what is alloy steel?

In addition to iron and carbon, other alloying elements are added to steel, which is called alloy steel. An iron-carbon alloy composed of one or more alloying elements added on the basis of ordinary carbon steel. According to the different added elements, and take appropriate processing technology, can obtain high strength, high toughness, wear resistance, corrosion resistance, low temperature resistance, high temperature resistance, non-magnetic and other special properties.

Introduce

The main alloying elements of alloy steel are silicon, manganese, chromium, nickel, molybdenum, tungsten, vanadium, titanium, niobium, zirconium, cobalt, aluminum, copper, boron, rare earth and so on.

Among them, vanadium, titanium, niobium, zirconium, etc. are strong carbide forming elements in steel, as long as there is enough carbon, under appropriate conditions, they can form their own carbides, when carbon is lacking or under high temperature conditions, they enter the solid solution in an atomic state; Manganese, chromium, tungsten and molybdenum are carbide forming elements, some of which enter the solid solution in the atomic state, and the other part forms a replacement alloy cementite. Aluminum, copper, nickel, cobalt, silicon, etc., do not form carbide elements, generally exist in an atomic state in solid solutions.

effect

1. carbon (C) : the carbon content in the steel increases, the yield point and tensile strength increases, but the plasticity and impact are reduced, when the carbon content exceeds 0.23%, the welding performance of the steel deteriorates, so the low alloy structural steel used for welding, the carbon content generally does not exceed 0.20%. The high carbon content will also reduce the atmospheric corrosion resistance of the steel, and the high carbon steel in the open air yard is easy to rust; In addition, carbon can increase the cold brittleness and aging sensitivity of steel.

2. silicon (Si) : in the steelmaking process to add silicon as a reducing agent and deoxidizer, so the sedative steel contains 0.15-0.30% silicon. If the silicon content of the steel exceeds 0.50-0.60%, silicon is considered an alloying element. Silicon can significantly improve the elastic limit, yield point and tensile strength of steel, so it is widely used as spring steel. When 1.0-1.2% silicon is added to the tempered structural steel, the strength can be increased by 15-20%. The combination of silicon and molybdenum, tungsten, chromium, etc., has the effect of improving corrosion resistance and oxidation resistance, and can manufacture heat-resistant steel. Low carbon steel containing 1-4% silicon, with very high magnetic permeability, used in the electrical industry to make silicon steel sheet. The increase of silicon will reduce the weldability of the steel.

3. manganese (Mn) : in the process of steelmaking, manganese is a good deoxidizer and desulfurizer, and the general steel contains 0.30-0.50% manganese. When more than 0.70% is added to carbon steel, it is considered as "manganese steel", which is not only sufficient toughness, but also has higher strength and hardness than the general steel, improving the quenching of steel and improving the thermal processing performance of steel, such as 16Mn steel is 40% higher than A3 yield point. Steel containing 11-14% manganese has extremely high wear resistance and is used in excavator buckets, ball mill linings, etc. The increase of manganese content weakens the corrosion resistance of steel and reduces the weldability.

4. Phosphorus (P) : under normal circumstances, phosphorus is a harmful element in steel, which increases the cold brittleness of steel, deteriorates the welding performance, reduces plasticity, and deteriorates the cold bending performance. Therefore, the phosphorus content in steel is usually required to be less than 0.045%, and the requirements for high-quality steel are lower.

5. sulfur (S) : Sulfur is also a harmful element under normal circumstances. The steel produces hot brittleness, reduces the ductility and toughness of the steel, and causes cracks during forging and rolling. Sulfur is also detrimental to welding performance, reducing corrosion resistance. Therefore, the sulfur content is usually required to be less than 0.055%, and the high-quality steel is required to be less than 0.040%. Adding 0.08-0.20% sulfur to the steel can improve the machinability, usually called free-cutting steel.

6. chromium (Cr) : in structural steel and tool steel, chromium can significantly improve strength, hardness and wear resistance, but at the same time reduce plasticity and toughness. Chromium can also improve the oxidation resistance and corrosion resistance of steel, so it is an important alloying element of stainless steel and heat-resistant steel.

