Steel is an alloy of iron with carbon and a variety of different elements, mainly metals. An alloy is a mixture of a metal with one or more other elements. The presence of other elements in a metal changes its properties and can often increase its strength. Brass is an alloy of copper and zinc and is stronger than either pure metal.
The properties of steel can be altered in a number of ways:
Nearly all new steel is made from impure iron made in the Blast Furnace. Iron is found as the ores, hæmatite, Fe2O3 and magnetite, Fe3O4. In the blast furnace iron ore is mixed with coke, limestone and air at a temperature of up to 2000K. The coke reacts with oxygen to produce carbon dioxide.
The carbon dioxide reacts with more coke to form carbon monoxide.
Finally, the carbon monoxide reduces the iron oxide to iron.
The limestone is there to remove the impurities as slag.
Steelmaking in the UK is made by the Basic Oxygen Steelmaking (BOS) process. In this process batches of about 300 tonnes of high-quality steel are produced in 40 minutes. Iron straight from the blast furnace contains impurities, mainly non-metals which make it too brittle for most uses. It contains about 4.4% carbon with some silicon, manganese, phosphorus and sulphur.
Sulphur is first removed by reacting the iron with magnesium, forming magnesium sulphide which floats on the surface and is raked off.
Carbon, manganese, silicon and phosphorus are removed by reaction with oxygen.
These oxides of silicon and phosphorus are acidic and are now reacted with a base, a mixture of calcium oxide and magnesium oxide. These products form a slag with manganese oxide and some iron oxide and is removed. Excess oxygen dissolves in the metal during the oxygen blow. This is removed by adding ingots of aluminium which form aluminium oxide that floats to the surface.
The composition of the steel is now adjusted to the required specification by addition of other elements such as manganese, carbon, nickel chromium, molybdenum, cobalt and tungsten. The composition can be quickly determined whilst in the furnace by atomic emission spectroscopy.
Many alloy steels are made in an electric arc furnace where steel scrap is recycled. The scrap is melted by the heat of an electric arc struck from the steel to carbon electrodes. Precise quantities of the other elements can be added to the molten scrap.
It is believed that steel made in this way has been responsible for guage corner cracking on railway lines as the steel contains abnormally large quantities of copper. This is still under in-depth investigation, however.
Rusting is a special name for the corrosion of iron. Iron rusts when it reacts with oxygen and water in the atmosphere. Rusting is an electrochemical process. Electrochemical cells are set up in the metal surface, where different areas act as sites of oxidation and reduction. If a drop of water is left in contact with iron, the concentration of dissolved oxygen in the water drop determines the regions of oxidation and reduction.
At the edges of the drop, where the oxygen concentration is higher, oxygen is reduced to hydroxide ions.
Where the concentration of dissolved oxygen is low, in the centre of the drop, iron passes into solution.
The electrons released flow in the metal surface to the edges of the drop. This explains why corrosion is greatest at the centre of a water drop.
Rust forms in a series of secondary processes within the solution, as Fe2+ and OH- ions diffuse away from the metal surface.
Further oxidation occurs producing a hydrated form of iron (III) oxide with variable composition, Fe2O3.xH2O. This oxide is permeable to air and water and does not form a protective layer on the metal surface.
The electrode potential for the oxygen half-reaction depends on pH; acid conditions accelerate rusting. Any ionic species, e.g. salt will promote rusting by increasing the conductivity of the water.
The simplest way of protecting iron and steel is to provide a barrier between the metal and the atmosphere. This can be achieved by oil, grease, paint, wax, plastic. Sometimes iron is covered with another metal. This is often carried out by electrolysis. Coating with zinc is called galvanising. If the zinc coating is scratched to reveal the steel, the zinc corrodes in preference to the steel because it has a more negative electrode potential (E = -0.76V). This is known as sacrificial corrosion.
If steel is coated with tin and the tin is scratched to reveal the steel, the steel corrodes in preference to tin because tin has a less negative electrode potential to iron (E = -0.14 V).
Stainless steel is steel which contains chromium and sometimes nickel. A surface layer of chromium oxide, Cr2O3, is formed which adheres to the surface. It is only a few nanometres thick so is invisible to the naked eye. It is impervious to air and water and protects the metal beneath. When scratched the oxide is quickly reformed.
Scrap steel makes up about 18% of every cast of 'new' steel. The electric arc process uses only scrap. Scrap from discarded products has to be carefully graded. Some elements in steel scrap can improve the properties of steel. Other elements, e.g. tin and copper can cause problems.
In tin cans, the tin has to be removed first. This can be achieved by dissolving the tin with hot sodium hydroxide solution in the presence of an oxidising agent such as sodium nitrate.
Part of this site was last updated on 21st January 2009.
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