Q390A is a low-alloy high-strength structural steel with high strength and good overall performance, widely used in engineering structures subjected to heavy loads. The designation is interpreted as follows: "Q" stands for yield strength (from the first letter of the Chinese pinyin "Qu"); "390" indicates a minimum upper yield strength of 390 MPa when the thickness is no more than 16 mm, significantly higher than the Q355 series, providing superior load-bearing capacity; "A" denotes the quality grade, indicating that this is a standard quality grade steel which does not require low-temperature impact testing—meaning impact toughness is not guaranteed. It is typically suitable for structures operating at higher ambient temperatures where high toughness is not a critical requirement.
Q390A steel plates are primarily used in large-scale building structures, industrial workshops, heavy machinery, lifting and transportation equipment, transmission towers, auxiliary bridge components, mining equipment, and general heavy-duty steel structures. Due to its high strength, good weldability, and relatively moderate cost, Q390A is often used to replace Q235 or Q355 steels to reduce structural weight and improve load efficiency. It is especially suitable for static or slowly varying load applications under moderate stress in environments where temperatures remain above 0 °C.
Key characteristics of this steel include high strength, good ductility and cold-forming performance, and excellent weldability, allowing for cutting, welding, and forming using conventional fabrication methods. Q390A is typically delivered in hot-rolled or controlled-rolled condition, ensuring a uniform microstructure ideal for mass production and on-site construction. Although low-temperature impact toughness is not specified, the steel still provides sufficient toughness and fatigue resistance under normal ambient conditions to meet the requirements of most general engineering applications.
The current applicable standard is GB/T 1591-2018, High-Strength Low-Alloy Structural Steels, published and implemented in 2018, replacing the previous GB/T 1591-2008. This updated standard clearly specifies the chemical composition, mechanical properties, carbon equivalent, and delivery conditions for Q390A. It integrates the grade into a unified, internationally aligned numbering system, enhancing the standardization of high-strength steels in China. The standard also defines thickness ranges (typically 6 mm to 150 mm), yield-to-tensile strength ratio requirements, and maximum carbon equivalent limits to ensure welding safety and structural reliability.
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.
