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1. Chromium (Cr): Chromium can increase the hardenability of steel and induce secondary hardening. It can increase the hardness and wear resistance of high-carbon steel without making the steel brittle ； When the content exceeds 12%. It confers on steel good high-temperature oxidation resistance and resistance to corrosion by oxidizing media. It also increases the heat strength of steel; chromium is the main alloying element in stainless acid-resistant steel and heat-resistant steel. Chromium can increase the strength and hardness of carbon steel in the rolled state. Reduce elongation and reduction of area. When the chromium content exceeds 15%, strength and hardness decrease, while elongation and reduction of area increase accordingly. Parts made of chromium-containing steel can achieve high surface finish quality easily through grinding. The main role of chromium in quenched and tempered structural steel is to improve hardenability. It enables steel to possess good comprehensive mechanical properties after quenching and tempering; in carburized steel, it also forms chromium-containing carbides, thereby improving the wear resistance of the material surface. Chromium-containing spring steel does not tend to decarburize during heat treatment. Chromium can improve the wear resistance, hardness, and hardenability of tool steel. It has good tempering stability. In electric heating alloys, chromium improves the alloy’s oxidation resistance, electrical resistance, and strength. (1) Effects on the microstructure of steel and heat treatment: A. Chromium forms a continuous solid solution with iron, reducing the area of the austenite phase. Chromium forms various carbides with carbon; its affinity for carbon is greater than that of iron and manganese, but lower than that of elements such as tungsten and molybdenum. Chromium and iron can form an intermetallic compound, the σ-phase (FeCr). B. Chromium reduces the carbon concentration in pearlite and the ultimate solubility of carbon in austenite. C. It slows down the decomposition rate of austenite, significantly improving the hardenability of the steel. However, it also increases the tendency for temper brittleness in the steel. (2) Effect on the mechanical properties of steel: A. It increases the strength and hardness of steel; the effect is more significant when other alloying elements are added. B. Significantly increases the brittle transition temperature of steel. C. In Fe-Cr alloys with high chromium content, the impact toughness drops sharply if the σ phase precipitates. (3) Effects on the physical, chemical, and mechanical properties of steel: A. It improves the wear resistance of steel, and higher surface finish can be achieved through grinding. B. Reduce the electrical conductivity of steel and lower the temperature coefficient of resistance. C. Increases the coercivity and residual magnetization of steel. Widely used in the manufacture of permanent magnets. D. Chromium promotes the formation of a passivation film on the surface of steel; when present in sufficient amounts, it significantly improves the corrosion resistance of steel (especially against nitric acid). The precipitation of chromium carbides reduces the corrosion resistance of the steel. E. Improve the oxidation resistance of steel. F. Dendritic segregation tends to form in chromium steel, reducing the plasticity of the steel. G. Since chromium reduces the thermal conductivity of steel, the temperature should be increased slowly during hot working, and slow cooling is required after forging and rolling. (4) Applications in steel: A. In alloy structural steel, chromium is primarily used to improve hardenability, and chromium-containing carbides can be formed on the carburized surface to enhance wear resistance. B. The comprehensive properties provided in spring steel by chromium and other alloying elements together. C. In bearing steel, the special carbides of chromium are primarily utilized for their contribution to wear resistance and the advantage of high surface smoothness after grinding. D. In tool steels and high-speed steels, chromium is primarily used to enhance wear resistance, and they also possess certain temper stability and toughness. In stainless steel and heat-resistant steel, chromium is often used in combination with elements such as manganese, nitrogen, and nickel. When it is necessary to create austenitic steel, there must be a certain ratio between the chromium, which stabilizes ferrite, and the manganese and nickel, which stabilize austenite; examples include Cr18Ni9. F. China has limited chromium resources, so its use should be conserved as much as possible. 