The Science of Forming


 

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Simple Graphics Can Send the Wrong Message

By: Stuart Keeler

Thursday, January 1, 2015
 

Fig. 1—Historical presentation of steel types as boxes designating levels of yield or tensile strength.
Fig. 1—Historical presentation of steel types as boxes designating levels of yield or tensile strength.
The world is full of pictures, graphs, charts, videos and other media types that improve communication when compared to the mere written word. The scientific world uses a similar but more complex approach. Here, print communication augmented by informative graphics is more powerful and easier to understand than a page full of printed technical information. However, too often authors will confuse things by interweaving groups of concepts into a single graph to show interactions—for example, “if A goes to B then C will change the value of D.”

A common graph found in the sheetmetal world illustrates how a steel’s mechanical properties change in relation to its strength (Fig. 1). This particular bar or box graph relates percent total elongation to yield strength. It shows three levels of yield strength; some graphs will provide medium strength as a fourth level.

Fig. 2—Problems above include two strength designations for the same steel coil, constant total elongation over a wide range of yield (or tensile) strengths, drastic total-elongation changes between boxes, and uncertainty for designating new, very high-strength steels.
Fig. 2—Problems above include two strength designations for the same steel coil, constant total elongation over a wide range of yield (or tensile) strengths, drastic total-elongation changes between boxes, and uncertainty for designating new, very high-strength steels.
At first glance, Fig. 1 provides the names and yield-strength ranges for each category of steel. The high-strength range originally was 35 to 80 ksi, matching the strength range for commercially available high-strength low-alloy (HSLA) steels. When steel mills introduced 30-ksi bake-hardenable steel, they wanted it classified as a high-strength alloy. Therefore, the lower boundary of the high-strength steel category was reduced to 30 ksi, indicating that these boundaries are more sales-based than technical.

Studying Fig. 1 reveals two problems that send the wrong message. First, each of the three steel types has a straight upper plateau that signifies a fixed value of percent total elongation for all steels that lie within that range of yield strengths. This does not happen in the real world. Instead, the percent total elongation for HSLA steels decreases as yield strength increases from 35 to 80 ksi. Second, crossing the line from low strength to high strength does not generate a precipitous drop in total elongation.

Fig. 3—Material properties change as a continuum of yield (or tensile) strength and should be shown as a continuous gradient.
Fig. 3—Material properties change as a continuum of yield (or tensile) strength and should be shown as a continuous gradient.
The problems become more threatening when actual steel types are overlaid onto Fig. 1. Fig. 2 illustrates the properties of three steel types. A typical yield-strength range for aluminum-killed draw-quality (AKDQ) steel is 20 to 40 ksi. Fig. 2 shows that this steel could be classified as low or high strength, even in the same coil. HSLA steel is now available with yield strengths of 100 and 110 ksi.

Note: HSLA steel can be high strength or ultra-high strength—again, within the same coil. Similar problems occur when the graphic is based on tensile strength instead of yield strength.

Fig.4—Data indicating the range of properties for different types of steels takes the shape of a banana—the Banana Curve.
 Fig.4—Data indicating the range of properties for different types of steels takes the shape of a banana—the Banana Curve.

Since the1980s, entirely new types of steels—advanced high-strength steels (AHSS)—have become increasingly popular. While mostly targeted to the automotive industry, these steels will become common components in many other industries as technical knowledge and experience spreads. All the while, the industry is struggling to categorize these new steels per Fig. 2. For example, since martensitic steels (MS) can have yield strengths around 200 ksi, should we extend the range for ultra-high-strength steel from 80 ksi to 200 ksi, or do we need to create a new category/box? And, what would we call the new box—super-ultra high-strength steel?

Around 2000, WorldAutoSteel decided that a continuum approach to categorizing steels would correct the problems described in Fig. 1. This continuum would be constructed around the dashed line in Fig. 3, with the exact shape of the curve depending on the actual properties of the different steel types. Instead of bars, lines or dots, an ellipse was drawn for each type of steel that encompasses available combinations of yield (or tensile) strengths and a formability property such as total elongation. From 2004 to 2009, WorldAutoSteel published this diagram in its AHSS Application Guidelines. The overlapping curves of ellipses (Fig.4) took on the nickname “Banana Curve.”

Fig. 5—The original Banana Curve has been extended to include two generations of new AHSS grades, as well as recognizing research underway to develop Generation 3 AHSS.
 Fig. 5—The original Banana Curve has been extended to include two generations of new AHSS grades, as well as recognizing research underway to develop Generation 3 AHSS.

Version 5.0 of the AHSS Application Guidelines, released in May 2014, welcomed a radically updated steel-property diagram called the Global Formability Curve (Fig. 5). It displays three generations of steel:

• Gen.1 is the region containing the DP, TRIP, CP and MS steels current in use.

• Gen. 2, TWIP steels, located near the stainless-steel group are undergoing press trials.

• Gen. 3 steels encompass all of the grades still in the research phase.

Download the AHSS Application Guidelines Version 5.0 (free of charge) at www.worldautosteel.org. MF

 

Related Enterprise Zones: Materials/Coatings


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