Page 40 - MetalForming-Sep-2018-issue

The Science of Forming By Daniel J. Schaeffler, Ph.D.
Steel Terminology Part I: Better Forming Grades
You’ve probably heard steel described using initials such as EDDS, IF, DS, DP, AHSS and the like. Some—but not all—of these terms mean the same thing.
Steel is composed mostly of iron and an assortment of other elements, either intentionally added to change certain properties such as strength or corrosion resistance (alloying elements) or too difficult to remove from steel- making (residual elements).
When alloying for additional strength, carbon (C) and manganese (Mn) provide the most bang for the buck. A typical low-carbon drawing steel (DS) may contain percentages of about 0.05 C and 0.25 Mn. Even with other alloying and residual elements totaling about 0.25 percent, drawing steel is more than 99-percent iron. (Fun fact: Due to carbon percentages, steel has more iron than cast iron.)
Reducing carbon and manganese lowers strength, increases formability and makes steel easier to weld. How- ever, 0.04 percent denotes the practical lower limit of carbon achievable when
Danny Schaeffler, with
30 years of materials and applications experi- ence, is co-founder of 4M Partners, LLC and founder and president of Engineering Quality Solu- tions (EQS). EQS provides product-applications assistance to materials and manufacturing com-
panies; 4M teaches fundamentals and practical details of material properties, forming technolo- gies, processes and troubleshooting needed to form high-quality components. Schaeffler, who also spent 10 years at LTV Steel Co., received his Bach- elor of Science degree in Materials Science and Engineering from the Johns Hopkins University in Baltimore, MD, and Master of Science and Doctor of Philosophy degrees in Materials Engineering from Drexel University in Philadelphia, PA.
Danny Schaeffler
248/66-STEEL • www.EQSgroup.com
E-mail ds@eqsgroup.com or Danny@learning4m.com
using conventional steelmaking tech- niques. With some special mill prac- tices, steelmakers can lower the carbon to 0.02 percent. This allows for the pro- duction of a more formable grade known as deep-drawing steel, or DDS.
A molten-steel heat must undergo an additional step of vacuum degassing to achieve even lower levels of carbon, hydrogen and oxygen. These dissolved gases bubble out of the melt at the lower pressures associated with a vacuum in the same way as popping a beer-can lid reduces pressure and forces the beer to bubble over. In a steelmaking plant, a vacuum- degassing unit may cost $50 million to install, which is why not every com- pany offers ultra-low-carbon (ULC) steels.
Any pure element is very soft. For this reason, wedding bands never are made from pure 24-carat gold. Other added elements give the ring strength and hardness, reducing the designation to 18-carat gold (75-percent gold and 25-percent alloying elements) or less.
Pure iron is similarly soft. Elements added to iron to change the properties include not only carbon and man- ganese, but also silicon, phosphorus, titanium and others. ULC steels typi- cally contain carbon levels between 10 and 50 ppm (0.001 to 0.005 percent), and half the manganese of drawing steels, making ULC steels very similar to pure iron. This is why ULC steels sometimes are described as extra-deep- drawing steel (EDDS).
At a microscopic level, pure iron (or any other element) looks like a 3D network of racked billiard balls, all of the same size. To make a steel alloy, some of the iron billiard balls are replaced with ones made of man- ganese, silicon, phosphorus, titanium, etc., similar but not identical in size to the iron balls (see the Atomic Struc- tures graphic). The disruption in the pure-iron atomic network caused by these alloying additions causes what is known as solid-solution hardening. As the alloying increases, the straining in the atomic 3D structure increases,
Extra-Deep-Drawing Steel
Drawing Steel
Atomic Structures
Atom of Iron Alloying element larger than iron
Alloying element smaller than iron
Interatomic bond
Interstitial alloying element such as carbon
38 MetalForming/September 2018
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