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Basic classification of carbon steel

Basic classification of carbon steel

1. Carbon steel

Carbon steel, also known as carbon steel, is an iron-carbon alloy with a carbon content of ωc less than 2%. In addition to carbon, carbon steel generally contains a small amount of silicon, manganese, sulfur, phosphorus.

According to the use of carbon steel can be divided into carbon structural steel, carbon tool steel and free cutting structural steel three categories. Carbon structural steel can be divided into two types: building structural steel and machine manufacturing structural steel. According to carbon content, carbon steel can be divided into low carbon steel (ωc≤0.25%), medium carbon steel (ωc=0.25%-0.6%) and high carbon steel (ωc> 0.6%)

According to the amount of phosphorus and sulfur, carbon steel can be divided into ordinary carbon steel (higher phosphorus and sulfur content), high-quality carbon steel (lower phosphorus and sulfur content) and senior high-quality steel (lower phosphorus and sulfur content). The higher the carbon content in general carbon steel, the higher the hardness, the higher the strength, but the lower the plasticity.

2. carbon structural steel

This kind of steel mainly guarantees mechanical properties. Therefore, its brand reflects its mechanical properties, and Q+ numbers indicate that "Q" is the Chinese pinyin prefix of the yield point "Qu" word.

The number indicates the yield point value, for example, Q275 indicates that the yield point is 275Mpa, if the letters A, B, C, D are marked after the grade, it means that the steel quality grade is different, the amount of S and P is reduced in turn, and the steel quality is increased in turn.

If the letter "F" is marked after the grade, it is boiling steel, the mark "b" is semi-killed steel, and the one who does not mark "F" or "b" is killed steel. For example, Q235-AF indicates grade A boiling steel with a yield point of 235MPa, and Q235-C indicates grade C sedative steel with a yield point of 235MPa.

Carbon structural steel is generally not heat treated, but directly used in the supply state. Usually Q195, Q215, Q235 steel carbon mass fraction is low, good welding performance, plasticity, toughness, has a certain strength, often rolled into sheet, steel bar, welded steel pipe and so on.

Used for Bridges, buildings and other structures and manufacture of common rivets, screws, nuts and other parts. Q255 and Q275 steel carbon mass fraction is slightly higher, higher strength, plasticity, toughness, can be welded, usually rolled into steel, strip steel and steel plate as structural parts and the manufacture of simple mechanical connecting rod, gear, coupling, pin and other parts.

3. high-quality structural steel

This kind of steel must ensure both chemical composition and mechanical properties. Its grade is a two-digit number representing the mass fraction of the average carbon in the steel (ω) *10000). For example, 45 steel indicates that the average mass fraction of carbon in steel is 0.45%; 08 steel indicates that the average mass fraction of carbon in steel is 0.08%.

High-quality carbon structural steel is mainly used in the manufacture of machine parts. It is generally heat treated to improve mechanical properties. Depending on the carbon mass fraction, it has different uses.

08, 08F, 10, 10F steel, high plasticity, toughness, with excellent cold forming performance and welding performance, often cold rolled into sheet, used to make instrument shell, car and tractor cold stamping parts, such as car body, tractor cab, etc.;

15, 20, 25 steel is used to make carburized parts with small size, light load, wear-resistant surface requirements and low core strength requirements, such as piston pins, templates, etc.

30, 35, 40, 45, 50 steel after heat treatment (quenching + high temperature tempering) has good comprehensive mechanical properties, that is, has high strength and high plasticity, toughness, for the production of shaft parts, such as 40, 45 steel is often used in the manufacture of automobiles, tractors crankshaft, connecting rod, general machine tool spindle, machine tool gear and other shaft parts with little force;

55, 60, 65 steel has a high elastic limit after heat treatment (quenching + medium temperature tempering), and is often used to make springs with small load and small size (cross section size less than 12~15mm), such as pressure regulation and speed regulation springs, plunger springs, cold coil springs, etc.

4. carbon tool steel

Carbon tool steel is basically no alloying elements of high carbon steel, carbon content in the range of 0.65%-1.35%, its production cost is low, raw material sources are easy to obtain, good machinability, heat treatment can get high hardness and high wear resistance, so it is widely used steel, used to manufacture a variety of cutting tools, molds, measuring tools.

