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Carbon, silicon, sulfur and phosphorus in iron and steel

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Carbon, silicon, sulfur and phosphorus in iron and steel

Steel mainly contains: carbon, silicon, sulfur, phosphorus, iron (the main elements), and other elements that change the properties of steel, such as chromium, manganese, nickel, etc., adding these elements can form high-quality carbon structural steel, alloy structural steel, easy cutting structural steel, spring steel, rolling bearing steel and other high-quality steel.

carbon (C)

The yield point and tensile strength increase with the increase of carbon content, but the plasticity and impact strength decrease 0.23% Therefore, the carbon content of low alloy structural steel used for welding generally does not exceed 0.20% High carbon content will also reduce the atmospheric corrosion resistance of steel, and the high carbon steel in open pit is easy to rust; in addition, carbon can increase the cold brittleness and aging sensitivity of steel.

sulfur (S)

The cutting performance of steel can be improved by increasing the content of sulfur and manganese. In free cutting steel, sulfur is added as a beneficial element. Sulfur segregation is serious in steel. Deterioration of steel quality, at high temperature, reduce the plasticity of steel, is a harmful element, it has a lower melting point FeS Form exists.

Alone FeS The melting point of 1190℃ However, the eutectic temperature of eutectic with iron in steel is lower than that in steel 988℃ When the steel solidifies, FES segregates at the primary grain boundary. steel one thousand and one hundred 1200℃ When rolling, the FeS Therefore, sulfur should be strictly controlled. It is generally controlled in 0.020% 0.050%

In order to prevent the brittleness caused by sulfur, enough manganese should be added to form a high melting point MnS If the flow rate in the steel is too high, due to SO2 The porosity and porosity will be formed in the welding metal.

silicon (Si)

Silicon can be dissolved in ferrite and austenite to improve the hardness and strength of steel, and its effect is second only to phosphorus, and stronger than manganese, nickel, chromium, tungsten, molybdenum, vanadium and other elements. But the silicon content is more than 3% The plasticity and toughness of the steel will be reduced significantly. Silicon can improve the elastic limit, yield strength and yield ratio of steel (σs/σb) And fatigue strength and fatigue ratio (σ-1/σb) Etc. This is because silicon or silicon manganese steel can be used as spring steel.

Silicon can reduce the density, thermal conductivity and electrical conductivity of steel. It can coarsen ferrite grains and reduce coercivity. It can be used to produce electrical steel, so the magnetic resistance of silicon steel sheet is low. Silicon can improve the permeability of ferrite, and make the steel sheet have higher magnetic induction strength in weak magnetic field.

However, under high magnetic field, silicon reduces the magnetic induction strength of steel. The magnetic aging effect of iron is reduced due to the strong deoxidizing force of silicon.

When the steel containing silicon is heated in oxidizing atmosphere, a layer will form on the surface SiO2 So as to improve the oxidation resistance of steel at high temperature.

Silicon can promote the growth of columnar crystals in cast steel and reduce the plasticity. If the silicon steel is cooled quickly during heating, the temperature difference between the inside and outside of the steel is large due to the low thermal conductivity, which leads to fracture.

Silicon can reduce the weldability of steel. Because silicon has stronger binding capacity with oxygen than iron, it is easy to form silicate with low melting point during welding, which increases the fluidity of slag and molten metal, causes splashing phenomenon and affects welding quality. Silicon is a good deoxidizer. The deoxidization rate can be significantly improved by adding a certain amount of silicon when deoxidizing with aluminum. Silicon has a certain amount of residual in steel, which is brought in as raw material in ironmaking and steelmaking.

In rimmed steel, silicon is limited to <0.07% The ferrosilicon alloy is added in steelmaking if it is intentionally added.

phosphorus (P)

Phosphorus has a strong effect on solution strengthening and cold work hardening in steel. As an alloying element, low alloy structural steel can improve its strength and atmospheric corrosion resistance, but reduce its cold stamping performance.

The combined use of phosphorus, sulfur and manganese can increase the machinability of steel and the surface quality of machined parts. It is used in free cutting steel, so the phosphorus content of free cutting steel is relatively high. Although the hardness and toughness of the steel can be increased significantly when tempered, the toughness and toughness of the steel can be increased significantly Cold crisp Phenomenon. Phosphorus also has an adverse effect on weldability.

Phosphorus is a harmful element, should be strictly controlled, the general content is not more than 0.03% 0.04%

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