7. nickel (Ni) : nickel can improve the strength of steel, while maintaining good plasticity and toughness. Nickel has high corrosion resistance to acid and alkali, rust prevention and heat resistance at high temperatures. However, because nickel is a scarce resource, it should try to use other alloying elements instead of nickel-chromium steel.

8. molybdenum (Mo) : molybdenum can refine the grain of steel, improve hardenability and thermal strength, and maintain sufficient strength and creep resistance at high temperatures (long-term stress at high temperatures, deformation, called creep). Adding molybdenum to structural steel can improve mechanical properties. It can also inhibit the brittleness of alloy steel caused by quenching. It can improve redness in tool steel.

9. titanium (Ti) : titanium is a strong deoxidizer in steel. It can make the internal structure of the steel dense, refine the grain strength; Reduces aging sensitivity and cold brittleness. Improve welding performance. Intergranular corrosion can be avoided by adding appropriate titanium to chromium-18 nickel-9 austenitic stainless steel.

10. Vanadium (V) : Vanadium is an excellent deoxidizer for steel. Adding 0.5% vanadium to steel can refine grain structure and improve strength and toughness. The carbides formed by vanadium and carbon can improve the resistance to hydrogen corrosion at high temperature and pressure.

11. tungsten (W) : tungsten melting point is high, the ratio is significant, is a precious alloy element. Tungsten carbide forms with carbon Tungsten carbide has a high hardness and wear resistance. Adding tungsten to tool steel can significantly improve the red hardness and thermal strength, and can be used as cutting tools and forging dies.

12. niobium (Nb) : Niobium can refine the grain and reduce the overheating sensitivity and tempering brittleness of steel, improve strength, but the plasticity and toughness have decreased. Adding niobium to ordinary low alloy steel can improve the resistance to atmospheric corrosion and hydrogen, nitrogen and ammonia corrosion at high temperature. Niobium can improve welding performance. Adding niobium to austenitic stainless steel can prevent intergranular corrosion.

13. cobalt (Co) : Cobalt is a rare precious metal, mostly used in special steel and alloys, such as heat-strong steel and magnetic materials.

14. copper (Cu) : WisCO with Daye ore refined steel, often contains copper. Copper can improve strength and toughness, especially atmospheric corrosion performance. The disadvantage is that it is easy to produce hot brittleness during hot processing, and the plastic content of copper exceeds 0.5% is significantly reduced. When the copper content is less than 0.50%, the weldability is not affected.

15. aluminum (Al) : Aluminum is a commonly used deoxidizer in steel. Adding a small amount of aluminum to the steel can refine the grain and improve the impact toughness, such as 08Al steel for deep drawing sheet. Aluminum also has oxidation resistance and corrosion resistance, aluminum and chromium, silicon combined, can significantly improve the high temperature non-peeling performance of steel and high temperature corrosion resistance. The disadvantage of aluminum is that it affects the thermal processing performance, welding performance and cutting performance of steel.

16. boron (B) : Adding a trace amount of boron to steel can improve the densification and hot rolling performance of steel, improve strength.

17. nitrogen (N): nitrogen can improve the strength of steel, low temperature toughness and weldability, increase aging sensitivity.

18. rare earth (Xt) : Rare earth elements are 15 lanthanides in the periodic table with atomic numbers of 57-71. These elements are all metals, but their oxides are very much like "earth", so they are commonly called rare earths. The addition of rare earths to steel can change the composition, form, distribution and properties of inclusions in steel, thereby improving various properties of steel, such as toughness, weldability, and cold working performance. Adding rare earth to share steel can improve wear resistance.

Effect of alloying elements on phase diagram of iron-carbon alloys

1, the effect of alloying elements on the A-phase zone: 1) expand the A-phase zone (Mn, Ni, Co); 2) Reduce the A-phase region (Cr, V, Mo, Si); 3) It is for this reason that we can produce austenitic steel and ferritic steel;

2, the influence of alloying elements on S and E points: all the elements that expand the A phase zone make S and E points move to the lower left; Any element that shrinks the A phase region shifts the S and E points to the upper left.

Effect of alloying elements on heat treatment of steel

1. the effect on austenitizing - most alloying elements (except nickel and cobalt) slow down the austenitizing process. Therefore, in the heat treatment, it needs a higher heating temperature and a longer holding time than carbon steel. -- Carbide is not suitable for decomposition.

