Effects of common impurity elements on carbon steel properties

Release time:

2025-01-08

In carbon steel, carbon is the main element that determines the performance of steel. However, steel also contains a small amount of common impurity elements such as manganese, silicon, sulfur, and phosphorus, which also have a certain influence on the performance of steel.

(I) The influence of manganese
Manganese is the residue in steel after ferromanganese is added for deoxidation during steelmaking. Manganese has good deoxidation ability and can remove FeO in steel and reduce the brittleness of steel; manganese can also form MnS with sulfur to reduce the harmful effects of sulfur. Therefore, manganese is a beneficial element. However, when it exists as an impurity, its content (Wmn) is generally not less than 0.8%, which has little effect on the performance of steel.

(II) The influence of silicon
Silicon is the residue in steel after ferrosilicon is added for deoxidation during steelmaking. Silicon has a stronger deoxidation ability than manganese. At room temperature, silicon can dissolve into ferrite to improve the strength and hardness of steel. Therefore, silicon is also a beneficial element. However, when it exists as an impurity, its content (Wsi) is generally less than 0.4%, which has little effect on the performance of steel.

(III) The influence of sulfur
Sulfur is brought into steel from ore and fuel during steelmaking. Sulfur forms a compound FeS with iron in steel, and Fes forms a low melting point (985°C) eutectic with iron distributed on the austenite grain boundary. When the steel is heated to 1100-1200°C for forging, the eutectic on the grain boundary has melted, causing the steel to crack during the forging process. This phenomenon is called "hot brittleness". Adding manganese to steel can form MnS with a high melting point (1620°C). MnS is distributed in the grains in a granular form and has a certain plasticity at high temperatures, thus avoiding hot brittleness. Therefore, sulfur is a harmful element, and its content (Ws) should generally be strictly controlled below 0.03%-0.05%.

(IV) Effect of phosphorus
Phosphorus is brought into steel from ore during steelmaking. Phosphorus can be completely dissolved in ferrite, producing strong solid solution strengthening, which increases the strength and hardness of steel, but significantly reduces plastic toughness. This embrittlement phenomenon is more serious at low temperatures, so it is called "cold brittleness". Phosphorus is also prone to segregation during crystallization. As a result, cold brittleness occurs locally. Therefore, phosphorus is also a harmful element, and its content must be strictly controlled below 0.035%-0.045%. However, when the sulfur and phosphorus content is high, due to the greater brittleness, it is easy to break during cutting and form broken chips, which improves the machinability of steel. This is the advantage of sulfur and phosphorus.

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