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Steel is a metal alloy comprised primarily of iron and carbon, with the carbon content typically below 2.06%. When the carbon content exceeds this threshold, and up to a maximum of 6.67%, the alloy is classified as cast iron. Carbon is always present as cementite, also known as iron carbide (Fe3C), while the presence of other chemical elements in significant proportions gives the steel unique properties.

Classification of steels

Steels are classified into two main groups based on their chemical composition: common carbon steel and special steel.


These steels consists solely of iron and carbon, with carbon content typically between 0.2% and 1%. The mechanical properties of this steel vary with the percentage of carbon present. In particular, increasing carbon content enhances mechanical strength, hardness, hardenability and wear resistance, but it also increases brittleness while decreasing malleability, ductility, toughness and weldability.

Different classifications based on carbon content include:

  • very low carbon steels: C < 0.008%;
  • low carbon steels: C between 0.008% and 0.1%;
  • medium low-carbon steels: C between 0.1% and 0.4%;
  • medium hard-carbon steels: C between 0.40% and 0.55%;
  • high carbon steels: C between 0.55% and 0.80%;
  • very high carbon steels: C between 0.80% and 2.06%.

These types of steel are widely produced and exhibit good resistance to static loads. They find applications in numerous fields such as the manufacture of knives, scissors, lifting ropes, high-tension cables, car parts and mechanical parts in general, etc.


Special steels are often referred to as alloy steels because, in addition to carbon, they contain other chemical elements that give the product unique properties. These elements typically include nickel, chrome, molybdenum, silicon, copper, manganese, and tungsten, among others.

Special steels find application in various fields that require superior properties than common steel, and the specific type of alloy varies based on their intended use. For instance, the presence of chromium (Cr) increases hardness and oxidation resistance, making this type of steel suitable for manufacturing bearings and components for thermal and chemical plants.

The presence of Silicon (Si) increases the yield strength, which makes them suitable for manufacturing springs.

The presence of Manganese (Mn) improves the penetration of hardening effects, thus enhancing hardness and wear resistance. This makes them suitable for producing parts requiring high mechanical properties in very deep areas of the part.

The presence of Tungsten (W) increases hardness, which makes them suitable for manufacturing lathes, milling tools, etc.

The presence of Lead (Pb) enhances machinability, allowing the material to be easily machined and to produce chips, which is why they are also known as automatic steels and used for mass-produced parts on automatic machines.