Round steel is categorized into three types: hot-rolled, forged, and cold-drawn. The specifications of hot-rolled round steel range from 5.5 to 250 millimeters. Among them, small round steel with diameters of 5.5 to 25 millimeters is typically supplied in bundles of straight bars and is commonly used for reinforcing bars, bolts, and various mechanical parts; round steel with diameters greater than 25 millimeters is mainly used for manufacturing mechanical parts and billets for seamless steel tubes. By chemical composition, carbon steel can be divided into low-carbon, medium-carbon, and high-carbon steel based on carbon content. (1) Low-carbon steel, also known as soft steel, contains carbon 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 products like killed steel, semi-killed steel, and沸腾钢(float 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 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, or after heat treatment. Quenched and tempered medium-carbon steel has good comprehensive mechanical properties. The achievable hardness is about HRC55 (HB538), and σb is 600 to 1100 MPa. Therefore, medium-carbon steel is widely used in various applications at medium strength levels, in addition to being used as building materials, it is also extensively used in the manufacture 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, crowbars, etc., 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%.
