What are the mechanical property indicators of Q235NH? - Knowledge

The mechanical property indicators of Q235NH (a low-alloy weathering structural steel, compliant with standards like China's GB/T 4171) are mainly defined to ensure its load-bearing capacity, toughness, and processability in atmospheric corrosion environments. These indicators are typically tested on hot-rolled or normalized steel plates/sheets and include the following core categories:

1. Strength Indicators (Core for Load-Bearing)

Strength indicators reflect the steel's ability to resist deformation and fracture under external loads, which is critical for structural safety.

Indicator Name	Symbol	Definition	Typical Requirement (Thickness ≤ 16mm, GB/T 4171)
Yield Strength (Rel)	Rel	The stress at which the steel begins to undergo plastic deformation (no longer elastic).	≥ 235 MPa (the "235" in Q235NH refers to this minimum yield strength)
Tensile Strength (Rm)	Rm	The maximum stress the steel can withstand before breaking during tension.	375 – 500 MPa

2. Plasticity Indicator (Resists Brittle Fracture)

Plasticity reflects the steel's ability to undergo permanent deformation before fracture, ensuring structural ductility and avoiding sudden brittle failure.

Indicator Name	Symbol	Definition	Typical Requirement (Thickness ≤ 16mm, GB/T 4171)
Elongation after Fracture (A)	A	The percentage of the sample's length increase after breaking (calculated by the gauge length).	≥ 22% (for a standard gauge length of 50mm, A50)

3. Toughness Indicator (Resists Low-Temperature/Impact Damage)

Toughness evaluates the steel's ability to absorb energy during impact loading, which is essential for resisting dynamic forces (e.g., wind, vibration) and avoiding low-temperature brittleness.

Indicator Name	Symbol	Test Condition	Typical Requirement (GB/T 4171)
Impact Absorbed Energy (KV2)	KV2	Charpy V-notch sample; tested at -40°C (standard low-temperature condition for weathering steels).	≥ 34 J (single sample value; average of 3 samples ≥ 39 J)

4. Additional Key Requirement: Bend Test (Processability & Uniformity)

The bend test assesses the steel's cold-forming performance and internal uniformity (no cracks after bending), which is crucial for fabrication processes like bending, rolling, or welding.

Test Item	Test Condition	Requirement
Cold Bend Test	Sample thickness ≤ 16mm; bending angle = 180°; inner bend radius = 1.5 × sample thickness (d).	No cracks, splits, or visible defects on the outer surface of the bent sample.

Important Notes on Variations

Thickness Dependence: For steel plates thicker than 16mm (e.g., 16–40mm), yield strength (Rel) and elongation (A) requirements may slightly decrease (e.g., Rel ≥ 225 MPa, A ≥ 21% for 16–40mm), while tensile strength (Rm) ranges remain similar-specifics follow GB/T 4171.

Mill Test Certificate (MTC): Actual mechanical properties of a specific batch of Q235NH must be verified via the MTC provided by the manufacturer, as minor variations may exist between production lots.

Comparison with Ordinary Steel: Compared to ordinary carbon steel Q235, Q235NH has similar basic strength but better low-temperature toughness (KV2 at -40°C vs. Q235's typical -20°C requirement) and atmospheric corrosion resistance (due to alloying elements like Cu, Cr).

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