Carbon steel

Carbon steel is one of the most commonly used materials in many fields. According to information published by worldsteel.org (the website of the World Steel Association, a non-profit organization based in Brussels), the production of crude steel reached 161.6 million tonnes by the fifth month of 2023.

Therefore, we can see that the demand for various types of steel is very high. In today’s article, let’s explore some useful knowledge related to carbon steel.

Understanding Carbon Steel

Carbon steel is a material that plays an important role in various industries, including automobile manufacturing, shipbuilding, construction, and mechanical engineering. According to information from World Steel, the total global crude steel production in 2021 reached 1,950.5 million tonnes.

What is Carbon Steel?

Carbon steel is an alloy primarily composed of iron (Fe) and carbon (C), with a common carbon content ranging from 0.05% to 2.1% by weight, along with some other elements.
The composition of carbon steel typically includes the following elements:

The composition of carbon steel includes carbon and some other elements. The main components of carbon steel are as follows:

• Iron (Fe): The basic component.
• Carbon (C): The carbon concentration in carbon steel usually ranges from about 0.05% to 2.1% by weight. The carbon content determines the hardness of the steel. The higher the carbon concentration, the harder the steel, but excessive carbon can make the steel brittle.
• Manganese (Mn): Manganese is a common element in carbon steel. It is usually present in concentrations ranging from 0.3% to 1.65%. Manganese helps improve the hardness and strength of the steel and aids in deoxidizing, removing harmful FeO from the steel.
• Silicon (Si): Silicon is commonly present in materials, typically in concentrations ranging from 0.15% to 0.35%.
• Sulfur (S): Sulfur may be present in carbon steel in low concentrations, usually below 0.05%. A small amount of sulfur remains within the material, and a high sulfur content is generally not beneficial for the material.

Other elements: In addition to the above elements, carbon steel may also contain other elements such as nickel (Ni), chromium (Cr), molybdenum (Mo), etc. These elements are often added in very small amounts to improve specific properties of the steel, such as corrosion resistance, heat resistance, strength, and workability, to meet the requirements of specific applications.

Properties of Carbon Steel

Physical properties:

• Density: The density of carbon steel typically ranges from about 7.8 g/cm³ to 8.1 g/cm³, depending on the specific chemical composition.
• Melting point: The melting point of carbon steel generally falls within the range of 1,370°C to 1,540°C, depending on the specific chemical composition.
• Thermal conductivity: Around 50-60 W/(m.K).
• Electrical conductivity: Relatively good electrical conductivity.
• Magnetism: Carbon steel is not naturally magnetic; however, it can exhibit magnetism if processed in a specific way.

Chemical properties:

• Oxidation: Carbon steel has the ability to oxidize, forming a layer of iron oxide on the surface when exposed to moist air. This can lead to the process of rusting if not protected.
• Corrosion resistance: Carbon steel is prone to corrosion, especially when exposed to corrosive substances such as acids or salts.
• Solubility: It has the ability to dissolve in acid and acid mixtures, depending on the acid concentration and processing conditions.

Classification of Carbon Steel

There are many types of carbon steel present in the market, as well as used in practical applications. Different types of steel will have different properties, and for the convenience of material selection, carbon steel is classified into distinct types based on various aspects.

Classification based on carbon content

Classification of carbon steel based on carbon content, or more specifically, the weight percentage of carbon in the material, is the most commonly used type of classification.

Low Carbon Steel

Low carbon steel contains a carbon content ranging from about 0.05% to 0.3%. It is considered to be softer compared to other variants, which makes it more easily bendable and increases its flexibility. The low carbon content also facilitates easier welding.
Low carbon steel is commonly supplied in the form of sheets, bars, with various cross-sectional shapes and sizes. It is often used as the reinforcing steel in construction, for manufacturing outer casings of machinery, and for creating containers, among other applications.

To enhance the hardness of this material, carbonization methods can be employed.

Some typical grades of low carbon steel include:

• Europe: S185, S235, S275, etc.
• Japan: JIS SS400, JIS G3131 SPHC, JIS G4051 S15C, etc.
• United States: A36, AISI 1080, AISI 1012, etc.

