Carbon (C)
Carbon is the main element after iron, and it directly affects the strength, plasticity, toughness, and weldability of steel. When the carbon content in steel is below 0.8%, increasing the carbon content improves the steel's strength and hardness while reducing its plasticity and toughness. However, when the carbon content is above 1.0%, increasing the carbon content actually decreases the steel's strength. As the carbon content increases, the steel's weldability worsens (steel with a carbon content greater than 0.3% sees a significant decrease in weldability), its cold brittleness and aging sensitivity increase, and its resistance to atmospheric rust decreases.
Silicon (Si)
Silicon is a deoxidizer, and its deoxidizing effect is stronger than manganese's. It is a beneficial element in steel. When the silicon content is low, it can increase the steel's strength without a significant effect on its plasticity and toughness. However, when the silicon content exceeds 0.8% to 1.0%, plasticity decreases, and impact toughness in particular is significantly reduced. Low-carbon steel with a silicon content of 1% to 4% has extremely high magnetic permeability and is often used in the electrical industry and for silicon steel sheets. However, increasing the silicon content will reduce the steel's weldability.
Manganese (Mn)
Manganese is added to steel as a deoxidizing and desulfurizing element and is a beneficial element in steel. Manganese has a strong ability to deoxidize and desulfurize. It can combine with sulfur to form MnS, thereby largely eliminating the harmful effects of sulfur and significantly improving the steel's hot workability. At the same time, manganese has a good effect on the mechanical properties of carbon steel; it can increase the steel's hardness, strength, and wear resistance. When the manganese content is less than 0.8%, it can significantly increase the yield strength and ultimate strength of carbon steel while maintaining (or only slightly lowering) the original plasticity and impact toughness. Manganese also affects the weldability of steel. When the manganese content is very low, manganese's main role is to eliminate hot brittleness, so at this point, its effect on weldability is beneficial, especially when the sulfur content is slightly high. However, when the manganese content is far more than what is necessary to eliminate hot brittleness, the excess manganese will significantly increase the supercooling ability of the austenite. At this time, manganese's main role is to increase the formation of cold cracks, which will worsen the steel's weldability.
Phosphorus (P)
Phosphorus is a harmful impurity in steel that is difficult to remove. It can increase the steel's cold brittleness and damage its weldability. The reason for "cold brittleness" is that phosphorus forms the hard and brittle compound Fe2P. Additionally, phosphorus can improve machinability and corrosion resistance, so the phosphorus content can be appropriately increased in free-cutting or weathering steels.
Sulfur (S)
Sulfur mainly comes from steelmaking raw materials and is difficult to remove completely during steelmaking. Sulfur exists in steel as sulfide inclusions and has a detrimental effect on the steel's plasticity, toughness, weldability, through-thickness properties, fatigue performance, and corrosion resistance. The most harmful effect is the formation of FeS with iron, which creates a low-melting-point Fe-FeS binary eutectic, causing the steel to become brittle and prone to cracking at temperatures of 800-1200°C, which is known as "hot shortness."
Oxygen (O)
Oxygen is a harmful impurity element. In steel, oxygen exists almost entirely in the form of oxides. The total amount of various oxides in steel increases with the oxygen content. These oxide impurities have a detrimental effect on various aspects of the steel's mechanical properties. As the oxygen content in steel increases, the steel's plasticity and impact toughness decrease. Oxide inclusions reduce the steel's corrosion resistance and wear resistance, and worsen its cold stamping, forging, and machining performance.
Nitrogen (N)
The effect of nitrogen on steel properties is similar to that of carbon and phosphorus. As the nitrogen content increases, it can significantly increase the steel's strength, while also significantly reducing its plasticity and especially its toughness. It worsens weldability and intensifies cold brittleness. It also increases the tendency for aging and both cold and hot brittleness, damaging the steel's weldability and cold-bending performance. Therefore, the nitrogen content in steel should be minimized and restricted. Generally, the nitrogen content should not be higher than 0.018%. Nitrogen can be used as an alloying element in low-alloy steel to reduce its negative effects and improve steel properties when used in conjunction with elements such as aluminum, niobium, and vanadium.
Hydrogen (H)
Hydrogen is also a harmful element in steel. Like oxygen and nitrogen, gaseous elements have extremely low solubility in solid steel. They dissolve in molten steel at high temperatures and, during cooling, they do not have enough time to escape and instead accumulate in the structure to form tiny, high-pressure pores. This drastically reduces the steel's plasticity, toughness, and fatigue strength. In severe cases, it can cause cracks and brittle fracture, making it a harmful element that must be strictly controlled.
Chromium (Cr)
Chromium can significantly increase the steel's strength, hardness, and wear resistance, but it also reduces its plasticity and toughness. Chromium can also improve the steel's oxidation and corrosion resistance, making it an important alloying element for stainless steel and heat-resistant steel.
Nickel (Ni)
Nickel can increase the steel's strength while maintaining good plasticity and toughness. Nickel has high corrosion resistance to acids and alkalis and provides rust and heat resistance at high temperatures.
Molybdenum (Mo)
Molybdenum can refine the steel's grains, increase hardenability and hot strength, and maintain sufficient strength and creep resistance at high temperatures (creep is the deformation that occurs when a material is stressed for a long time at a high temperature). Adding molybdenum to structural steel can improve its mechanical properties and also inhibit the brittleness caused by fire in alloy steel.
Titanium (Ti)
Titanium is a strong deoxidizer. It can make the steel's internal structure denser, refine the grains, reduce aging sensitivity and cold brittleness, and improve weldability. Adding an appropriate amount of titanium to some austenitic stainless steels can prevent intergranular corrosion.
Vanadium (V)
Vanadium is an excellent deoxidizer for steel. Adding 0.5% of vanadium to steel can refine the grain structure and increase strength and toughness. The carbides formed by vanadium and carbon can improve resistance to hydrogen corrosion at high temperatures and pressures.
Niobium (Nb)
Niobium can refine grains and reduce the steel's overheating sensitivity, increasing its strength, but at a slight cost to plasticity and toughness. Adding niobium to ordinary low-alloy steel can improve its resistance to atmospheric corrosion and high-temperature corrosion from hydrogen, nitrogen, and ammonia. Niobium can also improve weldability and prevent intergranular corrosion in austenitic stainless steels.
Copper (Cu)
Copper can increase strength and toughness and provides good resistance to atmospheric corrosion. A disadvantage is that it is prone to hot shortness during hot working, and when the copper content exceeds 0.5%, plasticity is significantly reduced. When the copper content is less than 0.5%, it has no effect on weldability.
Boron (B)
Adding a small amount of boron to steel can improve its compactness and hot-rolling properties and increase its strength.
Aluminum (Al)
Aluminum is a commonly used deoxidizer in steel. Adding a small amount of aluminum to steel can refine the grains and increase impact toughness. Aluminum also has oxidation and corrosion resistance. When used with chromium and silicon, it can significantly improve the steel's high-temperature resistance to scaling and high-temperature corrosion. A disadvantage of aluminum is that it affects the steel's hot workability, weldability, and machinability.
Tungsten (W)
Tungsten has a secondary hardening effect, giving steel red hardness and increasing wear resistance. Its effects on the steel's hardenability, temper stability, mechanical properties, and hot strength are similar to molybdenum's. It slightly reduces the steel's oxidation resistance.
Lead (Pb)
Lead can improve machinability. Lead-based free-cutting steels have good mechanical properties and heat treatability. However, due to environmental pollution and its harmful effects during the scrap steel recycling and melting process, lead is gradually being replaced.