2. Molybdenum (Mo): Molybdenum can improve the hardenability and thermal strength of steel. Prevents temper brittleness, increases residual magnetism and coercivity, as well as corrosion resistance in certain media. In quenched and tempered steel, molybdenum enables deeper and more thorough hardening of parts with larger cross-sections, improves the steel’s resistance to tempering or its tempering stability, allowing the parts to be tempered at higher temperatures. This facilitates more effective elimination (or reduction) of residual stresses and enhances plasticity. In carburized steel, in addition to the aforementioned functions, molybdenum can also reduce the tendency of carbides to form continuous networks at grain boundaries within the carburized layer, decrease the amount of residual austenite in that layer, and thereby increase the wear resistance of the surface layer. In die steels, molybdenum also helps maintain a relatively stable hardness in the steel, thereby increasing its resistance to deformation, cracking, and wear. In stainless acid-resistant steel, molybdenum further enhances resistance to acids such as formic acid, acetic acid, oxalic acid, etc., as well as to hydrogen peroxide, sulfuric acid, sulfurous acid, sulfates, acidic dyes, and bleaching solutions. In particular, the addition of molybdenum prevents the tendency to pitting corrosion caused by the presence of chloride ions. W12Cr4V4Mo high-speed steel, containing about 1% molybdenum, possesses high wear resistance, temper hardness, and red hardness. (1) Effects on the microstructure of steel and heat treatment: A. Molybdenum can be dissolved in ferrite, austenite, and carbides in steel; it is an element that reduces the austenite region. B. When the steel content is low, cementite that forms a compound with iron and carbon ； At high levels, it can form special carbides in steel. C. Molybdenum improves the hardenability of steel, with an effect stronger than that of chromium but weaker than that of manganese. D. Molybdenum improves the temper stability of steel; when present as a single alloying element, it increases the temper brittleness of steel ； In the presence of elements such as chromium and manganese, molybdenum further reduces or suppresses the temper brittleness caused by these other elements. (2) Effects on the mechanical properties of steel: A. Molybdenum exerts solid solution strengthening on ferrite; it also increases the stability of carbides, thereby enhancing the strength of the steel. B. Molybdenum plays a positive role in improving the ductility and toughness of steel, as well as its wear resistance. C. Since molybdenum raises the softening and recovery temperatures as well as the recrystallization temperature resulting from strain strengthening, and significantly enhances the creep resistance of ferrite, it effectively prevents the aggregation of cementite at 450–600°C. It also promotes the precipitation of special carbides; therefore, it is the most effective alloying element for improving the heat strength of steel. (3) Effects on the physical, chemical, and mechanical properties of steel: A. In magnetic steel with 1.5% carbon content, 2%-3% of steel increases the residual magnetism and coercivity. B. It can passivate the steel surface in both reducing acid and strongly oxidizing salt solutions. Therefore, molybdenum can generally improve the corrosion resistance of steel and prevent pitting corrosion in chloride solutions. C. A high molybdenum content (>3%) deteriorates the oxidation resistance of the steel. D. Steels with a molybdenum content of no more than 8% can still be forged and rolled, but at higher levels, the steel’s resistance to deformation during hot processing increases. (4) Applications in steel: It is widely used in quenched and tempered as well as carburized structural steels, spring steels, bearing steels, tool steels, stainless and acid-resistant steels, heat-resistant steels, and magnetic steels. B. Chromomolybdenum steel can replace chromium-nickel steel in many cases to manufacture important components. C. China is rich in molybdenum, but its reserves are not abundant on a global scale. Molybdenum-containing steel should be developed appropriately in China, but since molybdenum is an important strategic material, its use should be rational and economical. 