Control of Transformation Microstructure in Steel by use of Inclusions

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Proeutectoid ferrite (bcc α) nucleation on the Bl (NaCl)-type complex inclusions/precipitates (e.g. oxide, sulfide, carbide, nitride) dispersed in austenite (fcc γ), denoted as intragranular ferrite transformation, has been used to refine the microstructure in the heat affected zone to improve the toughness of low and medium carbon steels. The present study aims to study the ability of various complex inclusions/precipitates (MnS, MnS+VC, MnS+V(C,N)) in austenite as nucleation sites in proeutectoid ferrite transformations by electron microscopy.In the presence of MnS+V(C,N) complex precipitates, intragranular ferrite nucleation is markedly accelerated. V(C,N), which usually holds a cube-cube orientation relationship (<001>ィイD2γィエD2//<001>ィイD2vcィエD2) when they forms within austenite grains, does not exhibit any specific orientation relationship with respect to austenite matrix when they nucleate on incoherent MnS. Many of intragranular ferrite crystals apparently do not hold a specific orientation relationship with V(C,N) and austenite. These crystallographic analyses revealed that the randomization in the matrix/precipitate orientation relationships occurs in the precipitation at incoherent interphase boundaries.