2. the effect on austenite grain size - most alloying elements have the effect of hindering austenite grain growth. However, manganese and boron, on the contrary, can promote austenite grain growth, so, in addition to manganese steel, alloy steel is not easy to overheat when heated. This is beneficial to obtain fine martensite after quenching; It is also conducive to appropriately increasing the heating temperature, so that more alloying elements are dissolved in austenite to increase hardenability and improve the mechanical properties of steel. [2]

3. the influence of alloying elements on the over-austenitic transformation - except for cobalt, all alloying elements shift the C curve to the right, reduce the critical cooling rate of steel, and improve the hardenability of steel (Figure 7-4). Some alloying elements also change the shape of the C curve. In addition, most alloying elements also reduce the Ms point.

Effect on phase transformation of steel during heating and cooling

The main solid-state transformation of steel during heating is the transition from non-austenite to austenite phase, that is, the process of austenitizing. The whole thing has to do with the diffusion of carbon. Among alloying elements, non-carbide forming elements reduce the activation energy of carbon in austenite and increase the rate of austenite formation. The strong carbide forming elements strongly hinder the diffusion of carbon in steel and significantly slow down the austenitizing process.

The phase transition of steel during cooling refers to the decomposition of supercooled austenite, including perlite transition (eutectoid decomposition), bainite transition and martensite transition. Taking the effect of alloying elements on the isothermal transition curve of supercooled austenite as an example, most alloying elements, except cobalt and aluminum, all play a role in slowing down the isothermal decomposition of austenite, but the effects of various elements are different. Non-carbide forming elements (such as silicon, phosphorus, nickel, copper) and small amounts of carbide forming elements (such as vanadium, titanium, molybdenum, tungsten) have little difference in the effect of the transition from austenite to pearlite and the transition to bainite, so that the transition curve is shifted to the right.

Carbide forming elements (such as vanadium, titanium, chromium, molybdenum, tungsten) if the content is more, will significantly delay the transformation of austenite to perlite, but the delay of the transformation of austenite to bainite is not significant, so that the isothermal transition curve of these two transformations is separated from the "nose", and form two C-shapes. [3]

Effect on grain size and hardenability of steel

There are many factors affecting the grain size of austenite. The deoxidation and alloying of steel are related to the "austenite grain size". In general, some elements that do not form carbides, such as nickel, silicon, copper, cobalt, etc., have a weak effect on preventing austenite grain growth, while manganese and phosphorus have a tendency to promote grain growth. Carbide forming elements such as tungsten, molybdenum, chromium, etc. play a moderate role in preventing austenite grain growth. Strong carbide forming elements such as vanadium, titanium, niobium, zirconium, etc. strongly prevent austenite grain growth and play a role in refining grain. Although aluminum is an element that does not form carbides, it is the most commonly used element for refining grains and controlling the temperature at which grains begin coarsening.

The hardenability of steel (see quenching) depends mainly on the chemical composition and grain size. In addition to cobalt and aluminum and other elements, most of the alloying elements are dissolved into the solid solution to varying degrees to inhibit the transformation of supercooled austenite to pearlite and bainite, increase the number of martensitic structure, that is, improve the hardenability of steel. [4]

Influence on mechanical properties and tempering properties of steel

The properties of steel depend on the respective properties of iron solid solution and carbide and their relative distribution. The influence of alloying elements on the mechanical properties of steel is also related to this. The alloying element that is solidly dissolved in ferrite plays a solid solution strengthening role, which increases the strength and hardness, but at the same time reduces the toughness and plasticity relatively.

The toughness to brittleness transition temperature of tempered steel is an important index to evaluate the mechanical properties.

① The elements that increase the transition temperature are B, P, C, Si, Cu, Mo, Cr;

② The elements that reduce the transition temperature are Ni and Mn;

③ The elements that increase the transition temperature in a small amount and decrease the transition temperature in a large amount are Ti and V;

(4) The elements that reduce the transition temperature when a small amount and increase the transition temperature when a large amount are Al.

