What are the key mechanical properties (yield strength, tensile strength, elongation) for Q355B I-beams?

The mechanical properties form the core performance criteria for Q355B I-beams, as specified in GB/T 1591-2018 and the sectional dimension standard GB/T 706-2016 (for hot-rolled sections). The most critical properties are Yield Strength (ReH or Rp0.2), Tensile Strength (Rm), Elongation after Fracture (A), and Impact Toughness (KV2 at 20°C for Grade B). Yield strength is the stress at which the material begins to deform plastically; for Q355B, the minimum yield strength is 355 MPa but only for nominal thicknesses ≤ 16mm. It's crucial to note that the minimum yield strength decreases for larger thicknesses due to slower cooling rates during rolling, which results in a coarser microstructure. For example, for thicknesses >16 to 40mm, it's ≥ 345 MPa; >40 to 63mm, ≥ 335 MPa; >63 to 80mm, ≥ 325 MPa; >80 to 100mm, ≥ 315 MPa; >100 to 150mm, ≥ 305 MPa; and >150 to 250mm, ≥ 285 MPa. Tensile strength (Rm) must be between 470 to 630 MPa for all thickness ranges up to 250mm, ensuring a sufficient margin above yield strength. Elongation (A), measured on a standard test piece, is typically ≥ 20-22% (depending on thickness and test piece type), indicating good ductility. For Q355B, Charpy V-notch impact energy at +20°C must be ≥ 34J. These properties are verified through destructive testing of samples taken from the web or flange of the I-beam.

*Table: Key Mechanical Properties of Q355B Steel (GB/T 1591-2018)*