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Influence of chemical elements on properties of steel (ii)

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Influence of chemical elements on properties of steel (ii)

Effect of silicon on steel properties

Element 8: Si (silicon)


Si is an important reducing agent and deoxidizer in steelmaking: for many materials in carbon steel, Si is less than 0.5%, which is generally brought in as reducing agent and deoxidizer in steelmaking.


Silicon can be dissolved in ferrite and austenite to improve the hardness and strength of steel, which is only second to phosphorus and stronger than manganese, nickel, chromium, tungsten, molybdenum, vanadium and other elements. However, when the silicon content exceeds 3%, the plasticity and toughness of steel are significantly reduced. The silicon improves the elastic limit, yield strength and yield ratio of the steel (sigma/sigma b) and the fatigue strength and fatigue ratio (sigma/sigma b). This is because silicon or silicomanganese steel can be used as spring steel.


Silicon can reduce the density, thermal and electrical conductivity of steel. It can coarsene ferrite grains and reduce coercivity. It can reduce the anisotropy of crystal, make it easy to magnetize and reduce the magnetoresistance, which can be used to produce electrical steel, so the magnetic retardation loss of silicon steel sheet is low. Silicon can improve the magnetic permeability of ferrite and make steel sheet have higher magnetic sensitivity under weak magnetic field. However, under strong magnetic fields, silicon reduces the magnetic sensitivity of steel. Due to the strong deoxidation of silicon, the magnetic aging effect of iron is reduced.


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


Silicon promotes the growth of columnar crystals in cast steel and reduces plasticity. When silicon steel is heated, it cools more quickly. Due to the low thermal conductivity, the temperature difference between the inside and outside of the steel is larger, so it breaks.


Silicon can reduce the weldability of steel. Due to its stronger binding capacity with oxygen than iron, silicon is easy to form silicate with low melting point during welding, which increases the fluidity of molten slag and melted metal, causing splash phenomenon and affecting welding quality. Silicon is a good deoxidizer. When using aluminum deoxidization, adding a certain amount of silicon as appropriate can significantly improve the deoxidization rate. Silicon has a residual presence in steel because it is brought in as a raw material when making iron and steel. In ebullient steel, the silicon limit is <0.07%, and when it is intended to be added, ferrosilicon alloy is added in steelmaking.


Effect of phosphorus on properties of steel

Element 9: P (phosphorus)


P is carried into steel from ore, and phosphorus is generally a harmful element. Although phosphorus can increase the strength and hardness of steel, it can significantly reduce the plasticity and impact toughness. Especially at low temperatures, it makes steel significantly brittle, a phenomenon known as "cold brittle". Cold brittleness makes the cold processing and weldability of steel worse. The higher the phosphorus content is, the greater the cold brittleness is. High quality steel: P < 0.025%; High quality steel: P < 0.04%; Ordinary steel: P < 0.085%.


The solid solution strengthening and cold hardening of P are very good, which can be used in combination with copper to improve the atmospheric corrosion resistance of low-alloy high-strength steel, but reduce its cold stamping performance. It can be used in combination with sulfur and manganese to improve the machinability, increase temper brittleness and cold brittleness sensitivity.


Phosphorus can improve ratio resistance, and can make the coercive force due to easy to coarse grain and eddy current loss is reduced, the magnetic induction, is in the weak magnetic field under high phosphorus content of steel magnetic induction will improve, containing P silicon steel hot working is not difficult, but because it can make the silicon steel with cold brittleness, content 0.15% (such as cold rolled electrical silicon steel containing P = 0.07 ~ 0.10%).


Phosphorus is the most powerful element to strengthen ferrite. (the effect of P on recrystallization temperature and grain growth of silicon steel is 4 ~ 5 times greater than that of the same silicon content.)


Effect of sulfur on properties of steel

Element 10: S (sulfur)


Sulfur comes from the ore and fuel coke used to make steel. It is a harmful element in steel. Sulfur exists in steel in the form of iron sulfide (FeS), and FeS and Fe form a compound with low melting point (985). However, the thermal processing temperature of steel is generally above 1150 ~ 1200, so when the steel is thermal processing, the premature melting of FeS compounds leads to the workpiece cracking, this phenomenon is called "thermal brittleness". Reduces the ductility and toughness of steel, causing cracks during forging and rolling. Sulfur is also bad for welding performance, reducing corrosion resistance. High quality steel: S < 0.02% ~ 0.03%; High quality steel: S < 0.03% ~ 0.045%; Ordinary steel: S < 0.055% ~ 0.7% below.


Because its chips are brittle and give a very shiny surface, it can be used to make steel pieces with low load and high surface finish (called fast cutting steel) (e.g. Cr14) with a small amount of sulfur added intentionally (=0.2 ~ 0.4%). Some HSS tool steels are vulcanized on the surface.


Elements 11, 12: K/Na (potassium/sodium)


Potassium/sodium can be used as a modifier to pelletize the carbide in white iron, and increase the toughness of white iron (and leite steel) by more than two times while maintaining the original hardness. The microstructure refinement of ductile iron and the treatment of vermicular iron were stabilized. Is a strong austenitizing element, for example, it reduces the manganese/carbon ratio of austenitic manganese steel from 10:1 to 13:1 to 4:1 to 5:1.


Effect of calcium on properties of steel

Element 13: Ca (calcium)


Adding calcium in steel can refine grain, partly desulfurize, and change the composition, quantity and morphology of nonmetallic inclusions. The effect of adding rare earths to steel is basically similar.


Improve corrosion resistance, wear resistance, high temperature and low temperature resistance of steel; The impact toughness, fatigue strength, plasticity and welding properties of steel are improved. The cold upsetting, shock resistance, hardness and contact strength of steel are increased.


Adding calcium to cast steel greatly increases the fluidity of molten steel. The surface finish of casting was improved, and the anisotropy of casting structure was eliminated. Its casting properties, thermal cracking resistance, mechanical properties and machining properties have increased to different degrees.


Adding calcium in steel can improve the resistance to hydrogen crack and lamellar tear, and extend the service life of equipment and tools. Calcium can be used as deoxidizer, inoculant and microalloying agent in master alloy.


Effect of titanium on steel properties

Element 14: Ti (titanium)


Titanium has a strong affinity with nitrogen, oxygen and carbon, and a stronger affinity with sulfur than iron. It is a good deoxidizer and effective element to fix nitrogen and carbon. Although titanium is a strong carbide forming element, it does not combine with other elements to form complex compounds. Titanium carbide has strong binding force, stability, and is not easy to decompose. Only when heated to more than 1000 can it slowly dissolve into the solid solution in steel.


Titanium carbide particles have the effect of preventing grain growth before dissolution. Because the affinity between titanium and carbon is much greater than that between chromium and carbon, titanium is often used to fix the carbon in stainless steel to eliminate the dilution of chromium at the grain boundary, so as to eliminate or reduce the intercrystalline corrosion of steel.


Titanium is also one of the strong ferrite forming elements, which strongly increases the A1 and A3 temperatures of steel. Titanium can improve plasticity and toughness in ordinary low alloy steels. The strength of steel is enhanced by titanium fixation of nitrogen and sulfur to form titanium carbide. The plasticity and impact toughness of steel can be significantly improved by normalizing grain refinement and precipitation to form carbide.


It is necessary to add about 5 times of titanium in high chromium stainless steel, which can not only improve the corrosion resistance (mainly inter-crystalline corrosion resistance) and toughness of the steel; It can also organize grain growth tendency and improve welding property of steel at high temperature.

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