The Effects Of Strain Path Reversal On Austenite Grain Subdivision Recrystallisation And Phase Transformations In Microalloyed Steel

The Effects of Strain Path Reversal on Austenite Grain Subdivision, Recrystallisation and Phase Transformations in Microalloyed Steel

Effects of Strain on Austenite to Ferrite Transformation Behaviour of a NB-microalloyed Steel

Dynamic Transformation of Austenite to Ferrite Above the Ae3 Temperature

"The dynamic transformation (DT) of austenite to ferrite above the Ae3 temperature was studied in a plain C-Mn and a Nb microalloyed steel. Compression and torsion tests were carried out over the temperature range 880 °C to 1350 °C applied at a strain rate of 1 s-1. The deformation temperatures were all above the ortho and para-equilibrium Ae3 transformation temperatures. These experiments were performed in a 95%Ar - 5%H2 atmosphere (on the MTS machines) and in a vacuum (Gleeble 3800 system), and all the samples were water quenched immediately after straining. In this work, the optical and electron microscopy images provided evidence of the presence of dynamically formed ferrite plates. These plates have near identical orientations and coalesce into polygonal grains upon continued straining.The occurrence of dynamic transformation is explained here by introducing the concept of transformation softening. The net softening during transformation is considered as the driving force for the phase change. This is given by the difference between the flow stress of the work hardened austenite at the critical strain and the yield stress of the fresh Widmanstätten ferrite that takes its place. The latter is estimated from hardness data. The obstacle to thetransformation consists of three parts: i) the Gibbs energy difference between austenite and Widmanstätten ferrite, ii) the shear accommodation work that is imposed on the neighborhood by the transformation, and iii) the dilatation work associated with making room for the lower density ferrite. This approach was validated using test results on a plain C-Mn steel compressed at temperatures well above the Ae3. It is shown in this way that DT ferrite can be formed at any temperature within the austenite phase field. These observations are consistent with the calculated Gibbs energy difference between the austenite and ferrite, which displays a peak approximately midway between the Ae3 and the delta ferrite formation temperature.The present thermodynamic analysis can also be used to determine the effect of alloying element addition. In order to assess the effects of Mn and Si addition on the transformation, the high temperature flow stress data obtained by Wray (1984) by means of tensile tests on four Fe-Mn-Si alloys were employed to identify how these alloying additions affect the driving and opposing forces. The present model shows that the addition of Mn increases both the driving force for transformation as well as the energy barrier to ferrite formation. Conversely, while Si addition increases the driving force, it simultaneously decreases the free energy obstacle to the transformation. This then increases the temperature range over which DT can take place. Torsion simulations of strip rolling were performed on a C-Mn and a Nb microalloyed steel to determine the effect of DT on the rolling load. Pass strains of 0.4 were applied at a strain rate of 1 s-1 with interpass times in the range 0.5s to 5s. For additional realism, simulations were carried out under both isothermal and continuous cooling conditions. It was observed that the flow stress levels and MFS's decrease significantly during the simulations. This softening is associated with the formation of ferrite. Optical microscopy revealed that the volume fraction of DT ferrite increased continuously from pass to pass. The fraction of DT ferrite formed and retained was significantly higher when short interpass times were employed. When long intervals were used, the ferrite tends to retransform back into austenite. These observations indicate that the dynamic transformation of ferrite occurs displacively while the retransformation back into austenite takes place by a diffusional mechanism. The nucleation and growth of the ferrite reduce the rolling load and modify the microstructure. Comparison of the behaviors of the C-Mn and Nb steels indicates that Nb addition retards both the forward as well as the reverse transformation." --