What is the difference between pig iron and cast iron?

Pig iron is cast iron with a very high carbon content: 4% by weight or more. Cast iron has at least 2% carbon by weight. Less than that, and it's steel. Cast iron and pig iron are not called steel, despite being iron carbides, because they lack the structural strength of steel and are extremely brittle. In terms of the microstructure, cast iron and pig iron contain no cementite, austenite, or martensite.

Historically, "pig iron" refers to cast iron made by a particular process: the high-carbon molten iron made in a blast furnace was poured into moulds made of sand, which had a particular shape. Narrow trenches would run down the edge of the mould, and then branching off of them there would be the actual ingots. The whole setup looked to the ironworkers like a sow suckling piglets, so the product became known as pig iron.