Phases transformation textures in steels

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1 J. Phys. IV France 0 (004) EDP Sciences, Les Ulis DOI: 0.05/jp4: Phases transformation textures in steels C. Cabus,, H. Regle and B. Bacroix IRSID-CMC, Voie Romaine, BP. 3030, 5783 Maizières-lès-Metz Cedex, France LPMTM-CNRS, Institut Galilée Université Paris Nord, Avenue J-B. Clément, Villetaneuse, France Abstract. Low-carbon steels used for deep-drawability applications have properties which depend greatly on their crystallographic texture. It is therefore important to control the texture evolution during the thermomechanical processing. