1. Mechanical properties of steel
1. Yield point (σs)
When the steel or sample is stretched, when the stress exceeds the elastic limit, even if the stress does not increase, the steel or sample still continues to undergo obvious plastic deformation. This phenomenon is called yielding, and the minimum stress value when yielding occurs is for the yield point. Let Ps be the external force at the yield point s, and Fo be the cross-sectional area of the sample, then the yield point σs =Ps/Fo(MPa).
2. Yield strength (σ0.2)
The yield point of some metal materials is very inconspicuous, and it is difficult to measure. Therefore, in order to measure the yield characteristics of the material, the stress when the permanent residual plastic deformation is equal to a certain value (generally 0.2% of the original length) is stipulated, which is called the condition. Yield strength or simply yield strength σ0.2.
3. Tensile strength (σb)
The maximum stress value reached by the material from the beginning to the time of fracture during the stretching process. It represents the ability of steel to resist fracture. Corresponding to tensile strength are compressive strength, flexural strength, etc. Let Pb be the maximum tensile force reached before the material is broken, and Fo be the cross-sectional area of the sample, then the tensile strength σb=Pb/Fo (MPa).
4. Elongation (δs)
After the material is broken, the percentage of its plastic elongation to the length of the original sample is called elongation or elongation.
5. Yield ratio (σs/σb)
The ratio of the yield point (yield strength) of steel to the tensile strength is called the yield ratio. The greater the yield ratio, the higher the reliability of structural parts. Generally, the yield ratio of carbon steel is 0.6-0.65, that of low-alloy structural steel is 0.65-0.75, and that of alloy structural steel is 0.84-0.86.
6. Hardness
Hardness indicates the ability of a material to resist pressing a hard object into its surface. It is one of the important performance indicators of metal materials. Generally, the higher the hardness, the better the wear resistance. Commonly used hardness indicators are Brinell hardness, Rockwell hardness and Vickers hardness.
1) Brinell hardness (HB)
Press a hardened steel ball of a certain size (usually 10mm in diameter) into the surface of the material with a certain load (generally 3000kg) and keep it for a period of time. After the load is removed, the ratio of the load to the indentation area is the Brinell hardness value ( HB).
2) Rockwell hardness (HR)
When HB>450 or the sample is too small, Brinell hardness test cannot be used and Rockwell hardness measurement should be used instead. It uses a diamond cone with a vertex angle of 120° or a steel ball with a diameter of 1.59mm and 3.18mm to press into the surface of the material to be tested under a certain load, and the hardness of the material is obtained from the depth of the indentation. According to the hardness of the test material, it can be expressed in three different scales:
HRA: It is the hardness obtained by using a 60kg load and a diamond cone indenter, and is used for materials with extremely high hardness (such as cemented carbide, etc.).
HRB: It is the hardness obtained by using a 100kg load and a hardened steel ball with a diameter of 1.58mm. It is used for materials with lower hardness (such as annealed steel, cast iron, etc.).
HRC: It is the hardness obtained by using a 150kg load and a diamond cone indenter, and is used for materials with high hardness (such as hardened steel, etc.).
3) Vickers hardness (HV)
Use a diamond square cone indenter with a load of less than 120kg and a vertex angle of 136° to press into the surface of the material, and divide the surface area of the indentation pit by the load value to obtain the Vickers hardness value (HV).
2. Ferrous and non-ferrous metals
1. Ferrous metal
Refers to the alloy of iron and iron. Such as steel, pig iron, ferroalloy, cast iron, etc. Both steel and pig iron are alloys based on iron with carbon as the main additive element, collectively referred to as iron-carbon alloys.
Pig iron refers to the product made by smelting iron ore in a blast furnace, which is mainly used for steelmaking and casting.
Smelting cast pig iron in an iron melting furnace to obtain cast iron (liquid iron-carbon alloy with a carbon content greater than 2.11%), and casting the liquid cast iron into castings, this type of cast iron is called cast iron.