However, the red hardness of this kind of steel is poor, that is, when the working temperature is greater than 250 ° C, the hardness and wear resistance of the steel will drop sharply and lose the working ability. In addition, carbon tool steel, if made of larger parts, is not easy to harden, and it is easy to produce deformation and cracks.

5. free cutting structural steel

Free-cutting structural steel is to add some elements to the steel to make the steel brittle, so that the steel cutting is easy to break into debris, which is conducive to improving the cutting speed and extending the tool life. The elements that make steel brittle are mainly sulfur, and lead, tellurium, bismuth and other elements are used in common low-alloy free-cutting structural steel.

The sulfur content of this steel is in the range of 0.08%-0.30%, and the manganese content is in the range of 0.60%-1.55%. The sulfur and manganese in the steel exist in the form of manganese sulfide, which is very brittle and has lubricating effect, so that the chips are easy to break, and is conducive to improving the quality of the processed surface.

6. Alloy steel

In addition to containing iron, carbon and a small amount of unavoidable silicon, manganese, phosphorus, sulfur elements, but also contains a certain amount of alloying elements, the alloying elements in steel are silicon, manganese, molybdenum, nickel, chromium, vanadium, titanium, niobium, boron, lead, rare earth and one or several, this steel is called alloy steel.

The alloy steel system of various countries varies with their respective resources, production and use conditions, foreign countries have developed nickel and chromium steel systems in the past, and China has developed alloy steel systems based on silicon, manganese, vanadium, titanium, niobium, boron and rare earth.

Alloy steel accounts for about ten percent of the total output of steel, generally smelted in electric furnace according to the use of alloy steel can be divided into eight categories, they are: alloy structural steel, spring steel, bearing steel, alloy tool steel, high-speed tool steel, stainless steel, heat-resistant non-skin steel, electrical silicon steel.

7. ordinary low alloy steel

Ordinary low alloy steel is a kind of ordinary alloy steel containing a small amount of alloying elements (in most cases its total amount does not exceed 3%). The strength of this steel is relatively high, the comprehensive performance is better, and has corrosion resistance, wear resistance, low temperature resistance and better cutting performance, welding performance, etc. Under the condition of saving a large number of scarce alloying elements (such as nickel, chromium), usually 1t ordinary low alloy steel can be used on 1.2-1.3t carbon steel, and its service life and use range are far more than carbon steel. Ordinary low-alloy steel can be smelted in open-hearth and converter by ordinary smelting methods, and the cost is close to that of carbon steel.

8. Alloy steel for engineering structure

This refers to alloy steels used in engineering and building structures, including weldable high-strength alloy structural steel, alloy reinforced steel, alloy steel for railway, alloy steel for geological oil drilling, alloy steel for pressure vessels, high-manganese wear-resistant steel, etc. This kind of steel is used as engineering and building structural parts, in alloy steel, the total amount of this kind of steel alloy content is low, but the production and use of larger.

9. mechanical structure with alloy steel

This type of steel refers to the alloy steel suitable for the manufacture of machinery and mechanical parts. On the basis of high-quality carbon steel, it is appropriate to add one or several alloying elements to improve the strength, toughness and hardenability of steel. This type of steel is usually used after heat treatment (such as tempering treatment, surface hardening treatment).

It mainly includes two categories of commonly used alloy structural steel and alloy spring steel, including tempered alloy steel, surface hardening alloy steel (carburizing steel, nitriding steel, surface high-frequency quenching steel, etc.), cold plastic forming alloy steel (cold forging steel, cold extrusion steel, etc.).

According to the basic composition of chemical composition series can be divided into Mn series steel, SiMn series steel, Cr series steel, CrMo series steel, CrNiMo series steel, Ni series steel, B series steel and so on.

10. alloy structural steel

The carbon content of alloy structural steel is lower than that of carbon structural steel, generally in the range of 0.15%-0.50%. In addition to carbon, it also contains one or several alloying elements, such as silicon, manganese, vanadium, titanium, boron and nickel, chromium, molybdenum and so on. Alloy structural steel is easy to harden and not easy to deform or crack, which is easy to improve the performance of steel by heat treatment.

