Glossary
Basic oxygen steel making (BOS)
Electric arc furnace (EAF) steel making
Hot-rolled structural sections
‘Hot metal’ from the blast furnace is transferred to a BOS vessel to which scrap, lime and other fluxes are added. Oxygen is blown into the hot metal through a water-cooled oxygen lance which oxidises and removes most of the carbon from the metal.
Other unwanted elements are converted to acidic oxides that combine with the fluxes to produce slag that floats to the surface of the hot metal.
The steel is tapped from the furnace when it is at the correct temperature and composition.
A Bessemer converter is a large ovoid steel container lined with clay or dolomite in which molten iron is converted into steel.
The bottom is of the vessel is perforated with a number of channels through which air is forced.
The oxidation process removes impurities such as silicon, manganese, and carbon as oxides, these oxides either escape as gas or form a solid slag.
The Bessemer converter revolutionised steel production enabling far greater quantities of iron to be converted into steel much quicker than conventional techniques.
A blast furnace is a large steel structure about 30 metres high which is lined with refractory firebricks that can withstand temperatures approaching 2000oC. Pre-heated air at about 1000oC is blasted into the furnace through nozzles near its base as raw materials (coke and iron ore) are constantly charged into the top of the furnace.
The hot air blast to the furnace burns the coke and maintains the very high temperatures that are needed to reduce the ore to iron.
Iron making in the furnace usually continues for about ten years before the furnace linings have to be renewed.
In the EAF process, steel scrap (or other ferrous material) is first tipped into the furnace from an overhead crane. A lid is then swung into position over the furnace. This lid contains electrodes which are lowered into the furnace. An electric current is passed through the electrodes to form an arc. The heat generated by this arc melts the scrap.
During the melting process, other metals (ferro-alloys) can be added to the steel to give it the required chemical composition. As with the basic oxygen process, oxygen is blown in to the furnace to purify the steel and lime and fluorspar are added to combine with the impurities and form slag.
Structural sections or ‘long products’ are produced from semi-finished products (blooms or billets which), after heating to around 1200C, are passed through a succession of stands which reduce the size of the steel and simultaneously change its shape. In a universal mill, all faces of the piece of steel are rolled at the same time.
Sheet steel, normally called hot rolled coil, is produced from semi-finished slabs that are continuously passed through a series of finishing stands which progressively squeeze the steel to make it thinner. As the steel becomes thinner, it becomes longer, and starts moving faster. By the time it reaches the end of the mill, the steel is travelling at about 40 miles per hour. Finally the long strip of steel is coiled and allowed to cool. Hot rolled coil is used for many different applications including construction.
The construction industry is the largest market sector for steel worldwide. Around a quarter of all steel consumed in the UK goes into construction. This equates to around 3 million tonnes per year which is made up of approximately 1 million tonnes each of hot-rolled structural sections, sheet or strip steel products and bars and mesh used in reinforced concrete.
Advances in modern production and fabrication technologies and supported by evermore efficient design guidance, makes steel the material of choice in many construction applications. By way of example if the Eiffel Tower were to be rebuilt today using modern design codes it would only require around one third of the steel it was built with in the 1880s.
The economic and other sustainability advantages of steel are widely recognised by clients and designers alike. This is borne out by the market share that structural steel commands; for example steel has a 70% share in multi-storey buildings and 95% share in single-storey industrial buildings.
