What is steel made of? The short answer is: iron and carbon . The complete one opens up a whole world. Steel is a metal alloy composed of over 95% iron, but it's the remaining elements that determine its characteristics. Understanding the composition of steel means understanding why an S355 behaves differently from a 42CrMo4, why an AISI 304 resists corrosion while a C40 doesn't, and how to interpret a designation in order to choose the grade most suited to your component.