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Q345A, Q345B, Q345C, Q345D, and Q345E are five grades of low-alloy high-strength structural steel, with their main differences lying in impact performance, chemical composition, and applicable applications. The following is a detailed comparative analysis: 01 Impact performance and test temperature. Q345A: No impact test required. Q345B: Must pass the impact test at 20°C, with an impact energy of ≥27 J. Q345C: Must pass the impact test at 0°C, with an impact energy of ≥27 J. Q345D: Must pass the impact test at -20°C, with an impact energy of ≥27 J. Q345E: Must pass the impact test at -40°C, with an impact energy of ≥27 J. As the grade increases from A to E, the material’s toughness at low temperatures improves, while the suitable operating temperature decreases; in particular, Q345E is suitable for extremely cold environments. 02 Differences in chemical composition: The contents of phosphorus (P) and sulfur (S) decrease gradually as the grade increases, while grades C and above have an increased aluminum (Al) content to refine the grain structure. P and S are harmful elements; reducing their levels improves the purity of the material and reduces its cold brittleness ； The addition of Al further refines the grains and enhances toughness. 03 Mechanical properties: The tensile strength (490–675 MPa) and yield strength (≥345 MPa) are roughly the same across all grades, but there are slight differences in elongation; the elongation of Q345B and Q345A is ≥21% ； The elongation rate of Q345C, Q345D, and Q345E is ≥22%. 04 Application scenarios Q345A: Used for general structures in normal temperature environments, such as ordinary buildings and mechanical parts. Q345B: Structures such as bridges and vehicles that require impact resistance at room temperature. Q345C/D: Manufacturing of containers, pipelines, and ships in low-temperature environments (such as 0°C to -20°C). Q345E: Special equipment, pressure vessels, and critical engineering structures for extremely cold environments (-40°C). Comparison with the old grade 16Mn steel: The Q345 series replaces 16Mn steel; by adding trace amounts of alloying elements such as V, Ti, and Nb, the grain structure becomes finer. Its overall mechanical properties (especially low-temperature toughness) are significantly better than those of 16Mn steel, and its allowable stress is also higher. END Summary: The main differences between Q345A and Q345E lie in low-temperature toughness, chemical composition purity, and applicable environments. The choice should be made by balancing specific temperature requirements with cost considerations; for example, Q345E is required in extremely cold areas, while Q345A/B is suitable for normal environments.