The tempering stability of alloy steel is better than that of carbon steel, which is because the alloying elements hinder the diffusion of atoms in the steel during tempering, and thus play the role of delaying martensitic decomposition and resisting tempering softening at the same temperature. Carbide forming element, the delay effect of tempering softening is particularly significant. Although cobalt and silicon do not form carbide elements, they have a strong delay effect on the formation and growth of cementite crystal nuclei, and therefore, they also have a delayed tempering softening effect. [5]

Effect on weldability and machinability of steel

Weldability and machinability are the main aspects to measure the technological properties of steel. Any alloy element that can improve the hardenability is bad for the weldability of steel. Because hard and brittle structures such as martensite are easy to form when the heat affected zone of the weld is cooled near the fusion line, there is a risk of cracking. On the other hand, the grain near the fusion line of the heat affected zone is easy to coarsene due to high heat, so it is beneficial to contain elements such as titanium and vanadium that can refine the grain.

Adding an appropriate amount of sulfur, lead and other elements to steel can improve the machinability of steel (see free-cutting steel). Alloying elements in alloy steel generally increase the hardness of steel, thus increasing cutting resistance and aggravating tool wear. The machinability of steel can be affected by changing the matrix structure, type, quantity and shape of inclusions. [6]

Influence on corrosion resistance of steel

Chromium is the main alloying element of stainless acid-resistant and heat-resistant steels. If the chromium content of alloy steel reaches about 12%, dense chromium oxides are formed on the surface of the steel, which greatly improves the corrosion resistance of the steel in the oxidizing medium. Chromium, aluminum, silicon and other elements can improve the oxidation resistance of steel and the corrosion resistance of high temperature gases, but excessive aluminum and silicon will make the thermoplasticity of steel worse. Nickel is mainly used to form and stabilize austenitic structure, so that steel can obtain good mechanical properties, corrosion resistance and process properties. Molybdenum can make stainless acid-resistant steel quickly passivated, improve the corrosion resistance of solutions containing chloride ions and other non-oxidizing media. Titanium and niobium are usually used to fix the carbon in alloy steel to generate stable carbides to reduce the harmful effect of carbon on the corrosion resistance of alloy steel. When copper and phosphorus are used together, the atmospheric corrosion resistance of steel can be improved.

1. according to the content of alloying elements

1) The total content of alloying elements of low alloy steel is less than or equal to 5%;

2) The total content of alloy elements in alloy steel is between 5% and 10%;

3) The total content of alloy elements of high alloy steel is greater than or equal to 10%;

2. according to the type of alloy elements

There are chromium steel, manganese steel, chromium manganese steel, chromium nickel steel, chromium nickel molybdenum steel, silicon manganese molybdenum vanadium steel.

3. according to the main use

(1) Structural steel

1) Structural steel for construction and engineering

2) Structural steel for mechanical manufacturing

(2) Tool steel

(3) Special performance steel

There are many kinds of alloy steel, usually divided into low alloy steel (content <5%), medium alloy steel (content 5% ~ 10%), high alloy steel (content >10%) according to the content of alloying elements; According to the quality is divided into high quality alloy steel, special alloy steel; According to the characteristics and uses, it is divided into alloy structural steel, stainless steel, acid-resistant steel, wear-resistant steel, heat-resistant steel, alloy tool steel, rolling bearing steel, alloy spring steel and special performance steel (such as soft magnetic steel, permanent magnetic steel, non-magnetic steel).

Countries of alloy steel system, with their resources, production and use conditions are different, foreign countries have previously developed nickel, steel system, China is found to silicon, manganese, vanadium, titanium, niobium, boron, lead, rare earth based alloy steel system in the total output of steel accounted for about ten percent, generally in electric furnace smelting according to the use of alloy steel can be divided into 8 categories, They are: alloy structural steel, spring steel, bearing steel, alloy tool steel, high-speed tool steel, stainless steel, heat-resistant non-peeling steel, electrical silicon steel.