3. Silicon (Si): Silicon can dissolve in ferrite and austenite, thereby increasing the hardness and strength of steel. Its effect is second only to that of phosphorus, and it is stronger than elements such as manganese, nickel, chromium, tungsten, molybdenum, and vanadium. However, when the silicon content exceeds 3%, it significantly reduces the plasticity and toughness of the steel. Silicon can increase the elastic limit, yield strength, and yield ratio (σs/σb) of steel, as well as its fatigue strength and fatigue ratio (σ-1/σb). This is why silicon-containing or silicomanganese steel can be used as spring steel. Silicon can reduce the density, thermal conductivity, and electrical conductivity of steel. It can promote the coarsening of ferrite grains. Reduce coercivity. It has the tendency to reduce the anisotropy of the crystal, facilitating magnetization and reducing magnetic resistance; it can be used in the production of electrical steel. As a result, silicon steel sheets have low hysteresis losses. Silicon can increase the permeability of ferrite, enabling silicon steel sheets to achieve a high magnetic flux density even in weak magnetic fields. However, under a strong magnetic field, silicon reduces the magnetic susceptibility of steel. Silicon reduces the magnetic aging effect of iron due to its strong deoxidizing ability. When silicon-containing steel is heated in an oxidizing atmosphere, a SiO2 film forms on its surface, thereby improving the steel’s oxidation resistance at high temperatures. Silicon can promote the growth of columnar crystals in cast steel, reducing its plasticity. If silicon steel is heated or cooled rapidly, due to its low thermal conductivity, a large temperature difference arises between the interior and exterior of the steel, making it prone to cracking. Silicon can reduce the weldability of steel. Because silicon has a stronger affinity for oxygen than iron, low-melting-point silicates are easily formed during welding, increasing the fluidity of the slag and molten metal and causing spattering, which affects the quality of the weld. Silicon is a good deoxidizer. Adding an appropriate amount of silicon during aluminum deoxidation can significantly enhance the deoxidizing capacity of aluminum. Silicon is already present in steel to a certain extent, as it is introduced as a raw material during iron and steel production. In boiling steel, silicon is limited to <0.07%; when added intentionally, ferrosilicon is incorporated during steelmaking. (1) Effects on the microstructure of steel and heat treatment: A. As an alloying element in steel, its content is generally not less than 0.4 %. It exists in the form of a solid solution within ferrite or austenite, reducing the austenite phase region. B. Increase the annealing, normalizing, and quenching temperatures to improve hardenability in hypoeutectoid steels. C. Silicon does not form carbides, and it has a strong effect in promoting the graphitization of carbon. In medium and high carbon steels with a high silicon content, if there are no elements that promote the formation of strong carbides, graphitization can occur under certain temperature conditions. D. In carburized steel, silicon reduces the thickness of the carburized layer and the carbon concentration. E. Silicon has a good deoxidizing effect on molten steel. (2) Effect on the mechanical properties of steel: A. It increases the hardness and strength of ferrite and austenite, with an effect that is stronger than that of Mn, Ni, Cr, W, Mo, V, etc ； It significantly increases the elastic limit, yield strength, and yield strength ratio (σs/σb) of steel. It also enhances the fatigue strength and fatigue ratio (σ-1/σb). B. When the silicon content exceeds 3%, it significantly reduces the plasticity and toughness of the steel ； Silicon raises the plastic/brittle transition temperature. C. Silicon tends to cause banding in steel, resulting in lower transverse properties compared to longitudinal properties. D. Improve the wear resistance of steel. (3) Effects on the physical, chemical, and mechanical properties of steel: A. Reducing the density, thermal conductivity, electrical conductivity, and temperature coefficient of resistance of steel. B. The eddy current loss of silicon steel sheets is significantly lower than that of pure iron; their coercivity, magnetic reluctance, and hysteresis loss are also lower, while their permeability and magnetic flux density are higher. But in a strong magnetic field, silicon reduces the magnetic flux density. C. It improves the oxidation resistance of steel at high temperatures, but excessive silicon content exacerbates surface