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Carbon Steel Grade Guide
Carbon steel, as an extremely widely used metallic material, plays a crucial role in numerous fields such as industrial production, construction engineering, and the manufacturing of daily necessities. Its properties vary significantly due to differences in carbon content and other elements, which has led to the emergence of numerous grades of carbon steel. Understanding these different grades of carbon steel is essential for the correct selection and use of carbon steel materials.
Apr 21st,2025
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一、Basic knowledge of carbon steel
Carbon steel, in simple terms, is an alloy with iron and carbon as the main components, with a carbon content between 0.0218% and 2.11%. Unlike other alloy steels, carbon steel does not have elements such as chromium, cobalt, molybdenum, nickel, etc. that are added specifically to obtain specific alloying effects, or even if they are added, their content is within a specific standard range. For example, the American Iron and Steel Institute (AISI) stipulates that when there is no minimum content requirement for elements such as chromium, cobalt, and molybdenum, and the specific minimum content of copper does not exceed 0.40%, or the maximum content of manganese does not exceed 1.65%, silicon does not exceed 0.60%, and copper does not exceed 0.60%, this steel is considered carbon steel.
Carbon plays a core role in carbon steel. As the carbon content increases, carbon steel can obtain higher hardness and strength through heat treatment, but at the same time its ductility will decrease, its welding performance will deteriorate, and its melting point will decrease. This means that when selecting carbon steel materials, these properties need to be weighed according to the specific application scenario.
二、Carbon steel grade classification system
Classification by carbon content
1. Low carbon steel: The carbon content is generally less than 0.25%. This type of steel has good plasticity and weldability, relatively low cost, and is the largest type of carbon steel in production. However, its strength is relatively low, and it does not respond well to heat treatment, so cold working is often required to improve its strength. Common low carbon steels include AISI 1005, 1006, 1008, 1010, 1018, etc. Taking AISI 1018 as an example, it has good processing properties and is widely used in the manufacture of automotive parts, such as automotive body structural parts. While ensuring a certain strength, it can be made into various complex shapes through cold forming processes.
2. Medium carbon steel: The carbon content is between 0.25% and 0.6%. Medium carbon steel can be improved by heat treatment methods such as austenitization, quenching and tempering, and is widely used in the tempered state (forming tempered martensite structure). However, its hardenability is low, and the addition of elements such as chromium, nickel, and molybdenum can improve this situation. Medium carbon steel has higher strength after heat treatment, but its ductility will decrease. It is often used to manufacture mechanical parts such as railway wheels and tracks, gears, crankshafts, etc. For example, 45 steel (equivalent to AISI 1045) is a typical medium carbon steel. In the field of mechanical manufacturing, it is often used to manufacture parts that require certain strength and toughness, such as shaft parts of machine tools, etc. Through appropriate heat treatment processes, it can meet different usage requirements.
3. High carbon steel: The carbon content is between 0.6% and 1.4%. The high carbon content gives the steel high hardness and high strength, but at the same time makes it the hardest and least ductile type of carbon steel. High carbon steel is usually used in the quenched and tempered state, and strong carbide-forming elements such as chromium, vanadium, and tungsten are often added to form carbides of these metals to further improve hardness and wear resistance. Therefore, high carbon steel is often used to manufacture tools and molds, such as various knives, stamping dies, etc. For example, T10 steel (carbon content of about 1.0%) is often used to make woodworking knives, bench tools, etc. Its high hardness and wear resistance ensure the performance stability of the tools during long-term use.
4. Ultra-high carbon steel: The carbon content is about 1.25% - 2.0%. This type of steel can obtain extremely high hardness after tempering and is generally used for special purposes, such as making non-industrial knives, axles or punches. Due to its extremely high carbon content, most ultra-high carbon steels with a carbon content of more than 2.5% are manufactured using powder metallurgy.
Classification by use
1. Carbon structural steel
Ordinary carbon structural steel: This type of steel has relatively high phosphorus and sulfur content and low cost. It is usually rolled into steel bars, steel plates and steel pipes, etc., and is used for general structural components such as bridges and buildings. It can also be made into ordinary screws, nuts, rivets, etc. Its grade consists of the letter "Q" representing yield strength, the yield strength value (in MPa), the quality grade symbol (A, B, C, D), and the deoxidation method symbol (F means boiling, Z means calming, TZ means special calming, and the "Z" and "TZ" symbols can be omitted). For example, Q235AF represents boiling steel with a yield strength of 235MPa and a quality grade of A. It is often used for ordinary steel beams and steel columns in building structures, etc., where strength and corrosion resistance are not required.
