Iron and steel are among the most widely-used engineering materials. It is our common knowledge that iron and steel have magnetism and their magnetization varies significantly. Steel is mainly an alloy of iron and carbon, which may contain small amounts of other alloying elements. This paper gives a brief introduction and performs some preliminary discussions on the magnetism of iron-carbon alloys with different carbon content or metallographic structures, mainly based on the iron-carbon binary phase diagram. There are three ferromagnetic components in steel: ferrite, cementite and martensite. The latter is a metastable phase which does not appear in the phase diagram. This paper can serve as a teaching case for learning and application of the phase diagram.