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requiring more force to deform the sheetmetal. This results in what we call higher-strength steel. Alloys added at low levels result in an atomic net- work that is still mainly iron, and, therefore, relatively soft.
Even with all atoms touching, small gaps called interstices exist. Only much smaller atoms, such as carbon and nitrogen, can fit within these gaps, and thus are called interstitial elements. In ULC steels where the carbon level rests below 50 ppm, most of the interstices are not occupied, leading to the term interstitial-free (IF) steel. As noted, these ultra-low carbon levels result only from vacuum degassing, hence the term vacuum-degassed intersti- tial-free ( VD-IF) steel. Therefore, ULC, IF, VD-IF and EDDS all mean the same thing—the most formable and dent- prone grade of steel.
Unlike higher-strength steels, these
grades can be produced with the sur- face quality necessary for critical exposed parts. These grades may be temper-rolled if the surface must be further optimized—for paint appear- ance, for example. This results in a small increase in strength combined with a small loss in ductility. Some steelmakers may choose to make DDS grades by temper-rolling an EDDS product, due to production efficiencies gained when the steel is melted and rolled. MF
Learn about high-strength-steel grades and formability, tool steels and coatings, presses and die design, and effective lubricant strategies at PMA’s two-day Stamping Higher-Strength Steels Seminar in Nashville on Septem- ber 12-13. Visit www.pma.org for details and to register, or contact Marianne Sichi at msichi@pma.org for more information.
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MetalForming/September 2018 39
The Science of Forming