Ferroalloy is an alloy composed of iron, silicon, manganese, chromium, titanium and other elements. Ferroalloy is one of the raw materials for steelmaking. It is used as a deoxidizer and alloy element additive for steel during steelmaking.
Iron-carbon alloys with a carbon content of less than 2.11% are called steel, and steel is obtained by putting pig iron for steelmaking into a steelmaking furnace and smelting it according to a certain process. Steel products include steel ingots, continuous casting slabs and direct casting into various steel castings. Generally speaking, steel generally refers to steel rolled into various steel products.
2. Non-ferrous metals
Also known as non-ferrous metals, it refers to metals and alloys other than ferrous metals, such as copper, tin, lead, zinc, aluminum, and brass, bronze, aluminum alloys, and bearing alloys. In addition, chromium, nickel, manganese, molybdenum, cobalt, vanadium, tungsten, titanium, etc. are also used in industry. These metals are mainly used as alloy additions to improve the performance of metals. Among them, tungsten, titanium, molybdenum, etc. are mostly used to produce knives. hard alloy. The above non-ferrous metals are called industrial metals, in addition to precious metals: platinum, gold, silver, etc. and rare metals, including radioactive uranium, radium, etc.
3. Classification of steel
In addition to iron and carbon, the main elements of steel include silicon, manganese, sulfur and phosphorus.
There are various classification methods of steel, and the main methods are as follows:
1. Classified by quality
(1) Ordinary steel (P≤0.045%, S≤0.050%)
(2) High-quality steel (both P and S≤0.035%)
(3) High quality steel (P≤0.035%, S≤0.030%)
2. Classification by chemical composition
(1) Carbon steel: a. Low carbon steel (C≤0.25%); b. Medium carbon steel (C≤0.25～0.60%); c. High carbon steel (C≤0.60%).
(2) Alloy steel: a. Low alloy steel (total content of alloying elements ≤ 5%); b. Medium alloy steel (total content of alloying elements > 5-10%); c. High alloy steel (total content of alloying elements > 10% %).
3. Classified by forming method
(1) forged steel; (2) cast steel; (3) hot-rolled steel; (4) cold-drawn steel.
4. Classification according to metallographic structure
(1) Annealed state: a. hypoeutectoid steel (ferrite + pearlite); b. eutectoid steel (pearlite); c. hypereutectoid steel (pearlite + cementite); d. Tensitic steel (pearlite + cementite).
(2) Normalized state: a. pearlitic steel; b. bainite steel; c. martensitic steel; d. austenitic steel.
(3) No phase change or partial phase change
5. Classification by purpose
(1) Steel for construction and engineering: a. Ordinary carbon structural steel; b. Low alloy structural steel; c. Reinforced steel.
(2) Structural steel:
a. Steel for machinery manufacturing: (a) Quenched and tempered structural steel; (b) Surface hardened structural steel: including carburizing steel, ammoniated steel, and surface hardened steel; (c) Easy-cut structural steel; (d) Cold plasticity Steel for forming: including steel for cold stamping and steel for cold heading.
b. Spring steel
c. Bearing steel
(3) Tool steel: a. carbon tool steel; b. alloy tool steel; c. high-speed tool steel.
(4) Special performance steel: a. Stainless acid-resistant steel; b. Heat-resistant steel: including anti-oxidation steel, heat-strength steel, valve steel; c. Electric heating alloy steel; d. Wear-resistant steel; e. Low temperature steel ; f. Electrical steel.
(5) Steel for professional use—such as steel for bridges, steel for ships, steel for boilers, steel for pressure vessels, steel for agricultural machinery, etc.
