The low-carbon steels are those numbered AISI 1005 to 1030, medium-carbon from 1035 to 1055, and high-carbon from 1060 to 1095. The AISI 11xx series adds sulphur, principally to improve machinability. These are called free-machining steels and are not considered alloy steels as the sulphur does not improve the mechanical properties and also makes it brittle. The ultimate tensile strength of plain carbon steel can vary from about 60 to 150 kpsi.

SAE 1070-1090 high carbon blue tempered and polished spring steel is a standard material for conventional springs. It is the lowest cost material and best suited for applications that have a protected environment, as carbon steel corrodes if not lubricated or atmospherically sealed. Additional corrosion protection can be added with a special finish. Products are supplied with an oil dip finish providing adequate protection for shipment and shelf storage. Carbon steel is highly magnetic and is typically blue in color.