The World’s Most Recycled Material

What is steel? What makes it so important and useful? What makes it so recyclable? Here are some interesting things about the world’s most recycled material. Steel is a hard, strong grey or bluish-grey alloy of iron and carbon, with carbon content not exceeding 2%. If the alloy exceeds 2% carbon in content, it moves from steel to cast iron. In addition to metal characteristics of iron, carbon improves the physical-chemical properties of steel especially resistance. Iron in its pure form is relatively soft and prone to rust but interaction of the allotropes of iron with alloying elements, primarily carbon gives steel and cast iron their range of unique properties like toughness and resistance to corrosion. Its unique combination of durability, workability and cost makes steel the most popular construction material in the world. Ferromagnetic properties of many steel alloys allow it to easily be removed from the solid waste stream and diverted into recycling plants. Virtually, steel can be recycled indefinitely without losing its attributes, which makes it extremely sustainable.

According to the World Steel Association, some of the largest steel producing countries are China, Japan, US, and India, with China accounting for roughly 50 percent of this production. The high natural abundance of iron in the crust (mostly inform of oxides) together with abundance of carbon in nature sustains steel production and usage at low affordable costs, making it the most widely used of all metals and facilitates its production on large scale. Due to this availability and diversity, steel is applied across all industries from machinery and tools, aeronautics, automobiles, buildings to manufacture of different equipments, appliances etc.

Today, steel production makes use of recycled materials as well as traditional raw materials, such as iron ore, coal, and limestone using two processes; Basic Oxygen Steelmaking (BOS) and Electric Arc Furnaces (EAF).

The first step in both processes is called ironmaking, where raw inputs like iron ore, coke and lime are melted in a blast furnace. The resulting molten iron is referred to as hot metal and contains 4-4.5 percent carbon and other impurities that make it brittle. In the BOS method recycled scrap steel is added to the molten iron in a converter and at high temperatures oxygen is blown through the metal. This reduces the carbon content to between 0-1.5 percent. In EAF method recycled steel scrap is added through use of high-power electric arcs (temperatures up to 1650 C) to melt the metal and convert it into high-quality steel. The BOS process uses up to 30% recycled steel (scrap) while EAF process uses around 90-100% scrap.

The molten steel from either BOS or EAF is then treated to adjust the steel composition. This is done by adding or removing certain elements and/or manipulating the temperature and production environment to produce specific/required steel. This is achieved through either of the following processes: Stirring, ladle furnacing, ladle injection, degassing, CAS-OB (composition adjustment by sealed argon bubbling –oxygen blowing). Continuous casting sees the molten steel cast into a cooled mold causing a thin steel shell to solidify. The shell strand is withdrawn using guided rolls and fully cooled and solidified. The strand is cut into desired lengths depending on application; slabs for flat products (plate and strip), blooms for sections (beams), billets for long products (wires) or thin strips.

In primary forming, the steel that is cast is then formed into various shapes, often by hot rolling, a process that eliminates cast defects and achieves the required shape and surface quality. Hot rolled products are divided into flat products, long products, seamless tubes, and specialty products.

Finally, it’s time for manufacturing, fabrication, and finishing. Secondary forming techniques give the steel its final shape and properties. These techniques include: Shaping (cold rolling); which is done below the metal’s recrystallization point, meaning mechanical stress, not heat affects change. Other techniques include: Machining , Joining (welding), Coating (galvanizing), Heat treatment (tempering) and Surface treatment (carburizing).

There are many types of steel according to the content of alloying elements, each type allowing different uses and applications. Major ones include:

TYPES OF STEEL

• Carbon Steel: This is the most widely used kind of steel. Its carbon content is under 2 percent and is usually less than 1 percent. It often also contains a little manganese.
• Stainless Steel: This is the most corrosion-resistant kind of steel. It normally contains at least 12 percent (and sometimes up to 30 percent) chromium, and it usually also contains nickel. A very popular stainless steel formulation is 18-8, 18 percent chromium and 8 percent nickel.
• Alloy Steels: These contain a little carbon, and sometimes silicon, but they mainly contain added metals, such as manganese (hardness), nickel (strength), molybdenum (improved wear), tungsten (high temperature strength), chromium (corrosion resistance), and vanadium (toughness).
• Galvanized Steel: This steel is coated with zinc to protect against corrosion. The coating is usually done by a hot dip process.
• Electroplated Steel: This steel has a coating of another metal, usually tin, applied by the use of an electric current. Tin-plated steel is widely used for making cans and other containers.
• Tool Steel: This is very hard steel made by tempering (heating to a very high temperature and then quickly cooling).
• Damascus Steel: This was a very high quality ancient steel with a beautiful wavy surface pattern used in making sword blades. It seems to have come mainly from India.