decarburization. D. Steels with a silicon content of over 2.5% are difficult to deform. E. Silicon reduces the weldability of steel. (4) Applications in steel: A. It increases strength in ordinary low-alloy steels and improves resistance to local corrosion; it enhances hardenability and temper resistance in quenched and tempered steels. It is one of the main alloying elements in multi-element alloy structural steels. B. SiMn or SiMnB steels with a silicon content of 0.5%–2.8% (and a carbon content of 0.5%–0.7%) are widely used in high-load spring materials, with strong carbide-forming elements such as W, V, Mo, Nb, and Cr being added. C. Silicon steel sheets are low-carbon and ultra-low-carbon steels containing 1.0%–4.5% silicon, used in motors and transformers. D. In stainless steels and corrosion-resistant steels, it works in combination with elements such as Mo, W, Cr, Al, Ti, and N to enhance corrosion resistance and resistance to high-temperature oxidation. E. Graphite steel with a high silicon content is used as material for cold working molds. 4. Manganese (Mn): Manganese is an excellent deoxidizer and desulfurizer. Steel generally contains a certain amount of manganese, which can eliminate or reduce the thermal brittleness of steel caused by sulfur, thereby improving its hot workability. Manganese and iron form a solid solution, increasing the hardness and strength of ferrite and austenite in steel ； It is also a carbide-forming element that enters the cementite to replace some of the iron atoms. Manganese in steel reduces the critical transformation temperature. It serves to refine pearlite. It also indirectly contributes to increasing the strength of pearlitic steel ； Manganese’s ability to stabilize austenite is second only to nickel, and it also significantly increases the hardenability of steel. Various alloy steels have been produced by combining manganese, in amounts of no more than 2%, with other elements. Manganese is characterized by its abundance and diverse functionalities, which have led to its wide use in applications such as carbon structural steel and spring steel with high manganese content. In high-carbon, high-manganese wear-resistant steel. The manganese content can range from 10% to 14%; after solution treatment, it exhibits good toughness. When deformed due to impact, the surface layer becomes strengthened as a result of this deformation, granting it high wear resistance. Manganese forms MnS with a high melting point in combination with sulfur. It can prevent hot cracking caused by FeS. Manganese has a tendency to increase grain coarsening in steel and sensitivity to temper brittleness. If cooling after smelting, casting, forging, and rolling is not properly carried out, it is easy for white spots to form in steel. (1) Effects on the microstructure of steel and heat treatment: A. Manganese is an excellent deoxidizer and desulfurizer, and industrial steels generally contain a certain amount of manganese. B. Manganese is fully dissolved in ferrite and austenite. It expands the austenite region, raising the critical temperature A4 and lowering point A3; the (α+γ) region shifts downward. When the manganese content exceeds 12%, the upper critical temperature drops below room temperature, resulting in a single austenite structure in the steel at room temperature. While lowering the eutectoid temperature, the carbon content in the eutectoid is reduced. C: Manganese significantly lowers the Ar1 and martensitic transformation temperatures of steel (its effect is second only to that of carbon), as well as the rate of phase transformations in steel. It also improves the hardenability of steel and increases the content of retained austenite. D. It makes the quenched and tempered structure of steel uniform and refined, preventing the agglomeration of carbides in the carburized layer; however, it increases the steel’s susceptibility to overheating and its tendency toward temper brittleness. E. Manganese is a weak carbide-forming element. (2) Effect on the mechanical properties of steel: A. Manganese’s effect on strengthening ferrite or austenite is less than that of carbon; phosphorus and silicon, while increasing strength, have no effect on ductility. B. By refining the pearlite, it significantly increases the strength of low-carbon and medium-carbon pearlitic steels, resulting in a decrease in ductility. C. The mechanical properties of quenched and tempered sorbite steel have been improved by enhancing hardenability. D. Under strict control of the heat treatment process to prevent grain growth