Several ferro alloys can be added to molten steel to improve its strength. Some of the most commonly used ones are:

Ferrochrome: Ferrochrome is added to steel to increase its corrosion resistance, tensile strength, and hardness. It also improves the steel’s resistance to wear and tear.

Ferromanganese: Ferromanganese is added to steel to improve its strength and toughness. It also helps to remove sulfur from the steel, which can improve its ductility and weldability. 

Ferrosilicon: Ferrosilicon is added to steel to increase its strength and reduce its ductility. It is also used to deoxidize the steel and to remove impurities such as sulfur.

Ferrovanadium: Ferrovanadium is added to steel to increase its strength and toughness. It also improves the steel’s resistance to wear and tear and its ability to withstand high temperatures.

Ferromolybdenum: Ferromolybdenum is added to steel to improve its strength and toughness. It also improves the steel’s resistance to corrosion and its ability to withstand high temperatures.

Advantages of adding ferro alloys to molten steel include:
Improved strength, toughness, and hardness of the steel.
Improved resistance to corrosion, wear and tear, and high temperatures.
Removal of impurities from the steel, which can improve its ductility and weldability.

Disadvantages of adding ferro alloys to molten steel include:
Increased cost due to the addition of expensive alloys.
Possibility of over-alloying, which can lead to problems such as cracking or brittleness in the steel.
Possibility of introducing new impurities, which can negatively affect the properties of the steel.