steel production

Chances are, you've already used steel in several ways today - from the car you drove in to work to the knife and fork you used at dinner. Steel is all around us and the world would look very different without it, but most of us don't really know how it's made.

Well, that's where the metal experts at Clickmetal come in! In the guide below, we'll take you through the step-by-step process of how steel is created, from its raw materials to the many final forms it takes.

Before We Start...

Steel production is a complex process that can vary based on the type of steel and the techniques used. This guide will be focusing on the most common method, basic oxygen steelmaking, but keep in mind that not all steel is processed this way - some are refined using electric arc furnaces or other methods.

Now, let's walk through how the typical process works.

1. Raw Material Extraction

The steel-making process begins with gathering the essential raw materials: iron ore, coal, and limestone.

  • Iron Ore: This is the main source of iron, the key element in steel. The most commonly used types are hematite and magnetite.
  • Coking Coal: Coking coal is used to produce the intense heat needed to reduce the iron ore into a usable form.
  • Limestone: Limestone removes impurities such as silica, sulphur, and phosphorus during the process.

Alloying elements like magnesium or nickel may also be gathered for use later in the refining process.

These materials are typically mined from large deposits and transported to steel mills, where the transformation into steel begins.

2. Coke Making

The next step is turning coal into coke (no, not the drink). This involves heating coal in a low-oxygen environment, usually in a coke oven, to remove volatile compounds. This leaves behind coke, a carbon-rich fuel that burns hotter and cleaner than regular coal.

Coke is crucial because it provides the intense heat needed in the blast furnace, where the iron ore will be melted down.

3. Blast Furnace

Once the coke is ready, the next step is to produce molten iron in a blast furnace - a massive structure that can reach 100 to 200 feet tall. Here's how the process works:

  • Iron ore, coke, and limestone are layered inside the furnace, which is lined with heat-resistant bricks.
  • Hot air around 1,500--2,200°C is blasted into the furnace from the bottom. This extreme heat ignites the coke to create carbon monoxide gas, which reacts with the iron ore, reducing it to molten iron.
  • As the iron ore melts, impurities like silica form a liquid slag that floats on top of the molten iron. The purified iron, now in liquid form, sinks to the bottom of the furnace and is tapped off.

This molten iron, known as pig iron, still contains a high level of carbon and impurities, which makes it brittle. It needs to go through a refining process to turn it into steel.

Fun fact: Pig iron got its name from the way the moulds were arranged for casting. The moulds were set up in a pattern that resembled a mother pig with her piglets. The larger mould, where the molten iron was poured, was called the "sow," and the smaller ingots that flowed from it were called "pigs"!

4. Basic Oxygen Steelmaking (BOS)

The pig iron from the blast furnace is then taken to a basic oxygen furnace to be refined into steel. This process lowers the carbon content and removes impurities, making the iron stronger and more versatile.

  • Molten pig iron is poured into the furnace, and pure oxygen is blown in at high pressure. The oxygen reacts with the carbon in the pig iron, creating carbon dioxide and reducing the carbon content.
  • Fluxes like limestone are added to help remove impurities. As the oxygen and fluxes react with the carbon and other impurities, they form slag, which is then removed.
  • The amount of oxygen injected, and the timing of the process are carefully adjusted to make sure the carbon content reaches the desired level for the specific type of steel being produced (e.g. 0.3% for mild steel), while preserving the steel's other essential properties.
  • At this stage, alloying elements like manganese, chromium, or nickel can be added to give the steel specific properties, such as increased strength or corrosion resistance. This creates different types of steel, with stainless steel being one of the most well-known.

The result is high-quality liquid steel with a much lower carbon content than the original pig iron.

5. Casting

Once refined, the molten steel is ready to be cast. The steel is poured into moulds to form slabs, billets, or blooms, depending on the desired shape.

Most modern steel plants use a method called continuous casting. In this process, the molten steel is continuously poured into a water-cooled mould, where it solidifies as it moves downward. This produces long strands of steel that are cut to the needed length. Steel might still be cast into large ingots in some cases, but this method is less common today.

6. Forming and Shaping

After the steel has solidified, it goes through additional shaping to create the final product. This is usually done through hot or cold rolling:

  • Hot Rolling: The steel is heated above its recrystallisation temperature and passed through large rollers to reach the desired thickness. This method is used for products like steel sheets, beams, and rails.
  • Cold Rolling: For some applications, the steel is rolled at room temperature. This gives the steel a smoother surface and more precise dimensions, making it ideal for products that require a high-quality finish.

7. Finishing Processes

The final step in the actual steel-making process is a series of finishing treatments that prepare the steel for use in construction, manufacturing, and other industries. These processes include:

  • Soaking the steel in acid (known as 'pickling') to remove surface impurities like rust or scale.
  • Galvanisation, which involves coating the steel with zinc to protect it from rusting.
  • Heat-treating the steel to improve its strength, ductility, or toughness.
  • Adding a protective coating or a colourful painted finish.
  • Cutting to meet certain specifications.

8. Quality Control and Testing

Before steel products are shipped out, they go through strict quality control tests to make sure they meet the necessary mechanical properties, chemical composition, and dimensional tolerances. Common tests include:

  • Tensile testing to measure the steel's strength.
  • Hardness testing to check its resistance to indentation.
  • Ultrasonic testing to detect any internal flaws or cracks.

Once the steel passes these tests and meets the standards, it's ready to be used in a wide range of products - from tools and machinery to vehicle frames, furniture, medical equipment, and just about anything else you can imagine!

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