Steel knowledge

I. Mechanical properties of steel

1. Yield point ( σ S)
When steel or sample is stretched, when the stress exceeds the elastic limit, even if the stress does not increase any more, the steel or sample will continue to undergo obvious plastic deformation. This phenomenon is called yield, and the minimum stress value when yield occurs is the yield point. If Ps is the external force at the yield point s and Fo is the cross-section area of the sample, then the yield point σ S = Ps/Fo (MPa).

2. Yield strength ( σ 0.2)
The yield point of some metal materials is not very obvious and it is difficult to measure them. Therefore, in order to measure the yield properties of materials, it is stipulated that the stress producing permanent residual plastic deformation is equal to a certain value (generally 0.2% of the original length), which is called conditional yield strength or yield strength. σ 0.2.
3. Tensile Strength ( σ B)
The maximum stress a material achieves during tension from the beginning to the time it breaks. It indicates the strength of the steel against breaking. Corresponding to the tensile strength are also the compressive strength, flexural strength, etc. Set Pb as the maximum tensile force before the material is pulled apart and Fo as the cross-section area of the sample, then the tensile strength σ B= Pb/Fo (MPa).
4. Elongation ( δ S)
The percentage of the plastic elongation of a material after breaking to the original sample length is called elongation or elongation.
5. Yield-strength ratio ( σ S/ σ B)
The ratio of the yield point (yield strength) of steel to the tensile strength is called the yield strength ratio. The higher the yield-strength ratio, the higher the reliability of structural parts. The yield-strength ratio of general carbon steel is 0.6-0.65, and 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 material’s resistance to hard objects pressing into its surface. It is one of the important performance indexes of metal materials. The higher the general hardness, the better the wear resistance. Commonly used hardness indicators are Brinell hardness, Rockwell hardness and Vickers hardness.
1) Brinell Hardness (HB)
Hardened steel balls of a certain size (diameter is generally 10mm) are pressed into the surface of the material with a certain load (generally 3000kg) for a period of time. After unloading, the ratio of the load to the indentation area is called Brinell Hardness (HB).
2) Rockwell Hardness (HR)
When HB>450 or the sample is too small, Rockwell hardness measurement cannot be used instead of Brinell hardness test. It is a diamond cone with a top angle of 120 degrees or a steel ball with a diameter of 1.59 and 3.18 mm, which is pressed into the surface of the material under certain loads and the hardness of the material is determined by the depth of the indentation. There are three different scales to indicate the hardness of the tested material:
HRA: Hardness obtained with a 60 kg load and a diamond cone press-in for extremely hard materials such as cemented carbides.
HRB: Hardness obtained by hardening a steel ball with a load of 100kg and a diameter of 1.58mm. It is used for materials with lower hardness (e.g. annealed steel, cast iron, etc.).
HRC: Hardness obtained by using a 150 kg load and a diamond cone press-in for materials with high hardness such as hardened steel.
3) Vickers Hardness (HV)
The material surface is pressed by a diamond square cone press with a load less than 120 kg and a top angle of 136 degrees. The Vickers hardness value (HV) is defined by dividing the surface area of the material indentation recess by the load value.

