Characteristics of Hot Ductility in Steels Subjected to the Melting and Solidification

Hot ductility in steels was studied. Special emphases were placed on the effects of thermal history, strain rate and fracture mode in order to clarify the sensitivity of surface cracking during both continuous casting operation and direct hot rolling.There exist three temperature regions where typical embrittlement is noticed, i.e., Tm-1200°C (I), 1200-900°C (II), and 900-600°C (III). The cause of the embrittlement in the region I is the existence of residual liquid film along the dendritic interfaces. The ductility is found to be independent of the strain rate. In the region II, the precipitation of finely distributed oxy-sulfides at the austenite grain boundary weakens the boundary strength, and thus overaging treatments such as slow cooling, holding for certain time, or slow rate of straining result in good ductility. On the other hand, the embrittlement in the III region is manifested by the slower strain rate of test. Controlling factors of this embrittlement are precipitation of oxides, sulfides and nitrides, precipitation of proeutectoid ferrite film along austenite grain boundary as well as grain boundary sliding. Detailed mechanism is discussed.