Bubble chart. Banana diagram. Steel strength ductility diagram—it’s been called a lot of things over the years. But the 2017 chart shown in Figure 1 is the subject of hundreds of requests for use we receive from engineers and students all over the world and appears in thousands of presentations and papers. Because of that, we periodically update it to make sure it reflects the most current picture of both commercially available, as well as emerging steel grades. In this blog, we are providing our updated GFD for download as well as definitions of steel classifications, as agreed to by our member companies.
Steel Classifications
There are different ways to classify automotive steels. One is a metallurgical designation providing some process information. Common designations include lower-strength steels (interstitial-free and mild steels); conventional high strength steels, such as bake hardenable and high-strength, low-alloy steels (HSLA); and Advanced High-Strength Steels (AHSS) such as dual phase and transformation-induced plasticity steels. Additional higher strength steels include press hardening steels and steels designed for unique applications that have improved edge stretch and stretch bending characteristics.
A second classification method important to part designers is strength of the steel. This document will use the general terms HSLA and AHSS to designate all higher strength steels. The principal difference between conventional HSLA steels and AHSS is their microstructure. Conventional HSLA steels are single-phase ferritic steels with a potential for some pearlite in C-Mn steels.
AHSS are primarily steels with a multiphase microstructure containing one or more phases other than ferrite, pearlite, or cementite – for example martensite, bainite, austenite, and/or retained austenite in quantities sufficient to produce unique mechanical properties. Some types of AHSS have a higher strain hardening capacity resulting in a strength-ductility balance superior to conventional steels. Other types have ultra-high yield and tensile strengths and show a bake hardening behavior.
What are 3rd Gen Steels?
Third Generation, or 3rd Gen, AHSS builds on the previously developed 1st Gen AHSS (DP, TRIP, CP, MS, and PHS) and 2nd Gen AHSS (TWIP), with global commercialization starting around 2020. Third Gen AHSS are multi-phase 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.
Graphical Presentation
Generally, elongation (a measure of ductility) decreases as strength increases. Plotting elongation on the vertical axis and strength on the horizontal axis leads to a graph starting in the upper left (high elongation, lower strength) and progressing to the lower right (lower elongation, higher strength). This shape, as can be seen in Figure 1, led to the colloquial description of calling this the banana diagram.
With the continued development of advanced steel options, it is no longer appropriate to describe the plethora of options as being in the shape of a banana. Instead, with new grades filling the upper right portion (see Figure 2), perhaps it is more accurate to describe this as the football diagram as the options now start to fall into the shape of an American or Rugby Football. Officially, it is known as the steel Global Formability Diagram.
Figure 2: 2021 The Global Formability Diagram comparing strength and elongation of current and emerging steel grades.
Even this approach has its limitations. Elongation is only one measure of ductility. Other ductility parameters are increasingly important with AHSS grades, such as hole expansion and bendability. There are several other approaches that have been proposed by experts around the world. Have a look at our Defining Steels article, from which this article was drawn, to learn more about them. You will also find within Defining Steels a detailed explanation of the nomenclature used throughout the Guidelines to define steels. If you have questions, please use the Comments tool below or on the Defining Steels page.
Download the GFD
Because of its popularity, we provide high resolution image files of the GFD here for your download and use. Please source it “Courtesy of WorldAutoSteel” in your papers and presentations. We are happy for you to use it. If you require our signed permission, please write us at steel@worldautosteel.org. We’ll respond quickly.
* The Guidelines use the general terms HSLA and AHSS to designate all higher strength steels.
Merhabalar, Tez çalışmamda grafikleri kaynak göstermek istiyorum. URL adresini kullanarak kaynak göstermem yeterli mi? Yoksa yine izin almalı mıyım?
Hello, we do not speak Turkish so we are relying on Google translate to respond. Apologies in advance if there is anything wrong in what I say. If the thesis is yours and you have permission to link to it, then by all means you are welcome to post it if it is relevant to the topic.
Merhaba, Türkçe konuşmuyoruz, bu yüzden cevap vermek için Google çeviriye güveniyoruz. Söylediklerimde yanlış bir şey varsa şimdiden özür dilerim. Tez size aitse ve ona bağlanma izniniz varsa, o zaman elbette konuyla ilgiliyse yayınlayabilirsiniz.
Should 2nd Gen PHS be separately shown from first gen? Especially those with retained austenite? Just tossing it out for discussion…
Dear Mike,
Thanks for that point. At this time, we do not have enough data/suppliers to separate out 2nd Gen PHS. Over time as the number of suppliers increase, separate bubbles may be justified.
The ability to add new information in a timely manner is one to the aspects about the online format which we really like.
– Danny Schaeffler
Engineering Quality Solutions, Inc., http://www.EQSgroup.com
Glad you think so! I have been working in the steel industry for a long time and the GFD never ceases to amaze me. It speaks of the amazing flexibility of the iron atom.
That is very intersting!