interact both with the manufacturing process as well as the final vehicle performance. And that allows us now to understand which specific mechanical properties or steel attributes are needed for maximum weight reduction and performance in every different part of the automobile,” Zuidema explains to Megatrends.
One of the reasons for the emergence of so many new steel grades is that virtually every part in a car has a different optimum combination of properties. ArcelorMittal says that understanding the design has enabled it to target these properties and is now driving its product development.
The steel supplier also focuses on the relationships between powertrain improvements, weight reduction and fuel economy, in order to understand the levels of weight reduction required to meet future fuel economy standards. Steel, it says, possesses sufficient weight reduction potential to close the powertrain gap and help OEMs meet future fuel economy targets.
“We’ve put considerable effort into understanding the role of weight reduction and how much weight reduction is required. And our research, even today, continues to show that the weight reduction we can get from steel is adequate to close the powertrain gap. It can get vehicles to their future fuel economy standards, and it not only gets us there but it gets us there at a lower cost, and with a lower carbon footprint than if you tried to do it with some of the other competing materials,” says Zuidema.
Despite the many well-publicised advantages of steel, certain OEMs are looking to increasing use of aluminium, even if it means major investments to retool facilities. One question that crops up is whether, in moving towards more sophisticated high-strength steels, the steel industry has pushed up the cost of steel closer to aluminium.
Not so, says Zuidema. Even though higher strength steels are more sophisticated and therefore more expensive per tonne, in most cases the weight reduction means that less material is required, thus offsetting the higher cost per tonne. In the end, the advanced steel solutions are only marginally more costly than some of the base line solutions.
“Almost all of these grades work very well with the existing manufacturing base, meaning that automakers don’t have to invest millions if not billions of dollars in changing their manufacturing plants to be able to use these new materials. And for the consumer, steel, physically, is easier to repair and, therefore, the consumer is going to see lower repair and insurance costs with steel. So, steel has a clear cost advantage over these other materials,” he says.
An increasing interest in electric vehicles means an increasing need for lighter materials. However, rather than developing steel grades specifically for battery electric vehicles, the focus at ArcelorMittal is on understanding the design challenges and using the right grades of steel in the right applications. For instance, some parts of the car need to be very high strength; other parts need to absorb energy in the event of a crash. Other parts, such as exterior sheet metal, merely need to look good and not dent.
Battery electric vehicles present certain unique challenges, says Zuidema. “There is a huge need to protect the battery during a crash event, just like we have to protect the fuel tank in a conventional vehicle. Given the space a battery takes up, it also can be used as a structural element in the car. So we spend a lot of time looking at battery packaging, protecting it and using the protection cage as a structural part.
“Battery electric vehicles tend to weigh more than their gasoline engine counterparts,” he adds, “largely because of the weight of the batteries, and so things like managing the front and rear crash loads are more difficult. Managing the roof crush in rollover is quite a bit more difficult.”
On the matter of emissions regulations, many steel manufacturers and organisations have suggested the implementation of norms based on total life cycle analysis (LCA) rather than just tailpipe emissions. They believe that there is a danger of reducing tailpipe emissions in the use phase, but creating a bigger carbon footprint overall. One approach could be to set a standard that can be achieved with a low overall emissions technology. Another way would be to allow vehicle manufacturers to take credit for a vehicle which is low in its use phase emissions but is also very low from an overall life cycle.
There are numerous possible approaches, says Zuidema. “For example, we would not want to set the standard so high that it could only be met with a carbon fibre car that would end up creating a much greater carbon footprint. All you end up doing is putting more carbon into the environment. As long as the standards are set such that the low carbon emissions materials can be used, we will ensure we have a lowest overall life cycle emission…What we don’t want to do is create an unbearable task for both the material provider and the car maker. It has to be fair, it has to be manageable, but in the end it has to protect the environment,” says Zuidema.
With so much emphasis on advanced high-strength steels, will there be a place for mild and conventional steels in the vehicle of the future? There will, says Zuidema, for the simple reason that there are a number of parts in the vehicle structure that are already at their stiffness limit. Unless a higher modulus steel is developed, the automotive manufacturing industry will continue to use lower strength steel grades in stiffness-critical applications.
Looking to the future, Zuidema says he believes that design insights will drive the product development cycle, something which could result in a whole new series of steel grades. Also likely are higher strength outer panels, which will allow for a thinner gauge. The challenge here is to reduce the gauge of the steel without creating NVH problems. Zuidema reveals that the company is also working on composite solutions, by way of composite patches, to address noise vibration and harshness concerns.
David Isaiah
