It is still a big challenge to obtain excellent low-temperature toughness for bulk steel materials. Delamination is an effective method to improve low-temperature toughness. In the present study, delamination toughening in a low carbon microalloyed steel plate with elongated and ultrafine-grained microstructure rolled in the dual-phase region has been investigated in detail. When toughness was measured along normal direction, the steel plate had a high upper shelf energy and no delamination occurred in the upper shelf region. A large delaminated crack parallel to rolling plane started to appear and changed the propagation path of main crack when testing temperature was lower than −60 °C. We find this kind of delamination induces a second upper shelf in the Charpy transition–temperature curve. The second upper shelf, reaching up to 300 J in the temperature range of −60 °C to −140 °C, results in excellent low-temperature toughness for the steel plate, and the ductile-brittle transition temperature is lowered to −157 °C. The developed steel plate also has high low-temperature toughness measured along transverse direction due to delamination. The effect factors on upper shelf energy, delamination mechanism and delamination toughening are discussed.