Steel: The Material That Shaped Our World, and Its History

Steel is an iron and carbon alloy containing less than 2% carbon and small amounts of manganese, silicon, phosphorus, sulfur and oxygen. The development of steel can be traced to the beginning of the Iron Age, approximately 4000 years ago. Iron, previously the most widely used metal, proved to be harder and stronger than bronze, which had dominated before, and began to replace bronze in weapons and tools. However, for several thousand years, the quality of iron produced would depend as much on production methods as on the type of ore.

Traditional Method

This iron production method involved separating oxygen from iron oxide compounds, or in other words, reduction. Forged iron ore was loaded into furnaces or kilns heated by burning wood, along with charcoal. Coal, burning with oxygen in the air, provided the heat energy and carbon monoxide (reducing agent) needed to separate oxygen from the iron oxide compound (reduce it), while also raising the temperature. As a result, iron metal formed on the surface of ore particles. The resulting product was hammered and returned to the furnace, and this process was repeated several times until the iron-carbon alloy reached the required properties.

Blast Furnaces

A blast furnace is a type of metallurgical furnace generally used to produce pig iron and also other metals such as lead or copper. In blast furnaces, fuel (coal), iron ore and flux (usually limestone) are continuously fed from the top of the furnace, while hot air (sometimes oxygen-enriched) is blown into the lower part. Thus chemical reactions occur throughout the furnace as the material falls downward. The downward flow of ore, contacted by rising hot gases rich in carbon monoxide, is a countercurrent exchange and chemical reaction process. The final products—molten metal and slag phases—exit through channels at the bottom, while waste gases (flue gas) exit through the top, or chimney. The development of these blast furnaces, which began to be used more widely in Europe during the Middle Ages, increased iron production. Pig iron is molten iron from blast furnaces, cooled in the main channel and adjacent molds. This product, obtained at higher temperatures, is strong but contains more carbon and is therefore brittle. For this reason it is far from ideal for working and shaping. The metallurgists of the time realized that the high carbon content in iron was at the root of the brittleness problem. They therefore began to investigate various methods of reducing carbon content to make the material more workable.

The "Puddling" Method

Pig iron contains too much free carbon and is brittle; before use, it must be converted into a more malleable form. In puddling furnaces it is processed to become wrought iron. In these operations pig iron is melted, stirred or agitated while a strong air stream is directed onto it. This causes dissolved impurities (such as silicon) to oxidize thoroughly. Although it had been used much earlier, in 1784 Henry Cort perfected the method and obtained its patent. Cort managed to eliminate the risk of the iron overheating and "burning" by adding an adjustable lid to the flue. Cort's process consisted of stirring molten pig iron in an oxidizing atmosphere in a burning furnace and thereby decarburizing it. The paste-like iron resulting from the process was formed into balls weighing 4–5 kg. These balls were then taken and subjected to a hammering operation to remove the slag that remained in the structure, and formed into rod shape. This process, known as "puddling," which involved melting cast iron and separating it from slag in an oxidizing environment, was very slow, laborious and inefficient. Nevertheless, by 1860, more than 3000 puddling furnaces were still operating in England.

Blister and Crucible Steel

Another early form of steel was blister steel, production of which began in Germany and England in the 17th century. It was produced by increasing the carbon content in molten pig iron using a process known as cementation. In this process, wrought iron rods were coated with powdered coal in stone boxes and heated. After approximately one week, the iron absorbs carbon from the coal. Repeated heating distributes the carbon more evenly, and the resulting material after cooling is blister steel. In the 1740s, progress was made in blister steel production. English watchmaker Benjamin Huntsman sought to develop high-quality steel for watch springs. After years of searching, the method he developed became crucible, or cast steel. A crucible is a container that holds molten steel or other metals while they are being melted or transported. It is made of heat-resistant materials such as clay or porcelain. However, due to production costs, both blister and crucible steel were used only in specialized applications. As a result, iron from puddling furnaces remained the primary production material for industrializing England through much of the 19th century.

