Q355B is a widely used low-alloy high-strength structural steel, known for its excellent combination of mechanical properties and processability, making it suitable for a wide range of industrial and construction applications. The designation is interpreted as follows: "Q" stands for yield strength (from the first letter of the Chinese pinyin "Qu"); "355" indicates a minimum upper yield strength of 355 MPa (for thickness ≤ 16 mm), representing a significant strength improvement over traditional Q235 steel; "B" denotes the quality grade, meaning the steel must meet Charpy V-notch impact test requirements at +20 °C, with an average longitudinal impact energy of no less than 27 J, indicating good toughness and resistance to impact loading.
Q355B steel plates are extensively used in building structures, bridges, vehicle manufacturing, transmission towers, machinery, pressure vessels, shipbuilding, and general industrial steel structures. Due to its high strength, good ductility, excellent weldability, and cost-effectiveness, Q355B has become one of the mainstream materials replacing Q235 steel in modern engineering. It is particularly suitable for medium-load-bearing components such as factory columns, roof trusses, platform beams, supports, and crane booms.
Key features of Q355B include a balanced combination of high strength and good elongation, excellent weldability, and ease of fabrication through cold forming, cutting, and welding. It also offers moderate corrosion resistance and fatigue resistance. Typically delivered in hot-rolled or controlled-rolled condition, the steel has a uniform microstructure, making it ideal for mass production and on-site construction.
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 harmonizes the designation system for high-strength structural steels in China, upgrading the former Q345 grade to Q355, aligning it with the European S355 series (e.g., S355JR) in EN 10025, thereby enhancing international compatibility. The standard specifies clear requirements for chemical composition, carbon equivalent, mechanical properties, impact toughness, and through-thickness properties.
With the continuous development of infrastructure and manufacturing in China, Q355B steel plate, valued for its outstanding cost-performance ratio and reliable performance, has become an essential material in construction, transportation, and energy sectors, widely used in national key projects and civil engineering applications.
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.
