The Influence Of Elements On The Properties Of Steel

Introduction to elements in steel

Manganese (Mn)

Manganese is used to deoxidize ferromanganese in steelmaking and remains in steel. Manganese can remove FeO in steel, improve steel quality and reduce the brittleness of steel. Manganese and vulcanization combine to form MnS, eliminate the harmful effect of sulfur, and improve the hot working performance of steel. The content of manganese in carbon steel is generally controlled between 0.25 and 0.80%. Manganese can be dissolved in ferrite to form manganese ferrite, which plays the role of strengthening ferrite. Manganese can also be dissolved in Fe3C to form alloy cementing, thus improving the strength of carbon steel. Manganese is a beneficial impurity element, and a small amount of manganese has no significant effect on the performance of steel.

Silicon (Si)

Silicon is also added to the steel as a deoxidizer. The content of silicon in carbon steel is usually controlled between 0.03-0.4%, and most of it is dissolved in ferrite, forming silicon ferrite, which plays the role of strengthening ferrite and improving the strength and hardness of steel. But plasticity and toughness have decreased. The lack of silicon has no significant effect on the properties of the steel.

Sulfur (S)

Sulfur is brought into steel by ore and fuel in steelmaking, sulfur is insoluble in iron, and the form of different FeS exists, FeS and Fe can form a melting point of 985℃ eutectic, and are distributed on the grain boundaries of austenite when the steel is rolled and forged at 1000-1200℃, will melt the eutectic crystal on the grain boundaries, is the steel brittle, this phenomenon is called thermal brittleness, Sulfur is a harmful impurity, so the carbon content in steel must be strictly controlled. The harmful effect of sulfur can be eliminated by increasing the Mn content in the steel. Because manganese and sulfur can form MnS with a melting point of 1620℃, MnS has a certain plasticity at high temperatures, so the thermal brittleness of steel can be avoided.

Phosphorus (P)

Phosphorus is introduced into steel from ore, where it is completely dissolved in ferrite, improving the strength and hardness of ferrite. But at the same time, the plasticity of steel at room temperature drops sharply and becomes brittle. This phenomenon is called cold brittleness. Phosphorus is also a harmful impurity, and its content in steel must be strictly controlled.

Metallographic structure in steel

Ferrite: The interstitial solid solution of carbon in α-Fe is called ferrite. It is often represented by the symbol F (or α). Ferrite has a body-centered cubic structure, and its ability to dissolve carbon is poor because of its small gap. At 727℃, the maximum dissolved carbon content is 0.0218%. With the decrease in temperature, the dissolved carbon content gradually decreases, and at room temperature, the dissolved carbon content is only 0.0008%. Ferrite has low strength, δσb is 180-280MN/m2, HB is about 80, but it has good ductility, δ is 50%.

Austenitic: The interstitial solid solution of carbon formed in gamma-Fe is called its. It is often represented by the symbol A (or γ). Austenite has a face-centered cubic lattice structure. Due to its large effective lattice gap, its carbon solubility is relatively high. The maximum carbon solubility at 1148℃ is 2.11%, which gradually decreases with the decrease in temperature and reaches 0.77% at 727℃. The mechanical properties of austenite depend on the amount of dissolved carbon and the size of a grain. Generally, the hardness of austenite is 170-220HBS, the elongation of δ is 40-50%, and austenite exists in the solid temperature range above 727℃. Austenite is prone to plastic forming.

Cementite (Cementite): C and Fe compound (Fe3C) known as cementite, the carbon content of 6.69%, the melting point of cementite is 1227℃, its hardness is very high, about 800HB, plastic and impact toughness is almost zero, brittleness is great, so it can not be used as the matrix phase of carbon steel, is the main strengthening phase of carbon steel. Cementite is a metastable phase. Under certain conditions, it will decompose and form graphitic-free carbon.

Martensite: The use of a rapid cooling method, due to the degree of undercooling, iron, and carbon atoms can not be diffused, austenite can only be by non-diffused lattice shear, there is γ-Fe face-centered cubic lattice reshuffled to α-Fe body-centered cubic lattice. This austenite directly transforms into a saturated alpha-solid solution containing carbon, called martensite.

The grain size of austenite and its influencing factors

The austenitic grain size of steel has a great influence on the microstructure and properties after cooling. The finer the grain size of austenite, the finer the structure after cooling, and the better the strength, plasticity, and toughness. Therefore, it is of great significance to obtain fine austenite grains by heat treatment for the final performance and quality of the workpiece.

The higher the temperature of austenitization, the more obvious the grain size growth of steel. At a certain temperature, the longer the holding time, the more favorable the growth of austenite grains.

The grain growth tendency increases with the increase of carbon content in the steel. But when the cementite is in the austenite grain boundary, it will hinder austenite grain growth.

In addition to Mn, P and other elements increase the tendency of austenite grain growth, other elements forming carbide elements (Ti, Nb, Zb, etc.), forming oxides and nitrides (such as Al), when they form compounds distributed in austenite grain boundaries, will hinder austenite grain growth to varying degrees.