due to overheating as well as temper brittleness, manganese does not reduce the toughness of steel. (3) Effects on the physical, chemical, and mechanical properties of steel: A. As the manganese content increases, the thermal conductivity of steel decreases sharply, while its linear expansion coefficient rises; this leads to the formation of large internal stresses during rapid heating or cooling, increasing the tendency for the workpiece to crack. B. It causes the electrical conductivity of steel to decrease sharply, the resistivity to increase accordingly, and the temperature coefficient of resistance to decline. C. It increases coercivity, while reducing saturation magnetization, remanent magnetization, and magnetic permeability; therefore, manganese is beneficial for permanent magnet alloys but harmful for soft magnet alloys. D. The oxidation resistance of steel decreases when the manganese content is high. E. It causes sulfur in the steel to form MnS with a higher melting point, thereby preventing the formation of FeS films at the grain boundaries, eliminating the thermal brittleness of the steel, and improving its hot workability. F. High-manganese austenitic steel has high deformation resistance, and the columnar crystals in the ingot are prominent, making it prone to cracking during forging and rolling. G. The improved hardenability and reduced martensite transformation temperature have an adverse effect on weldability. The carbon content should be reduced within an appropriate range. (4) Applications in steel: A. Free-cutting steels often contain appropriate amounts of manganese and phosphorus; MnS inclusions facilitate the fragmentation of chips. B. In ordinary low-alloy steels, manganese is used to strengthen ferrite and pearlite, thereby increasing the strength of the steel; the manganese content is generally 1%–2%. C. Many series of carburized and quenched and tempered alloy structural steels contain no more than 2% manganese. D. In spring steel, bearing steel, and tool steel, the strong hardening effect of manganese is utilized, allowing for quenching processes using oil or air cooling, which helps to reduce cracking, warping, and deformation. E. Wear-resistant steel, non-magnetic steel, stainless steel, and heat-resistant steel, including high-carbon, high-manganese wear-resistant cast steel (C: 1.0%–1.4%, Mn: 10%–14%), medium-carbon, high-manganese non-magnetic steel (C: 0.3%–0.6%, Mn: 18%–19%), low-carbon, high-manganese stainless steel (with Cr, without Ni or with little Ni), and high-manganese heat-resistant steel (heat-resistant steel that uses Mn instead of Ni, or steel containing Al, Mo, V, etc.). 5. Sulfur (S): Sulfur is generally considered a harmful element in steel. It exists mainly in the forms of MnS and FeS, and can cause thermal brittleness in steel as well as reduce its mechanical properties, particularly its yield strength, plasticity, and wear resistance. The presence of sulfur also has an impact on the corrosion resistance and weldability of steel. Generally, it is required that ω(S) be ≤0.050% in ordinary steel, ≤0.030% in high-quality steel, and ≤0.020% in ultra-high-quality steel. However, in some types of steel (such as cutting steel and magnetic steel), the addition of an appropriate amount of sulfur – with ω(S) reaching up to 0.35% in some cases – can improve the steel’s machinability, workability, and magnetic properties. Increasing the content of sulfur and manganese can improve the machinability of steel; in free-cutting steels, sulfur is added as a beneficial element. Sulfur segregates severely in steel, deteriorating its quality. At high temperatures, it reduces the plasticity of steel; it is a harmful element that exists in the form of FeS, which has a low melting point ； Pure FeS has a melting point of only 1190°C, while the eutectic temperature of FeS forming an eutectic with iron in steel is even lower, at 988°C. When steel solidifies, iron sulfide accumulates at the primary grain boundaries. When steel is rolled at temperatures of 1100–1200°C, the FeS at the grain boundaries melts, **weakening the bonds between the grains and leading to the thermochronic fragility of the steel. Therefore, sulfur content must be strictly controlled, generally kept between 0.020% and 0.050%. To prevent brittleness caused by sulfur, sufficient manganese should be added to form MnS with a higher melting point. If the sulfur content in the steel is high, the formation of SO2 during welding will result in pores and porosity within the welded metal. (1) Effects of nitrogen on