Effect of microstructural types on toughness and microstructural optimization of ultra-heavy steel plate: EBSD analysis and microscopic fracture mechanism

The uniformity of toughness along thickness direction has long been the critical problem for producing ultra-heavy steel plate. To clarify the rule and mechanism of the influence of microstructures on toughness, different microstructures and their mixtures have been obtained through various heat treatment processes. The microstructures and substructures were characterized by means of optical microscope and transmission electron microscope. Furthermore, the correlations between misorientation, grain size, microscopic fracture propagation and toughness have been studied in detail using electron backscatter diffraction. Results suggest that after tempering, lath bainite can achieve better toughness property than martensite, whereas granular bainite is detrimental for toughness. Note that firstly generated lath bainite can effectively refine subsequent martensitic packets and blocks, and increase barriers for fracture propagation. Compared with single-phase martensite microstructure, the mixture of martensite+bainite obtains finer substructure and more percentage of large misorientation, which are favorable for hindering the propagation of microcrack, meanwhile it is an ideal microstructural type to achieve the optimal combination of toughness and strength. Besides, functional mechanism of packet boundaries and block boundaries for hindering crack propagation is different, as it is more difficult for crack propagation to bridge between different lattice planes than between different crystallographic orientations.