High-strength surface layers with ultrafine grain steels generated by the intermediate billet controlled cooling method

Surface layers with ultrafine grain (SUF) steels possess excellent crack arrestability. But the strength of the current SUF steels is low and the grain ultrarefinement mechanism is still in doubt. In this study, a high-strength SUF steel has been fabricated by the combination of adding microalloying elements and intermediate-billet controlled cooling method. Microstructures of the developed SUF steel are characterized by optical microscope, scanning electron microscope, electron back-scattered diffraction analyses, and transmission electron microscope. Tensile property and toughness are also determined. The results show that a microstructure dominated by bainitic ferrite forms in the surface layers of the intermediate billet after controlled cooling. A 3.5 mm-thick laminated and ultrafine-grained microstructure is generated after finishing rolling. Thickness ratio of the ultrafine-grained layers is 28.0%. At 2 mm thickness position from the surface, the average grain size is 2.0 mu m. And the area fraction of recovered microstructure is 66.4%, which means dynamic recovery is the main grain ultrarefinement mechanism. The microstructure in the mid-thickness position is coarse-grained ferrite and pearlite. Fine and dispersed (Nb, V)C particles are distributed in the grains of ultrafine-grained layers and mid-thickness positions, respectively. The yield strength of the ultrafine-grained layers is 494 MPa, mainly caused by grain ultrarefinement. For the mid-thickness position, the yield strength is 424 MPa. The total strength of the developed SUF steel is larger than 440 MPa. An unexpected phenomenon in which toughness of the ultrafine-grained layers is lower than that of the mid-thickness position is mainly caused by delamination and low elongation. (C) 2022 The Author(s). Published by Elsevier B.V.