Tempered and tempered steel

1. Medium carbon alloy steel with low alloying element content;

2. High strength;

3. Used for high temperature bolts, nuts and other materials. [8]

Spring steel

1. the carbon content is higher than the tempered steel;

2. After tempering treatment, the strength is higher and the fatigue resistance is higher;

3. For spring materials.

Rolling bearing steel

1. High-carbon alloy steel, high alloy content;

2. With high and uniform hardness and wear resistance;

3. For rolling bearings.

Alloy tool steel

Also known as measuring steel

1. High-carbon alloy steel, low content of alloying elements;

2. With high hardness and wear resistance, good machining performance, good stability;

3. For measuring tool materials.

Special performance steel

1. Low carbon high alloy steel;

2. Good corrosion resistance;

3. For corrosion resistance, some can be used as heat resistant materials.

Heat-resisting steel

1. Low carbon high alloy steel;

2. Good heat resistance; 3

For heat resistant materials, some can be used as corrosion resistant materials.

Low-temperature steel

1. Low carbon alloy steel, according to the degree of low temperature resistance alloy elements are high or low;

2. Good low temperature resistance;

3. For low-temperature materials (special steel is nickel steel).

Classified according to the tendency of carbides

Alloy steel can be divided into three categories according to the tendency of various elements to form carbides in steel:

① Strong carbide forming elements, such as vanadium, titanium, niobium, zirconium, etc.

As long as there is enough carbon in these elements, under appropriate conditions, they will form their own carbides; Only in the absence of carbon or high temperature conditions, it enters the solid solution in an atomic state.

② Carbide forming elements, such as manganese, chromium, tungsten, molybdenum, etc. Some of these elements enter the solid solution in an atomic state, and the other part forms a replacement alloy cementate, such as (Fe, Mn)3C, (Fe, Cr)3C, etc., if the content exceeds a certain limit (except manganese), it will form its own carbides, such as (Fe, Cr)7C3, (Fe, W)6C, etc.

③ Do not form carbide elements, such as silicon, aluminum, copper, nickel, cobalt and so on. Such elements generally exist in the atomic state in austenite, ferrite and other solid solutions. Some of the more active elements in the alloying elements, such as aluminum, manganese, silicon, titanium, zirconium, etc., are easy to combine with oxygen and nitrogen in steel to form stable oxides and nitrides, which generally exist in the form of inclusions in steel. Manganese, zirconium and other elements also form sulfide inclusions with sulfur. Steel containing sufficient amount of nickel, titanium, aluminum, molybdenum and other elements can form different types of intermetallic compounds. Some alloying elements such as copper, lead, etc., if the content exceeds its solubility in steel, it exists as a purer metal phase.

Classification according to phase transition points

The performance of steel depends on the phase composition of steel, the composition and structure of the phase, the volume components of various phases in steel and the distribution state relative to each other. Alloying elements act by influencing the above factors. The influence of the phase change point on steel is mainly to change the position of the phase change point in steel, which can be roughly summarized in the following three aspects:

① Change the temperature of the phase transition point. In general, expanding the elements of the γ phase (austenite) region, such as manganese, nickel, carbon, nitrogen, copper, zinc, etc., reduces the temperature of A3 point and increases the temperature of A4 point; On the contrary, reducing the elements of the gamma phase region, such as zirconium, boron, silicon, phosphorus, titanium, vanadium, molybdenum, tungsten, niobium, etc., will increase the temperature at A3 point and decrease the temperature at A4 point. Only cobalt increases the temperature at both A3 and A4 points. The role of chromium is relatively special, when the chromium content is less than 7%, the A3 point temperature is reduced, and when the chromium content is greater than 7%, the A3 point temperature is increased.

② Change the position of the eutectoid point S. The temperature of the eutectoid point S increases when the elements in γ phase region are reduced. The opposite is true for elements expanding the gamma phase region. In addition, almost all alloying elements reduce the carbon content of the eutectoid point S and shift the S point to the left. However, carbide forming elements such as vanadium, titanium, niobium, etc. (also including tungsten, molybdenum), after the content is high to a certain limit, the S point is shifted to the right.

③ Change the shape, size and position of the gamma phase region. This effect is more complex, and it can be significantly changed when the alloying element content is higher. For example, when the content of nickel or manganese is high, the γ phase region can be extended to below room temperature, so that the steel becomes a single-phase austenite structure; When the silicon or chromium content is high, the gamma phase region can be shrunk to a small or even completely disappeared, so that the steel is a ferrite structure at any temperature.

General naming principles for alloy gold MEDALS

The carbon content of alloy steel, the various types of alloying elements and the content of alloying elements should be reflected in the grade.

