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 no longer increases, the steel or the sample still continues to undergo significant plastic deformation. This phenomenon is called yielding, and the minimum stress value when the yielding phenomenon occurs is Is the yield point. Suppose Ps is the external force at the yield point s, and Fo is 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 extremely inconspicuous, and it is difficult to measure. Therefore, in order to measure the yield characteristics of the material, it is stipulated that the stress when the permanent residual plastic deformation is equal to a certain value (usually 0.2% of the original length) is called the condition Yield strength or yield strength σ0.2 for short.
3. Tensile strength (σb)
The maximum stress value reached by the material during the stretching process from the beginning to when it breaks. It indicates the ability of steel to resist fracture. Corresponding to the tensile strength are compressive strength, bending strength and so on. Suppose Pb is the maximum tensile force reached before the material is broken, and Fo is 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 the plastic elongation length to the original sample length is called the elongation or elongation.
5. Yield-strength ratio (σs/σb)
The ratio of the yield point (yield strength) of steel to the tensile strength is called the yield ratio. The larger the yield ratio, the higher the reliability of structural parts. Generally, the yield ratio of carbon steel is 0.6-0.65, and that of low-alloy structural steel is 0.65-0.75 and alloy structural steel is 0.84-0.86.
6. Hardness
Hardness refers to the ability of a material to resist the pressing of hard objects 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 (usually 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, the Brinell hardness test cannot be used and the Rockwell hardness measurement can be used instead. It uses a diamond cone with an apex angle of 120° or a steel ball with a diameter of 1.59 and 3.18mm, which is pressed 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 with a 60kg load and a diamond cone indenter, and is used for extremely hard materials (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 with a load of 150kg and a diamond cone indenter, and is used for materials with high hardness (such as hardened steel, etc.).
(3) Vickers hardness (HV)
With a load of less than 120kg and a diamond square cone indenter with an apex angle of 136°, it is pressed into the surface of the material, and the surface area of ​​the material indentation pit is divided by the load value to obtain the Vickers hardness value (HV).
2. Ferrous metals and non-ferrous metals
1. Ferrous metals
Refers to an alloy of iron and iron. Such as steel, pig iron, ferroalloy, cast iron, etc. Both steel and pig iron are alloys based on iron and 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, and is mainly used for steelmaking and castings.
The cast iron is smelted in an iron melting furnace to obtain cast iron (liquid, iron-carbon alloy with a carbon content of more than 2.11%), and the liquid cast iron is cast into castings. This cast iron is called cast iron.
Ferroalloy is an alloy composed of iron and silicon, manganese, chromium, titanium and other elements. Ferroalloy is one of the raw materials for steelmaking. It is used as steel deoxidizer and alloy element additive during steelmaking.
Iron-carbon alloys with a carbon content of less than 2.11% are called steels. The pig iron for steelmaking is smelted in a steelmaking furnace according to a certain process to obtain steel. Steel products include steel ingots, continuous casting billets and direct casting into various steel castings. Generally speaking, steel generally refers to steel rolled into various steels.
2. Non-ferrous metals
Also called non-ferrous metals, it refers to metals and alloys other than ferrous metals, such as copper, tin, lead, zinc, aluminum, brass, bronze, aluminum alloy, 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 properties of metals. Among them, tungsten, titanium, molybdenum, etc. are mostly used to produce tools. Of cemented carbide. These non-ferrous metals are called industrial metals, in addition to precious metals: platinum, gold, silver and other rare metals, including radioactive uranium and radium.
Three, the classification of steel
In addition to iron and carbon, the main elements of steel include silicon, manganese, sulfur, and phosphorus.
There are many ways to classify 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. Classified 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. Classified by metallographic organization
(1) Annealed state: a. Hypoeutectoid steel (ferrite + pearlite); b. Eutectoid steel (pearlite); c. Hypereutectoid steel (pearlite + cementite); d. Lai 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. Rebar steel.
(2) Structural steel:
a. Steel for machinery manufacturing: (a) Quenched and tempered structural steel; (b) Surface hardened structural steel: Including carburized steel, ammonia infiltrated steel, and surface hardening steel; (c) Free-cutting 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 oxidation-resistant steel, heat-strength steel, valve steel; c. Electric heating alloy steel; d. Wear-resistant steel; e. Low-temperature steel ; F. Electrical steel.
(5) Professional steel-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. General structural steel for specific purposes
(2) High-quality steel (including 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. Classification by smelting method
(1) According to furnace type
a. Converter steel: (a) Acid 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 method
The expression of product grades generally adopts a combination of Chinese pinyin letters, chemical element symbols and Arabic numerals. which is:
① The 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").
②Product name, use, smelting and pouring methods, etc., are generally represented by the abbreviated letters of Chinese pinyin.
③The main chemical element content (%) in steel is expressed by Arabic numerals.
When the Pinyin alphabet is used to indicate the product name, use, characteristics and process method, the first letter is generally selected from the Pinyin 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 currently no available Chinese characters and Pinyin, the symbols used shall be English letters.
