Ordinary carbon structural steel, also known as ordinary carbon steel, has wide limits on carbon content, property range and phosphorus, sulfur and other residual element content. In China and some countries, according to the delivery guarantee conditions, there are three categories: Class A steel (Class A steel) is the steel that guarantees the mechanical properties. Class B steel (Class B steel) is the steel to ensure chemical composition. Special steel (Class C steel) is a kind of steel that can ensure both mechanical properties and chemical composition, and is often used to manufacture more important structural parts. At present, A3 steel (Class A No. 3 steel) with a carbon content of about 0.20% is the most produced and used in China, mainly for engineering structures. Some carbon structural steels also add a small amount of aluminum or niobium (or other carbide forming elements) to form nitride or carbide particles to limit grain growth, strengthen steel and save steel. In China and some countries, in order to meet the special requirements of professional steel, the chemical composition and properties of ordinary carbon structural steel are adjusted, thus developing a series of professional steel for ordinary carbon structural steel (such as steel for bridges, buildings, reinforcement, pressure vessels, etc.).