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Medium Carbon Steel

Medium carbon steel has a carbon content ranging from 0.3% to 0.6%. It possesses higher hardness and durability compared to low carbon steel, but as a trade-off, welding and cutting this type of material can be more challenging.
Additionally, medium carbon steel has sufficient ductility and resilience, allowing it to be used in the manufacturing of machine parts, tools, and equipment that require the ability to withstand significant impact forces. In many cases, it is also utilized in load-bearing structures.

To enhance the mechanical properties of some grades of medium carbon steel, various heat treatment methods can be employed, including tempering, annealing, etc.

Some typical grades of medium carbon steel include:

• Europe: EN8, C45, C55, C60, etc.
• Japan: S45C, S50C, S55C, S58C, etc.
• United States: AISI 1040, AISI 1050, AISI 1060, etc.

Classification based on the deoxidation method

Boiling Steel

Boiling steel is a type of deoxidized steel that is not completely deoxidized. During the pouring of liquid metal into the mold, CO gas is released, resulting in the formation of gas bubbles similar to the boiling phenomenon of water. This is why this type of steel is called “boiling steel.”
The underlying cause of the boiling phenomenon is that during production, ferromanganese is used for deoxidation. However, ferromanganese cannot completely eliminate oxygen, so a certain amount of FeO still remains. Subsequently, under high temperature and oxygen-free conditions, FeO reacts with carbon to produce CO gas.

FeO + C -> Fe + CO

Boiling steel is relatively soft compared to other types of steel, and it has a lower cost. It is commonly used for manufacturing parts and processed products through methods like stamping (machine covers, machine casings).

Killed Steel

Killed steel is a type of steel that is more thoroughly deoxidized, resulting in almost complete elimination of FeO, and no CO gas is generated during the mold pouring process.
To fully remove oxygen, killed steel requires the use of more deoxidizing agents, leading to higher production costs. As a result, killed steel is more expensive than boiling steel. However, killed steel exhibits higher uniformity, and since no CO reaction occurs, it effectively minimizes the formation of gas bubbles inside the material.

This deoxidation method is used to produce high-quality steel and is often employed in the manufacturing of various machine parts and components.

Semi-Killed Steel

As the name suggests, semi-killed steel is a type of steel that undergoes a deoxidation process intermediate between killed steel and boiling steel. It utilizes deoxidizing agents such as ferromanganese and aluminum. Therefore, in terms of both cost and quality, semi-killed steel falls between boiling steel and killed steel.

Classification based on application

As mentioned earlier, carbon steel is applied in various fields, and specific applications require certain suitable steel grades. Therefore, classifying steel based on specific applications is also a common method of categorization, which includes the following three main types.

Structural Steel

Structural steel is a specialized type of steel used in construction-related activities, including its use in load-bearing structures such as beams, columns, supports, and as reinforcement for concrete columns. The steel structures used in construction surround us, including bridges, tunnels, residential buildings, and warehouses, among others.
They are typically supplied in the form of long bars with cross-sectional profiles in the shape of I-beams, circular, V-shaped, U-shaped, or rectangular sections.

Structural steel is widely used, partly due to the prevalence of construction applications and the higher demand for steel, and partly because structural steel often has a lower cost compared to the other two types.

Characteristics of this type of steel include:

• Good load-bearing capacity.
• Resistant to strong and sudden impacts.
• Can be bent and stretched.
• Lower hardness compared to the other two types.

Some popular structural steel grades in Vietnam include:

• CT3 Steel: Low-strength steel, commonly used in lightweight structural components.
• CT5 Steel: Medium-strength steel, suitable for general construction projects.
• CB240 Steel: High-strength steel, used for structures with high load-bearing requirements such as bridges and warehouses.
• CB400 Steel: High-strength steel, typically used in high-rise buildings, apartment complexes, and projects with high technical requirements.

Mechanical Engineering Steel

Mechanical engineering steel is a type of steel used to manufacture parts and components of machinery, often for parts that experience high and continuous impact forces. Examples include gears, gear shafts, crankshafts, and transmission rods. Additionally, it can also be used for mold making.
Mechanical engineering steel has higher hardness than structural steel but lower than tool steel. It has several characteristics, including:

• With a higher carbon content than structural steel, mechanical engineering steel has greater hardness.
• High fatigue strength, suitable for working conditions with repeated impact loads.
• High thermal stability, capable of working within a wide range of temperature fluctuations.
• Good wear resistance, as mechanical components are in continuous contact and subjected to mutual impact, the material used to manufacture them must have good resistance to wear.

