The Influence of Common Elements on Steel Properties

Carbon steel: Steel with a carbon content of less than 2.11% and without the addition of special alloying elements is referred to as carbon steel, often simply called “carbon steel.”

The Influence of Common Elements on Steel Properties

In addition to the two principal elements—iron and carbon—in carbon steel, steelmaking raw materials typically contain certain impurities, and further impurities are introduced during the smelting process, such as silicon, manganese, phosphorus, sulfur, and non-metallic inclusions, as well as gases like oxygen, nitrogen, and hydrogen. All these impurities can exert a significant influence on the mechanical properties and quality of the steel.

(1) Manganese (Mn). Manganese primarily originates as a deoxidizer used in the steelmaking process. The manganese remaining in the steel after deoxidation can dissolve in both ferrite and cementite, thereby enhancing the steel’s strength and hardness. Moreover, manganese reacts with sulfur to form MnS, which mitigates sulfur’s detrimental effects on the steel. Consequently, manganese is a beneficial alloying element, typically present at levels of 0.25% to 0.80%.

(2) Silicon (Si). Silicon is an element that remains in steel after undergoing a deoxidation reaction with ferrosilicon during the later stages of steelmaking. It can dissolve in ferrite and impart a certain strengthening effect to the steel; therefore, it is considered a beneficial element. However, due to its relatively low content, its strengthening effect is not particularly pronounced, and its concentration is typically maintained within the range of 0.17% to 0.37%.

(3) Sulfur (S). Sulfur is a harmful element that enters steel along with pig iron and fuel. In steel, it forms the compound FeS with iron. FeS and iron constitute a low-melting-point eutectic structure (Fe + FeS), whose melting point is only 985°C. When steel is rolled or forged at temperatures between 1000 and 1200°C, this eutectic phase distributed along grain boundaries melts, weakening the bonding between grains and causing the steel to crack. This phenomenon is known as hot shortness. Therefore, sulfur is a detrimental element in steel, and its content must not exceed 0.05%.

(4) Phosphorus (P). Phosphorus is a deleterious element that enters steel along with pig iron. Part of it dissolves in ferrite to form a solid solution, while the remainder forms the extremely brittle Fe₃P compound during solidification. This leads to a sharp decline in the steel’s ductility and toughness at low temperatures—typically below 100°C—a phenomenon known as cold brittleness. When the phosphorus content in steel reaches 0.10%, cold brittleness becomes particularly severe. Consequently, phosphorus is considered an undesirable impurity in steel. Under normal circumstances, the phosphorus content should be kept below 0.04%.

(5) Non-metallic inclusions. Non-metallic inclusions in steel include oxides, sulfides, and silicates. These inclusions are either formed during the steelmaking process and difficult to eliminate completely, or they originate from refractory materials such as slag, furnace linings, and casting equipment. Non-metallic inclusions reduce the strength and ductility of steel; therefore, the fewer inclusions present, the higher the steel’s quality.