6. Comprehensive classification
(1) Ordinary steel
a. Carbon structural steel: (a) Q195; (b) Q215 (A, B); (c) Q235 (A, B, C); (d) Q255 (A, B); (e) Q275.
b. Low alloy structural steel
c. Ordinary structural steel for specific purposes
(2) High-quality steel (including high-grade high-quality steel)
a. Structural steel: (a) high-quality carbon structural steel; (b) alloy structural steel; (c) spring steel; (d) free-cutting steel; (e) bearing steel; (f) high-quality structural steel for specific purposes.
b. Tool steel: (a) carbon tool steel; (b) alloy tool steel; (c) high-speed tool steel.
c. Special performance steel: (a) stainless acid-resistant steel; (b) heat-resistant steel; (c) electric heating alloy steel; (d) electrical steel; (e) high manganese wear-resistant steel.
7. Classified by smelting method
(1) According to furnace type
a. Converter steel: (a) acidic converter steel; (b) basic converter steel. Or (a) bottom-blown converter steel; (b) side-blown converter steel; (c) top-blown converter steel.
b. Electric furnace steel: (a) electric arc furnace steel; (b) electroslag furnace steel; (c) induction furnace steel; (d) vacuum consumable furnace steel; (e) electron beam furnace steel.
(2) According to the degree of deoxidation and pouring system
a. Boiling steel; b. Semi-killed steel; c. Killed steel; d. Special killed steel.
4. Overview of my country’s steel grade representation methods
The indication of the product grade is generally indicated by a combination of Chinese pinyin letters, chemical element symbols and Arabic numerals. Right now:
①The chemical elements in steel grades are represented by international chemical symbols, such as Si, Mn, Cr…etc. Mixed rare earth elements are represented by “RE” (or “Xt”).
②The product name, usage, smelting and pouring methods, etc. are generally represented by the abbreviated letters of Chinese Pinyin.
③The main chemical element content (%) in steel is represented by Arabic numerals.
When the Chinese phonetic alphabet is used to indicate the product name, usage, characteristics and process methods, the first letter is generally selected from the Chinese phonetic alphabet representing the product name. When it is repeated with the letter selected by another product, the second letter or the third letter can be used instead, or the first pinyin letter of the two Chinese characters can be selected at the same time.
If there are no Chinese characters and pinyin available for the time being, the symbols used are English letters.
Five, the subdivision of the representation method of steel grades in my country
1. Designation method of carbon structural steel and low-alloy high-strength structural steel
The steel used above is usually divided into two categories: general steel and special steel. The method of indicating the grade is composed of the Chinese pinyin letters of the yield point or yield strength of the steel, the value of the yield point or yield strength, the quality grade of the steel, and the degree of deoxidation of the steel, which is actually composed of 4 parts.
①General structural steel adopts the pinyin letter “Q” representing the yield point. The yield point value (unit is MPa) and the quality grades (A, B, C, D, E) and deoxidation method (F, b, Z, TZ) and other symbols specified in Table 1 form the grade in order. For example: carbon structural steel grades are expressed as: Q235AF, Q235BZ; low-alloy high-strength structural steel grades are expressed as: Q345C, Q345D.
Q235BZ means killed carbon structural steel with yield point value ≥ 235MPa and quality grade B.
The two grades of Q235 and Q345 are the most typical grades of engineering steel, the grades with the largest production and use, and the most widely used grades. These two grades are available in almost all countries in the world.
In the grade composition of carbon structural steel, the symbol “Z” of killed steel and the symbol “TZ” of special killed steel can be omitted, for example: for Q235 steel with quality grades C and D respectively, the grades should be Q235CZ and Q235DTZ, But it can be omitted as Q235C and Q235D.
Low-alloy high-strength structural steel includes killed steel and special killed steel, but the symbol indicating the deoxidation method is not added at the end of the grade.
②Special structural steel is generally indicated by the symbol “Q” representing the yield point of the steel, the value of the yield point, and the symbols representing the product use specified in Table 1, for example: the steel grade for pressure vessels is expressed as “Q345R”; the grade of weathering steel is Expressed as Q340NH; Q295HP steel grades for welding gas cylinders; Q390g steel grades for boilers; Q420q steel grades for bridges.