Some common carbon steel grades used for mechanical engineering applications in Vietnam include: C45 Steel, C50 Steel, AISI 1045 (equivalent to C45), and more.

Tool Steel

Tool steel is a type of steel with a higher carbon content than the other two types. As a result, it has higher hardness. Tool steel is used to manufacture various types of mechanical tools, including screws, hammers, pliers, turning tools, planing tools, drill bits, and more.
Excluding alloy tool steels, carbon tool steel typically contains a carbon content ranging from 0.6% to 1.5%. In Vietnam, tool steel is also known as “gió steel,” which has high heat resistance and can withstand high impact forces.

Identification of Carbon Steel

To facilitate the selection and assessment of the quality of carbon steel, a combination of letters and numbers is commonly used, or only uppercase letters combined with lowercase letters, to indicate the properties and mechanical characteristics of the steel. Based on Vietnamese standards (TCVN), we have the following types of designations.

Ordinary Quality Carbon Steel

They are supplied in the form of plates, solid bars, or tubes. They are primarily used in construction (concrete frame structures, steel frames for workshops, bridges, tunnels, etc.) and for manufacturing machine components that do not require high load-bearing capacity or have low working strength. They are divided into three main groups: A, B, C, based on Vietnamese standards (TCVN).
Group A:

The steel designation specifies the mechanical properties without specifying the chemical composition. The general format of the designation is: CTxxy (TCVN)

Where:

• CT: Indicates ordinary quality carbon steel.
• xx: Indicates the yield strength (kg/mm²).
• y: Indicates the method of oxygen removal (s – boiling, if left blank, it is conventionally considered ladle refining, n – ladle-to-ladle).

For example, the steel grades could be: CT38, CT38s, CT38n.

• CT: Indicates ordinary quality steel.
• 38: Indicates a yield strength of 38 (kg/mm²).
• The oxygen removal methods are respectively: ladle-to-ladle, boiling, ladle-to-ladle.

Group B:
The steel designation specifies the chemical composition without specifying the mechanical properties. The general format of the designation is: BCTxxy.

Similar to Group A, CT indicates ordinary quality carbon steel, xx indicates the yield strength, and y indicates the method of oxygen removal.

Example designation for Group B: BCT34.

Group C:

The steel designation specifies both the chemical composition and mechanical properties. The general format of the designation is: CCTxxy.

Example designation: CCT42n.

High-Quality Carbon Steel

High-quality carbon steel has higher quality compared to the CT group. Criteria are used to evaluate the quality of carbon steel based on the presence of S and P elements in the material. High-quality carbon steel typically has S content < 0.04% and P content < 0.035%.
High-quality carbon steel is specified for both chemical composition and mechanical properties, and it is commonly used for manufacturing machine components.

General designation format: Caay

Where:

• C: Indicates high-quality carbon steel.
• aa: Represents the carbon content in parts per thousand in the steel.
• y: Indicates the method of oxygen removal.

For example: Steel grade C45

• C: High-quality steel
• 45: 45/10000 equivalent to 0.45% carbon
• “y” index is left blank: ladle-to-ladle steel.

Tool Carbon Steel

Tool steel is a type of carbon steel with high hardness, and the carbon content in the material is usually greater than 0.65%. Based on the P and S content, tool carbon steel is further divided into two groups.

• Group 1: P < 0.03%; S < 0.03%.
• Group 2: P < 0.03%; S < 0.02% (higher quality than Group 1).

They are commonly used for manufacturing various types of mechanical and metal cutting tools.
General designation format: CDaa(A)

Where:

• CD: Indicates tool carbon steel.
• aa: Represents the carbon content in parts per thousand in the steel.
• To distinguish “Group 1” from “Group 2,” the letter A is added at the end of the material code (no A indicates “Group 1,” while A indicates “Group 2”).

For example: CD75, CD57A.

