Exception: Stainless steels require Cr ≥ 10.5% regardless of other elements.
According to EN 10020, steel is defined as a material in which iron is the predominant element, and where the carbon content is generally less than 2% by weight. It may contain other elements, but the carbon threshold is a primary differentiator from cast iron (which typically contains more than 2% carbon). en 10020 pdf