The Effects of 49 Elements on Steel Properties! Such a Comprehensive Summary!

The Effects of 49 Elements on Steel Properties! Such a Comprehensive Summary!

| H (Hydrogen) | Hydrogen is the most harmful element in general steel. Dissolved hydrogen in steel can cause defects such as hydrogen embrittlement and white spots. Like oxygen and nitrogen, hydrogen has extremely low solubility in solid steel. It dissolves into molten steel at high temperatures but doesn’t have time to escape during cooling, accumulating in the microstructure and forming high-pressure microscopic pores. This sharply reduces the steel’s plasticity, toughness, and fatigue strength, and in severe cases can lead to cracks and brittle fractures. “Hydrogen embrittlement” mainly occurs in martensitic steel and is less pronounced in ferritic steel, increasing alongside hardness and carbon content.

On the other hand, hydrogen can increase the magnetic permeability of steel but also raises coercive force and iron loss (after adding hydrogen, coercive force can increase by 0.5 to 2 times).

| B (Boron) | The main role of boron in steel is to increase the steel’s hardenability, thus saving other rarer and more precious metals like nickel, chromium, and molybdenum. For this purpose, its content is generally specified in the range of 0.001%–0.005%. It can replace 1.6% nickel, 0.3% chromium, or 0.2% molybdenum. When substituting boron for molybdenum, care should be taken because molybdenum can prevent or reduce temper brittleness, while boron slightly promotes temper brittleness. Therefore, boron cannot completely replace molybdenum.

In medium-carbon steel, adding boron can significantly improve the performance of steel materials over 20mm thick after quenching and tempering due to increased hardenability. Therefore, 40B and 40MnB steels can replace 40Cr, and 20Mn2TiB steel can replace carburized 20CrMnTi steel. However, since the effect of boron diminishes with increasing carbon content in steel and may even disappear, when selecting boron-containing carburized steel, it is necessary to consider that after carburizing, the hardenability of the carburized layer will be lower than that of the core.

Spring steel generally requires complete hardenability, and since the cross-sectional area of springs is not large, using boron-containing steel is advantageous. However, in high-silicon spring steels, the effect of boron varies greatly, making it inconvenient to use.

Boron has a strong affinity for nitrogen and oxygen. Adding 0.007% boron to killed steel can eliminate the aging phenomenon in steel.

| C (Carbon) | Carbon is the main element after iron and directly affects the strength, plasticity, toughness, and weldability of steel.

When the carbon content in steel is below 0.8%, as the carbon content increases, the strength and hardness of the steel increase, while plasticity and toughness decrease. However, when the carbon content exceeds 1.0%, the strength of the steel decreases with further increases in carbon content. As carbon content increases, the weldability of steel deteriorates (when carbon content is greater than 0.3%, weldability significantly decreases), cold brittleness and aging sensitivity increase, and atmospheric corrosion resistance decreases.

| N (Nitrogen) | Nitrogen’s effect on steel properties is similar to that of carbon and phosphorus. As nitrogen content increases, the strength of the steel significantly increases, but plasticity, especially toughness, markedly decreases. Weldability worsens, and cold brittleness intensifies. At the same time, the tendency for aging, cold brittleness, and hot brittleness increases, damaging the steel’s welding performance and cold bending performance. Therefore, the nitrogen content in steel should be minimized and limited. Generally, nitrogen content should not exceed 0.018%.

Nitrogen can reduce its adverse effects and improve steel properties in coordination with elements like aluminum, niobium, and vanadium, and can be used as an alloying element in low-alloy steel. In some stainless steels, appropriately increasing the nitrogen content can reduce the use of chromium, effectively lowering costs.

| O (Oxygen) | Oxygen is a harmful element in steel. It naturally enters steel during the steelmaking process. Although manganese, silicon, iron, and aluminum are added at the end of steelmaking for deoxidation, it cannot be completely removed. During steel solidification, oxygen in the solution reacts with carbon to generate carbon monoxide, which can cause gas bubbles. Oxygen mainly exists in steel in the form of inclusions like FeO, MnO, SiO₂, and Al₂O₃, reducing the strength and plasticity of steel. It has a severe impact on fatigue strength and impact toughness.

