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Steel is, by minimal definition, an alloy of iron and up to 2 percent carbon (if it is more than 2 percent, the alloy is cast iron). Carbon is small enough to fit into the interstices (gaps) of a primarily iron matrix, making it an “interstitial element” in steel. If the steel alloy has an ultralow carbon level (typically less than 50 parts per million), most of these gaps will not be occupied and, as such, can be called interstitial-free (IF) steel. These very formable IF steels are extra-deep-drawing steels (EDDS) with a microstructure that is 100% ferrite (nearly pure iron). Achieving this low carbon level does not occur using conventional steel processing. Instead, the molten steel must be put under a vacuum that decarburizes it by removing carbon monoxide, as well as other gases like hydrogen and nitrogen. This process is called vacuum degassing, and it is done in the production of vacuum degassed interstitial-free steels (VD-IF).
