Processes - Ferrous (Iron / Steel): Steel Making

BOF
The Basic Oxygen Furnace (BOF) steel making process converts molten iron from the blast furnace -- with about 10 to 40% steel scrap -- into refined steel. High-purity oxygen (usually vaporized liquid oxygen) is blown through the molten bath to react with and lower carbon, silicon, manganese, and phosphorous content of the iron, while various fluxes are used to reduce the sulfur and phosphorous levels. The impurities and a small amount of oxidized iron are carried off in the molten slag that floats on the surface of the hot metal. Some experimentation has occurred with natural gas bubbling in a BOF as well. Additionally, some BOF shops utilize (or could utilize) scrap preheating to increase yield.

Steps to complete a steel making "heat" in a BOF are as follows:

1. Scrap Charging
The first step for making a heat of steel in a BOF is to tilt the furnace and charge it with scrap. The furnaces can be rotated or tilted to accept scrap.

2. Molten Iron Charging
Hot metal from the blast furnace accounts for up to 80% of the metallic charge and is poured from a ladle into the top of the tilted furnace (see photo at right).

3. Furnace Operation
The charged furnace is returned to an upright position and a water-cooled oxygen lance is lowered from the top. Oxygen is blown into the bath at supersonic speeds causing rapid mixing and heat from the oxidation of iron and impurities. Fluxes (burnt lime, burnt dolomite, and fluorspar) are added to help carry off the impurities in the floating slag layer. This step requires about 15 minutes of an overall 45 minute "tap-to-tap" cycle time. The illustration shows how this process is completed.

4. Tapping
After the steel has been refined, a sample is taken to determine temperature and composition analysis. The furnace is tilted (opposite to the charging side) and molten steel is poured out into a preheated ladle. Alloys are added to the ladle during the pour to give the steel the precise composition desired. In some steel making applications, further refining (or degassing) is conducted in the ladle to remove oxygen and sulfur from the molten steel.