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Steel is an alloy mainly composed of iron and carbon, with a carbon content generally less than 2.11%. Steel is an extremely important metal material in economic construction.

Steel is divided into two categories based on its chemical composition: carbon steel (abbreviated as carbon steel) and alloy steel. Carbon steel is an alloy obtained by smelting pig iron. In addition to iron and carbon as its main components, it also contains small amounts of impurities such as manganese, silicon, sulfur, and phosphorus. Carbon steel has certain mechanical properties, good processability, and low price. Therefore, carbon steel has been widely used. But with the rapid development of modern industry and science and technology, the performance of carbon steel can no longer fully meet the needs, so people have developed various alloy steels. Alloy steel is a multi-element alloy obtained by intentionally adding certain elements (called alloying elements) to carbon steel. Compared with carbon steel, the performance of alloy steel has significantly improved, so its application is becoming increasingly widespread.

Due to the wide variety of steel products, they are classified for ease of production, storage, selection, and research. According to the different uses, chemical compositions, and qualities of steel, it can be divided into many categories:

I.Classified by purpose

According to the purpose of steel, it can be divided into three categories: structural steel, tool steel, and special performance steel.
Structural steel: Steel used for various machine parts. It includes carburizing steel, quenched and tempered steel, spring steel, and rolling bearing steel.
2. Steel used for engineering structures. It includes Class A, B, special steel, and ordinary low-alloy steel in carbon steel.

Tool steel: Steel used to manufacture various tools. According to the different purposes of the tools, they can be divided into cutting tool steel, mold steel, and measuring tool steel.

Special Performance Steel: Steel with special physical and chemical properties. It can be divided into stainless steel, heat-resistant steel, wear-resistant steel, magnetic steel, etc.

II.Classified by chemical composition

According to the chemical composition of steel, it can be divided into two categories: carbon steel and alloy steel.

Carbon steel: can be divided into low carbon steel (carbon content ≤ 0.25%) according to its carbon content; Medium carbon steel (0.25%<carbon content<0.6%); High carbon steel (carbon content ≥ 0.6%).

Alloy steel: can be divided into low-alloy steel according to the content of alloy elements (total alloy element content ≤ 5%); Medium alloy steel (total alloy element content=5% -10%); High alloy steel (total alloy element content>10%). In addition, according to the different types of main alloying elements contained in steel, it can also be divided into manganese steel, chromium steel, chromium nickel steel, chromium manganese titanium steel, etc.

III.Classified by quality

According to the content of harmful impurities phosphorus and sulfur in steel, it can be divided into ordinary steel (phosphorus content ≤ 0.045%, sulfur content ≤ 0.055%; or both phosphorus and sulfur content ≤ 0.050%); High quality steel (with phosphorus and sulfur content both ≤ 0.040%); High quality steel with phosphorus content ≤ 0.035% and sulfur content ≤ 0.030%.

In addition, according to the type of smelting furnace, steel is divided into open hearth steel (acidic open hearth, alkaline open hearth), air converter steel (acidic converter, alkaline converter, oxygen top blown converter steel), and electric furnace steel. According to the degree of deoxidation during smelting, steel is divided into boiling steel (incomplete deoxidation), killed steel (relatively complete deoxidation), and semi killed steel.

Steel mills often combine the three classification methods of usage, composition, and quality when naming steel products. If steel is referred to as ordinary carbon structural steel, high-quality carbon structural steel, carbon tool steel, high-grade high-quality carbon tool steel, alloy structural steel, alloy tool steel, etc.