In metals and metallic materials, purity describes the number of inclusions of substances that might impact the performance properties of the material. Inclusions are created in the course of the material\'s production process In steel production, inclusions happen during the smelting process in the furnace, through small amounts of incombustible materials getting into the steel melt (exogenous inclusions) or by means of the so-called deoxidation (endogenous inclusions). For the latter process, a deoxidation agent (such as aluminum waste) is added to the steel melt. It reacts with the oxygen contained in the steel melt, thus removing the oxygen and preventing air bubbles from forming in the steel. Large amounts of endogenous or exogenous inclusions are undesirable, because they have a negative impact on the performance properties of the steel.
Inclusions (such as aluminum or silicon) of stainless steel are removed from the steel melt either during the production process or with the subsequent heat treatment of the (firmed up) steel. For each material, the official bodies in charge of standardization have determined the maximum amount of remaining inclusions in order to denote a steel product as stainless steel. Within the European Union, stainless steel is defined by European Standard (EN) 10027-2, as enacted by the European Committee for Standardization (CEN). According to the standard, unalloyed stainless steel is marked with the steel group numbers (material numbers) 10 to 18, while alloyed stainless steel is associated with steel group numbers 20 to 89.
Although the name is commonly used, it is actually incorrect to equate high-grade steel with stainless steel (or rustproof steel). Not all high-grade steel is stainless, and not all stainless steel types are high-grade steel. The requirements for stainless steel are clearly defined and may be met by some high-grade steel types, even though this isn\'t always the case.