The Basic Oxygen Furnace
This type of steelmaking furnace is sometimes referred to as a BOF (Basic Oxygen Furnace), and it is one of the principle methods for making steel.

BOF furnaces are generally smaller than the open-hearth (O-H) furnace with a maximum capacity of 350 tons per hour. Steelmaking in this type of furnace is very fast compared to the O-H furnace, and the BOF has generally replaced the O-H.

The furnace is simply a refractory-lined, barrel-shaped vessel with two spigots on the side that will allow the furnace to pivot through approximately 120° when ready to pour.

The steelmaking procedure starts with liquid molten iron plus approximately 120 tons of scrap steel. This steelmaking process was the first to recycle scrap steel. Once the basic additions are made (scrap and molten iron), an oxygen probe is lowered into the top of the furnace and positioned just above the now liquid metal in the furnace.

An oxygen supply is made to the probe and a stream of high-pressure oxygen is blown into the furnace. The oxygen stream will assist in raising the temperature of the molten basic steel. The temperature of the liquid steel will increase to 3100-3200°F. As the liquid-metal temperature is rising, the action of the addition of oxygen to just above the liquid steel level causes a massive chemical reaction to occur. This results in the agitation of the liquid, ensuring a reasonable movement (and complete mixing) of the bath within the furnace.

Once the cycle time is completed and the liquid metal is thoroughly agitated, the oxygen supply is turned off and the lance removed. Once the lance is removed, the basic steel liquid mixture is then poured into another refractory-lined ladle. Great care is taken to ensure that the surface slag from the melt procedure in the BOF is not poured into the same ladle. The slag is comprised of impurities that have been released from the steel. These impurities float to the liquid-metal surface.

It is at this point, and after the slag has been skimmed off the surface of the liquid steel, that alloying elements are added according to the steel chemistry being made. The new steel chemistry (from the addition of carbon plus alloying elements) is now ready to be poured into an ingot mold. Remember, that as the steel is cooling down in the mold, the cooling rate is not uniform. For example, the top-exposed surface of the ingot mold will cool fastest, followed by the base of the mold, followed by the sides and eventually down to the center of the ingot.

This means that as the ingot is cooling, many different microstructures develop, each with a different volumetric size. If the cooling rate (after casting into the mold or post-heat treatment of the ingot) is not cooled slowly enough, there will most likely be stress raisers in the ingot.