Classification of carbon steel

1. Carbon steel

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

According to the application, carbon steel can be divided into three categories: carbon structural steel, carbon tool steel, and free-cutting structural steel. Carbon structural steel can be divided into building structural steel and machine-made structural steel. According to the 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 content of phosphorus and sulfur, carbon steel can be divided into ordinary carbon steel (higher phosphorus and sulfur), high-quality carbon steel (lower phosphorus and sulfur) and advanced high-quality steel (lower phosphorus and sulfur) Generally, the higher the carbon content in carbon steel, the higher the hardness and the higher the strength, but the lower the plasticity.

2. Carbon structural steel

This type of steel mainly guarantees mechanical properties. Therefore, its grade reflects its mechanical properties, and the Q+ number is used to indicate the Hanyu Pinyin prefix of the word “Q” for the yield point “Qu”.

The number indicates the yield point value. For example, Q275 indicates that the yield point is 275Mpa. If the letters A, B, C, and D are marked after the grade, it means that the steel quality grades are different.

If the letter “F” is marked after the grade, it is boiling steel, marked with “b” is semi-killed steel, and those without “F” or “b” are killed steel. For example, Q235-AF means A-grade boiling steel with a yield point of 235MPa, and Q235-C means C-grade killed steel with a yield point of 235MPa.

Carbon structural steel is generally not subjected to heat treatment but is used directly in the supply state. Usually, Q195, Q215, and Q235 steel have a low mass fraction of carbon, good welding performance, good plasticity, and toughness, and a certain strength. It is often rolled into thin plates, steel bars, welded steel pipes, etc.

It is used for bridges, buildings, and other structures and the manufacture of ordinary rivets, screws, nuts, and other parts. Q255 and Q275 steels have a slightly higher mass fraction of carbon, higher strength, better plasticity, and toughness, and can be welded. They are usually rolled into shaped steel, bar steel, and steel plate as structural parts, as well as connecting rods, gears, and couplings for simple machinery. parts, pins, etc.

3. High-quality structural steel

This type of steel must guarantee both chemical composition and mechanical properties. Its grade is ten thousand percent (ωс*10000) of the average carbon mass fraction in the steel using two digits. For example, 45 steel means that the average carbon mass fraction in the steel is 0.45%; 08 steel means that the average carbon mass fraction in the steel is 0.08%.

High-quality carbon structural steel is mainly used to manufacture machine parts. Generally, heat treatment is required to improve mechanical properties. Depending on the carbon content, there are different uses.

08, 08F, 10, 10F steel, high plasticity, toughness, excellent cold forming performance and welding performance, often cold-rolled into a thin plate, used to make instrument housing, cold stamping parts on automobiles and tractors, such as automobile bodies, tractors Cab, etc.;

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

30, 35, 40, 45, and 50 steels have good comprehensive mechanical properties after heat treatment (quenching + high-temperature tempering), that is, they have high strength, high plasticity, and toughness, and are used to make shaft parts, such as 40, 45 steel is often used in the manufacture of crankshafts, connecting rods of automobiles and tractors, general machine tool spindles, machine tool gears 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 a small load and small size (section size less than 12~15mm), such as pressure regulating and speed regulating springs, Plunger springs, cold coil springs, etc.

4. Carbon tool steel

Carbon tool steel is high carbon steel that does not contain alloying elements. The carbon content is in the range of 0.65% to 1.35%. Its production cost is low, the source of raw materials is easy to obtain, and the machinability is good. High wear resistance, so it is a widely used steel grade for manufacturing various cutting tools, molds, and measuring tools.

However, the red hardness of this type 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 the working ability will be lost. In addition, carbon tool steel is not easy to harden if made into larger parts and is prone to deformation and cracks.

5. Free cutting structural steel

Free-cutting structural steel is the addition of some elements that make the steel brittle so that the steel is easily brittle and broken into chips during cutting, which is beneficial to increase the cutting speed and prolong the tool life. The element that makes the steel brittle is mainly sulfur, and elements such as lead, tellurium, and bismuth are used in ordinary low-alloy free-cutting structural steel.

The sulphur 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 a lubricating effect so that the chips are easily broken, and it is beneficial to improving the quality of the machined surface.

6. Alloy steel

In addition to iron, carbon, and a small amount of inevitable silicon, manganese, phosphorus, and sulfur elements, steel also contains a certain amount of alloying elements. The alloying elements in steel include silicon, manganese, molybdenum, nickel, chromium, vanadium, titanium, niobium, boron, lead, rare earth, etc., and one or more of them, these steel is called alloy steel.

The alloy steel systems of various countries vary with their respective resource conditions, production, and use conditions. In the past, nickel and chromium steel systems have been developed abroad, while my country has developed alloys based on silicon, manganese, vanadium, titanium, niobium, boron, and rare earth. steel system.

Alloy steel accounts for about ten percent of the total output of steel. Generally, it is smelted in electric furnaces. Alloy steel can be divided into 8 categories according to its purpose. They are alloy structural steel, spring steel, bearing steel, alloy tools Steel, high-speed tool steel, stainless steel, heat-resistant non-skin steel, and silicon steel for electrical use.

7. Ordinary low alloy steel

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

8. Alloy steel for engineering structure

This refers to alloy steel used in engineering and building structures, including weldable high-strength alloy structural steel, alloy reinforced steel, alloy steel for railways, alloy steel for geological oil drilling, alloy steel for pressure vessels, high manganese wear-resistant steel, etc. . This type of steel is used for engineering and building structural parts. Among the alloy steels, the total content of this type of steel alloy is relatively low, but the production and use are relatively large.

