The charge in the furnace is made up of molten pig iron, followed by scrap steel and limestone to act as a fluxing agent. The heating medium is produced by using large volumes of natural gas mixed with air (for complete combustion). The melt will usually take somewhere around 10 hours to complete and be ready to pour.
When the heat is complete, a plug is removed from the base of the furnace (very much like a drain plug in a kitchen sink) and will be directed by channels to a ladle to accept the liquid steel. There will always be slag on top of the molten steel. This is skimmed off and loaded into what is known as a “slag thimble.”
Now the steel is ready to be alloyed and poured into ingot molds or into ladles for transportation to the next stage of steelmaking. When the freshly cast steel is poured into the ingot molds, it begins to solidify into the shape of the mold. When the freshly poured steel begins to cool, it will be stripped of the mold at a temperature of approximately 1100°F. Remember that it is only the outside/external walls of the new ingot that is at the temperature of 1100°F.
If one looks at the molecular structure of the cast billet, the external surfaces will be in the ferrite condition with a structure of BCC (Body Centered Cubic lattice), while the internal molecular structure of the billet will be at the austenite phase (or cooling down very slowly from the austenite phase in relation to the external surfaces, which is Face Centered Cubic lattice). So, there will be a volumetric difference between the core and the external surfaces of the billet.
Once the ingot/billet has cooled down, it can be used by reheating it and then rolling it to the appropriate cross section of plate, I beams, hot-rolled bar or to whatever shape is desired.