Oxygen can increase iron loss in silicon steel, reduce magnetic permeability and magnetic induction strength, and intensify magnetic aging.

| Mg (Magnesium) | Magnesium can reduce the number of inclusions in steel, decrease their size, make their distribution uniform, and improve their shape. Trace amounts of magnesium can improve the size and distribution of carbides in bearing steel; magnesium-containing bearing steel has fine and uniform carbide particles. When the magnesium content is 0.002%–0.003%, the tensile strength and yield strength increase by more than 5%, and plasticity remains basically unchanged.

| Al (Aluminum) | Aluminum is added to steel as a deoxidizer or alloying element. Aluminum’s deoxidizing ability is much stronger than that of silicon and manganese. The main role of aluminum in steel is to refine grain size and fix nitrogen in the steel, thereby significantly improving the steel’s impact toughness and reducing the tendency for cold brittleness and aging. For example, D-grade carbon structural steel requires that the acid-soluble aluminum content in the steel be not less than 0.015%; deep-drawing cold-rolled steel sheet 08Al requires an acid-soluble aluminum content of 0.015%–0.065%.

Aluminum can also improve the corrosion resistance of steel, especially when used in combination with molybdenum, copper, silicon, and chromium.

In chromium-molybdenum steel and chromium steel, the presence of aluminum can increase wear resistance. In high-carbon tool steel, the presence of aluminum can cause quench brittleness. The disadvantage of aluminum is that it affects the steel’s hot-working performance, weldability, and machinability.

| Si (Silicon) | Silicon is an important reducing agent and deoxidizer in the steelmaking process. In many grades of carbon steel, the silicon content does not exceed 0.5%; this silicon is generally introduced as a reducing agent and deoxidizer during steelmaking.

Silicon can dissolve in ferrite and austenite, increasing the steel’s hardness and strength; its effect is second only to phosphorus and stronger than manganese, nickel, chromium, tungsten, molybdenum, and vanadium. However, when the silicon content exceeds 3%, the steel’s plasticity and toughness decrease significantly. Silicon can increase the steel’s elastic limit, yield strength, yield ratio (σs/σb), fatigue strength, and fatigue ratio (σ-1/σb). This is why silicon or silicon-manganese steel can be used as spring steel.

Silicon reduces the steel’s density, thermal conductivity, and electrical conductivity. It can promote the coarsening of ferrite grains and reduce coercive force. It reduces the tendency for crystal anisotropy, making magnetization easier and reducing magnetic resistance, and can be used to produce electrical steel; thus, the hysteresis loss of silicon steel sheet is lower. Silicon can increase the magnetic permeability of ferrite, allowing steel sheets to have higher magnetic induction under weak magnetic fields. However, silicon reduces the steel’s magnetic induction in strong magnetic fields. Because it has a strong deoxidizing ability, silicon reduces the steel’s magnetic aging effect.

When steel containing silicon is heated in an oxidizing atmosphere, a thin layer of SiO₂ forms on the surface, improving the steel’s oxidation resistance at high temperatures.

Silicon can promote the growth of columnar crystals in cast steel, reducing plasticity. If silicon steel is cooled rapidly after heating, due to low thermal conductivity, the temperature difference between the inside and outside of the steel is large, which can lead to fractures.

