Innovation in the steel industry is continuous. In fact, steel’s role in automotive is constantly evolving as automakers face increased demands for performance, lightweighting, value and sustainability. To meet these challenges, the steel industry has been working with its automotive customers to develop and apply the next generation of advanced high-strength steels (AHSS) that provide high-value solutions to meet the fuel economy and performance requirements of future vehicles.
With more than 200 steel grades available, automakers are able to use the right grade for the right application to maximize performance. Today’s steel grades are as much as six times stronger than the steels of a decade ago, and three to four times stronger than the latest aluminum alloys on the market. The added strength of AHSS allows automakers to deliver vital performance and safety benefits with lightweight products using their existing manufacturing infrastructure, eliminating major manufacturing cost penalties associated with the introduction of alternative materials. Additionally, consumer cost of ownership (purchase price, insurance and repair) is lower for steel-intensive vehicles than alternative material-intensive vehicles.
Thus, automakers see the value in steel’s versatility, using 10 percent more AHSS in automotive applications each year than forecasted by Ducker Worldwide. As AHSS grades continue to evolve, even more chassis and suspension applications are being realized in addition to the many body structure and closure applications already designed with AHSS.
In the first article in this series, Steel 101: Grades and Repairability, we discussed the different steel grades shown in the diagram. These grades were categorized by conventional steels, high-strength steels and AHSS. A significant portion of steel innovation is in AHSS such that this is divided into its own distinct classifications or generations:
- First-generation AHSS include dual phase (DP), ferritic-bainitic (FB), complex phase (CP), martensitic (MS), transformation-induced plasticity (TRIP) and hot-formed (HF). They offer significantly higher strengths as compared to conventional steels and some have improved formability as well.
- Second-generation AHSS have mainly austenitic microstructures and include austenitic stainless steel and twinning-induced plasticity (TWIP). They are extremely strong and formable and can be used to provide extraordinary mass reduction for difficult-to-form parts.
- Third-generation AHSS (3rd Gen AHSS) are currently being introduced commercially. These grades will mainly be multi-phased (MP) steels with high strength and increased formability compared to first-generation AHSS.
Third Generation AHSS
Third Gen AHSS are being developed to provide a high-value steel solution to bridge the properties gap between the already developed first-generation AHSS and second-generation AHSS. This new generation of steel shares the high-strength properties of AHSS, while also having a higher total elongation (measure of formability) similar to high-strength steels, enabling automakers the continued use of their current stamping and assembly infrastructures.
There are many opportunities for 3rd Gen AHSS applications, including: lightweighting through direct material substitution and thickness reduction, improved energy absorption through enhanced strength/elongation, and optimized geometries and part consolidation enabled by enhanced formability. Potential applications identified for maximum performance and weight reduction benefit include a- and b-pillars, roof rails, roof bows and underbody reinforcements to name a few.
Nissan has been introducing new grades of 3rd Gen AHSS into their vehicles since 2015. For example, the 2015 Nissan Murano was able to cut 146 pounds from its redesigned model through AHSS applications, specifically replacing its high-strength 590 MegaPascals (MPa) and 780 MPa ultra high-strength steel with 1180 MPa. This was a six percent body-in-white mass reduction from the previous Murano.
|The 2015 Nissan Murano was able to cut 146 pounds from its redesigned model through AHSS applications.|
Another example is General Motors incorporation of 3rd Gen AHSS into its 2016 Chevy Sail by using a Quenched and Partitioned grade, with 980 MPa and 15 percent total elongation. According to Great Designs in Steel 2017, General Motors also has multiple additional 3rd Gen AHSS applications under consideration.