Billet steel is stronger than cast steel for a number of reasons. First, billet steel has a higher carbon content than cast steel, which gives it greater strength. Second, billet steel is hot worked, meaning that it is heated to high temperatures and then cooled rapidly. This process alters the microstructure of the metal, making it stronger. Finally, billet steel typically undergoes further processing steps such as forging or rolling, which also add to its strength.