Silicon can reduce the steel’s weldability. Because silicon has a stronger ability to combine with oxygen than iron, low-melting-point silicate is easily formed during welding, increasing the fluidity of slag and molten metal, causing spatter and affecting weld quality. Silicon is a good deoxidizer; when deoxidizing with aluminum, adding a certain amount of silicon can significantly improve the deoxidation efficiency. Silicon is originally present in steel in a certain amount because it is introduced as raw material during ironmaking and steelmaking. In rimmed steel, silicon is limited to <0.07%; when intentionally added, it is introduced into the steel during steelmaking as ferrosilicon alloy.

| P (Phosphorus) | Phosphorus is introduced into steel from ore and is generally considered a harmful element. Although phosphorus can increase the steel’s strength and hardness, it significantly reduces the steel’s plasticity and impact toughness. Especially at low temperatures, it makes the steel noticeably brittle, a phenomenon known as “cold brittleness.” Cold brittleness worsens the steel’s cold working and weldability; the higher the phosphorus content, the greater the cold brittleness. Therefore, the phosphorus content in steel is strictly controlled. High-grade quality steel: P < 0.025%; quality steel: P < 0.04%; ordinary steel: P < 0.085%.

Phosphorus has a strong effect on solid solution strengthening and strain hardening. Combined with copper, it improves the atmospheric corrosion resistance of low-alloy high-strength steel but reduces its cold stamping performance. Combined with sulfur and manganese, it improves machinability, increases temper brittleness and cold brittleness sensitivity.

Phosphorus can increase resistivity, and due to its tendency to promote grain coarsening, can reduce coercive force and eddy current losses. Regarding magnetic induction, in weak to medium magnetic fields, steel with high phosphorus content has higher magnetic induction; the phosphorus content in cold-rolled electrical silicon steel is 0.07%–0.10%. Phosphorus is the strongest element for strengthening ferrite. (The effect of phosphorus on the recrystallization temperature and grain growth in silicon steel exceeds that of the same silicon content by 4–5 times.)

| S (Sulfur) | Sulfur comes from ore and coke fuel during steelmaking. It is a harmful element in steel. Sulfur exists in steel mainly in the form of iron sulfide (FeS); FeS and Fe form a low-melting-point (985℃) compound. Since the hot working temperature of steel is generally above 1150–1200℃, during hot working, the premature melting of the FeS compound causes the workpiece to crack, a phenomenon known as “hot shortness.” Sulfur reduces the steel’s ductility and toughness, causing cracks during forging and rolling. Sulfur is also detrimental to weldability and reduces the steel’s corrosion resistance. High-grade quality steel: S < 0.02%–0.03%; quality steel: S < 0.03%–0.045%; ordinary steel: S < 0.055%–0.7% or less.

Because the chips are brittle, a very smooth surface can be obtained, so sulfur-containing steel can be used to make steel parts that do not require high loads but have high surface finish (known as free-cutting steel), such as Cr14, where a small amount of sulfur (0.2%–0.4%) is intentionally added. Some high-speed tool steels undergo surface sulfide treatment.

| K/Na (Potassium/Sodium) | Potassium and sodium can be used as modifiers to spheroidize carbides in white cast iron, doubling its toughness while maintaining the original hardness; they refine the structure in nodular cast iron and stabilize the treatment process of malleable iron. They are strong promoters of austenitization; for example, they can reduce the manganese/carbon ratio in austenitic manganese steel from 10:1–13:1 to 4:1–5:1.

| Ca (Calcium) | Adding calcium to steel can refine grains, partially desulfurize, and change the composition, quantity, and morphology of non-metallic inclusions. Its effect in steel is similar to that of rare earth elements.

It improves the steel’s corrosion resistance, wear resistance, high-temperature and low-temperature properties; increases the steel’s impact toughness, fatigue strength, plasticity, and weldability; enhances the steel’s cold heading ability, shock resistance, hardness, and contact fatigue strength.

Adding calcium to cast steel greatly improves the fluidity of molten steel; the surface quality of castings is improved, and anisotropy in the casting organization is eliminated; casting performance, resistance to thermal cracking, mechanical properties, and machinability are all improved to varying degrees.