V. Subdivision description of the representation method of steel grades in my country
1. The designation method of carbon structural steel and low-alloy high-strength structural grades
The steel used above is usually divided into two categories: general steel and special steel. The grade expression method consists of the Chinese phonetic alphabet of the yield point or yield strength of the steel, the yield point or yield strength value, the quality grade of the steel, and the degree of deoxidation of the steel, which is actually composed of 4 parts.
① General structural steel adopts the pinyin letter "Q" which represents the yield point. The yield point value (unit: MPa), the quality grade (A, B, C, D, E), deoxidation method (F, b, Z, TZ) and other symbols specified in Table 1 are formed in order to form the grade. For example: carbon structural steel grades are expressed as: Q235AF, Q235BZ; low-alloy high-strength structural steel grades are expressed as: Q345C, Q345D.
Q235BZ represents a killed carbon structural steel with a yield point value ≥ 235MPa and a quality grade of B.
The two grades Q235 and Q345 are the most typical engineering steel grades, the most produced and used, and the most widely used grades. These two grades are available in almost every country in the world.
In the grade composition of carbon structural steel, the symbol for killed steel "Z" and the symbol for special killed steel "TZ" can be omitted, for example: Q235 steel with quality grades of C and D, and its grades should be Q235CZ and Q235DTZ. But it can be omitted as Q235C and Q235D.
Low-alloy high-strength structural steels include killed steel and special killed steel, but no symbol indicating the deoxidation method is written at the end of the grade.
②Special structural steel is generally represented by the symbol "Q" representing the yield point of steel, the value of the yield point, and the symbol representing the use of the product specified in Table 1, for example: the steel grade for pressure vessels is expressed as "Q345R"; the weathering steel is its grade Expressed as Q340NH; Q295HP steel grade for welding gas cylinders; Q390g boiler steel grade; Q420q steel grade for bridges.
③According to needs, the grades of general low-alloy high-strength structural steels can also use two Arabic numerals (representing the average carbon content, in ten-thousandths) and chemical element symbols, expressed in order; for special low-alloy high-strength structural steels Grades can also be expressed in order using two Arabic numerals (representing the average carbon content in parts per 10,000) and chemical element symbols, as well as some prescribed symbols representing product uses.
2. Designation method of high-quality carbon structural steel and high-quality carbon spring steel
High-quality carbon structural steel adopts two Arabic numerals (expressing the average carbon content in a few ten thousandths) or a combination of Arabic numerals and element symbols to form a brand.
① For boiling steel and semi-killed steel, add the symbols "F" and "b" to the end of the grades respectively. For example: rimming steel with an average carbon content of 0.08%, its grade is expressed as "08F"; for semi-killed steel with an average carbon content of 0.10%, its grade is expressed as "10b".
② Killed steel (S and P ≤0.035% respectively) are generally not marked with symbols. For example: killed steel with an average carbon content of 0.45%, its grade is expressed as "45".
③High-quality carbon structural steel with higher manganese content, add the symbol of manganese after the Arabic numeral 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 designation is expressed as "50Mn".
④High quality carbon structural steel (S and 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 designation is expressed as "45A".
⑤Super high quality carbon structural steel (S≤0.020%, P≤0.025%), add the symbol "E" after the grade. For example: the super high-quality carbon structural steel with an average carbon content of 0.45%, its designation is expressed as "45E".
The expression 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 the two standards of GB/T1222 and GB/T699 respectively).
3. Designation method of alloy structural steel and alloy spring steel
①The 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 ten thousandths) and place it at the head of the brand.
The content of alloying elements is expressed as follows: when the average content is less than 1.50%, only the elements are indicated in the grade, 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%～ When 5.49%,......, write 2, 3, 4, 5...... after the alloying element accordingly.
For example: alloy structural steel with average content of carbon, chromium, manganese, and silicon of 0.30%, 0.95%, 0.85%, 1.05%, when the content of S and P are ≤0.035%, the grade is expressed as "30CrMnSi".
High-quality high-quality alloy structural steel (S and P content ≤0.025% respectively), indicated by adding the symbol "A" at the end of the grade. For example: "30CrMnSiA".
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 symbols specified in Table 1 should be added to the head (or tail) of the grades to represent the purpose of the product. For example, 30CrMnSi steel special for riveting screws, the steel number is expressed as ML30CrMnSi.
②The expression method of alloy spring steel grade is the same as that of alloy structural steel.
For example: spring steel with an average content of carbon, silicon, and manganese of 0.60%, 1.75%, and 0.75%, respectively, is designated as "60Si2Mn". For high-quality spring steel, add the symbol "A" at the end of the grade, and its grade is expressed as "60Si2MnA".
4. How to express free-cutting steel grades
Free-cutting steel is represented by standard chemical element symbols, prescribed symbols and Arabic numerals. Arabic numerals indicate the average carbon content (in a few ten thousandths).
①Sulphur-added free-cutting steel and sulfur-added and phosphorus-added free-cutting steel shall not add free-cutting element symbols after the symbol "Y" and Arabic numerals. For example: the average carbon content.