9. Alloy steel for mechanical structure

This type of steel refers to alloy steel suitable for the manufacture of machines and mechanical parts. It is based on high-quality carbon steel, appropriately adding 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 quenching and tempering treatment, surface hardening treatment).

It mainly includes two categories of commonly used alloy structural steel and alloy spring steel, including quenched and tempered alloy steel, surface hardened alloy steel (carburized steel, nitrided steel, surface high frequency quenched steel, etc.), cold plastic forming Use alloy steel (steel for cold upsetting, steel for cold extrusion, etc.).

According to the basic composition series of chemical components, it 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, etc.

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 combined carbon, it also contains one or several alloying elements, such as silicon, manganese, vanadium, titanium, boron and nickel, chromium, molybdenum, etc. Alloy structural steel is easy to harden and not easy to deform or crack, and it is convenient for heat treatment to improve the performance of steel.

Alloy structural steel is widely used in the manufacture of various transmission parts and fasteners for automobiles, tractors, ships, steam turbines, and heavy machine tools. Low carbon alloy steel is generally carburized, and medium carbon alloy steel is generally quenched and 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, and it is not easy to produce deformation and cracks. It is suitable for manufacturing cutting tools, molds, and measuring tools with large sizes and complex shapes. The carbon content of alloy tool steel is also different.

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

The carbon content of cold-worked die steel is relatively high. For example, the carbon content of graphite die steel is up to 1.5%, and the carbon content of high-carbon and high-chromium cold-worked die steel is as high as 2%.

12. High-speed tool steel

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

High-speed tool steel has high red hardness. Under the condition of high-speed cutting, the temperature is as high as 500-600 ℃, and the hardness does not decrease, to ensure good cutting performance.

13. Spring

Springs are used under shock, vibration, or long-term alternating stress, so 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 hardenability, not easy to decarburize, and good surface quality.

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

In addition, there are alloy spring steels of chromium, tungsten, and vanadium. In recent years, combined with our country’s resources and according to the requirements of new technologies for automobile and tractor design, new steel grades with boron, niobium, molybdenum, and other elements added to silicon-manganese steel have been developed, which prolongs the service life of the spring and improves the quality of the spring. Steel quality.

14. Bearing steel

Bearing steel is the steel used to make balls, rollers, and bearing rings. Bearings are subjected to great pressure and friction during operation, so bearing steel is required to have high and uniform hardness and wear resistance, as well as a high elastic limit. Content and distribution, carbide distribution, and other requirements are very strict.

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

15. Electrical silicon steel

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

16. Rail steel

The rail mainly bears the pressure and impact load of the rolling stock, therefore. Sufficient strength and hardness and certain toughness are required. The steel rails usually used are carbon-killed steel smelted by an open hearth and converter. This steel contains carbon ωC=0.6%-0.8%, which belongs to medium carbon steel and high carbon steel, but the manganese content in steel is higher, at 0.6% -1.1% range. In recent years, ordinary low-alloy steel rails, such as high silicon rails, medium manganese rails, copper-containing rails, titanium-containing rails, etc., have been widely used. Ordinary low alloy steel rails are wear-resistant and corrosion-resistant than carbon steel rails, and their service life is greatly improved.

17. Steel for shipbuilding

Shipbuilding steel refers to the steel used to manufacture seagoing ships and large inland hull structures. Since the hull structure is generally manufactured by welding methods, the shipbuilding steel is required to have good welding performance. In addition, a certain strength, toughness, and certain low temperature and corrosion resistance are also required. In the past, low-carbon steel was mainly used as steel for shipbuilding. Recently, a large number of ordinary low-alloy steels have been used, and the existing steel grades such as 12 manganese ships, 16 manganese ships, 15 manganese vanadium ships, and other steel grades. These steel grades have comprehensive characteristics such as high strength, good toughness, easy processing and welding, and seawater corrosion resistance, and can be successfully used to manufacture 10,000-ton ocean-going giant ships.

18. Bridge steel

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

19.Boiler steel

Boiler steel mainly refers to the materials used to manufacture superheaters, main steam pipes, and heating surfaces of boiler fire chambers. The performance requirements for boiler steel are mainly good welding performance, certain high-temperature strength, corrosion resistance of alkali parts, and oxidation resistance. Commonly used boiler steels are low carbon killed steel smelted by an open hearth or low carbon steel smelted by an electric furnace, and the carbon content ωc is in the range of 0.16%-0.26%. When manufacturing high-pressure boilers, pearlitic heat-resistant steel or austenitic heat-resistant steel is used. In recent years, ordinary low-alloy steels have also been used to build boilers, such as 12 manganese, 15 manganese vanadium, 18 manganese molybdenum niobium, etc.

20. Steel for welding rod

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

21. Stainless steel

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

22. Heat-resistant steel

Under high-temperature conditions, steel with oxidation resistance, sufficient high-temperature strength, and good heat resistance is called heat-resistant steel. Heat-resistant steel includes two categories: oxidation-resistant steel and heat-strength steel. Anti-oxidation steel is also called non-skinned steel. Heat-strength steel refers to steel with good oxidation resistance at high temperature and high high-temperature strength. Heat-resistant steel is mainly used for parts that are used for a long time at high temperatures.

23. High-temperature alloy

Superalloy refers to a thermally strong material with sufficient endurance strength, creep strength, thermal fatigue strength, high-temperature toughness, and sufficient chemical stability at high temperatures.

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

24. Precision alloys

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

Precision alloys are divided into 7 categories according to their different physical properties, namely: soft magnetic alloys, deformed permanent magnet alloys, elastic alloys, expansion alloys, thermal bimetals, resistance alloys, and thermoelectric alloys. 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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