Billet is round or square stock of metal that is produced either by hot rolling of ingots, or continuous rolling or extrusion. Ingots can be produced by casting.
For a micro metal mill for producing metal products from scrap, the preferred route is an (1) induction furnace, followed by pouring (2) ingots, which are then (3) rolled into various shapes including rounds or flats. Further rolling via can produce all other types of profiles. Rods and wire can be produced as the material is pressed down further. Tubes can be produce from flat metal which is then rolled into a tube and welded. Seamless tubing is made by poking a hole through the middle of a bar while the metal is hot. .
A process where a piece of metal moves back and forth between rolls can be a means to pack a metal rolling operation into a tight area. The goal for OSE is that an 8000 square foot facility has the capacity to produce steel products from scrap metal, up to 1"x8" bars, 1/2"x16", or 1/4"x32" stock steel as the limit of an advanced universal microfactory.
- Wikipedia - 
- Video expainer of casting followed by rolling for stainless steel - 
- Billet-making in an Indian billet factory - YouTube - 
- Pennsylvania die and forge casting - 
- Semi-continuous casting - 
- Permanent mold casting (reusable mold) - disadvantage of short mold life for steel. 
- Ingot - just a hot pour - 
Production - Aluminum Casting
Continuous casting of billet -
Extrusion Ingot Casting
Extrusion Ingots are billets.
See  - bu the extrusion billets refer to non-ferrous metal.