Mono steel is a term for blades that are made from one layer of steel.
Generally speaking, two types of steel are used to make knife blades – carbon steel and stainless steel. Carbon steel has a carbon content of 0.7-1.5% and few impurities, meaning inclusions of other substances that might (negatively) impact the properties or performance profile of the steel. Carbon steel is very hard but breaks easily and is not stainless. In order to make more shatterproof stainless steel, chromium, molybdenum, vanadium and other metals are added. The combination of steel with other substances is called an alloy. As long as the substances added to the steel are within certain ranges, the product can be called stainless steel. Within the European Union, these ranges and thresholds are defined by the European Committee for Standardization (CEN). Stainless steel is not as hard as carbon steel, but offers greater elasticity, strength and rust-resistance.
In order to combine the advantages of carbon steel with those of stainless steel, laminated blades were developed. They usually consist of two or three layers with an inner layer of hard carbon steel. It provides a sharp edge and good edge retention. The outer layer or layers consist of stainless steel that has a much lower carbon content. It is also significantly more shatterproof. Mono steel blades, on the other hand, do not consist of several layers but just one type of steel, which can be carbon or stainless.
Mono steel blades made from carbon steel are very common in Asia, for traditional Japanese cooking knives (hochos), for example. In Europe and the entire Western world, traditional mono steel blades were made from stainless steel.