Round steel is categorized 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 as straight bars and is commonly used for reinforcing 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 pipes. 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 in the production of 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 like 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 ordinary carbon structural steel and carbon structural steel based on product quality. It has good hot working and cutting properties but poor welding properties. It is stronger and harder than low-carbon steel but has lower plasticity and toughness. It can be used directly after hot rolling or cold drawing, or after heat treatment. Quenched and tempered medium-carbon steel has excellent comprehensive mechanical properties, with a maximum hardness of about HRC55 (HB538) and σb ranging from 600 to 1100 MPa. Therefore, medium-carbon steel is widely used in various applications at medium strength levels, not only as construction materials but also in the manufacture 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 like drills, taps, and reamers are made from steel with a carbon content of 0.90% to 1.00%.
