Steels are usually defined as alloys of iron and carbon, containing not more than 2% carbon, with or without other alloying elements. With more than 2% carbon, the material comes into the category of cast iron.
The following information has been extracted from
IMMA Handbook of Engineering Materials, 5th Edition, and should be used as a guide only.
Carbon Steels – steels containing only carbon as the specific alloying element. These steels can also contain up to 1.2% manganese and 0.4% silicone. Residual elements such as nickel, chromium, aluminium, molybdenum and copper, which are unavoidably retained from raw materials, may be present in small quantities, in addition to ‘impurities’ such as phosphorous and sulphur.
Mild Steel, Normal Strength or Ordinary Weldable Steel – these are trade terms often used interchangeably to describe standard carbon steels used for structural purposes, a typical grade being AS3679 grade 250 or grade 300. The term ‘mild steel’ is also applied commercially to carbon steels not covered by standard specifications. Carbon content of this steel may vary from quite low levels up to approximately 0.3%. Generally, commercial ‘mild steel’ can be expected to be readily weldable and have reasonable cold bending properties, but to specify ‘mild steel’ is technically inappropriate and should not be used as a term in engineering.
Carbon-Manganese Steels – the manganese content in carbon steels is often increased for the purpose of increasing depth of hardening and improving strength and toughness. Carbon steels containing over 1.2% up to approximately 1.8% manganese are referred to as carbon-manganese steels. Typical examples of this type of steel are AS3679 grade 300 and AS1442/1320.
Alloy Steels – various attempts have been made to distinguish between ‘low’ and ‘high’ alloy steels, but the definitions vary between countries and between standard-setting organisations. As a general indication, low alloy steel can be regarded as alloy steel (by the ISO definition) containing between 1% and less than 5% of elements deliberately added for the purpose of modifying properties.