Additional requirements for resistance to delamination tearing

For flat covers manufactured from rolled sheets, the additional requirements regarding resistance to delamination mainly include specific tests and standards for the material, in order to ensure that it possesses good delamination resistance during use; this is done to enhance the overall structural safety and reliability of the product. For specific details, refer to the relevant descriptions in the GB/T150-2024 standard document. .

In steel structures joined by welding, when the steel plate thickness is 40 mm or more and it is subjected to tensile forces in the direction of the plate thickness, to prevent layer tearing during welding, steel materials resistant to layer tearing (commonly referred to as “Z-direction steel”) must be used. Thick plates suffer from laminar tearing issues, hence Z-direction performance testing needs to be proposed. Steel plates and sections are formed through rolling; the steel used in most high-rise steel structures is produced by hot rolling. Hot rolling can disrupt the casting structure of the steel ingots and refine the grain structure of the steel. Bubbles and cracks formed during steel ingot casting can be welded together under high temperature and pressure, thereby improving the mechanical properties of the steel. However, this improvement is mainly evident in the rolling direction, as the non-metallic inclusions within the steel (primarily sulfides, oxides, silicates, etc.) are compressed into thin layers after rolling and remain in the steel plate (usually parallel to the surface of the plate), causing delamination (interlayer formation) in the plate. This phenomenon of non-metallic interlayers. It deteriorates the ability of the steel to withstand tension in the thickness direction. Therefore, the mechanical properties of the steel plate vary in three directions: they are best in the rolling direction ； The properties perpendicular to the rolling direction are slightly inferior ； Performance in the thickness direction is next to worst.

Additional requirements for resistance to layered tearing typically involve the following aspects: Material selection: Steel quality – It is necessary to use steel with excellent resistance to layered tearing, such as Z-directional steel; its Z-directional strain reduction rate must meet relevant standards (such as ASTM A770, EN 10164, etc.). Chemical composition: Control the levels of harmful elements such as sulfur and phosphorus; the sulfur content should be below 0.005%, while the phosphorus content should be below 0.015%. Welding process: Preheating and interpass temperature control: Preheat before welding and control the interpass temperature to reduce welding stress. Welding method: A low-hydrogen welding process is used, such as gas shielded welding or low-hydrogen electrodes, to reduce the risk of hydrogen-induced cracks. Welding sequence: Arrange the welding sequence properly to reduce residual stress. Design considerations: Joint design – stress concentration should be avoided, and a bevel design is used to reduce welding stresses. Stress in the thickness direction: Minimize stress in the thickness direction to prevent delamination. Testing and evaluation: Non-destructive testing: Ultrasonic testing (UT) or radiographic testing (RT) is carried out to ensure the absence of delaminated tear defects. Mechanical property testing: Z-direction tensile tests are conducted to ensure that the Z-direction reduction of area meets the specified requirements. Heat treatment: Post-weld heat treatment (PWHT): Post-weld heat treatment is carried out when necessary to eliminate residual stresses and improve the microstructural properties. Quality Control: A rigorous quality control system ensures that every stage, from material procurement to welding operations, meets the required standards. Standards and specifications: Comply with relevant standards such as AWS D1.1, EN 1993-1-10, etc., to ensure that design and construction meet the specified requirements. These requirements are designed to ensure that the structure possesses sufficient tear resistance in the thickness direction through controls in various aspects such as materials, manufacturing processes, design, and testing