Alloy structural steel is widely used in the manufacture of automobiles, tractors, ships, steam turbines, heavy machine tools of various transmission parts and fasteners. Low carbon alloy steel is generally carburized, and medium carbon alloy steel is generally tempered.

11. alloy tool steel

Alloy tool steel is medium and high carbon steel containing a variety of alloying elements, such as silicon, chromium, tungsten, molybdenum, vanadium, etc. Alloy tool steel is easy to harden, not easy to produce deformation and cracks, suitable for manufacturing large size, complex shape of cutting tools, molds and measuring tools for different purposes, the carbon content of alloy tool steel is also different.

The carbon content of most alloy tool steels is 0.5-1.5%, and the carbon content of steel for hot deformation dies is low, and the carbon content of ωc is in the range of 0.3-0.6%. Cutting tool steel generally contains about carbon ωc1%;

The steel used in cold working molds has a high carbon content, such as the carbon content of graphite die steel ωc of 1.5%, and the carbon content of high-carbon and high-chromium cold working die steel ωc of more than 2%.

12. high-speed tool steel

High-speed tool steel is a high-carbon high-alloy tool steel, the carbon content of steel is 0.7%-1.4%, steel contains alloy elements that can form high-hardness carbides, such as tungsten, molybdenum, chromium, vanadium.

High-speed tool steel has high red hardness, under the conditions of high-speed cutting, the temperature is as high as 500-600℃ hardness is not reduced, so as to ensure good cutting performance

13. Spring

The spring is used under shock, vibration or long-term alternating stress, so the spring steel is required to have high tensile strength, elastic limit and high fatigue strength. In the process, the spring steel is required to have a certain degree of hardenability, not easy to decarbonize, and good surface quality

Carbon spring steel is the carbon content of ωc in the range of 0.6%-0.9% of high-quality carbon structural steel (including normal and higher manganese content). Alloy spring steel is mainly silicon manganese series steel, their carbon content is slightly lower, mainly by increasing the silicon content of ωsi (1.3%-2.8%) to improve performance;

In addition, chromium, tungsten, vanadium alloy spring steel. In recent years, combined with China's resources, and according to the requirements of the new technology of automobile and tractor design, a new steel with boron, niobium, molybdenum and other elements has been developed on the basis of silicon-manganese steel, which extends the service life of the spring and improves the quality of the spring steel.

14. bearing steel

Bearing steel is the steel used to make balls, rollers and bearing rings. Bearings are subjected to extreme pressure and friction when working, so bearing steel is required to have high and uniform hardness and wear resistance, as well as a high elastic limit. The uniformity of the chemical composition of bearing steel, the content and distribution of non-metallic inclusions, the distribution of carbides and other requirements are very strict.

Bearing steel is also known as high carbon chromium steel, carbon ωc is about 1%, lead content ωcr is 0.5%-1.65%. Bearing steel is divided into six categories: high-carbon chromium bearing steel, non-chromium bearing steel, carburized bearing steel, stainless bearing steel, medium and high temperature bearing steel and anti-magnetic bearing steel.

15. electrical silicon steel

Silicon steel for electrical industry is mainly used to manufacture silicon steel sheets for electrical industry. Silicon steel sheet is a large amount of steel used in the manufacture of motors and transformers. According to the chemical composition of silicon steel can be divided into low silicon steel and high silicon steel. Low silicon steel silicon content ωsi=1.0%-2.5%, mainly used to manufacture motor; The silicon content of high-silicon steel is ωsi=3.0%-4.5%, which is generally used to manufacture transformers. Their carbon content is ωc=0.06%-0.08%.

16. rail steel

The rail mainly bears the pressure and impact load of the rolling stock, so. It requires sufficient strength and hardness and a certain toughness. Usually used rail is open-hearth and converter smelting carbon killed steel, this steel contains carbon ωC=0.6%-0.8%, belongs to carbon steel and high carbon steel, but the manganese content of ωMn in steel is higher, in the range of 0.6%-1.1%. In recent years, the common low alloy steel rail has been widely used, such as high silicon rail, medium manganese rail, copper rail, titanium rail and so on. Compared with carbon rail, ordinary low-alloy rail is wear-resistant and corrosion-resistant, and its service life is greatly improved.

