By the end of this article you are going to learn:
- what is the difference between iron and steel
- how iron is found in nature
- how iron is extracted from ore
- how steel is produced
Table of contents
- Differences between iron and steel
- Iron ores
- Iron production
- Steel production
- Summary table of differences between iron and steel
Differences between iron and steel
Of all the metallic elements, iron is the most important. Iron is very abundant on Earth, easy to extract from its ores, and can readily be made into our primary constructional material, steel. No other metal is even close to iron in terms of amount used for equipment manufacturing. In June 2021, the world production of steel was of 167.9 million tonnes (Mt). By comparison world primary estimated of aluminium production totalled just over 64.4 million tonnes in 2019.
The main difference between iron and steel is that iron is a metal that occurs naturally on Earth, whereas steel is a man-made alloy which contains mainly iron and carbon and some small quantities of other metals such as nickel and chromium.
Metallic iron (Fe) itself is a greyish-white metal that is highly ductile and malleable, and therefore easy to shape. Iron is magnetic; most magnets contain iron, along with other magnetic elements like nickel and cobalt. By itself, iron is neither hard nor particularly strong.
With the addition of carbon, iron becomes steel and gains greater value. With the addition of as little of 0.1 % of carbon to iron, transforms it into a metal of high tensile strength, which can be made even tougher and harder with heat treatment. Other metallic and non-metallic elements can be incorporated in steel to give them enhanced properties, for example, corrosion resistance and even greater strength and toughness.
After aluminium, iron is the most abundant metallic element in the Earth’s crust, being 5% of it. Being a relatively reactive element, iron is seldom found in a native state, though it can sometimes be found alloyed with nickel.
Iron compounds are widespread in the Earth’s crust, and they have become concentrated in workable mineral deposits in many parts of the world. Among the many iron bearing minerals (iron ores) are the:
- haematite (Fe2O3)
- magnetite (Fe3O4)
- hydrated oxyde
- limonite (FeO(OH)·nH2O)
- siderite (FeCO3)
- iron pyrites (FeS)
The latter is used as a source of sulphur, not iron, but the others are all important iron ores. Haematite, often called kidney iron ore because of the way it sometimes crystallises, is deep red when pure. Magnetite is black and is magnetic. Limonite is brown and it’s often called bog-iron ore. Most iron is obtained by open cast surface mining, though some is worked in deep-level underground mines.
Because they come from the ground, most iron ores are associated with gangue, worthless earthy material which must subsequently be removed. This may be done by washing; or it may be done magnetically. Often the ore is roasted to drive off water and, in the case of carbonate ores, carbon dioxide. The ore is usually crushed during preparation, resulting in a fine dust. To prevent trouble in later operations, the crushed roasted ore is heated strongly, or sintered. This treatment makes it combine into larger lumps. Sometimes fine ore dust is mixed with clay, formed into shapes, and baked, a process called pelletization.
Iron is produced from the ore in blast furnaces. These furnaces are tall structures, 30 m or more high and about 9 m in diameter at the base. The essential reaction that takes place in a blast furnace is one of reduction. The iron oxide is reduced to metallic iron by a series of reactions involving carbon monoxide (CO) and carbon (C), which is added to the furnace in the form of coke. The coke, burning fiercely in the air blasted into the furnace, also provides the heat to melt the charge. Limestone is introduced into the furnace to act as a flux. It combines with impurities associated with the iron ore, and enables them to melt at a lower temperature then they would otherwise, to form a molten slag. This floats on top of the molten iron. The furnace is tapped periodically to allow the molten metal and slag to be removed.
The iron produced by the blast furnace is often carried in huge travelling ladles to an adjacent steel-making plant. Or it can be poured into moulds called pigs, from which it gets its name, pig iron. Pig iron is by no means pure; it contains up to 4 % carbon and smaller amounts of other elements, which may include manganese (Mn), phosphorus (P), silicon and sulphur (S).
