What is Q550NH?

Q550NH is a high-strength weathering structural steel defined in GB/T 4171, offering excellent atmospheric corrosion resistance with a minimum yield strength of 550 MPa. With the addition of alloying elements such as copper, chromium, nickel, and micro-alloys (Nb, V, Ti), it forms a stable, protective rust layer over time—significantly reducing the need for painting and lowering lifecycle maintenance costs. Q550NH can be delivered in quenched + tempered condition by agreement. In real-world use, it's a robust, durable choice for bridges, railway vehicles, transmission towers, and other outdoor steel structures requiring very high strength under harsh environmental conditions.

📄 GB/T 4171—2008 🌱 High Atmospheric Corrosion Resistance 🔩 Weldable / Riveted / Bolted Connections
📊 7.1 Chemical Composition (Ladle Analysis)
Grade C Si Mn P S Cu Cr Ni Other Elements
Q550NH ≤0.16 ≤0.65 ≤2.0 ≤0.025 ≤0.030(d) 0.20~0.55 0.30~1.25 0.12~0.65 (a,b,c)
📌 Footnotes Explanation:
(a) Micro-alloys: Nb(0.015%~0.060%), V(0.02%~0.12%), Ti(0.02%~0.10%), Alt≥0.020%; at least one element reaches the lower limit
(b) May contain Mo≤0.30%, Zr≤0.15%
(c) Nb+V+Ti total ≤0.22% (for Q415NH~Q550NH)
(d) By agreement, S content can be ≤0.008%
(e) By agreement, Ni lower limit may be waived
(f) By agreement, C content can be ≤0.15% (for Q235NH/Q295NH)

🔹 7.1.2 Permissible deviation for finished product chemical composition shall comply with GB/T 222.

🔹 7.2 Melting Method — Steel shall be made by converter or electric furnace, killed steel. Method at supplier's option unless specified.

🔹 7.3 Delivery Condition — Hot-rolled products delivered as-rolled, controlled-rolled or normalized; Q550NH may also be delivered quenched + tempered by agreement.

⚙️ 7.4 Mechanical Properties & Process Performance
Grade Thickness (mm) Lower Yield Strength (MPa) Tensile Strength (MPa) Elongation A% 180° Bend Test
Mandrel Diameter
Q550NH ≤16 550 620~780 16 2a (3a)
>16~40 540 16
>40~60 530 15
📌 Mechanical Property Footnotes:
Note 1: "a" = steel thickness (mandrel diameter in bend test = 2a for thickness ≤16mm, 3a for >16mm)
Note 2: When yield point is not obvious, Rp0.2 may be used
Note 3: For thickness >60mm, mechanical properties subject to agreement
🔹 Impact Properties (Table 5) — Q550NH available quality grades: Grade A (+20°C ≥47J); Grade B (0°C ≥34J); Grade C (-20°C ≥34J); Grade D (-40°C ≥27J).
7.4.2.1 By agreement, high weathering steel may be exempt from impact test.
7.4.2.2 Average of three specimens; one single value may be lower but not less than 70% of specified value.
7.4.2.3 Impact test required for thickness ≥6mm or diameter ≥12mm; sub-size specimens require proportional reduction.
Note 4: By agreement, average impact energy can be ≥60J
🔍 7.5 Additional Requirements
Grain Size — Shall be not less than Grade 7, with uniformity within three adjacent grade levels.
Non-metallic Inclusions (GB/T 10561 Method A): A≤2.5, B≤2.0, C≤2.5, D≤2.0, DS≤2.0.
🧪 7.6 Surface Quality
  • 7.6.1 No cracks, blisters, folds, bubbles, inclusions or laminations. Removal depth ≤ half thickness tolerance.
  • 7.6.2 Hot-rolled surfaces may have minor defects that do not affect usability, provided minimum thickness is maintained.
  • 7.6.3 Cold-rolled sheets/strips may have slight scratches, oxidation color, light yellow film, local pitting, marks within tolerance.
  • 7.6.4 Strips may be delivered with defects, but defective portion ≤8% of total length.
🌿 Atmospheric Corrosion Resistance Assessment (Appendix C & D)
📘 Appendix C · Protective Patina Layer
Corrosion resistance depends on alloying elements (Cu, Cr, Ni, P) forming a dense rust layer. Alternating wet-dry cycles aid patina stability.
🧪 Appendix D · Corrosion Resistance Index (I)
I = 26.01(%Cu)+3.88(%Ni)+1.20(%Cr)+1.49(%Si)+17.28(%P) - 7.29(%Cu)(%Ni) - 9.10(%Ni)(%P) - 33.39(%Cu)²
Index ≥6.0 indicates excellent weathering performance.

💡 Based on ISO 5952:2005 Appendix A and ASTM G101 methods. Minimum acceptable corrosion index shall be agreed between supplier and purchaser.

📚 Grade Comparison & Available Dimensions (Table 2)
GB/T 4171-2008 EN 10025-5 JIS G3114 ASTM A871M Thickness/Diameter Range
Q550NH — — — Plate/strip ≤60mm, sections —

📐 Dimensional Tolerances: Hot-rolled plate/strip per GB/T 709, cold-rolled per GB/T 708.

* Technical data based on GB/T 4171—2008, fully covering all requirements of Chapter 7 (7.1~7.6.4) and appendices.