The superior low temperature toughness in a low-carbon microalloyed Q500 steel by finish rolling in dual-phase region

The superior low-temperature toughness in the low-carbon microalloed Q500 steel is obtained by finish rolling in the dual-phase (Austenite + Ferrite, A+F) region without sacrificing the yield strength. In comparison with Steel Ⅰ finish-rolled in the austenite non-recrystallization region (ANR region), the impact energy at -60 °C of Steel Ⅱ has been improved by 174 J, reaching 226 J and the yield strength has also been enhanced by 63 MPa. By visualization and quantification of crystallographic structures, the differences of low-temperature toughness can be ascribed to different variant selection mechanisms and uniformly dispersed M-A constituents. The weakening of variant selection occurs in Steel Ⅱ finish-rolled in the A+F region. More high-angle packet boundaries contributing to high toughness can be obtained, especially the boundaries of V1/V10 (&V14) and V1/V18 (&V22). Then again, the large density of high-angle grain boundaries (HAGBs) can serve as high-speed channels for carbon diffusion. Therefore, M-A constituents are dispersedly distributed within the ferrite with small size, which are also beneficial to enhance the low-temperature toughness.