What is carbon steel? Because I use carbon and tool steel, they do require a bit of maintenance Carbon steel is an alloy of iron and carbon, the carbon ensures that the steel is hardenable. The higher the carbon percentage, the harder you can harden the knife. It generally has a finer structure, which means it can be sharpened more finely, which also makes the knife sharper than a stainless steel knife (stainless steel = steel with more than 11% chromium)