Steel: Definition, Composition, Types, Properties, and Applications
Steel is a strong and versatile metal. It is an alloy consisting of iron with a small amount of carbon and occasionally other elements. The carbon directly influences the steel’s strength and durability. However, a vast array of different steels have been developed through the addition of other alloying elements such as chromium, nickel, molybdenum, and silicon. These can provide enhanced properties like abrasion resistance or corrosion resistance. There is a grade of steel for almost any application. This article will define steel, its composition, types, properties, and applications.
What Is Steel?
Steel, in its simplest form, is iron metal that’s been alloyed with less than 2% carbon. However, many other elements can be added as well to create multiple grades of steel alloys with varying properties. Common alloying elements include chromium, manganese, and nickel.
What Is the History of Steel?
The history of steel begins with the history of iron. Iron’s discovery and rise to prominence in what we now call the Iron Age began in about the 12th century BCE (though it varies depending on geographical location). It is only considered to have started in Europe around the 5th century BCE. Early steels (iron with added carbon to increase strength) were made in China around that time but generally had too low an iron content to be considered true steel. It was in India around 400 BCE that true steel was created by melting iron and charcoal together in small crucibles.
Variations of steel and cast iron (which have a higher carbon content) were developed throughout the world during medieval times and into modern history. A significant step forward was discovered by Benjamin Huntsman in England in 1751 when he used geologically-sourced coal rather than charcoal from timber to heat the crucibles. An even bigger breakthrough was made by Henry Bessemer, who was awarded a British patent in 1855 for a steel-manufacturing process. His process blew air directly through the molten iron (and its additives). This became fundamental to modern commercial steel production.
It wasn’t until about 1912-1914 that stainless steel was first created with the addition of chromium and nickel. These materials were gradually developed and refined through the end of World War Two.
What Is Steel Made of?
Fundamentally, steel is made of iron and carbon, but many other alloying elements also get added to create thousands of different grades of steel. Mild steel, or carbon steel, is generally more than 99% iron, containing less than 0.25% carbon, similar amounts of manganese, and traces of phosphorus and sulfur. By contrast, a common grade of stainless steel (304) has only about 70% iron with a minimum of 18% chromium and 8% nickel. Manganese, silicon, phosphorus, and of course carbon are also present in varying amounts within this type of steel. Other alloying elements for different steels include molybdenum, vanadium, and boron. Multiple grades of each type of steel exist, with variations in their composition meant to produce different characteristics.
How Are Steels Made?
Steel is made via one of two main smelting processes — either a blast furnace or an electric arc furnace.
For a blast furnace, iron ore and coke (coal that has been treated to remove volatile components) are added to the furnace, which is fired by air. Lime is also added to reduce the iron from the ore to its metallic form. This produces so-called pig iron, which is then sent to a direct oxygen furnace for the production of molten steel.
In an electric arc furnace, the iron ore is fired first by natural gas in a direct-reduction furnace. The iron metal is then sent to the electric arc furnace for steel production. Large electrodes are submerged into the furnace where electricity is used to create high-temperature arcs between the electrodes and thus melt the metal. Alloying elements are added to the electric arc furnace section.
After either process, the molten steel is then continuously cast by a hot strip mill and then rolled into different forms such as plates, bars, pipes, and others. This can be done by hot rolling or cold rolling. Other finishing processes such as tempering or annealing can also take place depending on the steel grade being produced.
What Are the Characteristics of Steel?
The common characteristics of steel are listed below:
- Strength: Steel is a high-strength material, particularly in tension, and can be used for structural loads.
- Durability: Steel is highly durable with a potential lifespan of over 100 years. It does not swell or creep, instead remaining very rigid.
- Versatility: Steel is an incredibly versatile material. Its many grades can be applied to thousands of uses.
- Machinability: Most steel is easily machinable, depending on the grade. Some specific grades of steel (free-cutting steels) are highly machinable.
- Weldability: Most grades of steel are easily weldable, although some need specialized welding procedures.
- Corrosion Resistance: Steel can be alloyed with other elements such as chromium, nickel, and molybdenum to better resist corrosion.
- Conductivity: Steel generally has lower thermal and electrical conductivity compared to other metals. It can be employed as a strong and heat-resistant shielding material.
- Recycling: Steel can be completely recycled, and due to its value, a large portion (>60%) of steel globally is recycled.
What is the Color of Steel?
Steel is generally silver-gray, but the appearance depends on the grade of the steel and the level of oxidation. For instance, some stainless steels, when polished, are reflective silver, almost with a mirror finish. Carbon steels are generally a dull gray to start with and turn a dark brown as they oxidize.
What Does Steel Look Like?
Steel looks like a dark gray or dark brown metal, often gaining a dull or rough appearance from oxidation or rust forming on its surface. Superficial rust is often visible on the surface of steel that has been exposed to the elements for an extended period. Figure 1 contains examples of steel:
Steel is produced via two main routes: the blast furnace-basic oxygen furnace (BF-BOF) route and electric arc furnace (EAF) route. Variations and combinations of production routes also exist.
The key difference between the routes is the type of raw materials they consume. For the BF-BOF route these are predominantly iron ore, coal, and recycled steel, while the EAF route produces steel using mainly recycled steel and electricity. Depending on the plant configuration and availability of recycled steel, other sources of metallic iron such as direct-reduced iron (DRI) or hot metal can also be used in the EAF route.
A total of around 70% of steel is produced using the BF-BOF route. First, iron ores are reduced to iron, also called hot metal or pig iron. Then the iron is converted to steel in the BOF. After casting and rolling, the steel is delivered as coil, plate, sections or bars.
Steel made in an EAF uses electricity to melt recycled steel. Additives, such as alloys, are used to adjust to the desired chemical composition. Electrical energy can be supplemented with oxygen injected into the EAF. Downstream process stages, such as casting, reheating and rolling, are similar to those found in the BF-BOF route. About 30% of steel is produced via the EAF route.
Another steelmaking technology, the open hearth furnace (OHF), makes up about 0.4% of global steel production. The OHF process is very energy-intensive and is in decline owing to its environmental and economic disadvantages.
Most steel products remain in use for decades before they can be recycled. Therefore, there is not enough recycled steel to meet growing demand using the EAF steelmaking method alone. Demand is met through the combined use of the BF-BOF and EAF production methods.
All of these production methods can use recycled steel scrap as an input. Most new steel contains recycled steel.
Check out our publication World Steel in Figures for more information.