Steel

Steel is an alloy composed of between 0.2 and 2.0 percent carbon, with the balance being iron.

• From prehistory through the creation of the blast furnace, iron was produced from iron ore as wrought iron, 99.82 - 100 percent Fe
  • The process of making steel then involved adding carbon to iron, usually in a serendipitous manner, in the forge, or via the cementation process.

• The introduction of the blast furnace reversed the problem. A blast furnace produces pig iron — an alloy of approximately 90 percent iron and 10 percent carbon.
  • When the process of steel-making is started with pig iron, instead of wrought iron, the challenge is to remove a sufficient amount of carbon to reduce it to the 0.2 to 2 percentage for steel.

Bloomery

Until the end of the 18th century, wrought iron was smelted from ore using charcoal, by the bloomery process.

The bloomery is preheated by burning charcoal, and once hot, iron ore and additional charcoal are introduced through the top, in a roughly one-to-one ratio. Inside the furnace, carbon monoxide from the incomplete combustion of the charcoal reduces the iron oxides in the ore to metallic iron, without melting the ore.

The small particles of iron produced in this way fall to the bottom of the furnace, where they combine with molten slag, often consisting of fayalite, a compound of silicon, oxygen and iron mixed with other impurities from the ore. The mixed iron and slag cool to form a spongy mass referred to as the bloom. Because the bloom is highly porous, and its open spaces are full of slag, the bloom must later be reheated and beaten with a hammer to drive the molten slag out of it. Iron treated this way is said to be wrought (worked), and the resulting iron, with reduced amounts of slag is called wrought iron or bar iron.

Blast furnace

In a blast furnace, fuel (coke), ores, and flux (limestone) are continuously supplied through the top of the furnace, while a hot blast of air (sometimes with oxygen enrichment) is blown into the lower section of the furnace through a series of pipes called tuyeres, so that the chemical reactions take place throughout the furnace as the material falls downward. The end products are molten metal and slag phases tapped from the bottom.

Before about 1860, steel was an expensive product, made in small quantities and used mostly for swords, tools and cutlery; all large metal structures were made of wrought or cast iron.

Bessemer process

The introduction of cheap steel was due to the Bessemer and the open hearth processes, two technological advances made in England. In the Bessemer process, molten pig iron is converted to steel by blowing air through it after it was removed from the furnace. The air blast burned the carbon and silicon out of the pig iron, releasing heat and causing the temperature of the molten metal to rise. Henry Bessemer demonstrated the process in 1856 and had a successful operation going by 1864.

• By 1870 Bessemer steel was widely used for ship plate. By the 1850s, the speed, weight, and quantity of railway traffic was limited by the strength of the wrought iron rails in use. The solution was to turn to steel rails, which the Bessemer process made competitive in price. Experience quickly proved steel had much greater strength and durability and could handle the increasingly heavy and faster engines and cars.

Open hearth

After 1890 the Bessemer process was gradually supplanted by open-hearth steelmaking.

The open hearth process is a batch process and a batch is called a "heat". The furnace is charged with light scrap, such as sheet metal, shredded vehicles or waste metal. The furnace is heated using burning gas. Once the charge has melted, heavy scrap, such as building, construction or steel milling scrap is added, together with pig iron from blast furnaces.

Once all the steel has melted, slag forming agents, such as limestone, are added. The oxygen in iron oxide and other impurities decarburize the pig iron by burning excess carbon away, forming steel. To increase the oxygen contents of the heat, iron ore can be added.

The regenerators are the distinctive feature of the furnace and consist of fire-brick flues filled with bricks set on edge and arranged in such a way as to have a great number of small passages between them. The bricks absorb most of the heat from the outgoing waste gases and return it later to the incoming cold gases for combustion.

The process is far slower than that of Bessemer converter and thus easier to control. This was an advantage in the early 20th century, as it gave plant chemists time to analyze the steel and decide how much longer to refine it. But by about 1975, electronic instruments such as atomic absorption spectrophotometers had made analysis of the steel much easier and faster.

Electric arc furnace

The crucible process remained important for making high-quality alloy steel into the 20th century. By 1900 the electric arc furnace was adapted to steelmaking and by the 1920s, the falling cost of electricity allowed it to largely supplant the crucible process for specialty steels.

Basic oxygen steelmaking

Basic oxygen steelmaking (BOS, BOP, BOF, or OSM), also known as Linz–Donawitz-steelmaking or the oxygen converter process is a method of primary steelmaking in which carbon-rich molten pig iron is made into steel. Blowing pure oxygen through molten pig iron lowers the carbon content of the alloy and changes it into low-carbon steel. The process is known as basic because fluxes of burnt lime or dolomite, which are chemical bases, are added to promote the removal of impurities and protect the lining of the converter.

The process reduced capital cost of the plants, time of smelting, and increased labor productivity. Between 1920 and 2000, labor requirements in the industry decreased by a factor of 1,000, from more than three man-hours per metric ton to just 0.003. The majority of steel manufactured in the world is produced using the basic oxygen furnace. In 2000, it accounted for 60% of global steel output.

Modern furnaces will take a charge of iron of up to 400 tons and convert it into steel in less than 40 minutes, compared to 10–12 hours in an open hearth furnace.

Steel is the main material used for wire ropes.