Carbon structural steel is carbon steel with carbon content less than 0.8%, which contains less sulfur, phosphorus and non-metallic inclusions than carbon structural steel, and has better mechanical properties. Carbon structural steels can be divided into three types according to their carbon content: low carbon steel (C≤0.25%), medium carbon steel (C 0.25-0.6%) and high carbon steel (C>0.6%). According to different manganese content, carbon structural steels can be divided into two groups: normal manganese content (0.25%-0.8%) and high manganese content (0.70%-1.20%). The latter has better mechanical properties and machining performance.