Steel is an alloy primarily composed of iron and carbon, although it often includes other elements to achieve specific properties. The addition of carbon, typically less than 2% by weight, significantly increases the material’s strength, hardness, and durability compared to pure iron, which is soft and ductile. This combination of strength, flexibility, and relatively low cost has made steel the most widely used engineering and construction material globally. Modern production is divided into two primary routes: the integrated route, which processes new iron ore, and the secondary route, which relies on recycled scrap metal.
Preparing the Feedstock
The initial stage of steel production focuses on preparing the raw materials for the high-temperature furnaces. The integrated route, which feeds the Basic Oxygen Furnace (BOS), begins with iron ore that must first undergo beneficiation to increase its iron concentration. Since fine iron ore powder is unsuitable for the blast furnace, it is converted into larger, permeable forms through sintering or pelletizing. Sintering involves fusing fine ore with flux and coke breeze into a porous material, while pelletizing creates small, uniform balls by mixing fine ore with a binder and then firing them for hardness.
The alternative production route, centered on the Electric Arc Furnace (EAF), relies mainly on scrap steel as its feedstock. Scrap metal is collected from various sources and must be rigorously sorted to ensure the final product meets specific chemical requirements. Large pieces are shredded, and the material is inspected to remove unwanted contaminants like non-ferrous metals such as copper or zinc, which can cause defects in the finished steel. The scrap is also preheated using hot off-gases from the furnace to improve energy efficiency before it is charged.
Primary Steelmaking
This stage involves the fundamental chemical transformation of the prepared feedstock into molten crude steel. The Basic Oxygen Steelmaking (BOS) process starts by charging a converter with liquid hot metal—the high-carbon iron produced in a blast furnace—along with steel scrap. High-purity oxygen is blown into the vessel at supersonic speeds, initiating a rapid, exothermic reaction that removes impurities. This intense oxidation quickly removes excess carbon, silicon, and manganese from the molten bath, with the unwanted oxides forming a slag layer that is later removed.
The second method, Electric Arc Furnace (EAF) steelmaking, uses powerful electric arcs generated by graphite electrodes to melt the scrap metal charge. The intense heat quickly liquefies the scrap. EAFs are highly flexible and can produce a wide variety of steel alloys using nearly 100% recycled material, although iron sources like Direct Reduced Iron (DRI) can also be added. Oxygen is often injected during the EAF process to refine the bath by oxidizing impurities, with fluxes added to form a slag that absorbs the oxidized elements.
Once primary steelmaking is complete, the molten steel is transferred to a ladle for secondary steelmaking, also known as ladle metallurgy. This step fine-tunes the steel’s chemical composition and temperature, which is essential for meeting precise customer specifications. Processes like vacuum degassing are used to remove dissolved gases, such as hydrogen and nitrogen, which can lead to material defects if not addressed. Alloying elements are added to adjust the final properties, ensuring the molten product is chemically homogenous before casting.
Solidification Through Continuous Casting
After the steel’s chemistry has been refined, it moves to the continuous casting stage, where the liquid metal is solidified into a semi-finished shape. The molten steel is poured from the ladle into a reservoir called a tundish, which ensures a constant, controlled flow into the water-cooled mold below. This copper-lined mold causes the outer shell of the steel strand to solidify almost instantly upon contact, forming a solid skin that contains the remaining liquid core.
The partially solidified strand is continuously withdrawn from the bottom of the mold and passes through a series of rollers and secondary cooling zones, where water sprays complete the solidification process. This continuous process eliminates the traditional, energy-intensive ingot casting method. The machine cuts the resulting solid strand to specific lengths, creating three primary semi-finished products.
These shapes are determined by their cross-sectional dimensions. Slabs are wide and rectangular for the eventual production of flat products like sheet metal. Billets are smaller, square cross-sections used for manufacturing long products such as wire rods and rebar. Blooms are intermediate in size, larger than billets, and are often used for structural shapes like beams and rails.
Forming and Finishing Processes
The solid, semi-finished shapes next undergo mechanical deformation to achieve their final dimensions and shape. Hot rolling is the first major shaping process, where the steel is reheated and passed through powerful rolls at temperatures above its recrystallization point. This high-temperature rolling reduces the thickness of slabs into plates or strips and shapes blooms and billets into structural components, while also refining the steel’s internal grain structure.
For products requiring a smoother surface finish, tighter dimensional tolerances, and increased strength, the steel undergoes cold rolling. This process occurs at or near room temperature and is subsequent to hot rolling, rolling the steel strip without reheating. Cold working the metal increases its hardness and yield strength, but it also introduces internal stresses that must often be managed.
Finishing operations are then applied to modify the surface and mechanical properties of the final product. Heat treatments such as annealing involve heating the steel and allowing it to cool slowly to soften the material and improve its ductility, relieving internal stresses. Surface treatments like galvanizing are also common, where the steel is coated with a protective layer of zinc in a hot-dip bath to provide cathodic protection against corrosion.