This is the microstructure of a high carbon steel. It contains about 0.8% C by weight, alloyed with iron. The steel has one major constituent, which is pearlite.
It is made up from a fine mixture of ferrite and iron carbide, which can be seen as a "wormy" texture.
The pearlite has a very fine structure, which makes the steel very hard. Unfortunately this also makes the steel quite brittle and much less ductile than mild steel.
The high carbon steel has good wear resistance, and until recently was used for railways. It is also used for cutting tools, such as chisels and high strength wires. These applications require a much finer microstructure, which improves the toughness.
You should compare this steel with medium and low carbon steels.
You can see similar alloys and images of this microstructure at other magnifications in Categories: - Metals.