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.