Q355NH is a typical high‑strength weathering steel grade defined in the Chinese national standard GB/T 4171‑2008 Weathering steel for welded structures. It is widely used in outdoor structures, bridges, containers, rail vehicles, and architectural components requiring both high mechanical performance and atmospheric corrosion resistance. At room temperature (20 °C), its yield strength requirements are strictly specified according to plate thickness ranges, as shown below:
For plate thickness ≤ 16 mm
The specified minimum yield strength (ReL or Rp0.2) is ≥ 355 MPa.
This is the nominal yield strength level of Q355NH and applies to thin plates commonly used in general structural and decorative applications.
For plate thickness > 16 mm and ≤ 40 mm
The specified minimum yield strength is ≥ 335 MPa.
With increasing thickness, the material's hardenability and internal uniformity change slightly during production, so the minimum yield strength is adjusted downward while still maintaining high strength.
For plate thickness > 40 mm and ≤ 60 mm
The specified minimum yield strength is ≥ 325 MPa.
This range covers medium‑thick plates used in load‑bearing members, support structures, and bridge components.
For plate thickness > 60 mm and ≤ 100 mm
The specified minimum yield strength is ≥ 315 MPa.
Thick plates in this range are mainly used in heavy‑duty structures, large equipment frames, and high‑load welded assemblies.
These values represent the minimum guaranteed yield strength in the delivery condition (hot‑rolled or normalized) at room temperature. Actual yield strength from qualified steel mills is usually higher than these minimum values to ensure safety margin in engineering design and application.
