Round steel is divided into three types: hot-rolled, forged, and cold drawn. Hot-rolled round steel is available in sizes ranging from 5.5 to 250 millimeters. Among them, small round steel with diameters of 5.5 to 25 millimeters is usually supplied in bundles of straight bars and is commonly used as reinforcing bars, bolts, and various mechanical parts; round steel larger than 25 millimeters is mainly used for manufacturing mechanical parts, seamless steel tube blanks, and so on. Carbon steel is categorized by chemical composition (i.e., carbon content) into low-carbon steel, medium-carbon steel, and high-carbon steel. (1) Low-carbon steel, also known as soft steel, contains carbon from 0.10% to 0.30%. It is easy to process by forging, welding, and cutting and is often used to make chains, rivets, bolts, shafts, etc. (2) Medium-carbon steel is carbon steel with a carbon content of 0.25% to 0.60%. It includes products such as killed steel, semi-killed steel, and rimmed steel. Besides carbon, it can also contain a small amount of manganese (0.70% to 1.20%). It is divided into plain carbon structural steel and carbon structural steel based on product quality. It has good hot working and cutting properties but poor welding properties. Its strength and hardness are higher than those of low-carbon steel, while its ductility and toughness are lower. It can be used directly without heat treatment, either as hot-rolled or cold-drawn material, or after heat treatment. Quenched and tempered medium-carbon steel has good comprehensive mechanical properties and can achieve a hardness of approximately HRC55 (HB538) with σb ranging from 600 to 1100 MPa. Therefore, medium-carbon steel is widely used in various applications with moderate strength levels, aside from being used as building materials, it is also extensively used in the manufacturing of various mechanical parts. (3) High-carbon steel, commonly referred to as tool steel, contains carbon from 0.60% to 1.70% and can be hardened and tempered. Hammers and crowbars are made from steel with a carbon content of 0.75%; cutting tools like drills, taps, and reamers are made from steel with a carbon content of 0.90% to 1.00%.