What are the basic classifications of steel? (1)
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.
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