Daniel Schaeffler Daniel Schaeffler
President

Coming Soon: AHSS Application Guidelines, Online and Updated

March 26, 2020


Advanced high-strength steels (AHSS) find use in nearly every vehicle on the market. Near-term projections indicate that these grades will soon comprise 60 percent of the typical automotive body structure. While these new steels offer many beneficial characteristics, they behave differently from other grades during forming and joining, and, therefore, require metal formers and other manufacturers to master the fundamentals and keep pace with process improvements being realized on the shop floor.

Fig1 total elongation tensile strengthTo guide users of these new and emerging products and help them navigate through these challenges, 22 steelmakers from around the world have been working together. Under the umbrella of WorldAutoSteel, they developed, nearly two decades ago, the Advanced High Strength Steels Application Guidelines, as early AHSS grades began transitioning from research to commercialization. In 2017, WorldAutoSteel released Version 6 of this document, which explains the unique characteristics of each grade, and presents global best practices for forming and joining AHSS.  

The next update of the guidelines, expected to be completed in 2021, will transform them into a mobile-friendly online database, which users can browse and search in order to quickly find specific information of interest. WorldAutoSteel will release the content to the public in several phases during the next 18 months. Even then, the knowledge database will never be truly complete. Having content posted online allows for uploading new and pertinent information as it becomes available, making the AHSS Application Guidelines a useful reference well into the future.

Updated Materials Portfolio

The new guidelines will contain a materials portfolio, listing available grades as well as those projected to be commercially available by 2021. The number of AHSS grades has more than doubled from the 20 listed in Version 4.1, released in 2009, expanding to higher levels of strength and ductility—as illustrated by the accompanying graph. The vertical axis of the graph represents total elongation—a measure of formability—with tensile strength represented on the horizontal axis.  

The lower arc of the graph compares first-generation AHSS to conventional steels. Second-generation Twinning Induced Plasticity (TWIP) steels generally are not available in North America. And, third-generation AHSS grades, the most cutting-edge grades in use today, are with many more on the cusp of becoming commercially available and promise to be more formable at a given strength level than first-generation grades. They’re also less costly and more weldable than TWIP steels. The boundaries of each ellipse indicate the approximate range of properties for each AHSS grade.

In addition to strength and elongation, advanced steels can be tuned to optimize characteristics important for a specific stamped part. At a given strength level, separate grades may be available, offering either enhanced stretched edge flangeability, improved bendability or increased tensile elongation.  

Worth noting: When HSLA sheet steels were commercialized more than four decades ago, they were considered automotive-only grades; many shops not focused on the auto industry ignored them. These grades now represent the most common types of high-strength sheet metal used in all industries.

Web-Enabled AHSS Guidelines

Metallurgy, forming and joining information continue to represent the main themes of this growing knowledge database. Users will be able to search by these topics, or directly for grade-specific information.
The guidelines explain yield strength, tensile strength and total elongation, among other properties available from conducting tensile tests, such as uniform elongation, n-value and r-value. The document also explains properties not acquired from a tensile test, including bending and hole expansion. It then relates these properties to each grade’s forming behavior. Conventional forming-limit-curve (FLC) calculations can be used for conventional and dual-phase steels to estimate the maximum amount of sheet metal motion available for different combinations of major and minor surface strains. However, most other grades do not follow these classical equations, discovered by Dr. Stuart Keeler (the founding author of this Science of Forming column); detailed experiments are needed to create FLCs for these grades.
Springback is one of the most significant challenges stampers face when working with higher-strength steels. The AHSS  Guidelines describe the causes of springback, and present different ways that users can correct for the associated dimensional problems.

Applications using press hardenable steels continue to expand. After hot forming and quenching, strengths can reach, or exceed, 2000 MPa. Complex shapes can be achieved owing to forming at high temperatures, and in-die quenching will minimize springback. The guidelines document offers process-design details describing the challenges associated with these products.

The proper use of simulation tools impacts AHSS stamping success, requiring inputs appropriate for each grade. The AHSS Application Guidelines provides generic stress-strain curves useful for early feasibility analysis before selecting a production supplier. Other key inputs are described, with useful data shown.

Lastly, the AHSS Application Guidelines addresses techniques for joining AHSS grades, which must be modified from those used for mild and conventional high-strength steels. Among the topics covered: challenges and solutions associated with resistance spot welding of variable thickness stack-ups; arc, laser, and hybrid welding; weld bonding; mechanical joining; paint baking effects related to joining; and liquid-metal and hydrogen embrittlement, and corrective actions for each.

Learn and Follow Along

Experience with advanced steel grades has dramatically increased since the AHSS Guidelines was last published.  This leads to one of the major additions coming with the next release: an increased number of case studies, which aim to take users through the thought process of creating successful, robust stampings made from AHSS grades.

While we’ve always referred to the lower arc of the figure as the “banana curve,” a better description, considering the options now available, might be the “football graph.” AHSS applications will continue to proliferate at automotive OEM and supplier plants; follow developments by signing up to receive updates at www.ahssinsights.org. MF

Danny Schaeffler is the technical editor for metallurgy and forming for the upcoming release of the AHSS Guidelines. Contact him to contribute a case study to the Advanced High Strength Steels Application Guidelines.  We’re looking for lessons-learned, where things may not have gone smoothly at first. We will be giving full attribution where requested, including links to your company.

Industry-Related Terms: Stack-Ups, Thickness, Surface, Tensile Strength, Bending, Case, Edge, Forming, Hydrogen Embrittlement, LASER, Quenching
View Glossary of Metalforming Terms

 

See also: Engineering Quality Solutions, Inc., 4M Partners, LLC

Technologies: Materials

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