the microstructure of steel and heat treatment: A. Like carbon, nitrogen can be dissolved in iron to form interstitial solid solutions. B, nitrogen expands the austenite region in steel; it is a highly effective element for forming and stabilizing austenite, with an effectiveness about 20 times that of nickel, and can replace a portion of nickel in steel up to a certain limit. C. Nitrogen that penetrates the steel surface can combine with elements such as chromium, aluminum, vanadium, and titanium to form extremely stable nitrides, which serve as elements for surface hardening and strengthening. D. Nitrogen makes the microstructure of high-chromium and high-chromium-nickel steels dense and firm. E. Excess residual nitrogen in steel can lead to a loose macrostructure or pores. (2) Effect on the mechanical properties of steel: A. Nitrogen has a solid solution strengthening effect. B. In nitrogen-containing ferritic steels, precipitation hardening can occur during tempering after rapid cooling or upon prolonged exposure at room temperature, due to the precipitation of ultra-microscopic nitrides. Nitrogen also induces strain aging in low-carbon steels. As strength and hardness increase, the toughness of steel decreases and its sensitivity to notches rises. Nitrogen causes steel to become brittle in a manner similar to phosphorus, and its effect is much greater than that of phosphorus; nitrogen is also a major cause of blue brittleness in steel. C. It increases the strength of high-chromium and high-chromium-nickel steels, without reducing their plasticity, and it also significantly improves their impact toughness. D. Nitrogen can also increase the creep and high-temperature endurance strength of steel. (3) Effects on the physical, chemical, and mechanical properties of steel. A. Nitrogen has no significant effect on the corrosion resistance of stainless steel. B, it also has no significant effect on the high-temperature oxidation resistance of steel; excessive nitrogen content (e.g., >0.16%) can degrade this oxidation resistance. C. Nitrogen-containing steels exhibit a high rate of hardening due to cold deformation, and this should be taken into account when using cold deformation processes. D. Nitrogen can reduce the grain growth tendency of high-chromium ferritic steel, thereby improving its weldability. (4) Applications in steel: A. Nitrogen, as an alloying element, is generally present in steel in amounts of less than 0.3%, though in special cases it can reach up to 0.6%. B. It is mainly applied to nitrided and quenched structural steels, ordinary low-alloy steels, stainless and acid-resistant steels, as well as heat-resistant steel that does not scale. The use of nitrogen as an alloying element in steel is continuing to expand. 6. Phosphorus (P): Phosphorus exerts a strong effect of solid solution strengthening and cold work hardening in steel. When added as an alloying element to low-alloy structural steel, it improves the strength of the steel as well as its resistance to atmospheric corrosion, but it reduces its cold stamping properties. The combined use of phosphorus with sulfur and manganese improves the machinability of steel and enhances the surface quality of the machined parts; it is used in easy-to-machine steels, which is why such steels also contain high levels of phosphorus. Phosphorus dissolves in ferrite; although it can increase the strength and hardness of steel, its greatest drawback is severe segregation, which increases temper brittleness and significantly reduces the plasticity and toughness of the steel. As a result, the steel is prone to brittle fracture during cold working, a phenomenon known as \"cold brittleness\". Phosphorus also has an adverse effect on weldability. Phosphorus is a harmful element that must be strictly controlled, with its content generally not exceeding 0.030%-0.040%. 7. Carbon (C): As the carbon content in steel increases, its yield strength and tensile strength rise, but its ductility and impact resistance decrease. When the carbon content exceeds 0.23%, the weldability of the steel deteriorates; therefore, in low-alloy structural steels used for welding, the carbon content is generally kept below 0.20%. A high carbon content also reduces the steel’s resistance to atmospheric corrosion; high-carbon steel in outdoor storage areas is prone to rusting ； Furthermore, carbon can increase the cold brittleness and aging sensitivity of steel. Typical examples are low-carbon steel, high-carbon steel, and the changes in the mechanical properties of high-carbon steel.