Example: Alloy spring steel 60Si2Mn

Carbon content ~0.6%; Silicon content ~2%; Manganese content Mn~1%.

Low alloy structural steel

1. performance characteristics high strength, sufficient plasticity and toughness, good welding performance. Widely used in buildings, Bridges, etc.

2. chemical composition characteristics Low carbon steel (carbon content <0.2%); The main alloying element is Mn(content is 1.25~1.5%).

3. heat treatment characteristics are generally not heat treatment.

4. commonly used steel 16Mn, 15MnTi, etc. [10]

Alloy carburizing steel

1. performance characteristics used to manufacture hard and wear-resistant surface, heart toughness and impact resistant parts, such as gears, cams, etc. (With good carburizing ability and hardenability)

2. chemical composition characteristics Low carbon steel (carbon content 0.1~0.25%); The main alloying elements are Cr, Mn, Ti, V, etc., whose main function is to improve hardenability and prevent overheating.

3. heat treatment characteristics pre-heat treatment for normalizing, carburizing for quenching and low temperature tempering. Taking 20CrMnTi as an example to produce automobile transmission gear, the process route is as follows: forging - normalizing - machining tooth profile - local copper plating - carburizing - pre-cooling quenching, low temperature tempering - shot peening - grinding teeth.

Alloy tempered steel

1. performance characteristics after the tempering treatment with high strength and good plasticity and toughness, that is, has good comprehensive mechanical properties.

2. chemical composition characteristics of carbon steel (0.3~0.5%), alloying elements are mainly Cr, Mn, Ti, Mo, etc., the main role is to improve hardenability, refine grains and prevent overheating.

3. heat treatment characteristics pre-heat treatment for annealing or normalizing, the final heat treatment for quenching + high temperature tempering.

4. commonly used steel 40Cr, 40CrMn, etc. The production process route of making tractor connecting rod bolts with 40Cr is as follows: forging, normalizing, roughing, tempering, finishing and assembly

Alloy spring steel

1. the performance characteristics of the manufacture of various elastic components such as coil springs, plate springs, etc. It requires high elastic limit, high yield to strength ratio, high fatigue strength and enough toughness.

2. chemical composition characteristics carbon content (0.5~0.7%), alloying elements are mainly Mn, Si, Cr, V, Mo, etc., the main role is to improve hardenability and tempering stability, to prevent tempering brittleness.

3. heat treatment characteristics

(1) Thermoforming spring (large spring size ≥8mm) feeding, heating (Ac3+~100℃), molding, waste heat quenching, medium temperature tempering (~430℃), products

(2) Cold forming spring (size ≤8mm small spring) feeding, cold drawing steel wire cold coil forming, low temperature annealing, products

4, commonly used steel: 60Si2Mn

Rolling bearing steel

1. the performance characteristics require high strength and hardness, high elastic limit and contact fatigue strength, enough toughness and hardenability, high wear resistance, but also should have a certain corrosion resistance.

2. chemical composition characteristics of high carbon (0.95%

3. heat treatment characteristics pre-heat treatment for nodular annealing, the final heat treatment for quenching + low temperature tempering. The production process is as follows: rolling, forging, spheroidizing annealing, machining, quenching and low temperature tempering, grinding processing, the metallographic structure of the finished product is: M+ granular carbide + A small amount of A residue

4. commonly used steel GCr15, GCr15SiMn (pay attention to the content of Cr, C content).

Alloy cutting tool steel

Cutting tool steel should have the following performance requirements:

(1) High hardness (above 60HRC)

(2) High wear resistance

(3) High heat hardness (red hardness)

(4) It has certain strength, toughness and plasticity

(A) low alloy cutting tool steel

1. chemical composition characteristics of high carbon content (0.75~1.5%); In order to improve the hardenability and tempering stability, add Cr, Mn, Si, V, W and other alloying elements;

2. heat treatment characteristics pretreatment for nodular annealing, the final heat treatment for quenching + low temperature tempering.

(2) High-speed steel

chemical composition characteristics

① High C: 0.7% ~ 1.5%;

② Add Cr to improve hardenability;

③ Adding W and Mo to improve thermal hardness;

④ Add V to improve wear resistance.