Microstructure evolution and strengthening-toughening mechanism of EH36 heavy steel plate for high-power wind power along the thickness direction

This study investigates an industrially trial-produced 90 mm thick EH36 heavy steel plate for wind power through experimental characterization and calculations of its thermodynamic properties, phase transformations, and mechanical properties. The evolution of microstructure and mechanical properties across the full thickness was systematically examined. The results indicate that the strengthening and toughening mechanisms are primarily attributed to the high density of dislocations introduced during finish rolling, combined with grain refinement via recrystallization and dynamic recovery, which significantly contribute to dislocation and fine-grain strengthening. Moreover, deformation generated abundant low-angle and high-angle grain boundaries, with the high boundary density effectively inhibiting crack propagation. The microstructure is predominantly composed of ferrite and bainite. The volume fraction of bainite, dislocation density, and the fraction of low-angle and high-angle grain boundaries resulting from dynamic recovery and deformation decrease from the surface to the mid-thickness. These lead to variations in mechanical properties along the thickness direction: the yield strength at the 1/4 and 3/4 thickness positions reaches approximately 450 MPa, with Charpy impact energy at -40 °C exceeding 250 J, while about 40 MPa and 100 J are reduced at the 1/2 thickness. The elongation after breaking consistently around 26%. Computational results of material properties demonstrate that optimizing austenite grain size in combination with cooling temperature and rate can effectively enhance the uniformity of microstructure and properties throughout the thickness. This study reveals that grain refinement through controlled rolling followed by extensive dynamic recovery and deformation is an effective approach for achieving high strength and toughness.