Adding calcium to steel can improve resistance to hydrogen-induced cracking and lamellar tearing, extending the service life of equipment and tools. Calcium added to master alloys can be used as a deoxidizer and inoculant, playing a role in microalloying.

| Ti (Titanium) | Titanium has a strong affinity for nitrogen, oxygen, and carbon, and its affinity for sulfur is stronger than that of iron; it is a good deoxidizer, degasser, and an effective element for fixing nitrogen and carbon. Although titanium is a strong carbide-forming element, it does not form complex carbides with other elements. Titanium carbide has a strong bond, is stable, and does not easily decompose; in steel, it slowly dissolves into the solid solution only when heated above 1000℃. Before dissolving, titanium carbide particles can prevent grain growth. Because titanium’s affinity for carbon is much greater than that of chromium, titanium is often used in stainless steel to fix carbon and eliminate chromium depletion at grain boundaries, thereby eliminating or reducing intergranular corrosion of the steel.

Titanium is one of the strong ferrite-forming elements, significantly increasing the steel’s A1 and A3 temperatures. Titanium in ordinary low-alloy steel can improve plasticity and toughness. Because titanium fixes nitrogen and sulfur and forms titanium carbide, it increases the strength of the steel. After normalizing, it refines the grains, and the precipitated carbides can significantly improve the steel’s plasticity and impact toughness. Structural steels containing titanium have good mechanical and technological properties; their main drawback is slightly reduced hardenability.

In high-chromium stainless steel, about five times the carbon content of titanium is usually added, which not only can improve the steel’s corrosion resistance (mainly intergranular corrosion) and toughness but also inhibit the grain growth tendency at high temperatures and improve the steel’s weldability.

| V (Vanadium) | Vanadium has a strong affinity for carbon, nitrogen, and oxygen, forming corresponding stable compounds. Vanadium mainly exists in steel in the form of carbides. Its main function is to refine the steel’s structure and grains, reducing the steel’s strength and toughness. When dissolved in the solid solution at high temperatures, it increases hardenability; conversely, if it exists in the form of carbides, it reduces hardenability. Vanadium increases the tempering stability of quenched steel and produces a secondary hardening effect. Except for high-speed tool steel, the vanadium content in steel generally does not exceed 0.5%.

Vanadium in ordinary low-carbon alloy steel can refine grains, increase strength and yield ratio after normalizing, and improve low-temperature properties, enhancing the steel’s weldability.

In alloy structural steel, vanadium often reduces hardenability under general heat treatment conditions; therefore, in structural steels, it is commonly used in combination with manganese, chromium, molybdenum, and tungsten. In quenched and tempered steels, vanadium mainly increases the steel’s strength and yield ratio, refines grains, and reduces overheating sensitivity. In carburized steels, because it can refine grains, the steel can be directly quenched after carburizing without secondary quenching.

In spring steel and bearing steel, vanadium can increase strength and yield ratio, especially increasing proportional limit and elastic limit, reducing decarburization sensitivity during heat treatment, thereby improving surface quality. Chromium-containing bearing steel with vanadium has a high degree of carbide dispersion and excellent service performance.

In tool steel, vanadium refines grains, reduces overheating sensitivity, increases tempering stability and wear resistance, thereby extending tool life.

| Cr (Chromium) | Chromium can increase the steel’s hardenability and has a secondary hardening effect; it can increase the hardness and wear resistance of carbon steel without making the steel brittle. When the content exceeds 12%, chromium gives the steel good high-temperature oxidation resistance and oxidizing corrosion resistance, and increases the steel’s heat strength. Chromium is the main alloying element in stainless steel, acid-resistant steel, and heat-resistant steel.

Chromium can increase the strength and hardness of carbon steel in the rolled state and reduce elongation and reduction of area. When the chromium content exceeds 15%, strength and hardness decrease, and elongation and reduction of area increase correspondingly. Steel parts containing chromium can easily achieve high surface finish after grinding.

The main role of chromium in quenched and tempered structural steel is to increase hardenability so that the steel has good comprehensive mechanical properties after quenching and tempering; in carburized steel, it can form chromium-containing carbides, thereby increasing surface wear resistance.

