S355G6+M is an ultra-high-strength, ultra-high-toughness steel plate designed for offshore and marine engineering applications. Its designation follows the European standard (EN) steel nomenclature system and is specifically engineered for structural components operating in the most extreme and harsh environments, such as polar regions, deepwater oil and gas platforms, ultra-large floating structures, and offshore wind foundations with high seismic demands. The "S" stands for "Structural steel," indicating its use in critical load-bearing structures. "355" denotes the specified minimum yield strength of 355 MPa at room temperature, classifying it as a high-strength grade suitable for components subjected to complex and heavy loads. "G6" is one of the highest quality grades in the series, representing the most stringent requirements for low-temperature toughness. Specifically, the G6 grade requires Charpy V-notch impact testing at -80°C, with an average absorbed energy of no less than 47 J for three specimens and no single specimen below 35 J, demonstrating exceptional resistance to brittle fracture. This makes it particularly suitable for service in Arctic regions or in cryogenic applications such as liquefied natural gas (LNG) storage and transportation facilities. The suffix "+M" indicates the delivery condition: thermomechanically controlled processed (TMCP), where precise control of rolling temperature, deformation, and accelerated cooling enables significant grain refinement and microstructural strengthening. This process achieves an optimal balance of high strength and high toughness without relying on high carbon content, while maintaining a low carbon equivalent (CEV), ensuring good weldability.
The main characteristics of S355G6+M steel plate include ultra-high strength, outstanding cryogenic toughness, excellent weldability, and superior mechanical uniformity. Its chemical composition is based on carbon and manganese, with additions of microalloying elements such as niobium, vanadium, and titanium. Combined with the TMCP process, it forms a fine, uniform microstructure—typically acicular ferrite or low-carbon bainite—significantly enhancing comprehensive mechanical properties. The tensile strength ranges from 470 to 630 MPa, with elongation (A5) ≥20%, providing good ductility and fatigue resistance. Its high impact toughness at -80°C makes it a key material for polar engineering, deepwater drilling vessels, floating liquefied natural gas (FLNG) units, and offshore wind projects in high-latitude regions, effectively preventing brittle fracture and ensuring long-term structural safety and reliability under extreme climatic conditions. Additionally, S355G6+M exhibits excellent thickness-direction properties (Z-direction properties), making it suitable for thick-section welded joints with high resistance to lamellar tearing.
S355G6+M is widely used in the most demanding applications, including jacket frames for fixed platforms in the Arctic, deepwater semi-submersible platforms, FLNG module supports, and foundations for ultra-large offshore wind turbines.
The current standard for S355G6+M steel plate is the European standard EN 10225:2023 "Weldable structural steels for use in offshore structures — Technical delivery conditions for fabricated or rolled products". This standard is specifically tailored for offshore and marine engineering, providing detailed specifications for chemical composition, mechanical properties, impact toughness, manufacturing processes, non-destructive testing, and inspection procedures. It serves as a core technical reference for modern high-end offshore engineering design and construction. Due to its exceptional performance, S355G6+M has become a strategic material essential for polar and deep-sea engineering projects.
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.
