Answer:
Radiation.
Step-by-step explanation:
convection: The movement of heat through liquids.
convection current: The motion of moving the convection's mass.
radiation: The transfer of heat in wave like movements through electromagnetic waves.
evaporation: When a substance reaches its boiling point and turns to a gas.
So, since the heated iron is not touching any sides of the metal box, but the sides are still getting hot, the iron is radiating throughout the box.