Round steel is categorized into three types: hot-rolled, forged, and cold-drawn. Hot-rolled round steel comes in sizes ranging from 5.5 to 250 millimeters. Among them, small round steel with diameters of 5.5 to 25 millimeters is typically supplied in bundles as straight bars and is commonly used for reinforcement bars, bolts, and various mechanical components; round steel with diameters greater than 25 millimeters is mainly used for manufacturing mechanical parts and billets for seamless steel tubes. Carbon steel is classified by chemical composition (i.e., carbon content) into low-carbon, medium-carbon, and high-carbon steel. (1) Low-carbon steel, also known as soft steel, has a carbon content ranging from 0.10% to 0.30%. It is easy to process through 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 various products such as镇静钢, semi-killed steel, and killed steel. Besides carbon, it can also contain a small amount of manganese (0.70% to 1.20%). It is divided into ordinary 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 excellent comprehensive mechanical properties and can achieve a high hardness of about HRC55 (HB538) and σb of 600 to 1100 MPa. Therefore, medium-carbon steel is widely used in various applications at medium 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, has a carbon content ranging 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 such as drills, taps, and reamers are made from steel with a carbon content of 0.90% to 1.00%.