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.



The CO2 emission of ‘green’ steel made with green hydrogen or renewable energy and electric arc furnaces, or produced from scrap, is significantly lower than conventional types. For example, Stegra, Boston Metal, SSAB, Hybrit, ArcelorMittal, Voestalpine, JSW Steel, Helios and Electra.

Mild steel is one of the most widely recycled materials and the majority of new steel contains recycled content, with some made up of 50% or more. It is magnetic, which helps to recover it from mixed waste. There are three principal waste streams feeding into the recycling process:
– offcuts and scrap from production that are fed straight back into the furnace.
– offcuts and scrap produced when manufacturing goods, which are typically sold back to the producer.
– post-consumer recycled (PCR), which comes from a wide range of applications including packaging, appliances, automotive, construction and so on.

Mild steel makes up the majority of steel in general use, and so it the most commonly recycled. As with other metals, it can can be continuously recycled without any damage or degradation to its properties — no matter the product or form it takes. Mild steel, which has low carbon content and only small amounts of alloy, can be turned back into the same steel, or used as a base for alloy steel and stainless steel.


Design properties
Carbon footprint kgCO2e/kg
0.05-0.4
Density kg/m3
7870
Tensile modulus GPa
190-210
Brinell hardness HB
119-180
Thermal conductivity W/mK
51.9-65.2
Thermal
conductive
Electrical
conductor
Electrical resistivity µΩ⋅m
0.16