The Modern Steel Manufacturing Process
Steel production involves removing carbon from iron
Steel is the world's most popular construction material because of its unique combination of durability, workability, and cost. It's an iron alloy that contains between 0.2 and 2 percent carbon by weight.
According to the World Steel Association, in 2016 global crude steel production reached a high of 1, 628.5 million tonnes. Of this, approximately three-quarters were produced using BOS plants, while EAF facilities accounted for the remaining quarter.
Some of the largest steel producing countries are China, Japan, US, and India. China accounts for roughly 50 percent of this production.
The 6 Step Modern Production Process
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 original Bessemer Process, which uses oxygen to lower the carbon content in iron.
Today, steel production makes use of both recycled materials, as well as the traditional raw materials, such as iron ore, coal, and limestone. Two processes; basic oxygen steelmaking (BOS) and electric arc furnaces (EAF) account for virtually all steel production.
Modern steelmaking 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: There are two primary steelmaking methods: BOS (Basic Oxygen Furnace) and the more modern EAF (Electric Arc Furnace) 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, however, 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. Continuous Casting: In this step, the molten steel is 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.
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)