Daniel Schaeffler Daniel Schaeffler
President

Advanced High Strength Steel Application Guidelines: Online, Expanded and Updated

October 1, 2021
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WorldAutoSteel, a consortium of leading global sheet-steel producers, has a long history of promoting products and techniques that allow for their optimized use in automotive-body structures. Nearly 30 years ago, the consortium acknowledged that its automotive-industry customers were facing the conflicting challenges of reduced weight for improved emissions and fuel-economy performance while meeting ever-increasing safety regulations, all while producing a cost-effective body structure.  New grades of steel helped attack these challenges, but these grades had different forming, joining and processing characteristics than the mild steels commonly processed at most companies.

This realization led to the launch of the Ultralight Steel Auto Body (ULSAB) program to demonstrate a lightweight steel auto-body structure that met a wide range of safety and performance criteria. Following ULSAB were projects that highlighted closures and suspensions, as well as other vehicle parts and systems. 

Complementing these vehicle developments was the 2003 initial publication of the Advanced High Strength Steel (AHSS) Application Guidelines, which showcased global best practices on how to form and join advanced steel grades.  By the 2014 release of Version 5, the AHSS Application Guidelines contained a portfolio of 50 different commercially available automotive steels. The 2017 release of Version 6 highlighted 38 unique AHSS grades alone. Dr. Stuart Keeler, the esteemed industry pioneer and former Science of Forming columnist for MetalForming magazine, served as technical editor for metallurgy/forming through the 6th release; I took this role over for the 7th version, released earlier this year.

Global Formability Diagram Oct 2021The latest version is available for free public use as a mobile-friendly online database (AHSSinsights.org) where users can browse and search to quickly find specific information of interest. The new online format enables timely updates to keep pace with the development of new technology and grades. Currently, the website includes some 150 articles with 1000 citations, many of which have links back to the original sources. Content divides into three primary sections: metallurgy, forming and joining. In addition, the website houses the AHSS Insights technical blog. 

The AHSS Guidelines presents an overview of 15 types of sheet-steel grades, ranging from ultra-low carbon steels to AHSS and the latest 3rd-Generation steels. Cold and hot stamping grades approaching 2000-MPa tensile strength are in routine production.

AHSS grades are martensitic and multiphase steels with microstructural phases other than ferrite, pearlite or cementite having a minimum specified tensile strength of at least 440 MPa. 3rd-Gen AHSS are multiphase steels engineered to develop enhanced formability as measured in tensile, sheared edge and/or bending tests. Typically, these steels rely on retained austenite in a bainite or martensite matrix, and potentially some amount of ferrite and/or precipitates, all in specific proportions and distributions to develop these enhanced properties. 

Steels with very high minimum specified tensile strength sometimes are referred to as ultra-high-strength steels (UHSS). Several companies choose 980 MPa as the threshold where “ultra” high strength begins, while others use higher thresholds of 1180 MPa or 1270 MPa. There is no generally accepted definition among the producers or users of these steels. However, the difference between AHSS and UHSS is in terminology only—they are not separate products. The actions taken by the manufacturing community to form, join or process ultimately depend on the steel grade, mechanical properties and sheet thickness. Calling these steels “advanced” or “ultra” does not impact the technical response. Aside from an explanation of this rationale, the term “ultra-high-strength steels” does not appear in the guidelines unless information is extracted from papers which used this term. 

The often-replicated global formability diagram (see the accompanying figure) compares the relative strength and total-elongation performance of many families of steel grades. The latest version now includes press-quenched steels and incorporates the latest production information to generate the size and placement of the bubbles.

In addition to this updated diagram, nearly every section of the forming content in the latest version of the AHSS Application Guidelines has been expanded, with many new topics added. True fracture strain and alternative related measures for local formability characterization are discussed in detail; guest authors contributed articles on Industry 4.0 and additive manufacturing; the discussion on simulation describes the importance of selecting appropriate yield criteria and hardening laws; and an extensive new section covers springback and springback-reduction strategies.

Dr. Eren Billur, Cutting Edge columnist for MetalForming magazine, contributed multiple articles related to press-hardening steels (PHS). These articles cover all aspects of PHS, including their history, the current and projected market, the numerous production processes and required forming equipment, the resultant properties, corrosion-protection alternatives, and a detailed exploration of the vehicles that use these grades. Several figures accompany each article, and most articles include more than 50 citations linked back to the original source.

(Editor’s note: Billur and Schaeffler will reference this material during their presentation at the 2021 Hot Stamping Experience and Tech Tour, produced by PMA and MetalForming magazine and scheduled for November 2-3 in Ann Arbor, MI).

The website also contains information on joining of AHSS to other steels and to other alloys. Highlighted are articles related to resistance welding, broken down by the process, performance, testing and modeling. Author and editor of this section, Menachem Kimchi from The Ohio State University, had a similar role in Version 6.  
Integrated into the website is the AHSS Insights blog, with new posts monthly. Topical content allows a global readership to comment and engage the community with questions. Within the blog are several articles written by guest authors on topics about which they have particular expertise.

The content throughout the AHSS Guidelines is written with the user and practitioner in mind and does not require an advanced degree to understand. Housed at AHSSinsights.org are best practices and lessons learned on how best to form and join new steel grades. MF

Industry-Related Terms: Alloys, Austenite, Bending, Edge, Form, Forming, Martensite, Tensile Strength, Thickness
View Glossary of Metalforming Terms

 

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

Technologies: Materials

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