Plain hi-carbon or hi-tenstle steel.  Used on low end bicycle frames since the earliest of days.

1020 is one of the very commonly used plain carbon steels. It has a nominal carbon content of 0.20% with approximately 0.50% manganese. It is a good combination of strength and ductility and may be hardened or carburized.  Other applications include cold headed bolts.  Heavy, thick tubing material.