2.High-quality carbon structural steel
It has a low content of sulfur, phosphorus and non-metallic impurities. It is often used to manufacture important mechanical parts. It generally needs to be heat treated before use to optimize mechanical properties. The first two digits of the grade indicate the nominal mass fraction of carbon in the steel (in parts per thousand). If the manganese content is high, the manganese element needs to be marked. For example, 45 steel (carbon content 0.45%) is often used to manufacture gears, shafts and other parts. Through quenching and tempering (quenching and high-temperature tempering), it can obtain good comprehensive mechanical properties and meet the use requirements of mechanical parts under complex working conditions.
2.Tool steel
All are high-carbon steel, and most of them are high-quality steel or high-quality steel, mainly used to manufacture various cutting tools, measuring tools, molds, etc. The grade consists of the letter "T" representing tool steel, the nominal mass fraction of carbon (in parts per thousand), and the quality grade symbol (not marked as ordinary grade, A means high-quality). For example, T8A indicates high-quality tool steel with a carbon content of 0.8%. T8A has high hardness and good toughness and is often used to make punches, chisels and other tools; T12 steel (carbon content 1.2%) has high hardness but low toughness and is suitable for making cutting tools such as files and scrapers, as well as measuring tools such as gauges and sample sets, because its high hardness can ensure the accuracy and stability of the measuring tools and the wear resistance of the cutting tools.
3.Cast steel
The carbon content is generally between 0.20% and 0.60%. If the carbon content is too high, it will lead to poor plasticity and easy cracking during casting. There are two ways to indicate cast steel grades. One is composed of the letter "ZG" representing cast steel, the minimum yield strength (in MPa), and the minimum tensile strength (in MPa), such as ZG200-400, which means cast steel with a yield strength of 200MPa and a tensile strength of 400MPa. It is often used to manufacture blanks for mechanical parts, such as casings and brackets of large machinery; the other is based on chemical composition as the main acceptance basis, with a group of numbers after "ZG", indicating the nominal mass fraction of carbon in parts per ten thousand, such as ZG25.
三、Other elements that affect the performance of carbon steel
In addition to carbon, carbon steel may contain other elements that also have an important impact on the performance of carbon steel:
Manganese: It can improve the strength and hardness of steel, but it will reduce ductility and weldability, and affect the hardenability of steel. In some carbon steels, an appropriate amount of manganese can improve the overall performance of steel. For example, in high-quality carbon structural steel, manganese can enhance the strength of steel while having little effect on toughness.
Phosphorus: It increases the strength and hardness of steel, but it will reduce ductility and notch impact toughness. Phosphorus is generally regarded as a harmful element in steel, which will increase the cold brittleness of steel, that is, it is easy to brittle fracture at low temperatures, so the phosphorus content needs to be strictly controlled during the production process of steel.
Sulfur: It exists in steel in the form of sulfide inclusions, which reduces the ductility, notch impact toughness and weldability of steel. Sulfur is also a harmful element, which will cause hot brittleness of steel, that is, it is easy to crack during hot working, so the sulfur content in steel also needs to be strictly controlled.
Silicon: It is one of the main deoxidizers in the steelmaking process. In low-carbon steel, excessive silicon content is generally not good for surface quality, but an appropriate amount of silicon can dissolve in ferrite, improve the strength and hardness of steel, and in some alloy steels, silicon can also improve the oxidation resistance and corrosion resistance of steel.
Copper: It is harmful to hot-working steel, but when the content exceeds 0.20%, it helps to improve the corrosion resistance of steel. In some weathering steels, a certain amount of copper is deliberately added to improve the corrosion resistance of steel in atmospheric environment.
Nickel: It can strengthen ferrite and improve the hardenability and impact strength of steel. In some high-strength alloy steels, nickel is an important alloying element that can significantly improve the comprehensive performance of steel.
Molybdenum: It improves the hardenability of steel and enhances the creep resistance of low-alloy steel. Steels used in high-temperature environments, such as boiler steel, often add molybdenum to ensure the performance stability of steel under long-term service at high temperatures.
Understanding the classification of carbon steel grades and the performance characteristics of each grade, and comprehensively considering the impact of other elements on the performance of carbon steel, can help engineers, manufacturers and other relevant personnel accurately select the most suitable carbon steel materials in practical applications, thereby ensuring the quality and performance of the products, while optimizing costs and promoting the efficient development of various industries.
