The main nonlinear feature of ferromagnetic materials is that the B field saturates at a certain value, which is around 1.6 to 2 teslas (T) for most high permeability core steels. The B field increases quickly with increasing current up to that value, but above that value the field levels off and becomes almost constant, regardless of how much current is sent through the windings. So the maximum strength of the magnetic field possible from an iron core electromagnet is limited to around 1.6 to 2 T.