Chromium-containing spring steel is not prone to decarburization during heat treatment. Chromium can increase the wear resistance, hardness, and red hardness of tool steel and has good tempering stability. In electrothermal alloys, chromium can improve the alloy’s oxidation resistance, electrical resistance, and strength.

| Mn (Manganese) | Manganese can increase steel strength: because manganese is relatively inexpensive and can form an unlimited solid solution with iron, increasing steel strength while having a relatively small impact on plasticity. Therefore, manganese is widely used as a strengthening element in steel. It can be said that practically all carbon steels contain manganese. Commonly seen stamping soft steel, dual-phase steel (DP steel), transformation-induced plasticity steel (TRIP steel), and martensitic steel (MS steel) all contain manganese. Generally, the manganese content in soft steel does not exceed 0.5%; in high-strength steel, the manganese content increases with the increase in strength level; for example, in martensitic steel, the manganese content can be as high as 3%.

Manganese increases the steel’s hardenability and improves the steel’s hot-working performance: a typical example is 40Mn and 40 steel. Manganese can eliminate the effect of sulfur: during steel smelting, manganese can form high-melting-point MnS with sulfur, thereby weakening and eliminating the adverse effects of sulfur.

However, manganese content is a double-edged sword. The manganese content is not necessarily better the higher it is. An increase in manganese content can reduce the steel’s plasticity and weldability.

| Co (Cobalt) | Cobalt is mostly used in special steels and alloys. High-speed steels containing cobalt have high high-temperature hardness; when cobalt is added together with molybdenum in martensitic aging steels, ultra-high hardness and good comprehensive mechanical properties can be obtained. In addition, cobalt is an important alloying element in heat-resistant steels and magnetic materials.

Cobalt reduces the hardenability of steel; therefore, adding it alone to carbon steel can reduce the comprehensive mechanical properties after quenching and tempering. Cobalt can strengthen ferrite; when added to carbon steel in annealed or normalized condition, it can increase the steel’s hardness, yield point, and tensile strength, but has adverse effects on elongation and reduction of area; impact toughness also decreases with increased cobalt content. Because cobalt has oxidation resistance, it is used in heat-resistant steel and heat-resistant alloys. Cobalt-based alloys show their unique properties more prominently in gas turbines.

| Ni (Nickel) | The beneficial effects of nickel are: high strength, high toughness, good hardenability, high electrical resistance, and high corrosion resistance.

On the one hand, nickel significantly increases the strength of steel; on the other hand, it maintains a very high level of toughness in iron. Its brittle transition temperature is very low. (When nickel content is less than 0.3%, the brittle transition temperature reaches below -100℃; when nickel content increases to about 4%–5%, the brittle transition temperature can drop to -180℃.) Therefore, it can simultaneously improve the strength and plasticity of quenched structural steel. Non-chromium steel containing 3.5% nickel can air-harden; chromium steel with 8% nickel can transform into martensite even at very low cooling rates.

Since the lattice constant of nickel is close to that of γ-iron, they can form a continuous solid solution. This is conducive to increasing the steel’s hardenability. Nickel can lower critical points and increase the stability of austenite, so the quenching temperature can be lowered, and hardenability is good. Steel with large cross-sectional dimensions are generally made with nickel-added steel. When combined with chromium, tungsten, or chromium and molybdenum, hardenability can be significantly increased. Nickel-molybdenum steel also has very high fatigue limits. (Nickel steel has good thermal fatigue resistance.)

In stainless steel, nickel is used to make the steel have a uniform austenitic structure, improving corrosion resistance. Nickel steel is generally not prone to overheating, so it can prevent grain growth at high temperatures and maintain a fine-grained structure.

| Cu (Copper) | Copper’s prominent role in steel is to improve the atmospheric corrosion resistance of ordinary low-alloy steel, especially when used in combination with phosphorus. Adding copper can also increase the steel’s strength and yield ratio without adversely affecting weldability. Copper-containing rail steel (U-Cu) with 0.20%–0.50% copper, in addition to being wear-resistant, has an atmospheric corrosion resistance life 2–5 times that of ordinary carbon steel rails.

