Chemical Factors affecting Steel Properties (1)

Steel is an alloy of iron and carbon as the main components, although the proportion of carbon and other elements is very small, but it determines the performance of steel.

Carbon: is one of the important elements in various steels. In carbon structural steel, it is the most important element besides iron. Carbon is the main component of steel strength, with the increase of carbon content, the strength of steel gradually increased, while the plasticity and toughness decreased, cold bending performance, welding performance and corrosion resistance also deteriorated. Carbon steel according to the content of carbon, less than 0.25% is low carbon steel, between 0.25% and 0.6% is medium carbon steel, greater than 0.6% is high carbon steel. When the carbon content exceeds 0.3%, the tensile strength of steel is very high, but there is no obvious yield point, and the plasticity is very small. When the carbon content exceeds 0.2%, the welding performance of the steel will begin to deteriorate. Therefore, the steel recommended by the code, the carbon content is not more than 0.22%, for welded structural steel is strictly controlled within 0.2%.

Sulfur: A harmful element, often included in steel in the form of iron sulfide. When the temperature reaches 800-1000 degrees Celsius, iron sulfide will culture to make the steel brittle, so welding or hot processing may cause hot cracks, called hot brittle. In addition, sulfur also reduces the impact toughness, fatigue strength, corrosion resistance and weldability of the material. Due to the inclusion of non-metallic fluid compounds, the local stratification phenomenon will be formed in the medium thick steel plate after hot rolling processing. In the joints connected by welding, lamellar tearing will occur when subjected to tension along the plate thickness direction, so the sulfur content in the steel should be strictly limited. With the improvement of steel grade and quality grade, the limit value of sulfur content decreased from 0.05% to 0.025%, and the sulfur content of the thickness direction performance steel plate (lamellar tear resistance steel plate) was limited to less than 0.01%.

Phosphorus: can improve the strength of steel and the ability to resist corrosion, but seriously reduce the plasticity, toughness, cold bending performance and welding performance of steel, especially at low temperatures will make steel brittle, called cold brittle. Therefore, the content of phosphorus should also be strictly controlled, with the improvement of steel grade and quality grade, the limit value of phosphorus content is reduced from 0.045% to 0.025%. However, when the special smelting process is adopted, phosphorus can be used as an alloying element to make low-alloy steel containing phosphorus, and its content can reach 0.12%-0.13%.