How is steel made?
Have you ever wondered just how steel is made? The raw materials undergo a complex series of processes to be transformed in the steel products and profiles that are widely used in domestic, commercial, and industrial settings; and here is a brief account of how that is achieved.
Steel is made from three main ingredients- limestone, coke, and iron ore and undergoes a series of processes to complete the journey from ore to project.
The iron used for making steel commonly consists of three main presentations.
- Raw ore- a compound found in nature containing iron, oxygen, and other minerals.
- Pellets- iron ore that has been crushed and the unwanted compounds removed to create a concentrated iron powder which is further processed to make pellets.
- Sinter- a mixture of iron ore, lime, and coke to create a product suitable for blast furnace charging
Sinter, iron ore, or pellets, coke, and limestone are added (charged) into the top of a furnace which blasts hot air, hence the term ‘blast furnace’, through the bottom of the furnace. This causes the coke to burn and reach temperatures of more than 2,000 °C to create hot metal. Hot metal is the name given to liquid iron during the process of being physically and chemically altered through the blast furnace process. The molten metal pools in the base of the furnace where the limestone combines with the impurities to form slag. Slag is less dense than the molten iron so rises to the top of the hot metal where it is removed; although seen as a waste product, the slag is repurposed to be used within the road building and cement manufacturing industries. The hot metal is then poured or ‘tapped’ from the furnace into containers (torpedoes) that can transport around 300 tonnes of liquid iron to the steel plant for the next step of its journey.
Primary steel manufacture.
The molten iron still contains other impurities that will need to be removed to ensure the final steel is not brittle, this starts with pouring the molten metal into charging ladles where impurities such as sulphur are eliminated.
A blend of scrap and molten metal (from the torpedoes) are then charged into the steel making furnace where high purity oxygen is blown at high pressure onto the surface of the iron through a water-cooled lance. This method of steel making is also referred to as Basic Oxygen Steelmaking, or BOS. Lime is also added at this stage to form slag containing the unwanted impurities, this slag is removed. After the oxygen blowing stage of the process the molten steel is tapped into ladles where alloying elements are added and the deoxidation process is controlled to create the desired steel chemistry. Each steel produced is awarded a grade or designation dependent on their chemical and physical properties, to find out more mild steel grades, read our in-depth technical guide here.
Secondary steel manufacture.
Secondary steel manufacturing processes are employed to ensure the strictest chemical and physical properties required of a particular grade of steel are met. For example, to create a structural steel, more carbon will be left in to increase the tensile strength, whereas, to produce a drawing steel, more carbon will be removed to lower the tensile strength but aluminium will be added to increase the ductility. Secondary manufacture processes are vital to ensure the uniformity and strict chemical and physical property conformity for each specific grade.
Secondary steel manufacturing techniques included processes of alloying, deoxidation, desulphurisation, vacuum processing to reduce levels of undesired dissolved gases, and temperature adjustment to ensure correct conditions for teeming. Teeming is when the liquid steel is poured into a continuous casting machine or moulds to produce ingots, this process is crucially important as this point of manufacture poses risks of particle contamination and oxidation from the (external) environment.
Continuous steel casting. (Concast)
The molten steel is teemed into the casting equipment where it is solidified into a continuous strand of metal; depending on the dimensions these may be referred to as blooms, billets, or slabs. The steel is drawn from the mould vertically passing through a sequence of rollers where it is water cooled, the slabs are straightened as they leave the casting machine and cut to length ready for rolling in light mills or used for forging, re-rolling, ring rolling, and hot forge applications.
Primary steel forging.
During primary steel forging, the billets, blooms, ingots, and slabs are forged into shapes through the process of hot rolling. Hot rolling ‘sets’ the final quality of the steel and eliminates defects in the shape. Primary forging is used to make profiles, seamless steel tubes, and bespoke products.
Secondary steel forming.
This is the last stage of shaping and preparing the metal for use in its final application. Techniques for secondary forming include galvanising, thermal treatments such as annealing, case or work hardening, pressing, machining, drilling, joining, welding, and manual or flame cutting.
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