Formation Behavior of Proeutectoid Ferrite and Hot Ductility in (α+γ) Two Phase Region in Low Carbon Steels

Hot ductility of low carbon steels containing small amount of Nb or B was investigated with a Gleeble tensile machine. Special emphases were placed on the relation between the formation behavior of proeutectoid ferrite and the ductility in (α+γ) two phase region in the specimens with different austenite grain sizes (100 μm and 350 μm). The morphology of statically precipitated ferrite changed with an austenite grain size. In the case of coarse austenite grains, film-like ferrite was formed along austenite grain boundaries, whereas globular ferrite was formed in the specimen with small austenite grain size. Even when the specimen was single phase austenite before tensile testing, ferrite was formed during tensile testing at a certain temperature range below Ae3. This dynamically precipitated ferrite exhibited very thin film-like network along austenite grain boundaries, irrespective of austenite grain size. These morphology and distribution of ferrite affected the hot ductility in (α+γ) two phase region. The ductility became minimum under the condition in which the network structure of very thin film-like ferrite was formed. Such a structure was commonly obtained by the occurrence of dynamic precipitation of ferrite. The ductility in (α+γ) two phase region was markedly influenced by austenite grain size, namely, it decreased with increase in austenite grain size.