Carbon Steel: Classification, Marking, Application

Modern metallurgy currently produces such a vast number of steel grades that listing them all is both complex and time-consuming. The affordable price, universal technological, and mechanical properties of carbon steel drive its widespread production and extensive use in engineering, energy, agriculture, municipal services, and other industrial sectors.

What is Carbon Steel

Carbon steel is an alloy obtained by combining iron with carbon, the latter constituting less than 2.14% of the alloy. It also contains small amounts of various impurities, such as manganese, silicon, sulfur, nickel, chromium, and copper. Their concentrations are minimal and do not significantly affect the key characteristics of the final product.

Classification by Carbon Content

Carbon content has a defining influence on the properties and applications of steel. Higher carbon concentrations result in stronger alloys but reduce ductility and weldability. Therefore, classification by carbon content is fundamental for steels.

Based on this principle, they are divided into the following categories:

1. Low-carbon steel. These alloys have a carbon content of 0.025–0.25%. They weld well, are not prone to brittleness, and increase in strength after heat treatment, though they may lose strength upon subsequent heating. Typically used for manufacturing loaded elements of welded structures, prefabricated frames, and products subjected to chemical-thermal treatments such as nitriding, carburizing, or nitrocarburizing (e.g., parts operating under friction in aggressive environments or at high temperatures, matrices and punches for hot stamping, molds, etc.).
2. Medium-carbon steel. The carbon mass fraction reaches up to 0.6%. These steels are less sensitive to flocculation but are prone to hardening, which complicates welding. Preliminary or concurrent heating of the metal is required for this process.
3. High-carbon steel. The carbon content reaches up to 2.14%, present in the form of cementite (Fe3C). These alloys are strong, elastic, and hard but exhibit a tendency toward aging and cold brittleness. The material is difficult to process and weld.

Classification by Degree of Deoxidation

During smelting, the steel melt contains a significant amount of gases, including oxygen, hydrogen, nitrogen, carbon monoxide, and secondary reaction products. This is due to the numerous chemical reactions occurring during the process, which negatively affect the chemical structure and homogeneity of the metal, potentially reducing impact toughness, fatigue strength, and ductility.

To mitigate this, a degassing process is employed, achieved through deoxidation in the steelmaking unit using potent deoxidizers, most commonly silicon and aluminum. These are strong reducing agents relative to iron oxide, actively binding with secondary gaseous products. Deep degassing can also be performed using a vacuum degasser.

During deoxidation, gas release is less intense, leading to a reduction in gas inclusions, sulfides, and other impurities.

Based on the intensity of the degassing process, steels are classified into three types:

1. Fully killed steels. These alloys undergo maximum deoxidation, resulting in a dense, fine-grained, homogeneous structure with improved physical, mechanical, and technological properties. However, the process is costly and yields a lower output of finished products.
2. Boiling steels. These are lower in quality due to their heterogeneous structure and increased tendency to form cracks after welding. They are not used in loaded or critical structures or for equipment designed for low-temperature operation. Suitable for manufacturing non-critical products used under normal conditions.
3. Semi-killed steels. These alloys are partially deoxidized, offering average quality characteristics. They are less costly to produce, more affordable, and, compared to boiling steels, are more convenient to process and reliable in operation.

Classification by Quality

Domestic DSTU and GOST standards categorize steels based on the smelting method, which determines the metal’s quality. The primary criteria are the quantitative characteristics of sulfur and phosphorus.

These elements affect structural and chemical homogeneity. Sulfur promotes crack formation during welding and reduces impact toughness, while phosphorus increases cold brittleness. The higher their content in the alloy, the lower its quality.

Carbon steels of ordinary quality have a carbon content of up to 0.49%, with sulfur and phosphorus at 0.05% and 0.04%, respectively.

According to DSTU 2651, such steels are produced in the form of structural shapes, hot-rolled bars, wires, sheets, strips, and forgings. These products are manufactured using hot-rolled or cold-rolled methods, but may undergo heat treatment to achieve specific properties.

High-quality carbon steels contain 0.04% sulfur and 0.035% phosphorus. Their quality, chemical composition, and mechanical properties are regulated by DSTU 7809, requiring strict control over the composition of raw materials and alloy refining methods.

These steels exhibit good endurance, ductility, and sufficient impact toughness. They are more wear-resistant after heat treatment and have hard surface layers.

Classification by Application

During production, it is critical to consider where and in what form the steel will be used. In Ukraine’s standardization system, steels are classified into the following subgroups:

• Structural — used for metal structures, mechanisms, devices, and household items, requiring good machinability, impact resistance, sufficient strength, ductility, and durability.
• Tool — suitable for manufacturing stamping equipment, manual and mechanical cutting tools, and sometimes measuring equipment, with quality regulated by DSTU 3833, typically used in a heat-treated state.
• Construction — ensures reliable and durable connections in metal structures.
• Specialized — produced for specific industries such as aviation, automotive, or weapons manufacturing, offering unique properties, high resistance to temperature effects, corrosion, or mechanical loads.

Marking

Currently, there is no unified international system for marking metal alloys, so each country uses its own and international interstate systems. The marking is based on the chemical composition or application of the alloy. In Ukraine, an alphanumeric system is used. The alloy number is conditionally assigned based on the chemical composition, with high-quality steels using two-digit numbers indicating the average carbon content in hundredths of a percent.

The degree of deoxidation is indicated by corresponding letter indices:

• kp – boiling;
• ps – semi-killed;
• sp – fully killed.

If the steel has an elevated manganese content (from 0.8%), it is marked with the letter G. The letter A indicates that the steel has good machinability, often used in metal-cutting machines and automated production lines. Tool steels are marked with the letter U at the beginning of the designation.

In European Union countries, steel classification and marking are more complex. The EN 10027 standard regulates naming based on alphanumeric designations of physical and mechanical properties. For example, ordinary quality carbon steel grades are numbered 1.00XX, high-quality steels are 1.01XX–1.09XX, and tool steels are 1.15XX–1.18XX. Carbon steel offers optimal strength and ductility. It is more affordable but prone to rust under the influence of atmospheric precipitation, electrolytes, or other aggressive substances and polishes poorly. To minimize corrosion, it can be painted or coated with protective layers via galvanic methods or hot dipping.

Carbon steel is widely used in construction, industry, agriculture, forestry, and energy sectors.

Ordinary quality carbon steel is used for non-critical welded structures, washers, fences, anchor bolts, reinforcement, elements of wagon frames, bridge trusses, rollers, and more.

High-quality structural steel is used for manufacturing pressure vessel body parts, equipment casings and linings, valve flanges, traverses, levers, wagon wheels, clutch discs, and wires for cables.

Tool steels are used for producing tools for woodworking, stamps, lathes, needles, dies, metal saws, files, and scrapers.

For modern metal structures, selection is based on a thorough analysis of properties and characteristics. It is essential to consider the structure’s durability and reliability under the influence of aggressive substances, temperature, pressure, and dynamic loads.

The company UTMK will assist you in selecting the necessary rolled metal products. It offers a wide range of steel products. Orders can be placed at warehouses in Kyiv, Odesa, Dnipro, Poltava, Krakow, Warsaw, and Łódź. We deliver across Ukraine and Europe. Contact us, and we will help you choose the right rolled metal products. Bulk customers can expect discounts based on order volumes.