Q355NHA is a high-strength, low-alloy weathering structural steel plate, with its designation strictly conforming to the Chinese national standard GB/T 4171—2008 "Weathering structural steels". This standard is the authoritative technical document in China for specifying atmospheric corrosion-resistant structural steels.
The designation "Q355NHA" has a clear meaning:
"Q" stands for the first letter of the Chinese pinyin "Qu," representing the yield strength of the steel.
"355" indicates the specified minimum lower yield strength of the steel plate is 355 N/mm² (i.e., 355 MPa), placing it in the medium-to-high strength category and widely used in modern steel structure projects.
"NH" represents the first letters of the Chinese pinyin "Nai Hou," clearly identifying its resistance to atmospheric corrosion.
"A" is the quality grade symbol, indicating that the steel meets the requirements for internal cleanliness (such as non-metallic inclusions) at Grade A as defined by the standard.
Q355NHA steel plates are primarily used in the fabrication of load-bearing steel structures exposed to the atmosphere, where both high strength and good corrosion resistance are required. Its key advantage lies in combining high strength with excellent weathering performance, making it suitable for outdoor environments with relatively harsh service conditions. Typical applications include:
Construction and Bridges: Exposed components in high-rise buildings, large-span space structures, main girders, and auxiliary structures of railway and highway bridges.
Transportation: Manufacturing heavy-duty truck bodies, containers, and port machinery.
Power and Energy: Transmission towers, substation frameworks, and non-critical components or internal structures of wind turbine towers.
Industrial Facilities: Frames, platforms, pipe supports, etc., in chemical plants and power stations.
Its main characteristics include:
Excellent Atmospheric Corrosion Resistance: By adding alloying elements such as copper (Cu), chromium (Cr), and nickel (Ni) into the steel, it can form a dense and stable rust layer (corrosion product) in the atmosphere. This layer effectively prevents oxygen, water, and harmful ions (e.g., Cl⁻) from further penetrating the base metal, thereby significantly slowing down the corrosion rate. Its corrosion resistance is far superior to that of ordinary carbon steels.
High Strength: With a yield strength of 355 MPa and tensile strength ranging from 470–630 MPa, it offers strong load-bearing capacity, enabling lightweight structural design and reducing material consumption.
Good Comprehensive Performance: Possesses good ductility, toughness, and weldability, making it easy to process and construct on-site.
Cost-effectiveness and Environmental Friendliness: Although its initial cost is slightly higher than ordinary steel requiring regular painting and maintenance, its maintenance-free or low-maintenance nature can significantly reduce lifecycle costs while minimizing environmental pollution from paint usage.
According to GB/T 4171—2008, specifications for thickness range, chemical composition, delivery condition, surface quality, and testing rules for Q355NHA steel plates are clearly defined, ensuring product quality consistency and reliability. Therefore, Q355NHA is a high-performance, widely applied premium weathering structural steel.
Ultrasonic Testing (UT)
A key non-destructive testing technique that uses high-frequency sound waves to detect internal flaws in steel plates. The probe emits sound waves, which reflect when encountering defects such as cracks or inclusions. The receiver captures the echoes, enabling precise determination of defect location and size. With high sensitivity, strong penetration, and fast inspection speed, UT effectively ensures internal quality, widely used in the production of heavy plates, pressure vessel plates, and other high-end products to guarantee safety and reliability.
Magnetic Particle Testing (MT)
A common surface inspection method that magnetizes the workpiece, causing leakage magnetic fields at surface or near-surface defects like cracks or inclusions, which attract magnetic particles to form visible indications. Simple to operate and highly sensitive, MT is suitable for rapid inspection of surface and near-surface flaws in ferromagnetic materials, widely used for online or offline inspection of plate edges, ends, and welds, ensuring product quality and safety.
Penetrant Testing (PT)
A non-destructive method for detecting surface-breaking flaws. A penetrant liquid is applied to the cleaned steel surface, allowing it to seep into defects such as cracks or pores. After removing excess penetrant, a developer is applied, causing the trapped penetrant to bleed out and form visible indications. Simple and cost-effective, PT is suitable for inspecting surface defects in various non-porous materials, commonly used for welds, castings, and complex components, effectively ensuring surface quality of steel plates.
