Basic oxygen steelmaking (BOS, BOF, Linz-Donawitz-Verfahren, LD-converter) is a method of steelmaking in which carbon-rich molten iron is made into steel. The process is an improvement over the historically important Bessemer process. The LD-converter is named after the Austrian placenames Linz and Donawitz (a district of Leoben).
By blowing oxygen through molten pig iron, the carbon content of the alloy is lowered and changes the material into low-carbon steel.
A typical BOS vessel holds about 280 tonnes of steel. The vessel is lined with heat-resistant refractory bricks that can withstand the high temperature of molten metal.
The basic oxygen steel-making process is as follows:
- Molten iron from a blast furnace is poured into a large refractory-lined container called a ladle;
- The metal in the ladle is sent directly for basic oxygen steelmaking or to a pretreatment stage. Pretreatment of the blast furnace metal is used to reduce the refining load of sulfur, silicon, and phosphorus. In desulfurising pre treatment, a lance is lowered into the molten iron in the ladle and several hundred kilograms of powdered magnesium are added. Sulfur impurities are reduced to magnesium sulfide in a violent exothermic reaction. The sulfide is then raked off. Similar pretreatment is possible for desiliconisation and dephosphorisation using mill scale(iron oxide) and lime as reagents. The decision to pretreat depends on the quality of the blast furnace metal and the required final quality of the BOS steel.
- Filling the furnace with the ingredients is called charging. The BOS process is autogenous: the required thermal energy is produced during the process. Maintaining the proper charge balance, the ratio of hotmetal to scrap, is therefore very important. The BOS vessel is one-fifth filled with steel scrap. Molten iron from the ladle is added as required by the charge balance. A typical chemistry of hotmetal charged into the BOS vessel is: 4% C, 0.2-0.8%Si, 0.08%-0.18%P, and 0.01-0.04%S.
- The vessel is then set upright and a water-cooled lance is lowered down into it. The lance blows 99% pure oxygen onto the steel and iron, igniting the carbon dissolved in the steel and burning it to form carbon monoxide and carbon dioxide, causing the temperature to rise to about 1700 °C. This melts the scrap, lowers the carbon content of the molten iron and helps remove unwanted chemical elements. It is this use of oxygen instead of air that improves upon on the Bessemer process, for the nitrogen (and other gases) in air do not react with the charge as oxygen does.
- Fluxes (burnt lime or dolomite) are fed into the vessel to form slag which absorbs impurities of the steelmaking process. During blowing the metal in the vessel forms an emulsion with the slag, facilitating the refining process. Near the end of the blowing cycle, which takes about 20 minutes, the temperature is measured and samples are taken. The samples are tested and a computer analysis of the steel given within six minutes. A typical chemistry of the blown metal is 0.3-0.6%C, 0.05-0.1%Mn, .01-0.03%Si, 0.01-0.03%S and P.
- The BOS vessel is tilted again and the steel is poured into a giant ladle. This process is called tapping the steel. The steel is further refined in the ladle furnace, by adding alloying materials to give the steel special properties required by the customer. Sometimes argon or nitrogen gas is bubbled into the ladle to make sure the alloys mix correctly. The steel now contains 0.1-1% carbon. The more carbon in the steel, the harder it is, but it is also more brittle and less flexible.
- After the steel is removed from the BOS vessel, the slag, filled with impurities, is poured off and cooled.
The first basic oxygen steelmaking process was the LD process developed in 1952 by voestalpine AG in Linz, Austria. Some major steelmaking companies in the US did not convert to this process for decades, with the last Bessemer converter still operating commercially until 1968.
The LD process replaced both the previously common Siemens-Martin process, also known as the open-hearth process, and the Bessemer process.