When copper content exceeds 0.75%, after solution treatment and aging, aging strengthening can occur. At low contents, its effect is similar to nickel but weaker. At high contents, it adversely affects hot deformation processing, causing copper embrittlement during hot deformation. Copper at 2%–3% in austenitic stainless steel can improve corrosion resistance to sulfuric acid, phosphoric acid, hydrochloric acid, etc., and the stability against stress corrosion.

| Ga (Gallium) | Gallium in steel is an element that closes the γ-region. Trace gallium easily dissolves in ferrite, forming a substitutional solid solution. It is not a carbide-forming element and also does not form oxides, nitrides, or sulfides. In the γ + α two-phase region, trace gallium easily diffuses from austenite to ferrite, with a high concentration in ferrite. Trace gallium’s impact on the mechanical properties of steel is mainly solid solution strengthening. Gallium slightly improves steel’s corrosion resistance.

| As (Arsenic) | Arsenic from ore can only be partially removed during the sintering process; it can also be removed by chloridizing roasting. During blast furnace smelting, arsenic is completely reduced and enters pig iron. When arsenic content in steel exceeds 0.1%, it makes the steel more brittle and worsens its weldability. Arsenic content in the ore should be controlled, requiring that arsenic content should not exceed 0.07%. Arsenic tends to increase the yield point σs and tensile strength σb of low-carbon round steel and reduce elongation δ5; it has a more noticeable effect on decreasing the room temperature impact toughness Akv of ordinary carbon round steel.

| Se (Selenium) | Selenium can improve the machinability of carbon steel, stainless steel, and copper, resulting in a smooth surface on parts. In high magnetic induction oriented silicon steel, MnSe₂ is commonly used as an inhibitor. Beneficial MnSe₂ inclusions have a stronger inhibitory effect on primary recrystallization grain growth than beneficial MnS inclusions and are more conducive to promoting preferential grain growth during secondary recrystallization, thereby obtaining a highly oriented {110}[001] texture.

| Zr (Zirconium) | Zirconium is a strong carbide-forming element, and its role in steel is similar to that of niobium, tantalum, and vanadium. Adding a small amount of zirconium has deoxidizing, purifying, and grain refining effects, which is beneficial to the steel’s low-temperature properties and improves stamping performance. It is often used in ultra-high-strength steels and nickel-based superalloys used in gas engines and ballistic missile structures.

| Nb (Niobium) | Niobium often coexists with tantalum, and their roles in steel are similar. Niobium and tantalum partially dissolve into solid solution, playing a solid solution strengthening role. When dissolved in austenite, they significantly increase the steel’s hardenability. However, when present as fine particles of carbides and oxides, they refine grains and reduce the steel’s hardenability. They can increase the steel’s tempering stability, producing a secondary hardening effect. Trace amounts of niobium can increase the steel’s strength without affecting its plasticity or toughness. Due to its grain refining effect, it can improve the steel’s impact toughness and reduce its brittle transition temperature. When the content exceeds eight times the carbon content, it can almost fix all the carbon in the steel, giving the steel good resistance to hydrogen corrosion. In austenitic steel, it can prevent intergranular corrosion in oxidizing media. By fixing carbon and through precipitation hardening, it can improve high-temperature properties of heat-resistant steel, such as creep strength.

In ordinary low-alloy structural steel used in construction, niobium can increase yield strength and impact toughness, lower brittle transition temperature, and be beneficial for weldability. In carburized and quenched-tempered alloy structural steel, while increasing hardenability, it also improves steel’s toughness and low-temperature properties. It can reduce the air hardening tendency of low-carbon martensitic heat-resistant stainless steel, avoiding hardening and temper brittleness, and improving creep strength.

| Mo (Molybdenum) | Molybdenum in steel can increase hardenability and heat strength, prevent temper brittleness, increase residual magnetism and coercive force, and improve corrosion resistance in certain media.

In quenched and tempered steel, molybdenum can make large cross-sectional parts deep-hardenable, increasing the steel’s tempering resistance or tempering stability, allowing parts to be tempered at higher temperatures, thereby more effectively eliminating (or reducing) residual stresses and improving plasticity.

