Metallurgy of Ultra-Low Carbon Steels (DDS – EDDS)

ULC, IF, VD-IF, and EDDS are interchangeable terms that describe the most formable (high n-value) and lowest strength grade of steel.

Ultra-low-carbon (ULC) steels typically carbon levels less than 0.005%, or 50 parts per million.  At these low alloying levels, the atomic structure is primarily iron, with unfilled spaces or gaps (called interstices) between the atoms – the origin of the term “interstitial-free” or IF.  Molten steel needs an additional process prior to casting called vacuum degassing (VD) to reach these carbon levels.   Because this steel alloy is mainly iron and all pure elements are very formable, it is also referred to as either deep drawing steel (DDS) or extra-deep-drawing steel (EDDS).  Specifications which contain ULC grades are listed within the Mild Steels page.

Adding phosphorus to an IF grade increases the strength due to solid solution strengthening, precipitation of carbides and/or nitrides, and grain refinement. These higher strength IF-HS grades are widely used for both structural and closure applications.  Work hardening from forming will increase panel strength, and is sometimes called a dent resistant steel grade.  However, this alloying approach is not capable of producing a bake hardenable grade.

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