③According to the needs, the designation of general-purpose low-alloy high-strength structural steel can also use two Arabic numerals (indicating the average carbon content, in parts per ten thousand) and chemical element symbols, expressed in order; the special low-alloy high-strength structural steel The brand name can also be expressed in sequence by using two Arabic numerals (indicating the average carbon content in parts per ten thousand), chemical element symbols, and some specified symbols representing the use of the product.
2. Representation method of high-quality carbon structural steel and high-quality carbon spring steel
High-quality carbon structural steel adopts a combination of two Arabic numerals (indicating the average carbon content in ten-thousandths) or Arabic numerals and element symbols.
① For boiling steel and semi-killed steel, symbols “F” and “b” are respectively added at the end of the grade. For example, the grade of boiling steel with an average carbon content of 0.08% is expressed as “08F”; the grade of semi-killed steel with an average carbon content of 0.10% is expressed as “10b”.
② Killed steel (S, P≤0.035% respectively) is generally not marked with symbols. For example: killed steel with an average carbon content of 0.45%, its grade is expressed as “45″.
③ For high-quality carbon structural steels with higher manganese content, the manganese element symbol is added after the Arabic numerals indicating the average carbon content. For example: steel with an average carbon content of 0.50% and a manganese content of 0.70% to 1.00%, its grade is expressed as “50Mn”.
④ For high-grade high-quality carbon structural steel (S, P≤0.030% respectively), add the symbol “A” after the grade. For example: high-grade high-quality carbon structural steel with an average carbon content of 0.45%, its grade is expressed as “45A”.
⑤Super-grade high-quality carbon structural steel (S≤0.020%, P≤0.025%), add the symbol “E” after the grade. For example: super high-quality carbon structural steel with an average carbon content of 0.45%, its grade is expressed as “45E”.
The representation method of high-quality carbon spring steel grades is the same as that of high-quality carbon structural steel grades (65, 70, 85, 65Mn steels exist in both standards GB/T1222 and GB/T699 respectively).
3. Designation method of alloy structural steel and alloy spring steel
① Alloy structural steel grades are represented by Arabic numerals and standard chemical element symbols.
Use two Arabic numerals to indicate the average carbon content (in parts per ten thousand), and place it at the head of the grade.
The expression method of alloy element content is as follows: when the average content is less than 1.50%, only the element is indicated in the brand, and the content is generally not indicated; the average alloy content is 1.50%～2.49%, 2.50%～3.49%, 3.50%～4.49%, 4.50%～ 5.49%, …, correspondingly written as 2, 3, 4, 5 … after the alloying elements.
For example: the average content of carbon, chromium, manganese, and silicon are respectively 0.30%, 0.95%, 0.85%, and 1.05% of alloy structural steel. When the content of S and P is ≤0.035%, the grade is expressed as “30CrMnSi”.
High-grade high-quality alloy structural steel (S, P content ≤0.025% respectively), indicated by adding the symbol “A” at the end of the grade. For example: “30CrMnSiA”.
For special-grade high-quality alloy structural steel (S≤0.015%, P≤0.025%), add the symbol “E” at the end of the grade, for example: “30CrM nSiE”.
For special alloy structural steel grades, the symbol representing the product use specified in Table 1 should be added to the head (or tail) of the grade. For example, the 30CrMnSi steel specially used for riveting screws, the steel number is expressed as ML30CrMnSi.
②The representation method of the grade of alloy spring steel is the same as that of alloy structural steel.
For example: the average content of carbon, silicon, and manganese are respectively 0.60%, 1.75%, and 0.75% of spring steel, and its grade is expressed as “60Si2Mn”. For high-grade high-quality spring steel, add the symbol “A” at the end of the grade, and its grade is expressed as “60Si2MnA”.
4. The grade of free-cutting steel
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Post time: Jun-21-2023
