It is a common wisdom that metallic materials become brittle once being deformed quickly. However, here we reveal an abnormal strain-rate-induced brittle-ductile-delamination transition in a widely used pearlitic steel with unique structure of alternative arrangement of nanoscale ductile ferrite and brittle cementite through extensive molecular dynamics simulations. In contrast to the brittle cleavage fracture in conventional crystalline alloys, the brittle fracture in pearlitic steel at relatively low strain rate is mediated by the nanoscale cavitation ahead of crack tip, akin to the widely observed fracture mode in metallic glasses. As the strain rate increases, fracture mode transforms to a dislocation nucleation mediated ductile mechanism. At extremely high strain rate, it is found that the fracture mode turns to be collective delamination at the interfaces, leading to a surprising "delamination toughening". The abnormal brittle-to-ductile transition with increasing deformation rate is physically rationalized by a mechanistic model, which is based on a scenario of energetic competition between the interface cleavage and the dislocation nucleation in the vicinity of crack tip. Once the strain rate exceeds a critical value, fracture transitions to dislocation nucleation dominated. When strain rate increases to extremely high values, there is no enough time for either crack propagation or dislocation nucleation, and the collective delamination of interfaces occurs which involves only instantaneous bond breaking at weakly bonded regions, i. e. the interface. The unravelled phenomenon challenges the conventional knowledge of materials deformation and failure which might shed light on coordinating unanticipated utilities of the ultrastrong pearlitic steels in extreme environments.