In carburized steel, in addition to the above, molybdenum can reduce the tendency for continuous network formation of carbides at grain boundaries in the carburized layer, reduce retained austenite in the carburized layer, relatively increasing the surface layer’s wear resistance.

In forging die steel, molybdenum can maintain relatively stable hardness, increase resistance to deformation, cracking, and wear.

In stainless acid-resistant steel, molybdenum can further improve resistance to organic acids (such as formic acid, acetic acid, and oxalic acid), as well as hydrogen peroxide, sulfuric acid, sulfurous acid, sulfates, acid dyes, and bleaching solutions. Especially due to the addition of molybdenum, the tendency for pitting corrosion in the presence of chloride ions is prevented. W12Cr4V4Mo high-speed steel containing about 1% molybdenum has wear resistance, tempering hardness, and red hardness.

| Sn (Tin) | Tin has always been considered a harmful impurity in steel, affecting the quality of steel products, especially the quality of continuous casting billets, causing hot brittleness, temper brittleness, crack formation, and fractures, affecting the steel’s weldability; it is one of the “five harmful elements” in steel. However, tin plays a very important role in electrical steel, cast iron, and free-cutting steel.

The grain size in silicon steel is related to tin segregation; tin segregation hinders grain growth. The higher the tin content, the greater the amount of segregation, effectively preventing grain growth. The smaller the grain size, the lower the hysteresis loss. Tin can change the magnetic properties of silicon steel, increasing the strength of the favorable texture {100} in oriented silicon steel, significantly increasing magnetic induction.

When a small amount of tin is added to cast iron, it can improve wear resistance and affect the fluidity of molten iron. Pearlitic malleable cast iron has high strength and wear resistance; to obtain as-cast pearlite, tin is added during smelting. Since tin is an element that hinders graphite spheroidization, its amount should be controlled, generally limited to ≤0.1%.

Free-cutting steels can be divided into sulfur-based, calcium-based, lead-based, and composite free-cutting steels. Tin has a significant tendency to segregate around inclusions and defects. Tin does not change the shape of sulfide inclusions in steel but increases brittleness by segregating at grain boundaries and phase boundaries, improving the steel’s machinability. When tin content is >0.05%, the steel has good machinability.

| Sb (Antimony) | In high magnetic induction oriented silicon steel, adding antimony results in refinement of primary and secondary recrystallization grains; the secondary recrystallized structure is more perfect, and magnetic properties improve. In antimony-containing steel, after cold rolling and decarburization annealing, favorable texture components {110}〈115〉 or {110}〈001〉 that promote secondary recrystallization are enhanced, increasing the number of secondary grains.

In antimony-containing constructional welding steel, at austenite temperature, antimony in steel precipitates at MnS inclusions and along original austenite grain boundaries, increasing enrichment and precipitation at MnS inclusions, refining the steel’s structure, and improving toughness.

| W (Tungsten) | Tungsten, in addition to forming carbides in steel, partially dissolves into iron, forming a solid solution. Its effect is similar to molybdenum but generally less significant by mass percentage. The main role of tungsten in steel is to increase tempering stability, red hardness, heat strength, and, due to carbide formation, increase wear resistance. Therefore, it is mainly used in tool steels, such as high-speed steel and hot forging die steels.

In high-quality spring steel, tungsten forms refractory carbides; when tempered at higher temperatures, it can slow down the coagulation process of carbides, maintaining high-temperature strength. Tungsten can also reduce the steel’s overheating sensitivity, increase hardenability, and improve hardness. 65SiMnWA spring steel has high hardness after hot rolling and air cooling; springs made from 50mm² cross-section steel can be oil-quenched, suitable for making important springs bearing heavy loads, heat-resistant (not exceeding 350℃), and subjected to impact. 30W4Cr2VA high-strength heat-resistant spring steel has high hardenability; after quenching at 1050℃–1100℃ and tempering at 550℃–650℃, tensile strength reaches 1470–1666 MPa. It is mainly used to make springs operating at high temperatures (not exceeding 500℃).

