Q550NH Atmospheric Corrosion Resistant Weathering Steel Plate

GB/T 4171 | Patina-Forming Steel with 550 MPa Minimum Yield Strength

What is Q550NH Weathering Steel?

Q550NH is a high-strength Chinese-grade structural weathering steel standardized under GB/T 4171. The "NH" stands for "Nai Hou" (耐候), directly translating to "weather resistance". As the highest strength grade in the NH series, it delivers exceptional mechanical performance while forming a dense, stable, and adherent protective patina layer in natural atmospheric environments. This self-protecting characteristic provides 2 to 8 times the corrosion resistance of conventional carbon steel, making it ideal for critical heavy-duty outdoor structures requiring maximum load-bearing capacity and long-term durability.

Decoding the Grade: Q550NH

The naming follows the Chinese steel grade system, where each part reveals key performance characteristics:

Q Yield Strength Designator — Structural steel where "Q" stands for "Qu" (屈), representing Yield Point.
550 Minimum Yield Strength — Guarantees a minimum yield strength of 550 MPa (for thickness ≤16mm).
NH Weathering Quality Grade — "Nai Hou" (耐候) indicates atmospheric corrosion resistance with alloying elements (Cu, Cr, Ni) enabling patina formation.

Domestic Grades Comparison

Q460NH

Yield Strength: ≥460 MPa
Strength Level: Extreme High Strength
Application: Extreme heavy-load structures
Key Alloy Elements (GB/T 4171)
Cu: 0.25-0.55% | Cr: 0.30-1.25% | Ni: ≤0.65%

Q500NH

Yield Strength: ≥500 MPa
Strength Level: Maximum Strength
Application: Maximum load structures
Key Alloy Elements (GB/T 4171)
Cu: 0.25-0.55% | Cr: 0.30-1.25% | Ni: ≤0.65%

Q550NH

Yield Strength: ≥550 MPa
Strength Level: Ultra Maximum Strength
Application: Critical heavy-load structures
Key Alloy Elements (GB/T 4171)
Cu: 0.25-0.55% | Cr: 0.30-1.25% | Ni: ≤0.65%

Core Characteristics:
✔ All three grades share excellent weathering alloy composition (Cu, Cr, Ni) for superior corrosion resistance.
✔ Strength increases with grade number, while workability and cost efficiency decrease slightly.
✔ Q550NH is the highest strength grade in the NH series, designed for critical engineering projects.

Chemical Composition (GB/T 4171)

Grade C Si Mn P S Cu Cr Ni Others*
Q550NH ≤0.12 ≤0.65 ≤2.00 ≤0.025 ≤0.030 0.25-0.55 0.30-1.25 ≤0.65 ①,②,③
📌 Notes on chemical composition:
① Micro-alloying: may contain Nb (0.015-0.060%), V (0.02-0.12%), Ti (0.02-0.10%), Alt ≥0.020%. At least one element meets lower limit.
② Optional: Mo ≤0.30%, Zr ≤0.15% may be added by agreement. Also Ni lower limit may be waived.
③ Total amount of Nb, V, Ti added shall not exceed 0.22%.
④ Permissible deviations for product analysis according to GB/T 222. Sulphur can be agreed ≤0.008%.
⑤ Smelting: converter or electric furnace, fully killed steel. Delivery states: hot-rolled / controlled-rolled / normalized / quenched + tempered.

Mechanical Properties & Bending Test

Grade Lower yield strength ReL (≥ MPa) Tensile strength Rm (MPa) Elongation after fracture A/% ≥ 180° Bending
mandrel diameter
≤16 >16-40 >40-60 >60 N/mm² ≤16 >16-40 >40-60 >60 ≤6 >6-16 >16
Q550NH 550 540 530 — 630 – 780 16 16 15 — a 2a 3a
550/540/530 = Yield strength by thickness (MPa) — = not applicable for >60mm 630-780 = Tensile strength range (MPa) a/2a/3a = Bending mandrel diameter (a = specimen thickness)

Note: 'a' = specimen thickness. Rp0.2 applies if no yield plateau. Bending specimen width b=35mm for arbitration. "—" indicates the property is not specified for that thickness range. Q550NH may be delivered in quenched and tempered condition as per 7.3.

550
Min Yield (MPa) ≤16mm
630-780
Tensile Rm (MPa)
16%
Elongation A (≤40mm)
0.25-0.55
Copper Cu %

Available Supply Range

Thickness Range

  • Hot-rolled plates: 3mm - 60mm
  • Sheets / Coils: 1.5mm - 20mm
  • Custom thickness: Available upon request

Width & Length

  • Width: 900mm - 4000mm
  • Length: 2000mm - 18000mm
  • Custom sizes: Cut-to-length available

Product Forms

  • Steel Plates (hot-rolled, normalized, Q+T)
  • Steel Sheets & Coils
  • Sections: H-beams, I-beams, Channels, Angles
  • Flat bars & Round bars

Surface & Edge Options

  • As-rolled surface / Shot blasted
  • Primer coated (optional)
  • Mill edge / Trimmed edge
  • Beveled edge for welding
Stock availability: Regular sizes in stock for quick delivery | Mill order: Custom sizes available with 20-35 days lead time | Q+T condition: Available upon request

Custom Dimensions & Processing

Custom Sizes

Non-standard dimensions

Laser/Plasma Cutting

CNC precision cutting

Edge Beveling

Single/double bevel

Drilling & Punching

Custom hole patterns

Bending & Forming

Plate rolling, profiling

Surface Treatment

Shot blasting, priming

Engineering Support: Free CAD assistance, nesting optimization, technical consultation

Advantages & Typical Applications

Key Advantages

  • Highest strength design in NH series
  • Excellent weathering and corrosion resistance
  • 2-8x corrosion resistance vs carbon steel
  • Good weldability with proper preheating
  • No painting required, reduces lifecycle costs
  • Can be supplied in quenched and tempered condition

Typical Applications

  • Large-span bridges & high-rise buildings
  • Critical infrastructure & offshore structures
  • Heavy engineering machinery & vehicle frames
  • Transmission towers & extreme high-load facilities
  • Railway rolling stock & heavy containers

Frequently Asked Questions (FAQ)

1. What does "NH" mean in Q550NH?
2. What is the difference between Q550NH and Q460NH?
3. What is the difference between Q550NH and Q500NH?
4. Can Q550NH be supplied in quenched and tempered condition?
5. Is Q550NH suitable for welding?

Supply Information

Available Forms

  • Plates & Sheets
  • Hot-rolled Coils
  • Sections (H-beams, Channels, Angles)
  • Bars

Delivery Condition

  • Hot-rolled: As-rolled, controlled-rolled, or normalized
  • Quenched + Tempered (for high strength grades)
  • Cold-rolled: Annealed condition
  • Mill test certificate (EN 10204 3.1) available

MOQ & Lead Time

  • Standard MOQ: 5 tons
  • Stock items: 30-35 days
  • Custom production: 35-45 days

Start Your Q550NH Project

Emails go directly to customize@cehesteel.com — reply within 12 hours.