Basic oxygen steelmaking

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Basic oxygen steelmaking (BOS, Linz-Donawitz-Verfahren, LD-converter) is a method of converting molten iron to 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:

1. Molten iron from a blast furnace is poured into a large container called a ladle
2. A lance is lowered into molten iron and several hundred kilograms of powdered magnesium are then added to the molten iron in the ladle. Sulphur impurities are reduced by the magnesium in a violent exothermic reaction to magnesium sulphide, which is then raked off.
3. The BOS vessel is one-fifth filled with steel scrap. Molten iron from the ladle is added until the vessel is full. Filling the furnace with the ingredients is called charging.
4. The vessel is then stood upright and a lance is lowered down into it. The lance blows 99 % pure oxygen onto the steel and iron, 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 the use of oxygen instead of air that makes for the improvement on the Bessemer process, for the nitrogen (and other gases) contained in air does not react with the charge as oxygen does.
5. Fluxes (burnt lime or dolomite) are fed into the vessel to form slag which absorbs impurities of the steelmaking process. Near the end of the blowing cycle, which takes about 20 minutes, a temperature reading and samples are taken. The samples are tested and a computer analysis of the steel given within six minutes.
6. The BOS vessel is tilted again and the steel is poured into a giant ladle. This process is called tapping the steel. In the ladle furnace, the steel is further refined by adding alloying materials which 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.
7. 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 VOEST ALPINE 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 the previously common Siemens-Martin process, also known as the open-hearth process.de:Linz-Donawitz-Verfahren ja:転炉