Cast iron is made by remelting pig iron, often along with substantial quantities of scrap iron, and taking various steps to remove undesirable contaminants such as silica, phosphorous and sulfur, which weaken the material. Carbon content is carefully reduced to the desired level, which may be anywhere from 2% to 3.5% depending on the application.
The salient feature of its manufacture is that the mix of about 3.5% carbon with iron makes an alloy that has a minimum melting point (ca 1420-1470 K), and therefore can be created at temperatures at least 300 K lower than the melting point of pure iron.