Heterogeneous Nucleation on Inclusions/Precipitates in Ferrite Transformation of Steel

摘要

Intragranular ferrite nucleation on inclusions/precipitates in austenite has been used to refine ferrite+pearlite structure for improvement of toughness in low and medium carbon steels when deformation of the austenite matrix before transformation is hardly applicable [1] such as in the heat affected zone of a welded part. B1(NaCl)-type sulfides, carbides and nitrides have been used as nucleation sites in the intragranular ferrite transformation [2,3]. Inclusions are often formed before solidification and a steel containing precipitates sometimes undergoes forward and reverse transformations during cooling and reheating. The orientation relationship of such inclusions or precipitates with respect to the austenite matrix is non-specific and irrational in general. Thus, the intragranular ferrite transformation occurs by the ferrite nucleation on austenite/inclusion (or austenite/precipitate) boundaries with irrational orientation relationships. The present authors studied the microstructures of proeutectoid ferrite [4-6] and pearlite [7] nucleated on the irrational precipitate/austenite boundaries in alloys containing MnS and V(C,N). In this presentation, the kinetics, morphology and crystallography of intragranular ferrite transformation in Fe-0.2mass%C-(1.5 or 2) mass%Mn alloys containing small amounts of S, V and N are summarized.