Q355 steel is a high-strength low-alloy (HSLA) structural steel widely used in China. It's defined by the Chinese national standard GB/T 1591 (Low-alloy high strength structural steels).
Here's a breakdown of its key characteristics:
Naming:
Q: Stands for "Qu" in Chinese (屈), meaning "Yield". This indicates the primary classification is based on minimum yield strength.
355: Indicates the minimum yield strength of the steel at a thickness of ≤ 16mm is 355 Megapascals (MPa). (Yield strength decreases slightly with increasing thickness).
Key Properties:
High Strength: The primary advantage. Minimum yield strength of 355 MPa and minimum tensile strength of 470-630 MPa (depending on grade/subtype).
Good Ductility: Maintains reasonable elongation (≥ 21% for thickness ≤ 16mm, grade B), allowing it to deform without breaking under stress.
Good Weldability: Due to its controlled carbon content (typically ≤ 0.20%) and alloying elements, it's readily weldable using standard methods. Preheating might be necessary for thicker sections or lower ambient temperatures.
Good Machinability: Generally machines well, though its hardness requires appropriate tooling.
Impact Toughness: Available in different grades offering varying levels of impact toughness at low temperatures (see grades below).
Composition: Primarily alloyed with Manganese (Mn), often with additions like Silicon (Si), Vanadium (V), Niobium (Nb), and sometimes Titanium (Ti) or Aluminum (Al) for grain refinement and strengthening. Carbon content is kept low.
Grades (Quality Classes / Impact Toughness): Q355 is further subdivided based on impact toughness (Charpy V-notch test) and deoxidation practice. The common grades are:
Q355B: Minimum impact toughness of 34J at +20°C. (Most common general-purpose grade).
Q355C: Minimum impact toughness of 34J at 0°C. (Suitable for cold environments).
Q355D: Minimum impact toughness of 34J at -20°C. (Good for moderately cold environments).
Q355E: Minimum impact toughness of 34J at -40°C. (For very cold environments, like northern regions).
Higher grades (C, D, E) generally also have lower maximum Phosphorus (P) and Sulfur (S) limits for improved weldability and toughness.
Primary Applications:
Structural frameworks for buildings (industrial, commercial, high-rise)
Bridges
Towers (transmission, communication)
Cranes and lifting equipment
Heavy machinery and mining equipment
Construction vehicles
Ships (hull structures requiring higher strength than ordinary hull plate)
Pressure vessels (specific grades meeting additional standards)
Pipe racks, platforms, and other structural steelwork.
Equivalent Standards:
Q355 is the successor to the widely known Q345 steel in the updated GB/T 1591 standard (2018 revision), offering slightly improved properties and better alignment with international standards.
European (EN): S355 series (e.g., S355J2 is roughly equivalent to Q355B; S355K2+N is roughly equivalent to Q355D). This is the closest equivalent.
American (ASTM): A572 Grade 50 (Minimum Yield 50 ksi = ~345 MPa) is the most common equivalent, though strength differs slightly. ASTM A992 (for structural shapes) is also often comparable.
In Summary:
Q355 is a versatile, high-strength structural steel from China, offering a good balance of strength, ductility, weldability, and impact toughness. Its specific properties (especially impact toughness at low temperatures) are defined by its grade (B, C, D, E). It's the cornerstone steel for demanding structural applications across various industries in China and is increasingly recognized globally through its equivalence to S355 steel.
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