Crystallographic Reconstruction Study of the Effects of Finish Rolling Temperature on the Variant Selection During Bainite Transformation in C-Mn High-Strength Steels

Abstract

The effect of finish rolling temperature (FRT) on the austenite-(  ) to-bainite ( ) phase transformation is quantitatively investigated in high-strength C-Mn steels. In particular, the present study aims to clarify the respective contributions of the  conditioning during the hot rolling and the variant selection (VS) during the phase transformation to the inherited texture.To this end, an alternative crystallographic  reconstruction procedure, which can be directly applied to experimental electron backscatter diffraction (EBSD) mappings, is developed by combining the best features of the existing models: the orientation relationship (OR) refinement, the local pixel-by-pixel analysis and the nuclei identification and spreading strategy. The applicability of this method is demonstrated on both quenching and partitioning (Q&P) and as-quenched lath-martensite steels. The results obtained on the C-Mn steels confirm that the sample finish rolled at the lowest temperature (829°C) exhibits the sharpest transformation texture. It is shown that this sharp texture is exclusively due to a strong VS from parent brass {110}<1-12>, S {213}<-3-64> and Goss {110}<001> grains, whereas the VS from the copper {112}<-1-11> grains is insensitive to the FRT. In addition, a statistical VS analysis proves that the habit planes of the selected variants do not systematically correspond to the predicted active  slip planes using the Taylor model. In contrast, a correlation between the Bain group to which the selected variants belong and the FRT is clearly revealed, regardless of the parent orientation. These results are discussed in terms of polygranular accommodation mechanisms, especially in view of the observed development in the hot-rolled samples of high-angle grain boundaries with misorientation axes between <111>  and <110>  .

In Situ Synchrotron X-ray Diffraction and Microstructural Studies on Cold and Hot Stamping Combined with Quenching & Partitioning Processing for Development of Third-Generation Advanced High Strength Steels

“…Figure 30 shows the results of the nanoindentations superimposed on the results of EBSD combining the quality image, the grain orientation map (Figure 30a), and the Schmid factor (Figure 30b,c), for HSQ&P sample, intercritically austenitized at 800 • C, deformed at 800 • C by 30%, quenched at 318 • C and partitioned at 400 • C for 100 s. The Schmid factor [106] was calculated since this criterion indicates the deformation resistance of a particular region for a specific state of stress. Similar to the observations in Figure 27, ferrite and martensite grains oriented in the [107] direction show higher nanohardness than the grains oriented in the [001] and [101] directions. In Figure 30b,c, the grains with the highest Schmid factor are red, while the grains with the lowest Schmid factor are blue.…”

Texture formation in metal alloys with cubic crystal structures

“…31 As can be observed from the microstructure of Fig. 6, 32 high resolution reconstructed austenite phase maps can be generated, which reveal crucial information on the type of parent phase microstructure on the point of transformation. Such maps will allow for more accurate control of the hot rolling process, and hence a better control of the transformation texture.…”

“…4 Transformation potential versus the experimental transformation rate of 11 randomly chosen austenite orientations in a TRIP steel under tensile loading 19 6 Reconstructed austenite parent structure from EBSD scan of a product phase, finished rolled a below and b above the austenite Tnr temperature 32 5 Schematic representation of the process of texture memory. a two austenite grains with different KS relationships nucleating along a ferrite grain boundary, b the structure after complete transformation to austenite, c nucleation of a ferrite grain on cooling with YKS relationship to both neighbouring austenite grains, d final structure after displacive growth of the ferrite-the original grain orientation α 2 is restored 24 Kestens and Pirgazi Texture formation in metal alloys with cubic crystal structures Materials Science and Technology 2016 VOL 32 NO 13 in tension the active slip direction moves in the direction parallel to the tensile axis 32,33 (cf. Fig.…”

Nucleation Sites in the Static Recrystallization of a Hot-Deformed Ni-30 Pct Fe Austenite Model Alloy

“…[2] In accordance with this, SRX of austenite has been the subject of a wealth of experimental studies. These have mainly focused on the effect of deformation parameters on SRX kinetics and final grain size, [3][4][5][6] the evolution of crystallographic texture, [7][8][9][10][11] and that of substructure. [7] In the same way as phase transformations, recrystallization has often been understood as a combination of nucleation and growth.…”