| An investigation was performed to examine the deformation behavior and the fracture mechanisms of Bi-S free machining steels at elevated temperatures. Especially, focus was paid on the effects of hot rolling and Bi content on the hot ductility and the fracture behavior of the present free machining steels. In order to find the effect of a hot rolling. experiments were conducted on the as-casted state and the as-rolled state, and their high temperature deformation behavior was compared Under the present experimental conditions, the as-rolled state exhibits better hot ductility than the as-casted state due to its homogeneous matrix microstructure. However, the hot ductility in both the as-rolled steels and the as-casted steels decreased with decreasing temperature. This ductility loss would be attributed to the embrittlement which is induced by the existence of a liquid metal, Bi, at testing temperatures. It was found that the hot ductility in the as-rolled steels at the temperature range from 1100℃ to 1050℃ primarily depended on the ductility of matrix, at temperature range from 1050℃ to 950℃ primarily depended on the absolute Bi content, and below 900℃ primarily depended on the liquid Bi film on the grain boundary rather than the absolute Bi content. The as-rolled state exhibited the transition of fracture mode from ductile dimple fracture to brittle fracture at a reduction of area of about 60%. The above results were discussed in the light of the mechanisms of a liquid metal induced embrittlement as well as the fracture mechanisms of low carbon steels. |
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