Round steel is divided into three types: hot-rolled, forged, and cold drawn. The specification of hot-rolled round steel is 5.5-250 millimeters. Among them, 5.5-25mm small round steel is mostly supplied in bundles of straight bars, commonly used as steel bars, bolts, and various mechanical parts; Round steel larger than 25 millimeters is mainly used for manufacturing mechanical parts, seamless steel pipe blanks, etc.

Carbon steel can be divided into low carbon steel, medium carbon steel, and high carbon steel according to its chemical composition (i.e. carbon content).

(1) Low carbon steel

Also known as soft steel, low-carbon steel with carbon content ranging from 0.10% to 0.30% is easy to accept various processing such as forging, welding, and cutting. It is commonly used in the manufacture of chains, rivets, bolts, shafts, etc.

(2) Medium carbon steel

Carbon steel with a carbon content of 0.25% to 0.60%. There are various products such as killed steel, semi killed steel, boiling steel, etc. In addition to carbon, it may also contain a small amount of manganese (0.70% to 1.20%). According to product quality, it is divided into ordinary carbon structural steel and high-quality carbon structural steel. Good thermal processing and cutting performance, but poor welding performance. Strength and hardness are higher than low carbon steel, while plasticity and toughness are lower than low carbon steel. Hot rolled and cold drawn materials can be used directly without heat treatment, or they can be used after heat treatment. The quenched and tempered medium carbon steel has good comprehensive mechanical properties. The highest achievable hardness is approximately HRC55 and HB538, with a σ b of 600-1100MPa. So among various applications with moderate strength, medium carbon steel is the most widely used, not only as a building material, but also extensively used in the manufacture of various mechanical parts.

(3) High carbon steel

Commonly known as tool steel, with a carbon content ranging from 0.60% to 1.70%, it can be quenched and tempered. Hammer, pry bar, etc. are made of steel with a carbon content of 0.75%; Cutting tools such as drill bits, wire taps, reamers, etc. are made of steel with a carbon content of 0.90% to 1.00%.