With slight refining, however, pig iron becomes suitable for making iron castings. At the foundry pig iron is remelted in a cupola, which is a kind of miniature blast furnace. iron scrap and other metals may be added to the pig iron to enhance its properties. The iron produced, known as cast iron, is very fluid when molten and is thus easy to cast. The most common cast iron, called grey cast iron, is strong, easy to machine and resistant to shock, but it brittle.
Another form of iron, sometimes produced from pig iron, is wrought iron. It consists of almost pure iron mixed with threads of slag. It is made by remelting pig iron in a furnace with iron ore. Impurities in the iron combine with the iron ore to form a slag. The temperature is raised, and the carbon in the iron reacts with the iron ore. The resulting pasty iron-slag mix is then hammered or pressed to squeeze out excess slag. Although not very used today, wrought iron has excellent properties. It is tough and it is highly ductile and malleable; it is resistant to shock and more resistant to corrosion than ordinary iron and steel.
Steel was produced cheaply and in large quantities for the first time in 1856 when Henry Bessemer invented the steel-refining process that now bears his name. In the process, air was blown through pig iron to remove impurities. Today most steel refining is done in an open-hearth furnace. Pig iron, iron ore, steel scrap, and limestone are heated on a shallow hearth by the furnace flames. The flames burn out the excess carbon, while other impurities pass into the lime slag.
Another, more efficient type of furnace is the basic-oxygen furnace. The basic-oxygen furnace is shaped somewhat like the drum of a concrete mixer. it is filled with steel scrap and molten pig iron, on to which pure oxygen is blown at very high speed. Carbon and other impurities are burnt out. Limestone is added during the “blow” and combines with some impurities to form a slag. After some time the converter is tilted on its side and the steel is poured off.
The electric-arc furnace is fed a charge of steel scrap rather than hot metal. Three carbon electrodes are lowered into the furnace, and an electric ark is struck between the electrodes and the charge. This gives out great heat, which causes the charge to melt. Limestone, flourspar, and iron ore are added to the molten metal to absorb impurities. The electric-arc furnace can produce steel of higher quality than other processes because the metal is not contaminated by combustion products.
Depending on the amount of carbon and other alloys, steel can be classified as :
|Type of steel||Typical composition [wt%]||Typical use|
|Low-carbon (“mild”) steel||Fe + (0.04 % … 0.3 %) C + (~ 0.8 %) Mn||Low-stress uses, general construction steel, suitable for welding.|
|Medium-carbon steel||Fe + (0.3 % … 0.7 %) C||Medium-stress uses, machinery parts, nuts and bolts, shafts, gears.|
|High-carbon steel||Fe + (0.7 % … 1.7 %) C + (~ 0.8 %) Mn||High-stress uses: springs, cutting tools, dies.|
|Low-alloy steel||Fe + 0.2 % C + 0.8 % Mn + 1 % Cr + 2 % Ni||High-stress uses: pressure vessels, aircraft parts.|
|High-alloy (“stainless”) steel||Fe + 0.1 % C + 0.5 % Mn + 18 % Cr + 8 % Ni||High temperature or anti-corrosion uses: chemical or steam plants|
|Cast iron||Fe + (1.8 % … 4 %) C + (~ 0.8 %) Mn + (~ 2 %) Si||Low-stress uses: cylinder blocks for internal combustion engines, drain pipes.|
where: Fe – Iron, C – Carbon, Mn – Manganese, Cr – Chromium, Ni – Nickel. Si – Silicon
Steel is basically made of Iron and Carbon, where the Carbon content is less than ~2 wt%. If the Carbon content exceeds 2 wt% then the alloy is called cast iron.
Summary table of differences between iron and steel
|Where is it found||Iron is found in nature in forms of ores, in the Earth’s crust||steel is man-made, by mixing Iron with Carbon and other metals|
|Properties||greyish-white metal that is highly ductile and malleable||metal of high tensile strength, which can be made even tougher and harder with heat treatment, resistant to corrosion|
|What other primary elements does it contain||Ony Iron||Iron + Carbon + other metals (Manganese Chromium, Nickel, etc.)|
 Michael F. Ashby, David R. H. Jones, Engineering Materials 2 – An Introduction to Microstructures, Processing and Design, 2nd Edition, Butterworth-Heinemann, 1998.