The Bessemer Process and Modern Steel Production

The growth of railways throughout the 19th century in both Europe and America placed great pressure on the iron industry, which still struggled with inefficient production processes. During this period, steel had yet to prove itself as a structural material, and its production was slow and costly. However, this changed completely in 1856 when Henry Bessemer found a more effective way to give oxygen to molten iron to reduce its carbon content. Henry Bessemer designed a pear-shaped vessel called a "converter" through which iron could be heated while oxygen was blown through it. As oxygen passed through the molten metal, it reacted with carbon, releasing carbon dioxide and producing purer iron. The process was fast and cheap, capable of removing carbon and silicon from iron within minutes. However, this high efficiency of the process had a drawback: too much carbon was removed from the iron and too much oxygen remained in the final product. At the same time, English metallurgist Robert Mushet began testing spiegeleisen, an iron, carbon and manganese compound. It was known that manganese removed oxygen from molten iron, and when spiegeleisen was added in the correct amounts, with the help of its carbon content it would solve Bessemer's problems. Bessemer successfully added this material to his method with great success. However, one problem remained: Bessemer had not found a way to remove phosphorus, a harmful impurity that made steel brittle. For this reason, only naturally phosphorus-free iron ore from Sweden and Wales could be used. In 1876, Sidney Gilchrist Thomas provided a solution by chemically adding a basic flux, limestone, to the Bessemer process. Limestone drew phosphorus from pig iron into the slag, enabling the removal of this unwanted element. This innovation finally demonstrated that iron ore obtained from anywhere in the world could be used to make steel. As expected, steel production costs began to fall significantly. Steel rail prices fell more than 80% between 1867 and 1884, years when the growth of the global steel industry began thanks to new steel production techniques.

The Open Hearth Process

In the 1860s, German engineer Karl Wilhelm Siemens further increased steel production with his creation of the open hearth process. The open hearth process produced steel from pig iron in large shallow furnaces. The process, which used high temperatures to burn excess carbon and other foreign materials, relied on heated brick chambers beneath the furnace. These regenerative furnaces used exhaust gases from the furnace to provide high temperatures in the brick chambers. This method allowed production in much larger quantities (50–100 metric tons could be produced in one furnace), enabled specific testing of molten steel, meeting particular specifications, and allowed scrap steel to be used as raw material. Although the process itself was slower, by 1900, the open hearth process had primarily replaced the Bessemer process.

The Birth of the Steel Industry

The revolution in steel production that provided cheaper, higher-quality material was seen as an investment opportunity by many businessmen of the time. Capitalists in the late 19th century, including Andrew Carnegie and Charles Schwab, invested millions, and in Carnegie's case billions, of dollars in the steel industry. U.S. Steel, founded by Carnegie in 1901, became the first company to be worth more than one billion dollars.

Electric Arc Furnace Steel Production

Shortly after the beginning of the century, another development occurred that would have a strong impact on the evolution of steel production. Paul Heroult's electric arc furnace (EAF) was designed to pass an electric current through charged material, thus enabling exothermic oxidation and steel production at temperatures of up to 3272°F (1800°C). Because EAFs can produce steel from 100% scrap or cold iron, production requires less energy per unit. Unlike basic oxygen furnaces, operations can be stopped and restarted at low cost. For these reasons, production via EAF has been increasing steadily for more than 50 years and currently accounts for approximately 25% of global steel production.

Oxygen Steel Production

Today, the majority of global steel production, approximately 75%, is produced in blast furnace–basic oxygen furnace (BF-BOF) facilities. Basic oxygen furnaces blow oxygen in large amounts into molten iron and scrap steel and can complete a charge much faster than open hearth methods. Large iron vessels of up to 350 metric tons can be converted to steel in less than an hour. The fundamental difference between these methods is the type of raw materials they consume. In BF-BOF for steel production, iron ore, coal and recycled steel are mainly used. The EAF method produces steel primarily using recycled steel and electricity.

Steel is 100% Recyclable

Steel products naturally contribute to resource conservation through their light potential, durability and recyclability. The 100% recyclability of steel ensures that the resources invested in its production are not lost and can be reused indefinitely. Due to their magnetic properties, steels are easily separated from waste streams, providing high recovery rates. Many of the products we use are actually made from steel that is up to 100% recycled. Prepared by: B. Serhat Cengiz

Sources • Barraclough, K. (1990), Steelmaking 1850–1900, Institute of Metals, London. • Gale, W. K. V. (1969), Iron and Steel, Longmans, London. • The Making, Shaping, and Treating of Steel: Ironmaking volume (PDF). AISE Steel Foundation. 1999. • Carr, J. C.; Taplin, W. (1962). History of the British Steel Industry. Harvard University Press Cambridge. • https://www.thebalance.com/steel-history-2340172 • https://www.worldsteel.org/