The growth of railroads during the 19th century in both Europe and America put great pressure on the iron industry, which still struggled with inefficient production processes. Yet steel was still unproven as a structural metal and production was slow and costly. That was until 1856, when Henry Bessemer came up with a more effective way to introduce oxygen into molten iron in order to reduce the carbon content.
Now known as the Bessemer Process, Bessemer designed a pear-shaped receptacle - refered to as a 'converter' - in which iron could be heated while oxygen could be blown through the molten metal. As oxygen passed through the molten metal, it would react with the carbon, realasing carbon dioxide and producing a more pure iron.
The process was fast and inexpensive, removing carbon and silicon from iron in a matter of minutes but suffered from being too successful. Too much carbon was removed and too much oxygen remained in the final product. Bessemer ultimately had to repay his investors until he could find a method to increase the carbon content and remove the unwanted oxygen.
At about the same time, British metallurgist Robert Mushet acquired and began testing a compound of iron, carbon and manganese - known as speigeleisen. Manganese was known to remove oxygen from molten iron and the carbon content in the speigeleisen, if added in the right quantities, would provide the solution to Bessemer's problems. Bessemer began adding it to his conversion process with great success.
Yet, one problem still remained. Bessemer had failed to find a way to remove phosphorus - a deleterious impurity that makes steel brittle - from his end product. Consequently, only phosphorus-free ores from Sweden and Wales could be used.
In 1876 Welshman Sidney Gilchrist Thomas came up with the solution by adding a chemically basic flux - limestone - to the Bessemer process. The limestone drew phosphorus from the pig iron into the slag, allowing the unwanted element to be removed.
This innovation meant that, finally, iron ore from anywhere in the world could be used to make steel. Not surprisingly, steel production costs began decreasing significantly. Prices for steel rail dropped more than 80% between 1867 and 1884, as a result of the new steel producing techniques, initiating growth of the world steel industry.
The Open Hearth Process:
In the 1860s German engineer Karl Wilhelm Siemens further enhanced steel production through his creation of the open hearth process. The open hearth process produced steel from pig iron in large shallow furnaces.
Using high temperatures to burn off excess carbon and other impuriites, the process relied on heated brick chambers below the hearth. Regenerative furnaces later used exhaust gases from the furnace to maintain high temperatures in the brick chambers below.
This method allowed for the production of much larger quantities (50-100 metric tons could be produced in one furnace), periodic testing of the molten steel so that it could be made to meet particular specifications and the use of scrap steel as a raw material. Although the process itself was much slower, by 1900 the open hearth process had largely replaced the Bessemer process.
Birth of the Steel Industry:
The revolution in steel production that provided cheaper, higher quality material, was recognized by many businessmen of the day as an investment opportunity. Capitalists of the late 19th century, including Andrew Carnegie and Charles Schwab, invested and made millions (billions in the case of Carnegie) in the steel industry. Carnegie's US Steel Corporation, founded in 1901, was the first corporation ever launched valued at over one billion dollars.
Electric Arc Furnace Steelmaking
Just after the turn of the century, another development occurred that would have a strong influence on the evolution of steel production. Paul Heroult's electric aric furnace (EAF) was designed to pass an electric current through charged material, resulting in exothermic oxidation and temperatures up to 3272°F (1800°C), more than sufficient to heat steel production.
Initially used for specialty steels, EAFs grew in use and, by World War II, were being used for the manufacturing of steel alloys. The low investment cost involved in setting up EAF mills allowed them to compete with the major US producers like US Steel Corp. and Bethlehem Steel, especially in carbon steels, or long products.
Because EAFs can produce steel from 100% scrap - or cold ferrous - feed, less energy per unit of production is needed. As opposed to basic oxygen hearths, operations can also be stopped and started with little associated cost. For these reasons, production via EAFs has been steadily increasing for over 50 years and now accounts for about 33% of global steel production.
The majority of global steel production - about 66% - is now produced in basic oxygen facilities. The development of a method to separate oxygen from nitrogen on an industrial scale in the 1960s allowed for major advances in the development of basic oxygen furnaces.
Basic oxygen furnaces blow oxygen into large quantities of molten iron and scrap steel and can complete a charge much more quickly than open hearth methods. Large vessels holding up to 350 metric tons of iron can complete conversion to steel in less than one hour.
The cost efficiencies of oxygen steelmaking made open hearth factories uncompetitive and, following the advent of oxygen steelmaking in the 1960s, open-hearth operations began closing. The last open-hearth facility in the US closed in 1992 and in China in 2001.
Sources:Spoerl, Joseph S. A Brief History of Iron and Steel Production. Saint Anselm College.
The World Steel Association. Website: www.steeluniversity.org
Street, Arthur. & Alexander, W. O. 1944. Metals in the Service of Man. 11th Edition (1998).