While steel itself is manufactured, its raw materials—iron ore, coal (for COKE), and limestone—are sourced from mines and quarries worldwide. Recycling scrap steel is also an increasingly important part of steel production.
The process of making steel involves the following steps:
1. Mining Iron Ore
• It is iron ore (mainly iron oxide) that contains iron chemically bonded to oxygen, is extracted from the earth through mining. It is found in various forms, such as hematite and magnetite. Major deposits are found in countries like Australia, Brazil, China, and India.
2. Processing Iron Ore
• The ore is crushed and ground to separate impurities, resulting in concentrated iron ore.
3. Smelting in a Blast Furnace
• The iron ore is combined with coke (a carbon-rich material derived from coal) and limestone and heated in a blast furnace.
• The intense heat (up to 3,000 C or 5,432 F) melts the iron, separating it from impurities like slag.
• The intense heat melts the ore and causes chemical reactions that reduce the iron oxide to pure iron, forming what's called pig iron or sinter.
4. What Is Refining into Steel
• This pig iron contains too much carbon (about 4%) to be useful as steel. To create steel, the carbon content is reduced to typically between 0.1% and 2%, and other elements like manganese, chromium, or nickel may be added to create different types of steel with specific properties. The pig iron is further processed in steelmaking furnaces, such as:
- Basic Oxygen Furnaces (BOF): Oxygen is blown through molten pig iron to reduce carbon and other impurities.
- Electric Arc Furnaces (EAF): Scrap steel and pig iron are melted using electric arcs, commonly used in recycling steel.
- Direct Reduced Iron (DRI): also called sponge iron, is produced from the direct reduction of iron ore (in the form of lumps, pellets, or fines) into iron by a reducing gas which contains elemental carbon (produced from natural gas or coal) and/or hydrogen.
- This step creates steel, which is essentially iron with controlled levels of carbon and other elements like manganese, chromium, and nickel.
5. Alloying and Shaping
• Additional elements are added to create specific types of steel (e.g., stainless steel, high-strength steel).
• The steel is then cast into shapes like sheets, bars, or coils for various applications.
Modern steel production is highly efficient and heavily recycled - about 40% of global steel production comes from recycled scrap metal rather than new iron ore. This recycling process typically uses electric arc furnaces and helps reduce the environmental impact of steel production.
Electric Arc Furnace
Electric Arc Furnaces stand at the forefront of modern steelmaking, offering a versatile and efficient alternative to traditional steelmaking methods. Harnessing the power of electricity, these furnaces have revolutionized the industry by providing a more sustainable, cost-effective, and agile approach to producing high-quality steel.
Specifically, most refining utilizes an Electric Arc Furnace (EAF) process. EAF uses an electric arc to melt materials, primarily for steelmaking. It's widely used in the metallurgical industry for recycling scrap metal and producing high-quality steel.
Key Components of an EAF:
• Refractory-lined Vessel: The furnace is lined with heat-resistant material to contain the molten metal.
• Electrodes: Graphite electrodes are used to create an electric arc that generates intense heat, melting the scrap metal. The electrodes serve as conductors, delivering electricity to the charge material.
• Charging System: A piece of equipment that moves materials into a furnace for melting. Charging systems can be designed for blast furnaces, electric furnaces, and submerged electric arc furnaces.
• Tilting Mechanism: Allows the furnace to tilt for pouring molten metal or slag removal.
Key advantages of EAFs include:
• The ability to recycle steel scrap efficiently.
• Compared to traditional steelmaking methods, EAF's consume less energy, primarily because they do not require the high temperatures needed to melt iron ore in blast furnaces.
• Provide greater flexibility in production can process a wide variety of materials, including scrap and DRI.
• Produce faster melting times.
• Produce lower environmental impact by reducing reliance on raw materials like iron ore and coke, lowering carbon emissions.
• The electric arc process provides precise control over the steelmaking process, resulting in high-quality steel with consistent properties.
• They can be operated intermittently.
• Production of special quality steel.
• Units are much smaller and cheaper to build.
• Have a small size that allows them to be built near the point of use.
• Highly efficient and automated.
How It Works:
1. Charging the Furnace:
• The EAF is charged with raw materials, typically a combination of scrap steel, pig iron, or direct reduced iron (DRI).
• Alloying elements may also be added to achieve specific steel properties.
2. Starting the Electric Arc:
• Once charged, the furnace roof is closed, and the three electrodes are lowered.
• When they contact the metal, high-voltage electricity passes through the electrodes, creating an electric arc.
• The arc generates intense heat (up to 3,000 C or 5,432 F), which melts the metal.
3. Melting Process:
• The intense heat from the electric arcs melts the scrap and other materials. Additional scrap may also be added during this phase.
• Fluxes (like lime or dolomite) are added to remove impurities by forming a slag, which floats on the surface of the molten metal.
4. Refining:
• Oxygen may be injected to oxidize impurities like carbon, silicon, phosphorus, and sulfur to remove impurities.
• Carbon and other alloying elements are injected to achieve desired composition.
5. Tapping the Furnace:
• When the desired temperature and composition are reached, the molten steel is poured (tapped) into a ladle for further processing.
• A Tundish is used in the continuous casting process to distribute molten steel into the mold while the ladle is used for transporting and pouring molten steel from the furnace to the tundish.
• The slag is skimmed off separately.
6. Casting Process
• The ladle of molten metal is bottom tapped into a tundish which then feeds directly into the billet caster.
• The billet of steel will then later be reheated and rolled into sheets, bars, or coils for various applications.
7. Cooling and Maintenance:
• After tapping, the furnace is cooled and prepared for the next cycle. Electrodes are inspected and replaced, as necessary.
