Steel

What Is Steel?

Steel is a general term for iron-carbon alloys with carbon content between 0.02% and 2.11% by mass. The chemical composition of steel can vary greatly. Steel containing only carbon elements is called carbon steel (carbon steel) or ordinary steel; in actual production, steel often contains different alloying elements, such as: manganese, Nickel, vanadium, etc. Mankind has a long history of application and research on steel, but until the invention of the Bayesian steelmaking method in the 19th century, the production of steel was a high-cost and low-efficiency task. Nowadays, steel has become one of the most used materials in the world due to its low price and reliable performance. It is an indispensable ingredient in the construction industry, manufacturing industry and people’s daily life. It can be said that steel is the material basis of modern society.

The National Standard of the People’s Republic of China GB/T 13304-91 “Steel Classification” describes: “Materials with iron as the main element, carbon content generally below 2%, and other elements.” Generally refer to materials other than chrome steel For other steel grades, the carbon content of some chromium steels is allowed to be greater than 2%. Iron alloys with a carbon content of more than 2% are cast iron. The definition of steel in other international standards such as ISO 4948 or EN 10020 is similar.
Strictly speaking, steel is an iron-carbon alloy with a carbon content of 0.0218%-2.11 [2]%. We usually call it steel together with iron. In order to ensure its toughness and plasticity, the carbon content generally does not exceed 1.7%. In addition to iron and carbon, the main elements of steel include silicon, manganese, sulfur, and phosphorus. The other components are to differentiate the properties of the steel. The following lists the important steel materials in alphabetical order. They contain the following ingredients, and their functional characteristics are now introduced together:

• Carbon:Exist in all steels, it is the most important hardening element. It helps to increase the strength of steel. We usually hope that tool-grade steel has more than 0.6% carbon, which is also called high-carbon steel.
• Chromium:Increase wear resistance, hardness, and most importantly, corrosion resistance. More than 13% of them are considered to be stainless steel. Despite its name, all steel will rust if it is not properly maintained.
• Manganese:An important austenite stabilizing element, helps to generate a texture structure, increase firmness and strength and wear resistance. Deoxidize the inside of the steel during the heat treatment and coil pressing process, which appears in most of the steel for knife and scissors, except A-2, L-6 and CPM 420V.
• Molybdenum:Carbonization agent prevents the steel from becoming brittle and maintains the strength of the steel at high temperatures. It appears in many steels. Air hardening steels (such as A-2, ATS-34) always contain 1% or more molybdenum, so that they can Harden in the air.
• Nickel:Maintain strength, corrosion resistance, and toughness. Appears in L-6\AUS-6 and AUS-8.
• Silicon:Helps increase strength. Like manganese, silicon is used to maintain the strength of steel during the production of steel.
• Tungsten:Enhance wear resistance. Tungsten is mixed with appropriate proportions of chromium or manganese to make high-speed steel. A large amount of tungsten is contained in high-speed steel M-2.
• Vanadium:Enhance wear resistance and ductility. Vanadium is contained in many kinds of steel, among which M-2, Vascowear, CPM T440V and 420VA contain a large amount of vanadium. The biggest difference between BG-42 and ATS-34 is that the former contains vanadium.
• Phosphorus:It is a harmful element, which reduces the plasticity and toughness of steel, and appears cold brittleness, which can significantly increase the strength of steel and at the same time improve the stability of atmospheric corrosion. The content should be limited to less than 0.05%.
• Sulfur:Sulfur is usually a harmful element, which makes steel hot brittle, and its content is limited to less than 0.05%. But the sulfur content of free-cutting steel is high, up to 0.08%~0.40%.

Steel refers to an iron-carbon alloy with a carbon content of less than 2%. According to the different composition, it can be divided into carbon steel and alloy steel. According to different performance and use, it can be divided into structural steel, tool steel and special performance steel.

According to chemical composition:

(1) Carbon steel refers to an iron-carbon alloy containing a small amount of manganese, silicon, sulfur, phosphorus and other elements in addition to iron and carbon.

According to its carbon content, it can be divided into:

• Low carbon steel-carbon content wc≤0.25%
• Medium carbon steel-carbon content wc0.25%～0.60%
• High carbon steel-high carbon steel with carbon content wc>0.60% is generally more common in the military industry and industrial medical industry

(2) Alloy steel In order to improve the performance of steel, on the basis of smelting carbon steel, steel made by adding some alloying elements, such as chromium steel, manganese steel, chromium-manganese steel, chromium-nickel steel, etc.

According to the total content of alloying elements, it can be divided into:

• Low alloy steel-the total content of alloying elements ≤ 5%
• Medium alloy steel-the total content of alloying elements is 5% to 10%
• High alloy steel-the total content of alloying elements> 10%

The Classification Of Steel

Classified by carbon content

• Low carbon steel: the carbon content is generally less than 0.25% (mass fraction);
• Medium carbon steel: The carbon content is generally 0.25%~0.60% (mass fraction);
• High carbon steel: The carbon content is generally higher than 0.60% (mass fraction).

Classified by quality

• (1) Ordinary steel (P≤0.045%, S≤0.050%)
• (2) High-quality steel (P and S both ≤0.035%)
• (3) High-quality steel (P≤0.035%, S≤0.030%)

According to the forming method

• (1) Forged steel;
• (2) Cast steel;
• (3) Hot rolled steel;
• (4) Cold drawn steel.

Metallographic organization

Annealing state:

• a, hypoeutectoid steel;
• b, eutectoid steel;
• c, hypereutectoid steel (pearlite + cementite);
• d, ledeburite steel (pearlite + cementite).

Normalized state:

• a. Pearlitic steel;
• b. Bainite steel;
• c. Martensitic steel;
• d. Austenitic steel.

Classified by purpose

• Alloy structural steel, reinforced steel.
• Carburizing steel, ammonia steel, steel for surface hardening; free-cutting structural steel; steel for cold plastic forming: including steel for cold stamping and steel for cold heading.
• Spring steel, bearing steel
• Anti-oxidation steel, thermal strength steel, valve steel, electric heating alloy steel, wear-resistant steel, low temperature steel, electrical steel.
• Steel for bridges, steel for ships, steel for boilers, steel for pressure vessels, steel for agricultural machinery, etc.

Comprehensive classification

Tool steel:

• (a) carbon tool steel;
• (b) alloy tool steel;
• (c) high-speed tool steel.

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;
• (f) high-quality structural steel for specific purposes .

According to smelting method

Open hearth steel:

• (a) acid open hearth steel;
• (b) basic open hearth steel.

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.

The Related Explanation 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, Fo is the cross-sectional area of ​​the sample, then the yield point σs = Ps/Fo (MPa), MPa is called MPa equal to N (Newton)/mm2, (MPa=10^6Pa, Pa ：Pascal=N/m2)

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.

⑴Brinell 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), the unit is kilogram force/mm2 (N/mm2).

⑵Rockwell 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.).

⑶ 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 load value is divided by the surface area of ​​the material indentation pit, which is the Vickers hardness value (HV).