Internal fracture, known as Mannesmann effect, is a phenomenon regularly occurring in metal forming processes such as rotary tube piercing, cross-wedge rolling, etc. It is the main reason of reduced production quality and precision. Although there have been a number of publications on this topic, the scientific knowledge about fracture mechanisms leading to the phenomenon is still limited. In this study, the Johnson-Cook damage model is applied to investigate the internal material fracture in the 2D cross-wedge rolling process. The fracture mechanisms related to the Mannesmann effect are discussed in this article. Besides, it is also shown that the rolling velocity highly affects the appearance of surface defects and the occurrence of internal fracture in the billet.