The new Audi A8, scheduled for release in 2018, features a multi-material body structure (earlier post) consisting of more than 40% steel. That’s a marked turnaround from the all-aluminum body-in-white which Audi developed for the A8 in 1994, in which steel was essentially used for the B-pillars alone, and accounted for about 8% of the body structure.
Since then, steel has evolved significantly. Steel’s strength has multiplied by almost 10 times over the past 20 years, from 270 to 2000 MPa tensile strength. More than 80 new steel products are under development at steelmaker ArcelorMittal, with an automotive steel grade portfolio of almost 200 unique steel grades, half of which were introduced in just the past decade.
|Top. Body structure of the outgoing A8. Bottom: Body structure of the new A8 arriving in 2018. Click to enlarge.|
Around 17% of the new A8 body structure will comprise press hardenable steel (PHS), some of which will be supplied by ArcelorMittal. These steels have yield strengths up to 1500 MPa after press hardening. The strength-to-weight ratio of these grades outperforms even the most advanced—and costlier—aluminum grades. With the advent of new joining technologies, automakers can easily incorporate advanced high-strength steels into their vehicles. As these technologies mature and are adopted by carmakers, the use of PHS in the multi-material vehicles of the future is set to grow rapidly, according to ArcelorMittal.
here will be no cars made of aluminum alone in the future. Press hardened steels (PHS) will play a special role in this development. PHS grades are at the core of a car’s occupant cell, which protects the driver and passengers in case of a collision. If you compare the stiffness-weight ratio, PHS is currently ahead of aluminum.—Dr. Bernd Mlekusch, head of Audi’s Leichtbauzentrum (Lightweight Construction Center)
ArcelorMittal’s Usibor is compatible with standard press hardening technologies and processes. Combining Usibor with Ductibor into laser-welded blanks offers several significant advantages including weight savings, improved crash behavior and cost savings through material and manufacturing optimization, the steelmaker says
Usibor is our key product in hot stamping and has been a major commercial and technical success in the global automotive industry. Looking ahead, the scope of hot stamping products in vehicles will continue to increase with the release of more advanced products like Usibor 2000, which offers 10 to 15 percent weight savings when compared to existing hot stamping solutions.—Brian Aranha, executive vice president, global automotive, ArcelorMittal
Audi’s switch back to steel is part of a growing trend which is even surpassing the expectations of steelmakers, according to data released by the Steel Market Development Institute (SMDI).
Between 2006 and 2015, the use of advanced high-strength steels in vehicles has grown from an average of 81 pounds per vehicle (36.7 kg) to 275 pounds (124 kg)—a threefold increase in 10 years. Over the period 2012 to 2015, the use of AHSS has increased by around 10% each year—well above steel industry forecasts.
LCA. ArcelorMittal and the steel industry as a whole have also been working collaboratively to educate automakers and stakeholders on the importance of life cycle analysis (LCA).
Right now, regulations only consider tailpipe emissions generated during the drive phase. However, each material used in vehicle production contributes to lightweighting and improves fuel economy, but each does so at a different cost to the manufacturer—and to the environment. If we want to know how “green” a vehicle really is, we have to measure emissions over its entire life cycle.—Brad Davey, chief marketing officer, NAFTA and global automotive for ArcelorMittal
There have been numerous scientific LCA studies done in the last decade, including a 2016 Production Phase Emissions study. This study found aluminum produced in North America emits four to five times more GHGs than steel. Additionally, aluminum requires seven times more energy to produce than steel.
Dr. Roland Geyer developed the University of California Santa Barbara Automotive Materials GHG Comparison Model (UCSB Model). This 2007 study calculated GHG emissions and energy over the entire life cycle of a vehicle. Peer-reviewed and publicly available, the study found the majority of aluminum-intensive vehicles result in higher overall lifetime GHG emissions and significantly higher production phase emissions in every vehicle class tested.
In 2016, the study was broadened to include LCA model parameters that ArcelorMittal said favored aluminum-intensive vehicles. It also included a Monte Carlo assessment which ran the LCA analysis 5,000 times with different parameters, using assumptions from both the steel and aluminum industries.
Those findings showed steel-intensive vehicles had lower total emissions versus aluminum in approximately 70% of the potential scenarios.