Steel is an alloy, a material made by combining two or more elements, and it is the world’s most recycled and widely used structural material. The question of what two metals make up steel contains a slight inaccuracy, as steel is primarily composed of one metal, iron, and one non-metal, carbon. It is the controlled addition of carbon to iron that defines this alloy and transforms the soft metal into steel. The proportion of these two elements, along with the addition of other elements, determines the final properties, making steel a versatile material used across construction, manufacturing, and transportation.
Iron: The Foundational Element
Iron (Fe) is the base metal of all steels and is the fourth most abundant element in the Earth’s crust. Pure iron is characterized by its high ductility and malleability, meaning it can be easily deformed, hammered into sheets, or drawn into wires. The metal is also highly ferromagnetic and an excellent conductor of heat and electricity.
However, pure iron is relatively soft and lacks the necessary yield strength and hardness for most demanding structural applications. Its low strength means it is easily deformed under stress, making it unsuitable for building bridges, car bodies, or high-performance tools. This inherent softness necessitates the addition of other elements to modify its atomic structure and enhance its mechanical properties.
Carbon: The Essential Non-Metal
The second fundamental component of steel is carbon (C), which is a non-metal, not a metal. Carbon is the most important element for steel’s properties, and its presence in a precise concentration is what converts iron into steel. The amount of carbon in steel typically ranges from a few hundredths of a percent up to about 2.1% by weight, which is the upper limit that distinguishes steel from cast iron.
Carbon atoms are much smaller than iron atoms, allowing them to fit into the microscopic spaces within the iron’s crystal lattice structure, a phenomenon called an interstitial solid solution. These small, strategically placed carbon atoms distort the iron lattice and interfere with the movement of dislocations, which are defects that allow the metal to deform. By impeding this movement, carbon increases the steel’s hardness, tensile strength, and resistance to wear. Even a small change in carbon content, such as increasing the percentage from 0.3% to 0.6%, shifts the material from a medium-carbon steel used in structural components to a high-carbon steel used in tools requiring exceptional hardness.
Alloying Elements and Specialized Steels
While iron and carbon form the foundation of all steel, additional alloying elements are often introduced to achieve specialized characteristics beyond basic strength and hardness. These elements are added to tailor the steel for specific environments, such as those requiring corrosion resistance or stability at high temperatures. The purpose of these additions is to improve properties like toughness, heat resistance, and wear resistance that carbon alone cannot provide.
Chromium (Cr) is perhaps the most well-known alloying element, as the addition of at least 10.5% creates stainless steel, which is highly resistant to corrosion and oxidation. Nickel (Ni) is often combined with chromium to further enhance corrosion resistance and improve the steel’s toughness and ductility. Other elements serve specialized functions:
- Manganese (Mn) is included as a deoxidizer and to increase hardenability.
- Molybdenum (Mo) and Vanadium (V) are used in tool steels to provide strength at elevated temperatures and improve wear resistance.