Impact of C (Carbon), N (Nitrogen), and O (Oxygen) on Steel Properties

C (Carbon) is the second most significant element after iron, directly influencing steel’s strength, plasticity, toughness, and weldability.

When the carbon content in steel is below 0.8%, an increase in carbon content enhances steel’s strength and hardness while reducing its plasticity and toughness. However, when the carbon content exceeds 1.0%, an increase in carbon content paradoxically leads to a decrease in steel’s strength.

As carbon content rises, steel’s weldability diminishes (weldability significantly drops for steel with a carbon content above 0.3%), cold brittleness and aging sensitivity increase, and atmospheric corrosion resistance declines.

The influence of N (Nitrogen) on steel properties resembles that of carbon and phosphorus. An increase in nitrogen content significantly boosts steel’s strength but notably reduces plasticity, especially toughness, and impairs weldability, exacerbating cold brittleness. It also heightens aging tendencies, both cold and hot brittleness, damaging steel’s weldability and cold bending properties. Therefore, nitrogen content in steel should be minimized and limited. Typically, nitrogen content should not exceed 0.018%.

In combination with elements like aluminum, niobium, and vanadium, nitrogen can mitigate its adverse effects, improve steel properties, and serve as an alloying element in low-alloy steels. In some stainless steels, a moderate increase in nitrogen content can reduce chromium usage, effectively lowering costs.

O (Oxygen) is a deleterious element in steel. It naturally enters steel during the steelmaking process and cannot be completely removed, even with the addition of manganese, silicon, iron, and aluminum for deoxidization at the end of the process. During steel solidification, oxygen reacts with carbon in the melt to produce carbon monoxide, which can create bubbles. Oxygen primarily exists in steel in the form of inclusions such as FeO, MnO, SiO₂ and Al₂O₃, reducing steel’s strength and plasticity. This has a severe impact on fatigue strength, impact toughness, and other properties.

Oxygen increases iron loss in silicon steel, weakens magnetic permeability and magnetic induction intensity, and intensifies magnetic aging effects.