Due to the addition of tungsten, the wear resistance and cutting properties of steel can be significantly improved, making tungsten a main alloying element in alloy tool steels.

| Pb (Lead) | Lead can improve machinability. Lead-based free-cutting steels have good mechanical properties and heat treatment properties. Due to environmental pollution and harmful effects during scrap steel recovery smelting, lead is gradually being replaced.

Lead and iron hardly form solid solutions or compounds and easily segregate at grain boundaries in spherical form, which is the root cause of steel’s brittleness at 200℃–480℃ and weld cracking.

| Bi (Bismuth) | Adding 0.1%–0.4% bismuth to free-cutting steel can improve its machinability. When bismuth is uniformly dispersed in steel, bismuth micro-particles melt upon contact with the cutting tool, acting as a lubricant, and cause chip breakage, preventing overheating, thereby increasing cutting speed. Recently, bismuth has been added in large quantities to stainless steel to improve its machinability.

In free-cutting steel, bismuth exists in three forms: individually in the steel matrix, encapsulated by sulfides, or between the steel matrix and sulfides. In S-Bi free-cutting steel ingots, the deformation rate of MnS inclusions decreases with increasing bismuth content. Metallic bismuth can suppress sulfide deformation during steel ingot forging.

Adding 0.002%–0.005% bismuth to cast iron can improve the casting properties of malleable cast iron, increase the white cast iron tendency, and shorten annealing time; the elongation properties of parts improve. In nodular cast iron, adding 0.005% bismuth can improve its vibration resistance and tensile strength. Adding bismuth to steel and iron is somewhat difficult because bismuth volatilizes significantly at 1500℃, making it hard to uniformly incorporate into steel and iron. Currently, abroad, Bi-Mn alloy with a melting point of 1050℃ is used instead of bismuth as an additive, but bismuth utilization still only reaches about 20%.

Nippon Steel, POSCO, and Kawasaki Steel have successively proposed the addition of bismuth to significantly improve the B8 value of oriented silicon steel. According to statistics, the total number of inventions by Nippon Steel and JFE on producing high-magnetic-induction oriented silicon steel by adding bismuth has exceeded 100. After adding bismuth, magnetic induction reaches over 1.90T, with a maximum of 1.99T.

| Re (Rare Earth Elements) | Generally, rare earth elements refer to the 17 elements from atomic numbers 57 to 71 in the periodic table, namely the lanthanides (lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium), plus scandium (21) and yttrium (39). They have similar properties and are difficult to separate. Unseparated, they are called mixed rare earths, which are relatively inexpensive. Rare earths in steel can deoxidize, desulfurize, perform microalloying, and can change the deformability of rare earth inclusions. Especially, they can modify brittle Al₂O₃ inclusions to a certain extent, improving the fatigue performance of most steel grades.

Rare earth elements, like Ca, Ti, Zr, Mg, Be, etc., are the most effective modifiers of sulfides. Adding an appropriate amount of rare earth elements to steel can turn oxide and sulfide inclusions into fine dispersed spherical inclusions, eliminating the harmful effects of inclusions like MnS. In production practice, sulfur in steel exists as FeS and MnS; when Mn content is high, the tendency to form MnS increases. Although MnS has a higher melting point and can prevent hot brittleness, MnS can elongate into strips during plastic deformation, significantly reducing the steel’s plasticity, toughness, and fatigue strength. Therefore, adding rare earth elements for modification is considered necessary.

Rare earth elements can also improve the steel’s oxidation resistance and corrosion resistance. Their effect on oxidation resistance surpasses that of silicon, aluminum, and titanium. They can improve steel’s fluidity, reduce non-metallic inclusions, making the steel’s structure dense and pure. The role of rare earths in steel is mainly purification, modification, and alloying. As oxygen and sulfur content are gradually controlled, the traditional roles in steel purification and modification are diminishing, replaced by more advanced purification techniques and alloying roles.

In Fe-Cr-Al alloys, rare earth elements increase the alloy’s oxidation resistance, maintain fine grains at high temperatures, and improve high-temperature strength, thereby significantly extending the life of electrothermal alloys.