The alloy elements commonly added in alloy steel are Mn, Si, Cr, Mo, W, V, Ti, Nb, Ni, Al, and so on. Among them, Ni, Si, Al, and other elements cannot form compounds with carbon. Elements such as Nb, Ti, V, W, Mo, Cr, and Mn can form compounds with carbon. Alloying elements exist in different forms in steel. Therefore, the main role of steel is also different. The main role has the following aspects:

1. When the strength of steel is increased, the elements that can not form carbide are mainly dissolved into ferrite to form alloy ferrite, and the solid solution is strengthened. Carbide-forming elements form alloy carbide with carbon, which has higher hardness and greater dispersion strengthening effect. Whatever element is added to the steel increases its strength.

   
   

2. Improve hardenability in addition to Co, most alloy elements added to steel, slow down the decomposition of supercooled austenite, so that the C curve to the right, reduce the critical cooling rate, improve the hardenability of steel, (Ni, Cr, Co) this is one of the main purposes of alloy elements added to steel. But it should be pointed out that this effect can only be achieved when the alloying elements are dissolved into austenite.

   
   

3. Prevent austenite grain growth except for Mn, the alloy austenite grain growth tendency is small, especially the carbide generated by the strong carbide forming elements (Ti, V, Zr, Nb, etc.) can strongly prevent the austenite grain growth, and thus play the role of refining grain.

   
   

4. Increase the tempering stability of steel. The resistance of steel to the softening process during tempering is called tempering stability. Many alloying elements can increase and slow down the temperature of carbide precipitation and residual austenite decomposition in martensite. Therefore, compared with carbon steel, alloy steel has higher hardness and strength when tempered at the same temperature. On the contrary, when tempering to the same hardness, the tempering temperature of alloy steel is high, so its internal stress elimination is more thorough, and plasticity, and toughness is higher. Carbide-forming elements Cr, Mo, Nb, and V have strong effects on tempering stability.

   
   

5. Secondary hardening when W, Mo, Cr, V content is higher, in the 500-600℃ tempering temperature range, the hardness does not reduce, but increases the phenomenon known as secondary tempering hardening. There are two reasons for this: first, when tempering at this temperature, fine, dispersed special carbides, such as Mo2C, W2C, and VC, will precipitate out of martensite, which plays the role of dispersion hardening; Second, when tempering at this temperature, part of carbide is precipitated in the residual austenite, which reduces the carbon content and increases the temperature of Ms point, so that the residual austenite can be converted into martensite (secondary quenching) at a higher temperature during the cooling process to increase the hardness of steel.

   
   

6. Tempering brittleness occurs when the alloy steel is tempered in a certain temperature range after quenching, which is called tempering brittleness. The decrease in toughness at 350-400℃ is called the first type of temper brittleness. This brittleness occurs regardless of carbon steel or alloy steel and is independent of the rate of cooling. This kind of temper brittleness cannot be eliminated after production, also known as irreversible temper brittleness. To avoid the occurrence of such tempering brittleness, tempering is generally not done within this temperature range.

   
   

The decrease in toughness that occurs after slow cooling after 500-600℃ tempering is called Type II tempering brittleness (high-temperature tempering brittleness). The alloy steels containing Cr, Mn, and Ni are most prone to this kind of temper brittleness. This kind of tempering brittleness will not occur if it cools quickly after tempering; If slow cooling after tempering, brittleness has occurred, as long as reheating to tempering temperature and fast cooling, then tempering brittleness can be completely eliminated, so it is also called reversible tempering brittleness.

Heat resistant steel

The heat resistance of steel includes high-temperature oxidation resistance and high-temperature strength.

The oxidation resistance of a metal, usually refers to the formation of a dense oxide film after rapid oxidation at high temperatures, covering the metal surface, so that the steel will not continue to oxidize. The oxidized surface of carbon steel at high temperatures generates loose porous FeO, which is easy to flake off, and oxygen atoms continue to diffuse through FeO so that the steel continues to oxidize. Alloy elements such as Cr, Si, and Al are added to the steel, so that when the steel is in contact with oxygen at high temperature, dense oxidation films such as Cr2O3, SiO2, and Al2O3 with high melting points are formed on the surface, which makes it difficult to continue the oxidation process of steel in high-temperature gas.

The high-temperature strength (thermal strength) of steel is the resistance to creep at high temperatures. To improve the thermal strength of steel, Cr, Mo, V, Ni, and other alloying elements are usually added to the steel, which can dissolve into the matrix to strengthen the solid solution and improve the recrystallization temperature, thus enhancing the high-temperature strength of steel. Adding Nb, V, Ti and other alloying elements can also form carbides with high hardness and good thermal stability, which are distributed on the matrix and play a role in dispersion strengthening. In addition, austenite structure has better creep resistance than ferrite and better grain size than fine grain. Of course, it can not be too coarse, which will affect the strength.

Martensitic heat-resistant steel

Cr and Si were added to improve oxidation resistance at high temperatures. Adding Mo can strengthen the matrix and avoid tempering brittleness. Adding V can form dispersed carbide and improve the high-temperature strength. Adding W can precipitate stable carbide and increase the recrystallization temperature significantly. The operating temperature of this kind of steel does not exceed 700℃.

Austenitic heat-resistant steel

Austenitic heat-resistant steel has high Cr content, which can improve its high-temperature strength and oxidation resistance. Ni content is also high and can form a stable austenite structure. The heat resistance of this kind of steel is better than that of martensitic heat-resistant steel, and the cold deformation and welding performance are very good, generally working at 600-700℃.