Chemical composition of steel

　　In addition to the main chemical composition Fe iron, steel also contains a small amount of carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), oxygen (0), nitrogen (N), titanium (Ti), vanadium (V) and other elements, although the content of these elements is small, they have a great influence on the properties of steel.

　　Carbon is the most important element to determine the performance of steel, which affects the mechanical properties of steel such as strength, plasticity and toughness. When the carbon content in the steel is below 0.8%, as the carbon content increases, the strength and hardness of the steel increase, and the plasticity and toughness decrease; but when the carbon content is greater than 1.0%, as the carbon content increases, the steel's Intensity decreased instead. The carbon steel used in general engineering is low carbon steel, that is, the carbon content is less than 0.25%, and the carbon content of low alloy steel for engineering is less than 0.52%.

　　Beneficial elements in steel include manganese, silicon, vanadium, titanium, etc., which can be smelted into low-alloy steel by controlling the amount of incorporation. The main harmful elements in steel are sulfur, phosphorus and oxygen, and special attention should be paid to controlling their content. Phosphorus is one of the most harmful elements in steel, and it is mainly dissolved in ferrite to strengthen it. As the phosphorus content increases, the strength and hardness of the steel increase, but the plasticity and toughness decrease significantly. Especially the lower the temperature, the greater the impact on plasticity and toughness, thus significantly increasing the cold brittleness of steel. Phosphorus also significantly reduces the weldability of steel, but phosphorus can improve the wear resistance and corrosion resistance of steel. Sulfur is also a very harmful element, which exists in steel as non-metallic sulfide inclusions, reducing various mechanical properties of steel.

　　Due to the low melting point of sulfide, the steel will cause the separation of grains during thermal processing, causing the steel to fracture and forming a hot brittle phenomenon called hot brittleness. Sulfur reduces the weldability, impact toughness, fatigue resistance and corrosion resistance of steel. Oxygen is a harmful element in steel, which mainly exists in non-metallic inclusions, and a small amount is melted in ferrite. Non-metallic inclusions reduce the mechanical properties of steel, especially toughness. Oxygen has the effect of promoting aging tendency. The low melting point caused by oxides also deteriorates the weldability of steel.