Study on Forming Mechanism of Lamination Defect of AH36 Shipbuilding Plate Steel

Abstract

The lamination defect makes the mechanical properties deterioration along the thickness of steel plate and therefore finding out the forming mechanism is of great significance for production of shipbuilding plate steel. The tensile fracture and microstructure characteristics on AH36 shipbuilding plate steel of lamination defect were studied with Scanning Electron Microscope (SEM), Transmission Electron Microscopy (TEM), etc. The results show that the strength-toughness properties vary widely at different thickness of the steel plate due to different ferrite grain size, and the center zone firstly yields and steps into plastic deformation stage due to coarse ferrite grain subjected to tensile stress, while the surface zone is still in elastic deformation stage due to relatively fine ferrite grain. The inconsistency of deformation and fracture leads to fracture separation, namely lamination. The continuous banded Widmanstätten structure, strip-shaped sulphide inclusions and mixed ferrite grain distributed at the center of shipbuilding plate steel create conditions for the lamination defect, which are important reasons for fracture separation. Widmanstätten structure at the center of steel plate has relationship with coarse austenite grain, and strip-shaped sulphide inclusions have relationship with centerline segregation of continuous casting slab during solidification process.