Currently, there are nearly 2,000 special steel grades and approximately 50,000 varieties worldwide. Beyond the wide range of special steels, their specifications also exhibit distinct characteristics compared to ordinary steel. In addition to common forms such as plates, tubes, wires, strips, bars, and profiles, there are also composite materials, surface alloys, surface treatments, precision forgings, precision castings, and powder metallurgy products.
Overview of China's Steel Number Representation Method
The steel grade, often referred to as the steel number, is the name assigned to each specific type of steel product. It serves as a universal language for people to identify and understand different types of steel. China’s system for representing steel numbers follows the national standard "Method for Representing Steel Product Grades" (GB221-79). This method combines Chinese pinyin letters, chemical element symbols, and Arabic numerals to create a comprehensive classification system.
1. Chemical elements in the steel number are represented by international chemical symbols, such as Si, Mn, Cr, etc. The mixed rare earth elements are denoted by "RE" (or "Xt"). 2. The product name, application, smelting, and casting methods are usually expressed using abbreviated Chinese pinyin letters. 3. The content of major chemical elements in the steel is indicated by Arabic numerals, showing the percentage of each element present.
Classification Description of China's Steel Number Representation Method
Carbon Structural Steel
1. The steel number format is Q + number + quality level symbol + deoxidation method symbol. The letter "Q" stands for the yield point of the steel, and the following number indicates the yield strength value in MPa. For example, Q235 refers to a carbon structural steel with a yield point (σs) of 235 MPa.
2. If necessary, the quality grade and deoxidation method can be added after the steel number. Quality levels are denoted by A, B, C, D, while deoxidation methods are marked as F (boiling steel), b (semi-killed steel), Z (killed steel), and TZ (special killed steel). Killed steel may not require the Z or TZ marking. For instance, Q235-AF represents Class A boiling steel.
3. Special-purpose carbon steels, such as bridge steel and marine steel, generally follow the same naming convention as carbon structural steel but include additional letters at the end to indicate their specific use.
High-Quality Carbon Structural Steel
1. The first two digits in the steel number represent the average carbon content, expressed in tenths of a percent. For example, a steel with an average carbon content of 0.45% is labeled “45â€. Note that this is not a sequence number, so it should not be read as “45 steelâ€.
2. High-quality carbon structural steel with high manganese content should have “Mn†added to the steel number, such as 50Mn.
3. Boiling steel, semi-killed steel, and special-purpose high-quality carbon structural steel are marked at the end of the steel number. For example, a semi-killed steel with an average carbon content of 0.1% would be labeled “10bâ€.
Carbon Tool Steel
1. Carbon tool steel is prefixed with “T†to differentiate it from other types of steel.
2. The number in the steel number indicates the average carbon content, expressed in thousandths of a percent. For example, “T8†means an average carbon content of 0.8%.
3. If the manganese content is higher, “Mn†is added at the end of the steel number, such as “T8Mnâ€.
4. High-quality carbon tool steel has lower phosphorus and sulfur content than regular carbon tool steel. To distinguish it, the letter “A†is added at the end of the steel number, like “T8MnAâ€.
Free-Cutting Steel
1. Free-cutting steel is prefixed with “Y†to distinguish it from high-quality carbon structural steel.
2. The number following the “Y†indicates the average carbon content, expressed in tenths of a percent. For example, a free-cutting steel with an average carbon content of 0.3% is labeled “Y30â€.
3. If the manganese content is higher, “Mn†is added after the steel number, such as “Y40Mnâ€.
Alloy Structural Steel
1. The first two digits in the steel number represent the average carbon content, expressed in ten-thousandths of a percent. For example, “40Cr†indicates a carbon content of 0.40%.
2. The main alloying elements in the steel are generally expressed as percentages. When the average alloy content is less than 1.5%, only the element symbol is used without specifying the exact percentage. However, in some cases, the number “1†may be added to avoid confusion. For example, “12CrMoV†and “12Cr1MoV†both contain chromium, but the former has 0.4–0.6% chromium, while the latter has 0.9–1.2%. If the alloy content is 1.5% or higher, the percentage is explicitly stated, such as “2†for 2%, “3†for 3%, etc. An example is “18Cr2Ni4WAâ€.
3. Microalloying elements such as vanadium (V), titanium (Ti), aluminum (Al), boron (B), and rare earth elements (RE) are always included in the steel number due to their significant impact on properties, even though they are present in small amounts. For instance, in “20MnVBâ€, vanadium is present at 0.07–0.12%, and boron at 0.001–0.005%.
4. High-quality alloy structural steel should have “A†added at the end of the steel number to distinguish it from regular high-quality steel.
5. Special-purpose alloy structural steels are marked with a prefix or suffix indicating their intended use. For example, “30CrMnSi†used for rivet screws is labeled as “ML30CrMnSiâ€.
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