Advantages and Disadvantages of Carbon Steel

Like any other material, carbon steel is a type of material that is applied for specific purposes, not for all purposes. Each type of material has its own strengths and weaknesses. Understanding the strengths and limitations of the material helps us choose the appropriate type of material for specific applications.

Advantages

High strength: Carbon steel has the ability to withstand high tension and pressure, while also exhibiting good tensile strength.
Good hardness: Carbon steel can be heat-treated to achieve high hardness, depending on the specific machining and heat treatment processes.

Good machinability and weldability: Carbon steel has good machinability and weldability, making it easy to produce complex products and structures.

Cost-effective: Compared to alloy steels, carbon steel has lower production costs, making it a popular and economical choice.

Versatile applications: With various grades of carbon steel, as well as the ability to adjust the composition of its constituents or employ specific heat treatment methods, the material can have varied mechanical properties, thereby expanding its application possibilities for specific working conditions.

Good technological properties: Carbon steel can be shaped through various machining methods such as rolling, forging, casting, welding, and other cutting processes.

Disadvantages

Susceptible to corrosion: Carbon steel is prone to corrosion when exposed to moisture and certain environments, requiring proper maintenance and protection to prevent rust.
Lower resistance to impact and wear: Compared to some other materials, carbon steel may have lower resistance to impact and wear under certain conditions.

Limited strength at high temperatures: Carbon steel may experience a reduction in strength when exposed to high temperatures, limiting its suitability for applications requiring high-temperature resistance.

Sensitive to heat treatment: Carbon steel requires precise control of the heat treatment process to achieve desired mechanical properties, as improper heat treatment can lead to variations in hardness and other characteristics.

Overall, carbon steel is a widely used material with notable strengths in terms of strength, hardness, machinability, weldability, cost-effectiveness, and versatility. However, it also has limitations such as susceptibility to corrosion, lower impact resistance, limited high-temperature strength, and sensitivity to heat treatment. These factors should be considered when selecting carbon steel for specific applications.

Applications of carbon steel

Carbon steel has numerous applications in various industries. Here are some common applications of carbon steel:

• Construction: Carbon steel is widely used in construction, including bridges, high-rise buildings, commercial buildings, transportation infrastructure, etc. It is used to create load-bearing frameworks and structural systems such as columns, floors, and other components of construction projects.
• Manufacturing of transportation components: Used in the production of automobiles, trucks, bicycles, and other vehicles. It is used in vehicle frames, suspension parts, load-bearing components, and other mechanical parts.
• Machining and cutting operations: Carbon steel is used to manufacture cutting tools such as knives, scissors, saw blades, and cutting tools. Not only conventional cutting tools, but also high-carbon steel types are used to make metal cutting tools.
• Applications in manufacturing various types of pipeline equipment: In industries such as oil refining, power generation, and many other industrial production sectors. Carbon steel is used to manufacture various types of pipes, pipe fittings (steel flanges, steel elbows, etc.), steel pipes, and various types of valves (ball valves, gate valves, butterfly valves, etc.). These components are used to create a fluid circulation system and perform specific functions.
• Precision engineering: Used in the production of mechanical parts and components with high precision requirements, such as shafts, gears, screws, bearings, and other parts, to assemble complete machines.

Conclusion

Carbon steel is an important material used in various industries and fields, produced by the alloying of iron and carbon. Carbon steel has many characteristics and diverse applications.
Here are the key points:

• Mechanical properties: High strength, good hardness, resistance to tension and high pressure, and resistance to wear and abrasion.
• Wide-ranging applications: Used in many industries such as construction, automotive, maritime, aerospace, cutting and drilling tools, energy, and many other fields.
• Diverse steel grades: There are different codes and standards, depending on specific chemical composition and mechanical properties.
• Advantages: Carbon steel has high strength, good hardness, good machinability and weldability, and it is cost-effective compared to alloy steel.
• Disadvantages: Carbon steel is susceptible to rust, more prone to breakage compared to other types of steel, and has limitations in corrosive environments.

The information shared in this article is referenced from various sources, including worldsteel.org, youtube vaquhu, TCVN 1765:1975, and the author’s knowledge and experience in materials.
As always, I welcome questions and feedback to improve future articles. Thank you for your interest in the article!