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.
Steel Grades
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.
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.
Other automakers are also evaluating this new classification of AHSS and how to best utilize the weight reduction potential and increase performance in their next vehicle designs. This is no surprise and we estimate 3rd Gen AHSS will capture roughly 10 percent of the automotive market as quickly as 2020.
Integrated Computational Materials Engineering Project
The steel industry in partnership with the United States Department of Energy (DOE) and automotive customers completed the Integrated Computational Materials Engineering (ICME) project in March 2017. This four year, $8.5 million project was conducted with the support of five universities, three steel companies, two engineering firms, three automotive OEMs, and one national laboratory. The project developed two 3rd Gen AHSS grades for model validation and design optimization to meet aggressive DOE weight reduction targets. The new grades were applied to a body side system which resulted in 30 percent mass saved at an estimated cost of $0.32 per pound of mass saved (calculated by a technical cost model). In addition, the number of parts was reduced from 46 to 28 over the baseline.
As demonstrated by this project, these 3rd Gen AHSS both provide higher strength and enhanced formability, offering the automotive designer an additional suite of grades to help reduce mass and continue to keep steel the material of choice in future vehicles. This project reinforces why the steel and automotive industries must continue to work together, as further development and availability of 3rd Gen AHSS will provide an excellent economic path forward in meeting 2025 Corporate Average Fuel Economy standards and greenhouse gas regulations.
Third Gen AHSS Repairability
As with first- and second-generation AHSS, 3rd Gen AHSS applications will require different repair techniques. However, the automotive and steel industries are fully committed to working with the repair industry to inform these professionals on where in the vehicle AHSS are being applied and the best ways to work with those steels based on their properties. Partnering with repair professionals will continue as 3rd Gen AHSS becomes more prominent in vehicle design.
Additionally, the Auto/Steel Partnership (A/SP), a consortium of auto- and steel-makers, addresses the needs of the repair community in their AHSS Repairability study. This study aims to provide standardized safe and resilient procedures for AHSS use in vehicles. Currently, Phase I and Phase II of the study are complete. Phase I examined a 600 MPa tensile strength (TS) dual phase (DP) steel and a 1300 MPa TS martensitic steel after exposure to typical repair arc welding and flame straightening temperature cycles. Phase II studied 780 MPa TS dual phase and 600 and 780 MPa TS transformation induced plasticity (TRIP) steels after typical repair exposure. Both phases resulted in OEMs publishing guidelines in the form of automotive OEM repair matrices. The A/SP will soon start Phase III of the study to develop automotive repair guidelines for AHSS grades having tensile strengths between 780 MPa and 1800 MPa.
Conclusion
As the automotive industry works to meet increasingly stringent fuel economy and safety regulations, the steel industry continues to innovate and introduce new grades of 3rd Gen AHSS. This next generation of steel opens automakers’ doors to the ability to continue to lightweight their vehicles with steel thus avoiding expensive alternative materials. It will provide higher strength and formability choices, increasing the value of steel as a weight reduction solution. It’s safe to say steel will remain the material of choice in the automobile for years to come, especially as 3rd Gen AHSS is implemented into more and more vehicles.
