Crystallographic orientation relationships between intragranular ferrites, the MnSϩV(C, N) complex precipitates acting as ferrite nucleation sites, and austenite matrix were studied in Fe-Mn-C alloys by scanning electron and transmission electron microscopy. VC holds a cube-cube orientation relationship (͗001͘ g // ͗001͘ VC ) when it is formed directly within austenite grains in an Fe-12Mn-0.8C-0.3V alloy. When VC precipitates nucleate on incoherent MnS particles dispersed in austenite, there is no specific orientation relationship between the three phases. Intragranular ferrite idiomorphs nucleating on the MnSϩV(C, N) complex precipitate in austenite in Fe-1.5Mn-0.2C and Fe-2Mn-0.2C alloys often hold the Baker-Nutting orientation relationship ((001) a //(001) V(C, N) , [110] a //[100] V(C, N) ). Although several irrational ferrite/ V(C, N) orientation relationships were observed, misorientation for either low-index planes or directions are relatively small between ferrite and V(C, N) for those relationships. The orientation relationships between intragranular ferrite and austenite were estimated by examining the misorientations between the ferrite and the neighboring martensite lathes from the Kurdjumov-Sachs inter-variant relationships. There is no specific orientation relationship between the intragranular ferrite idiomorph and the austenite matrix because of the low-energy orientation relationships between ferrite and V(C, N).KEY WORDS: phase transformation; precipitation; steel; austenite; ferrite; carbide; nitride; sulfide; inclusion; crystallography; interface.involved for intragranular ferrite transformation.In the present study, the detail of multiphase relationship between ferrite, MnSϩV(C, N) complex precipitate and austenite is discussed based on the observation using scanning and transmission electron microcopy. Table 1 shows the chemical composition of the alloys used in the present study. In an Fe-20Cr-10Ni austenitic alloy which contains a small amount of Mn and S, MnS is fully dissolved in austenite by the solutionizing at 1 473 K and precipitate during aging at 1 273 K based on the calculation with the equation proposed by Turkdogan. Experimental Procedure10) The solution temperatures of MnS in austenite for the other three alloys are well above the melting temperature of austenite, and thus, MnS cannot be dissolved in austenite after solidification. In an Fe-12Mn-0.8C-0.3V austenitic alloy which was used in the study on intragranular pearlite transformation, 7) V(C, N) can be dissolved into austenite by solutionizing treatments at high temperatures and precipitate during aging at lower temperatures. Thus, in this alloy, the measurement of precipitate/austenite orientation relationship was made both for coherent VC which precipitate directly in austenite as well as incoherent MnSϩVC complex precipitate. The solution temperatures of V(C, N) were estimated from the Thermo-Calc calculation. The specimens were homogenized at 1 473 K for 86.4 ks, cold rolled by 70 % and solution treated at 1 473 K...

Kinetics and crystallography of intragranular pearlite nucleated at the surface of (MnSϩVC) complex precipitate were studied in hypereutectoid Fe-Mn-C steels. The incoherent MnS embedded in the austenite does not act as a strong nucleation site of pearlite unless the transformation time is prolonged. The intragranular pearlite transformation is promoted effectively by the addition of vanadium (V). EPMA analysis showed that the intragranular pearlite nucleates on the (MnSϩVC) complex precipitate in the V-added alloy. As the transformation temperature decreases, the intragranular pearlite formation occurs more frequently. A single intragranular pearlite is composed of several colonies, indicating that multiple pearlite colonies nucleate on a (MnSϩVC) complex precipitate for intragranular pearlite transformation. There is no specific orientation relationship (OR) between ferrite in intragranular pearlite and austenite matrix while there is a specific OR (Pitsch-Petch OR) between pearlitic ferrite and cementite in the intragranular pearlite.

The effect of plastic deformation of austenite on the nucleation of pearlite was investigated using an Fe–12Mn–0.8C (mass%) alloy. Slight warm deformation of austenite prior to pearlite transformation effectively accelerates the intragranular nucleation of pearlite although intragranular pearlite is hardly formed without deformation. Formation of pearlite at annealing twin boundaries is promoted in the warm‐rolled specimens. Additionally MnS particles are activated as intragranular nucleation sites of pearlite. Cold rolling of austenite introduces many deformation twins in the alloy used. Subsequent isothermal transformation heat treatments result in nucleation of pearlite at intersections of the deformation twins. It is concluded that incoherent portions introduced by deformation onto the annealing or deformation twin boundaries are effective nucleation sites in the pearlite transformation.

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