Steel — the iron–carbon alloy and its varieties
Steel is an alloy of iron and carbon, often with other elements added. It combines strength, ductility, and versatility, and underpins construction, transport, tools, and many industrial applications.
Overview: Steel is an alloy primarily of iron and carbon, with carbon content lower than that of cast iron. Its controlled composition and processing give it a balance of strength, toughness and formability that makes it one of the most widely used engineering materials.
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Composition and main types
At its simplest, steel is iron with a small percentage of carbon. Manufacturers also add other elements to modify properties: manganese, silicon, chromium, nickel, and vanadium are common alloying agents. The addition of chromium produces stainless grades that resist corrosion, while elements such as tungsten or molybdenum improve hardness at high temperatures.
- Carbon steels: primarily iron and carbon, varied by carbon level.
- Alloy steels: contain deliberate additions of other metals (alloying elements).
- Stainless steels: include chromium for corrosion resistance.
- Tool steels: tailored for wear resistance and hardness.
History and development
Steelmaking traces back millennia, from early crucible and bloomery techniques to historic high-quality steels such as wootz and Damascus. Modern mass production began in the 19th century with industrial processes like the Bessemer converter and later basic oxygen and electric furnace methods, which made steel affordable for building railways, ships and cities.
Uses and importance
Steel is central to infrastructure, transportation, machinery, and household goods. It is used for structural beams, reinforcing bars, automobile bodies, pipelines, appliances, and surgical instruments. Its combination of recyclability and diverse properties means scrap steel is widely reused in new production.
Distinctions and notable facts
Steel differs from cast iron, which has higher carbon (typically above about 1.7%) and is relatively brittle, and from wrought iron, which contains very little carbon and historically had a fibrous texture. The precise carbon limit separating cast iron from steel is a useful guideline rather than a sharp chemical boundary; materials are also distinguished by microstructure and processing. For further technical details on iron and carbon relationships, see related resources.
Notable: Steel's adaptability—through alloying and heat treatment—lets engineers tailor strength, hardness, ductility and corrosion resistance for specific applications, making it a foundational material of modern industry.
Word Origin
The word 'steel' developed from Middle High German stahel, stāl, Old High German stahal, Middle Low German stāl, Middle Dutch stael, and Old Norse stál; besides the j-formation in Old Saxon stehli 'axe' and Old English stīle.
Definitions
EN 10020 - Definitions for the classification of steels
| |||
| Area | Materials | ||
| Title | Definitions for the classification of steels | ||
| Short Description: | Steel, definitions | ||
| Last issue | 2000–03-22 | ||
| Classification | 01.040.77, | ||
| National standards | DIN EN 10020:2000-07, | ||
In EN 10020:2000-07 Definitions for the classification of steels, point 2.1 states the following:
"[Steel is a] material whose iron content is greater by mass than that of any other element, whose carbon content is generally less than 2%, and which contains other elements. A limited number of chromium steels may contain more than 2% carbon, but 2% is the usual limit between steel and cast iron."
The limiting carbon content of 2 % is derived directly from the iron-carbon diagram. Up to a content of 2.06 %, the material can be in the form of austenite, which can be easily formed.
Trade literature
The definitions commonly used in the technical literature are based on DIN EN 10020 and all contain the definition as ferrous material, usually with reference to the carbon content of less than 2 %. In addition, however, it is always emphasised that steels are formable, i.e. can be processed by forging, rolling and similar processes.
- "...Ferrous alloys are those containing less than 2% carbon and which are suitable for hot working."
B. Ilschner, R. F. Singer Materials science and manufacturing engineering: properties, processes, technologies. 5th ed. 2010, S. 431.
- "Iron-carbon alloys with a carbon content generally below 2% by weight which are cold or hot formable (forgeable), ..."
H. Czichos, B. Skrotzki, F.-G. Simon: Hütte - Das Ingenieurwissen: Materials. 2014, S. 24.
- "Iron-carbon alloys which are forgeable without further finishing treatment are steels (C ≤ 2 %)." The addition of "without further finishing treatment" here excludes malleable cast iron, a type of cast iron that becomes malleable after heat treatment (tempering).
Hans-Jürgen Bargel, Günter Schulz (Eds.): Werkstoffkunde. 11th ed. 2012, S. 181.
- "Steel is malleable iron..."
Böge: Handbook of mechanical engineering. 21st edition. S. E14.
- "Steels are practically all malleable engineering alloys of iron."
Erhard Hornbogen, Hans Warlimont Metals: Structure and Properties of Metals and Alloys. 6th ed. 2016, S. 291.
- "...are ferrous materials generally suitable for hot working..."
Dubbel, 24th ed, p. E34.
- "...which are generally suitable for thermoforming."
VDEh: Werkstoffkunde Stahl - Band 1: Grundlagen, 1984, p. 21.
Use of terms in the history of technology
This general definition, which has been in use since the early 20th century, also includes wrought iron, which was hardly produced at that time and has a low carbon content of usually less than 0.3 %. This means that it cannot be hardened and has a different area of application. However, despite having a similar composition to steel, it is not identical to steel due to various impurities. When modern works (from the 20th century onwards) refer to "steel" in a historical context, they usually mean "wrought iron". Even in works on the history of technology, the term steel is used for earlier forgeable iron materials. In antiquity, the term wrought iron served to distinguish it from pig iron, which was still very heavily contaminated, and from the Middle Ages onwards additionally from non-forgeable cast iron.
Questions and answers
Q: What is steel?
A: Steel is a metal that is made by mixing iron with carbon and possibly other metals.
Q: How is steel different from iron?
A: Steel is harder and stronger than iron.
Q: What is cast iron?
A: Cast iron is iron that contains more than 1.7% carbon by weight.
Q: How is cast iron different from steel?
A: Cast iron contains more carbon than steel, and it is more brittle.
Q: What is wrought iron?
A: Wrought iron is a form of iron that contains little or no carbon.
Q: How is wrought iron different from steel?
A: Wrought iron is softer and less strong than steel.
Q: Why is steel a popular metal?
A: Steel is a popular metal because it is hard, strong, and versatile. It can be used for a wide range of applications, from construction to manufacturing.
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