Classification of carbon steel and corresponding properties


Basic classification of carbon steel


Carbon steel, also called carbon steel, is an iron-carbon alloy with a carbon content ωc less than 2%. 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 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 phosphorus and sulfur content. Carbon steel is divided into ordinary carbon steel (higher phosphorus and sulfur content), high-quality carbon steel (lower phosphorus and sulfur content), and high-grade high-quality steel (lower phosphorus and sulfur content). The higher the hardness, the higher the strength, but the lower the plasticity.


Carbon structural steel


This type of steel mainly guarantees mechanical properties. Therefore, its brand name reflects its mechanical properties, and the Q+ number is used to indicate the Chinese pinyin prefix of the word "Qu" in which "Q" is the yield point. The number indicates the value of the yield point. For example, Q275 means that the yield point is 275Mpa. If the letters A, B, C, and D are marked after the grade, it means that the quality of the steel is different, and the amount of S and P decreases in turn and the quality of the steel increases in turn. If the letter "F" is marked after the grade, it is boiling steel, if it is marked with "b", it is semi-killed steel, and if it is not marked with "F" or "b", it is a killed steel. For example, Q235-AF means grade A boiling steel with a yield point of 235MPa, and Q235-C means grade C killed steel with a yield point of 235MPa. Carbon structural steel is generally not heat-treated and is used directly in the supplied state. Usually, Q195, Q215, and Q235 steel have low carbon mass fraction, good welding performance, good plasticity, toughness, and certain strength. Screws, nuts, and other parts. The mass fraction of carbon in Q255 and Q275 steel is slightly higher, the strength is higher, the plasticity and toughness are better, and it can be welded. It is usually rolled into section steel, bar steel, and steel plate as structural parts and to manufacture connecting rods, gears, and couplings of simple machinery: sections, pins, and other parts.


carbon steel


High quality structural steel


This type 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 steel in tens of thousands (ωс*10000). For example, 45 steel means that the average carbon mass fraction in steel is 0.45%; 08 steel means that the average carbon mass fraction in steel is 0.08%. High-quality carbon structural steel is mainly used in the manufacture of machine parts. Generally, heat treatment is required to improve mechanical properties. Depending on the carbon mass fraction, there are different uses. 08, 08F, 10, and 10F steels have high plasticity and toughness and excellent cold forming and welding properties. They are usually cold-rolled into thin plates and used to make instrument casings, and cold stamping parts on automobiles and tractors, such as automobile bodies and tractors. Cab, etc.; 15, 20, and 25 sheets of steel are used to make carburized parts with small size, light load, wear-resistant surface, and low core strength requirements, such as piston pins, sample plates, 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. For example, 40 and 45 steels are often used in manufacturing Crankshafts, connecting rods of automobiles and tractors, general machine tool spindles, machine tool gears, and other shaft parts with little stress; 55, 60, and 65 steels have high elastic limits after heat treatment (quenching + medium temperature tempering) and are often used in production 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.


Carbon tool steel


Carbon tool steel is a high-carbon steel that basically does not contain alloy elements. The carbon content is in the range of 0.65%-1.35%. Its production cost is low, the source of raw materials is easy to obtain, and it has good machinability. High wear resistance, so it is widely used in the manufacture of various cutting tools, molds, and measuring tools. But the red hardness of this kind of steel is poor, that is to say, when the working temperature is higher than 250°C, the hardness and wear resistance of the steel will drop sharply and lose the working ability. In addition, if carbon tool steel is made into larger parts, it is not easy to harden, and it is prone to deformation and cracks.


carbon steel coil


Free cutting structural steel


Free-cutting structural steel is to add some elements that make the steel brittle to the steel, so that the steel is easily brittle and broken into chips when cutting, which is beneficial to increase the cutting speed and prolong the tool life. The element that makes steel brittle is mainly sulfur, and elements such as lead, tellurium, and bismuth are used in ordinary 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%. Sulfur and manganese in steel exist in the form of manganese sulfide. Manganese sulfide is very brittle and has a lubricating effect, which makes chips easy to break and improves the quality of the machined surface.