Steel is an alloy of iron and carbon. ALL steel is carbon steel. BUT due to sloppy use of the language, High carbon steel is sometimes called just “carbon steel.” Mild steel usually has carbon below 0.3% and High carbon steel above 0.6% Medium carbon steel falls between. Most people Only think of high and medium carbon steel as “Carbon Steel,” but ALL steel is carbon steel. The term “Plain Carbon Steel” is often used the diffirenetiate the simple steel in which carbon is the principal alloying element from “low alloy steels” in which other elements are very important.