Steel

Steel is a material consisting mostly of iron with a low carbon content. Steel can be formed hot or cold, i.e. it can be rolled, bent, drawn and forged.

Steel is often defined as an iron-carbon alloy with a maximum carbon content of 2% by mass. Iron-carbon alloys with higher carbon contents are called cast iron and are not plastically formable, i.e. cannot be forged or rolled. In addition to carbon, steel always contains residues of undesirable phosphorus, sulfur and some other impurities. Modern steel grades contain additional elements that have been alloyed to improve the properties of the steel. Decisive for the properties of the steel are the proportions of the accompanying and subsequently added alloying elements as well as the crystal structure after deformation and the heat treatment condition.

Steel is one of the most versatile construction materials and can be recycled almost indefinitely. Its production (in 2016: 1629 million tonnes) exceeds the amount of all other metallic materials combined by more than ten times. Steel is available in large quantities and at low cost. Its properties can be varied over a wide range by alloying and heat treatment.

Steel can be processed by casting and especially well by rolling, forging, milling and welding and has a high strength (simple steel 180 to 350 N/mm², high-strength steel up to well over 1200 N/mm²), good hardenability, stiffness (E-modulus) and elongation at break.

Coal and steel (coal and steel industry) were for a long time the main pillars of heavy industry and the basis for the political power of a state. Modern steel production is being further developed from the point of view of _{CO2 emissions.}

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

In EN 10020:2000-07 Definitions for the classification of steels, point 2.1 states the following:

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.