The Role of Silicon Element In Steel and The Influence of Heat Treatment

Silicon can be dissolved in ferrite and austenite to improve the hardness and strength of steel pipes, its role is second only to phosphorus, and stronger than manganese, nickel, chromium, tungsten, molybdenum and vanadium. However, when the silicon content exceeds 3%, the plasticity and toughness of the steel pipes will be significantly reduced. Silicon can improve the elastic limit, yield strength and yield ratio (σs/σb), and fatigue strength and fatigue ratio (σ-1/σb) of steel pipe, which is the reason that silicon or silicon-manganese steel can be used as spring steel.

Silicon can reduce the density, thermal conductivity and electrical conductivity of steel pipe. It can promote the coarsening of ferrite grains. Reduce coercive force. It has the tendency to reduce the anisotropy of the crystal, so that the magnetization is easy, the magnetic resistance is reduced, and it can be used to produce electrical steel, so the hysteresis loss of the silicon steel sheet is low, and the silicon can improve the permeability of the ferrite, so that the silicon steel sheet has a higher magnetic induction strength under the weak magnetic field. However, under strong magnetic field, silicon reduces the magnetic induction strength of steel pipe. Silicon has a strong deoxidation, thus reducing the magnetic aging effect of iron.

When the steel containing silicon is heated in an oxidizing atmosphere, a layer of SiO2 film will be formed on the surface, thereby improving the oxidation resistance of the steel at high temperatures.

Silicon can promote the growth of columnar crystals in cast steel and reduce plasticity. If the silicon steel is heated or cooled faster, due to the low thermal conductivity, the temperature difference between the inside and outside of the steel is large, and it is easy to crack.

The role of silicon element in steel and the influence of heat treatment.

Silicon can reduce the weldability of steel pipe. Because the affinity of silicon with oxygen is stronger than iron, it is easy to generate low melting point silicate during welding, which increases the fluidity of molten slag and molten metal, causing splashing phenomenon and affecting the quality of weld. Silicon is a good deoxidizer. The deoxidation capacity of aluminum can be significantly improved by adding a certain amount of silicon when deoxidizing aluminum. Silicon has a certain residual in steel, which is brought in as a raw material by iron and steel making. In boiling steel, silicon is limited to < 0.07%, and when it is intended to be added, ferrosilicon alloy is added in steelmaking.

(1) The effect of microstructure and heat treatment of steel

A, as an alloying element in steel, its content is generally not less than 0.4%. It exists as a solid solution in ferrite or austenite, reducing the austenite phase region.

B, increase the annealing, normalizing and quenching temperature, improve the hardenability in hypoeutectoid steel.

C, silicon does not form carbide, has a strong role in promoting the graphitization of carbon, in the silicon content of high carbon and high carbon steel, such as does not contain strong carbide forming elements, easy to occur at a certain temperature graphitization.

D, in carburized steel, silicon reduces the thickness of the carburized layer and the concentration of carbon.

E, silicon has a good deoxidation effect on molten steel.

(2) Effect on mechanical properties of steel

A, improve the hardness and strength of ferrite and austenite, its effect is stronger than Mn, Ni, Cr.W, Mo, V, etc.; The elastic limit, yield strength and yield to strength ratio (σs/σb) of steel are significantly improved. The fatigue strength and fatigue ratio (σ-1/σb) are also improved.

B, when the silicon content exceeds 3%, the plasticity and toughness of steel are significantly reduced; Silicon increases the plastic/brittle transition temperature.

C, silicon is easy to form a ribbon structure in the steel, so that the transverse performance is lower than the longitudinal performance.

D, improve the wear resistance of steel.

(3) The effect on the physical, chemical and technological properties of steel

A, reduce the density of steel, thermal conductivity, electrical conductivity and resistance temperature coefficient.

B. The eddy current loss of silicon steel sheet is significantly lower than that of pure iron, and the coercive force, magnetoresistance and hysteresis loss are lower. High permeability and magnetic induction. But in strong magnetic fields, silicon reduces the magnetic sensitivity.

C, improve the oxidation resistance of steel at high temperature, but when the silicon content is high, the surface decarbonization is intensified.

D, silicon content of more than 2.5% of the steel, its deformation processing is more difficult.

E, silicon reduces the weldability of steel.

(4) Application in steel

A, in the ordinary low alloy steel pipe to improve the strength, improve the local corrosion resistance, in the tempered steel to improve the hardenability and tempering resistance, is one of the main alloy components in the multi-alloy structural steel.

B, SiMn or SiMnB steel with silicon content of 0.5%-2.8% (carbon content of 0.5%-0.7%) is widely used in high-load elastic yellow materials, while adding W, V, Mo, Nb, Cr and other strong carbide forming elements.

C, silicon steel sheet is low carbon and ultra-low carbon steel containing silicon 1.O % -4.5%, used in motors and transformers.

D, in stainless steel and corrosion resistant steel, with Mo, W, Cr, Al, Ti, N, etc., improve the corrosion resistance and high temperature oxidation resistance.

E, graphite steel with high silicon content is used for cold working mold materials.

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