- Ferrous (Iron / Steel): Steel Making
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.
to complete a steel making "heat" in a BOF are as follows:
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.
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
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.
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.