Carbon steel

0.5-1.1 usd/kg
Circularity potential
Very high
Strength
High
Production energy
Low
Stiffness
Very high
Embodied CO2
Low
Density
Extreme

Steel is an immensely important material. Made up of iron (Fe) and 0.06-2% carbon (C), plain carbon steel is the basis upon which all steels are built, including stainless steel and alloy steel. From relatively ductile and formable low carbon steels – referred to as mild steel, they make up the majority of all steel produced – through ultra strong high carbon steel, this group offers something for every type of application. Indeed, it surrounds us in our daily lives in applications including tinplate packaging and food cans, toys, buildings and bridges, automotive bodywork and components, and industrial components. The difficulty with carbon steels is that they have very high corrosion rate, in the order of 20-50 micrometres per year, when exposed to weathering. Therefore, if used outdoors or exposed to an oxidising environment, they must be protected, such as with galvanising, metal plating, powder coating or painting.

While steel has some incredible attributes, and is irreplaceable in many applications, its downfall is its relatively high density. Compared to engineering plastics, and nonferrous alloys, such as aluminium, it is quite heavy for the same thickness; and it can be expensive to form and fabricate. Plastics are injection moulded into complex and lightweight parts in a single step, which is hard to beat in many engineering applications. And aluminium can be extruded into complex shapes with very low investment costs. Even so, steel remains the most significant engineering and construction material, second only to concrete in terms of global production volumes, and has the highest recycling rates of all – its magnetic properties make it relatively easy to recover from mixed waste. Today, new steel products contain on average around 30% recycled steel.

There are two main processes used to produce steel: basic oxygen steelmaking (BOF), which accounts for the majority, and electric arc furnace (EAF) steelmaking. While the BOF process is used to make primary steel from iron ore and coal and produces the highest quantity of emissions, EAF uses recycled material and be powered with renewable energy. The Steel Manufacturer’s Association claims that EAF operations produce twice the steel with 75% less greenhouse gas emissions compared to BOF production.

According to World Steel Association, carbon steel production has an average of 1.91 kgCO2e/kg, with basic oxygen steelmaking (BOF) at 2.33 and accounting for around 70% of global production, and electric arc furnace (EAF) at 0.66. On average, 20 MJ of energy is required for every kg of crude steel.



Medium carbon steels have between 0.3-0.6% carbon (C) and 0.6-1.65% manganese (Mn), by weight. These grades offer a good balance of strength and formability, falling between low and high carbon steels in performance. They have good machining and welding characteristics. Often other alloys, such as chromium (Cr), nickel (Ni), and molybdenum (Mo) are added in order to increase its stress resistance. They are prone to corrosion and require protection (such as painting, powder coating or galvanising).

Increasing the carbon content to approximately 0.5% with an accompanying increase in manganese allows medium-carbon steels to be heat treated – such as by quenching and tempering – which provides higher hardness and toughness without compromising machinability. Quenching and tempering is a two-step heat treatment process. Quenching involves heating the steel to between 800-900 degC, then rapidly cooling in water. In tempering, the steel is re-heated to below critical temperature, between 150-350 degC, and air-cooled. This process alters the crystal grain structure of steel to enhance hardness and other mechanical properties.

Applications include machine building, hydraulics, fasteners, strapping, gears, shafts, axels, couplings, construction and transportation – parts that will be subjected to more stress or wear than is suitable for low carbon grades, but require more formability or ductility than high carbon grades. Steels with carbon ranging from 0.4-0.6% are used for rails, railway wheels, and rail axles. Grades are available to military (MIL) specification, which are used in armoured vehicles and building protection, for example.


Design properties
Carbon footprint kgCO2e/kg
0.66-2.33
Density kg/m3
7870
Tensile modulus GPa
190-210
Brinell hardness HB
150-183
Thermal conductivity W/mK
51.9
Thermal
conductive
Electrical
conductor
Electrical resistivity µΩ⋅m
0.16