Great question, your son perhaps has the makings of a future materials scientist :-) Vessels made to contain liquid iron or steel are usually made of steel, but are lined with refractory bricks, made of sintered sand. The bricks obviously have a much higher 'melting point' than the iron or steel and are made not by casting but by a process called sintering. Sand, carefully chosen to have the right high temperature properties, and chemical properties (so it doesn't dissolve in the liquid iron) is formed up into a brick and held for a lengthy period at high temperature. This does not actually melt the sand grains, but makes them bond together in the solid state.
The next thing your son might like to ponder is that when iron is refined into steel in the steelmaking process, impurities are extracted as metal oxides which normally float as slag on the top of the steel. A major factor in steel making is to make sure the brew is hot enough to keep the slag liquid, as this usually has a higher melting point that the metal itself.