II. Black Metals and Non-ferrous Metals

1. Ferrous Metals
It refers to the alloy of iron and iron. Such as steel, pig iron, ferroalloy, cast iron, etc. Steel and pig iron are alloys based on iron and mainly added with carbon. They are collectively called FERROCARBON alloys.
Pig iron is a product made by smelting iron ore into a blast furnace and is mainly used for steelmaking and casting.
Cast pig iron is melted in an iron melting furnace to obtain cast iron (liquid iron with carbon content greater than 2.11%). Cast liquid cast iron into cast iron, which is called cast iron.
Ferroalloy is an alloy composed of iron and elements such as silicon, manganese, chromium and titanium. Ferroalloy is one of the raw materials for steel making and is used as deoxidizer and additive for alloy elements in steel making.
Iron-carbon alloy with carbon content less than 2.11% is called steel. Steel is obtained by putting pig iron for steelmaking into the steelmaking furnace and smelting it according to a certain process. Products of steel include ingots, continuous casting billets and direct casting of various steel castings. Generally speaking, steel refers to the steel rolled into various steels. Used for manufacturing hot forged and hot pressed mechanical parts, cold drawn and cold headed forged steel, seamless steel pipe mechanical manufacturing parts, cnc machining parts, casting parts.
2. Non-ferrous metals
Also known as non-ferrous metals, refers to metals and alloys other than ferrous metals, such as copper, tin, lead, zinc, aluminium and brass, bronze, aluminium alloy and bearing alloys.For example, CNC lathe can process various materials, including 316 and 304 stainless steel plates, carbon steel, carbon steel, aluminum alloy, zinc alloy materials, aluminum alloy, copper, iron, plastic, acrylic plates, POM, UHWM and other raw materials, and can process into CNC turning parts and CNC milling parts as well as some complex parts with square and cylindrical structures. In addition, chromium, nickel, manganese, molybdenum, cobalt, vanadium, tungsten and titanium are also used in industry. These metals are mainly used as alloy additives to improve the properties of metals, in which tungsten, titanium, molybdenum and other cemented carbides are used to produce cutting tools. These nonferrous metals are referred to as industrial metals. In addition, there are precious metals such as platinum, gold, silver and rare metals, including radioactive uranium and radium.

III. Classification of Steel

Besides iron and carbon, the main elements of steel include silicon, manganese, sulfur and phosphorus.
There are various classification methods for steel, and the main ones are as follows:
1. Classify by Quality
(1) Common steel (P < 0.045%, S < 0.050%)
(2) High quality steel (P, 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 alloy elements < 5%); B. Medium alloy steel (total content of alloy elements > 5-10%); C. High alloy steel (total alloy element content > 10%).
3. Classification by forming method
(1) Forged steel; (2) Cast steel; (3) Hot rolled steel; (4) Cold drawn steel.
4. Classification by Metallographic Organization
(1) Annealed state: a. Hypoeutectoid steel (ferrite + pearlite); B. Eutectic steel (pearlite); C. Hypereutectoid steel (pearlite + cementite); D. Ledeburite steel (pearlite + cementite).
(2) Normalized state: A. pearlitic steel; B. Bainitic steel; C. martensitic steel; D. Austenitic steel.
(3) No phase transition or partial phase transition
5. Classify by Use
(1) Construction and engineering steel: a. Common carbon structural steel; B. Low alloy structural steel; C. Reinforced steel.
(2) Structural steel:
A. Machinery steel: (a) tempered structural steel; (b) Surface hardening structural steels: including carburized, ammoniated and surface hardening steels; (c) Easy-cutting structural steel; (d) Cold plastic forming steel: including cold stamping steel and cold heading steel.
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 and valve steel; C. Electrothermal alloy steel; D. Wear-resistant steel; E. Low temperature steel; F. Electrical steel.
(5) Professional steel – such as bridge steel, ship steel, boiler steel, pressure vessel steel, agricultural machinery steel, etc.
6. Comprehensive Classification
(1) Common 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) Easy-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 and acid-resistant steel; (b) Heat-resistant steel; (c) Electric heat 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) Alkaline 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 deoxidization degree and pouring system
A. Boiling steel; B. Semi-calm steel; C. Killed steel; D. Special killed steel.

IV. Overview of Steel Number Representation Method in China

The product brand is generally represented by combining Chinese alphabet, chemical element symbol and Arabic number. That is:
(1) The chemical elements in steel numbers are represented by international chemical symbols, such as Si, Mn, Cr, etc. Mixed rare earth elements are represented by RE (or Xt).
(2) Product name, use, smelting and pouring methods, etc., are generally expressed by abbreviations of Chinese phonetics.
(3) The content of main chemical elements (%) in steel is expressed by Arabic numerals.
When using Chinese alphabet to represent product name, use, characteristics and process method, the first letter is usually selected from Chinese alphabet to represent product name. When repeating with the selected letter of another product, the second or third letter can be used, or the first alphabet of two Chinese characters can be selected at the same time.
Where there is no Chinese character or Chinese alphabet available for the time being, the symbols shall be English letters.