Steel

Steel is a vital alloy made primarily of iron and carbon. An alloy is a mixture of different chemical elements. By combining iron with small amounts of carbon, engineers create a material with much better mechanical properties than pure iron. These properties include high yield strength and a high elastic modulus, which means the metal can resist deformation. Because it is strong and relatively inexpensive to produce, steel is one of the most common manufactured materials on Earth.

The process of making steel begins with iron ore. This ore is usually found in the Earth's crust as iron oxides, like magnetite or hematite.

To get the metal, we use a process called smelting. In a low-oxygen environment, carbon fuel reacts with oxygen to create carbon monoxide. This gas then reduces the iron oxide into metallic iron. While smelting creates pig iron, this material contains too much carbon to be called steel. To reach the correct composition, the excess carbon and other impurities must be removed through further processing.

To make steel, the carbon content must be carefully controlled. For plain carbon steel, the carbon weight must be between 0.02% and 2.14%. If the carbon content rises above 2.14%, the material becomes cast iron. Cast iron has a lower melting point and is easy to cast into shapes, but it is not malleable, even when hot. In contrast, steel is often strengthened by adding carbon to the iron lattice. This carbon inhibits the metal from deforming under mechanical stress. However, adding more carbon usually decreases the ductility and elongation compared to pure iron.

Scientists can change how steel behaves by adding other alloying elements. For example, adding at least 11% chromium creates stainless steel. This addition allows a hard oxide to form on the surface, which prevents corrosion and oxidation.

Other elements like nickel and manganese increase tensile strength. Tungsten can be added to create high-speed steel by slowing the formation of cementite. Some elements, such as phosphorus, sulfur, and nitrogen, are considered contaminants. These p-block elements are usually removed because they make the steel more brittle.

The internal structure of steel, or its microstructure, changes based on temperature and cooling. At room temperature, stable iron has a body-centered cubic (BCC) structure called alpha iron. When heated to 910 °C, it transforms into a face-centered cubic (FCC) structure called austenite. Austenite can hold much more carbon than alpha iron. If the steel is cooled slowly, the carbon can form a layered structure called pearlite. If the steel is cooled very quickly through a process called quenching, the carbon is trapped. This creates a highly stressed, very hard, but brittle phase called martensite.

Steel production has undergone massive changes throughout history. For thousands of years, people used bloomery furnaces to make small amounts of metal. Large-scale industrial production only began after the 17th century with the blast furnace. In the mid-19th century, the Bessemer process in England allowed for the mass production of steel. This era saw mild steel replace wrought iron. Later, the open-hearth furnace and modern basic oxygen steelmaking (BOS) further improved quality and lowered costs.

Today, the steel industry is a massive global system. In the 19th century, production was centered in the German states and American cities like Pittsburgh and Cleveland. Currently, China is the world leader, producing 54% of the world's steel in 2023.

The world produces more than 1.6 billion tons of steel every year. While the industry contributes 8% of global greenhouse gas emissions, it is also highly sustainable. Steel is one of the most recycled materials in the world, with a global recycling rate of over 60%.