Steel, aside from its primary chemical component iron (Fe), also contains trace amounts of carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), oxygen (O), nitrogen (N), titanium (Ti), and vanadium (V). Although these elements are present in small quantities, they significantly influence the properties of steel.

Carbon is a critical element in determining steel properties, influencing its mechanical properties such as strength, plasticity, and toughness. When the carbon content in steel is below 0.8%, its strength and hardness increase with rising carbon content, while plasticity and toughness decrease. However, when the carbon content exceeds 1.0%, the strength of the steel actually decreases with further carbon increase. Carbon steel used in general engineering applications is typically low-carbon steel, containing less than 0.25% carbon, while low-alloy steel for engineering purposes contains less than 0.52% carbon.

Beneficial elements in steel include manganese, silicon, vanadium, titanium, and others; controlling the amount added allows for the production of low-alloy steel. The main harmful elements in steel are sulfur, phosphorus, and oxygen, which require special attention to their content. Phosphorus is one of the most harmful elements in steel, primarily dissolved in ferrite to enhance strength.

Increased phosphorus content enhances the strength and hardness of steel, while significantly reducing its ductility and toughness, especially at lower temperatures, which exacerbates the impact on these properties, thereby significantly increasing the steel's cold brittleness.

Phosphorus also significantly reduces the weldability of steel, but it enhances the wear and corrosion resistance of steel. Sulfur is a highly harmful element, existing in steel as non-metallic sulfide inclusions, which lowers the various mechanical properties of steel.

Due to the low melting point of sulfides, they cause grain separation in steel during the hot processing, leading to steel fracture and the phenomenon known as hot brittleness. Sulfur reduces the weldability, impact toughness, fatigue resistance, and corrosion resistance of steel. Oxygen is a harmful element in steel, primarily found in non-metallic inclusions, with a small amount dissolved in the ferrite.

Non-metallic inclusions degrade the mechanical properties of steel, particularly its toughness. Oxygen has a tendency to promote aging, and the low melting points caused by oxides also worsen the steel's weldability.