17. shipbuilding steel

Shipbuilding steel refers to the steel used in the manufacture of sea ships and large inland river hull structures. Because the hull structure is generally manufactured by welding method, the shipbuilding steel is required to have good welding performance. In addition, it also requires a certain strength, toughness and a certain low temperature and corrosion resistance. In the past, mild steel was mainly used as shipbuilding steel. Recently, a large number of ordinary low-alloy steel has been used, and our own steel grades such as 12 manganese ships, 16 manganese ships, 15 manganese vanadium ships and other steel grades. These steel grades have high strength, good toughness, easy processing and welding, seawater corrosion resistance and other comprehensive characteristics, can be successfully used to manufacture tons of ocean ships.

18. bridge steel

Railway or highway Bridges bear the impact load of vehicles, bridge steel requires a certain strength, toughness and good fatigue resistance, and the surface quality of steel requirements are high. Bridge steel is often used basic open-hearth killed steel. Recently, common low alloy steels such as 16 Mn and 15 Mn vanadium N have been successfully used.

19. boiler steel

Boiler steel mainly refers to the material used to make superheater, main steam pipe and boiler fire chamber heating surface. The performance requirements of boiler steel are mainly good welding performance, certain high temperature strength and alkali corrosion resistance, oxidation resistance and so on. Commonly used boiler steel is low carbon killed steel smelted in open furnace or low carbon steel smelted in electric furnace, the carbon content of ωc is in the range of 0.16% to 0.26%. In the manufacture of high-pressure boilers, pearlitic heat-resistant steel or austenitic heat-resistant steel is used. In recent years, ordinary low-alloy steel has also been used to build boilers, such as 12 manganese, 15 manganese vanadium, 18 manganese molybdenum niobium and so on.

20. welding rod steel

This kind of steel is specially used for the manufacture of arc and gas welding electrode wire. The composition of steel varies with the material being welded. According to the needs of roughly divided into carbon steel, alloy structural steel and stainless steel three categories. These steel sulfur, phosphorus content of ωs, ωp is not more than 0.03%, higher than the general steel requirements. These steels are not required to have mechanical properties, but only to be tested for chemical composition.

21. stainless steel

Stainless steel is referred to as stainless steel, which is composed of two parts of stainless steel and acid-resistant steel. In short, steel that can resist atmospheric corrosion is called stainless steel, and steel that can resist the corrosion of chemical media (such as acids) is called acid-resistant steel. Generally speaking, steel with chromium content of ωcr greater than 12% has the characteristics of stainless steel; According to the microstructure of stainless steel after heat treatment, it can be divided into five categories: ferritic stainless steel, martensitic stainless steel, austenitic stainless steel, austenitic ferritic stainless steel and precipitation-hardened stainless steel.

22. heat-resistant steel

Under high temperature conditions, steel with oxidation resistance and sufficient high temperature strength as well as good heat resistance is called heat resistant steel. Heat resistant steels include oxidation resistant steels and heat strength steels. Antioxidant steel is also known as non-peeling steel. Thermal strength steel refers to the steel with good oxidation resistance and high high temperature strength at high temperatures. Heat-resistant steel is mainly used for parts that are used for a long time at high temperatures.

23. Superalloy

Superalloy refers to a thermal strength material with sufficient durable strength, creep strength, thermal fatigue strength, high temperature toughness and sufficient chemical stability at high temperatures, which is used for thermal power components working under high temperature conditions of about 1000℃.

According to the different basic chemical composition, it can be divided into nickel based superalloy, iron nickel based superalloy and cobalt based superalloy.

24. precision alloy

Precision alloys are alloys with special physical properties. It is an indispensable material in the electrical industry, electronics industry, precision instrument industry and automatic control system.

Precision alloys are divided into 7 categories according to their different physical properties, namely: soft magnetic alloy, deformed permanent magnet alloy, elastic alloy, expansion alloy, thermal bimetal, resistance alloy, thermoelectric Angle alloy. The vast majority of precision alloys are based on ferrous metals, and only a few are based on non-ferrous metals.


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