Abstract: During the process of punching in a hot rolling plant,concave defect was found on the surface of a piercing plug which was used for only 50 times.The piercing plug was compared with another intact one used for 200 times from the aspects of physical testing and chemical anslysis.The results show that the structure became loose and the hardness declined due to the granular ferrite existing in the oxidation layer,which led to the concave defect on the surface of the piercing plug.