This odd name is given to iron that has just come from the blast furnace, before it goes through any of the refining and hardening processes that transform the metal into steel. In this state, it is very crude iron, only one step removed from the earth from which it was taken as ore. (See photo.)
The term ‘pig’ is an old expression in the industry and comes from the sand molds into which the streaming, molten iron is poured. As the liquid iron is poured from the furnace, it runs into a central trough or runner and then into small molds arranged on each side of the trough. The central runner is called the sow and the whole is thought to resemble a family of pigs. Hence the name.
Pig iron (Wikipedia)