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Carbon steel
0.5-1.1 usd/kg
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.
Low carbon steels, also called mild steel, have a good balance of toughness, strength and ductility. They are classified by having less than 0.3% carbon (C) by weight. They are easier to form, machine and weld than high-carbon steel. However, they are not as strong or hard, and they have poor resistance to corrosion – they are painted, powder coated, galvanised or plated for protection. In applications that do not require the strength of medium or high-carbon steel, but require higher surface hardness – such as forged motor shafts, hydraulic parts and pump shafts – carburising is used to produce a higher surface hardness, while leaving the core of the metal relatively soft. This helps improve drilling, machining, threading and punching of the metal, and prevent cracking in severe bends. They are commonly used without hardening in a range of everyday applications from large-scale shipbuilding and construction, through to mass-produced packaging, automotive and appliance parts.
Mild steel is the most common types of steel used in packaging. It is plated with tin on both sides to produce a bright finish that is resistant to corrosion and good for food contact. As well as providing a good barrier, it provides excellent soldering and welding properties. It can be drawn into cans (tins) and containers with the tin layer fully intact. It is fully recyclable end of life. As well as packaging, tinplate is used in electrical parts (connectors) and toys, for example.
Some notable types and grades:
– 1008 (UNS G10080), with 0.08% C, has the lowest strength (285-340 MPs), and highest thermal conductivity (65.2 W/mK) of the plain carbon steels. It has excellent formability and weldability, and is used in relatively low strength structural applications in construction and transportation.
– 1018 (UNS G10180, DIN 1.0453), with 0.18% C, is one of the most commonly used cold-rolled steels. It is used in a range of everyday applications, such as mounting plates and brackets, pins, bushing and tie rods. AISI 1117 (UNS G11170), 1118 and 1119, also known as a resulphurised free machining steel (or free cutting steel), is almost identical but contains more manganese (1.2%) in order to ensure good hardenability, and refined sulphur (up to 0.4%) for better machinability and short chip formation. They are used to make small and complex parts that are otherwise hard to machine with other metals. ASTM A350 LF2 is similar again, but with 1.1-1.35% manganese.
– 1030 (UNS G10300, DIN 1.0528), with 0.3% C, and ASTM A36 are virtually identical in chemistry. The difference is that A36 is designated by mechanical properties. These low carbon steels have a good balance of strength and toughness, which is utilised in construction and engineering applications (pylons, bridges, building, construction).
