What is Q235NH?

Q235NH is a weathering structural steel defined in GB/T 4171, developed to resist atmospheric corrosion while maintaining reliable strength and weldability. With the addition of elements like copper, chromium, and nickel, it forms a stable, protective rust layer over time—reducing the need for painting and lowering maintenance costs. In real-world use, it's a practical, durable choice for bridges, containers, and outdoor steel structures exposed to the elements.

📄 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
Q235NH ≤0.13f 0.10~0.40 0.20~0.60 ≤0.030 ≤0.030 0.25~0.55 0.40~0.80 ≤0.65a,b
📌 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%

🔹 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; cold-rolled products generally annealed.

⚙️ 7.4 Mechanical Properties & Process Performance
Grade Thickness (mm) Lower Yield Strength (MPa) Tensile Strength (MPa) Elongation A% 180° Bend Test
Mandrel Diameter
Q235NH ≤16 235 360~510 25 aNote1 (2a)
>16~40 225 24
>40~60 215 23
>60 215 23
📌 Mechanical Property Footnotes:
Note1 "a" = steel thickness (mandrel diameter in bend test = 2a for Q235NH)
Note2 When yield point is not obvious, Rp0.2 may be used
🔹 Impact Properties (Table 5) — Q235NH available quality grades: Grade A (+20°C ≥47J); Grade B (0°C ≥34J); Grade C (-20°C ≥34J); Grade D (-40°C ≥27J)Note3.
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.
Note3 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 A588M Thickness/Diameter Range
Q235NH S235J0W / S235J2W SMA400AW/BW/CW Grade K Plate/strip ≤100mm, sections ≤100mm

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

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