Ferrous Metal is one of the base metals added by Thermal Foundation. It can be obtained from Ferrous Ore and, less commonly, Iron Ore.
Ferrous Ore and Iron Ore can also be processed in various ways in an Induction Smelter, with varying byproducts.
Like most base metals in the mod, Ferrous Metal can be used to craft tools and armor. Other than that, it does not serve much purpose, besides being a critical component in creating the much more useful alloy Invar.
The reason the metal is not called Nickel is because it is not part of the seven metals of antiquity. In the setting of the game, the modern name of the metal wouldn't have been defined yet. This results in the player calling the metal Ferrous Metal; a name that describes the metal rather than the modern scientific name.