Currently, there are nearly 2,000 special steel grades and about 50,000 varieties available globally. Beyond the wide range of special steels, their specifications also exhibit unique characteristics compared to ordinary steel. In addition to common forms like plates, tubes, wires, bands, rods, and profiles, there are also composites, surface alloys, surface treatments, precision forgings, precision castings, and powder metallurgy products.
Overview of China's Steel Grade Representation Method
Steel grades, also known as steel numbers, serve as a naming system for specific steel products, acting as a universal language for understanding different types of steel. In China, the representation method follows the national standard "Method of Representing Steel Product Grades" (GB221-79). This system combines Chinese Pinyin letters, chemical element symbols, and Arabic numerals. Specifically:
1. The chemical elements in the steel grade are represented using international chemical symbols, such as Si, Mn, Cr, etc. Rare earth elements are denoted by "RE" or "Xt".
2. The product name, usage, smelting, and casting methods are usually expressed through abbreviated Chinese Pinyin letters.
3. The percentage content of major chemical elements is indicated by Arabic numerals.
Classification of China's Steel Grade Representation Method
Carbon Structural Steel
1. It is composed of Q + number + quality level symbol + deoxidation method symbol. The letter "Q" stands for the yield point of the steel, while the following number indicates the yield point value in MPa. For example, Q235 refers to a carbon structural steel with a yield point (σs) of 235 MPa.
2. If needed, the quality grade symbol and deoxidation method can be added after the steel number. Quality levels are marked as A, B, C, and D. Deoxidation method symbols include F (boiling steel), b (semi-killed steel), Z (killed steel), and TZ (special killed steel). Killed steel may not require marking, so Z and TZ might be omitted. For example, Q235-AF denotes Class A boiling steel.
3. Special-purpose carbon steels, such as bridge or marine steel, typically follow the same format as carbon structural steel but add a letter indicating the use at the end of the grade.
High-Quality Carbon Structural Steel
1. The first two digits in the steel number indicate the carbon content, expressed as a fraction of the average carbon content. For instance, a steel with an average carbon content of 0.45% has the grade “45â€, which should not be read as “45 steelâ€.
2. High-quality carbon structural steel with higher manganese content should include “Mn†in the grade, such as 50Mn.
3. Boiling steel, semi-killed steel, and special-purpose high-quality carbon structural steel should have specific markings at the end of the grade. For example, a semi-killed steel with an average carbon content of 0.1% would be labeled “10bâ€.
Carbon Tool Steel
1. These steels are prefixed with “T†to differentiate them from other types of steel.
2. The number following “T†represents the carbon content, expressed as a fraction of the average carbon content. For example, “T8†means an average carbon content of 0.8%.
3. If the manganese content is high, it is marked at the end of the grade, such as “T8Mnâ€.
4. High-quality carbon tool steel contains lower phosphorus and sulfur levels than regular ones. An “A†is added at the end of the grade to distinguish it, such as “T8MnAâ€.
Free-Cutting Steel
1. These steels are prefixed with “Y†to distinguish them from high-quality carbon structural steel.
2. The number following “Y†indicates the carbon content, expressed as a fraction of the average carbon content. For example, a free-cutting steel with an average carbon content of 0.3% is labeled “Y30â€.
3. Steels with higher manganese content also have “Mn†marked at the end, such as “Y40Mnâ€.
Alloy Structural Steel
1. The first two digits in the steel number represent the carbon content, expressed as a fraction of the average carbon content. For example, 40Cr indicates a carbon content of 0.4%, with chromium as the main alloying element.
2. The main alloying elements, except for microalloying elements, are generally shown as percentages. When the average alloy content is less than 1.5%, only the element symbol is used. However, in some cases, the number “1†may be added to avoid confusion. For example, “12CrMoV†and “12Cr1MoV†differ in chromium content, with the former having 0.4–0.6% and the latter 0.9–1.2%. When the average alloy content is 1.5% or more, the exact percentage is indicated, such as in 18Cr2Ni4WA.
3. Microalloying elements such as vanadium (V), titanium (Ti), aluminum (Al), boron (B), and rare earth elements (RE) are always included in the steel grade due to their significant impact on properties, even though their content is low. For example, in 20MnVB, vanadium is 0.07–0.12% and boron is 0.001–0.005%.
4. High-quality steel grades should end with “A†to distinguish them from ordinary high-quality steel.
5. Special-purpose alloy structural steel includes a prefix or suffix that indicates its intended use. For example, 30CrMnSi steel used for rivet screws is labeled as ML30CrMnSi.
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