Common round steel mainly consists of iron (Fe), with carbon (C) content varying according to different grades. Low - carbon round steel usually has a C content of less than 0.25%, which gives it good ductility and weldability, suitable for general - purpose applications. Medium - carbon round steel has a C content between 0.25% - 0.6%, balancing strength and ductility, and is used for parts requiring higher strength, like shafts. High - carbon round steel has a C content above 0.6%, providing high hardness and wear resistance, often used for tools. Additionally, it may contain small amounts of other elements such as manganese (Mn), silicon (Si), sulfur (S), and phosphorus (P), which affect its properties.