Methods for manufacturing steel have evolved significantly since industrial production began in the late 19th century. Modern methods, however, are still based the same premise as the Bessemer Process, namely, how to most efficiently use oxygen to lower the carbon content in iron.
Today, steel production comes from both recycled as well as the tradition raw materials, iron ore, coal and limestone. Two processes; basic oxygen steelmaking (BOS) and electric arc furnaces (EAF) account for virtually all steel production.
According to the World Steel Association, in 2011 global crude steel production reached a new record high at 1.527 billion metric tonnes. Of this, approximately two-thirds was produced using BOS plants, while EAF facilities accounted for the remaining third.
The largest steel producing countries in 2011 were China, Japan, US, and India. China currently supplies about 45% of the world's steel. The world's largest steel producers include ArcelorMittal, Baosteel, POSCO and Nippon Steel.
Modern Production Process:
Steel production can be broken down into six steps:
1. Ironmaking: In the first step, the raw inputs iron ore, coke and lime are melted in a blast furnace. The resulting molten iron - also referred to as 'hot metal' - still contains 4-4.5% carbon and other impurities that make it brittle.
2. Primary Steelmaking: Primary steelmaking methods differ between BOS and EAF methods. BOS methods add recycled scrap steel to the molten iron in a converter. At high temperatures, oxygen is blown through the metal, which reduces the carbon content to between 0-1.5%. EAF methods, alternatively, feed recycled steel scrap through use high power electric arcs (temperatures up to 1650 °C) to melt the metal and convert it to high quality steel.
3. Secondary Steelmaking: Secondary steelmaking involves treating the molten steel produced from both BOS and EAF routes to adjust the steel composition. This is done by adding or removing certain elements and/or manipulating the temperature and production environment.Depending on the types of steel required, the following secondary steelmaking processes can be used:
- ladle furnace
- ladle injection
- CAS-OB(Composition Adjustment by Sealed argon bubbling with Oxygen Blowing).4
4. Continuous Casting: In this step, the molten steel is cast into a cooled mould 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 The shell strand is withdrawn using guided rolls and fully cooled and solidified.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.
5. 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.
6. Manufacturing, Fabrication, and Finishing: Finally, secondary forming techniques give the steel its final shape and properties. These techniques include:
- shaping (e.g. cold rolling)
- machining (e.g. drilling)
- joining (e.g. welding)
- coating (e.g. galvanizing)
- heat treatment (e.g. tempering)
- surface treatment (e.g. carburizing)