1. "355" – Minimum Yield Strength
Represents the minimum yield strength of the steel: ≥355 MPa (megapascals).
This value is measured at room temperature under standardized testing (e.g., GB/T 4171).
2. "NH" – Weathering Resistance Code
"N": Abbreviation for "Nài Hòu" (Chinese term for weathering-resistant).
"H": Denotes "Nài Hòu Gāng" (Chinese for weathering steel).
Together: "NH" classifies the steel as atmospheric corrosion-resistant structural steel per the Chinese standard GB/T 4171.
3. Full Meaning of "Q355NH"
Q: Stands for "Qū Fú" (Yield) in Chinese, indicating yield strength as the primary mechanical property.
355: Minimum yield strength ≥355 MPa.
NH: Weathering steel grade with enhanced corrosion resistance via alloy elements (e.g., Cu, Cr, P).
4. International Equivalents
| Property | Q355NH (China GB/T 4171) | EU Equivalent | U.S. Equivalent |
|---|---|---|---|
| Yield Strength | ≥355 MPa | S355J0W (EN 10025-5) | ASTM A588 Grade A (≥345 MPa) |
| Corrosion Resistance | Protective patina formation | S355J0W | ASTM A588 |
5. Key Characteristics
Alloy Composition: Typically contains Copper (Cu: 0.25–0.55%), Chromium (Cr: 0.40–1.25%), and Phosphorus (P: 0.07–0.15%) to enable self-protecting rust layer formation.
Applications: Widely used in bridges (e.g., coastal viaducts), shipping containers, and architectural facades where painting is undesirable.
Advantage: Achieves 50+ years of service life in temperate environments with minimal maintenance.
6. Comparison with Non-Weathering Steel
Q355NH vs. Q355B:
Both share identical mechanical strength (≥355 MPa yield strength).
Q355NH forms a dense, adherent rust layer that inhibits further corrosion; Q355B requires protective coatings to avoid rapid degradation outdoors.
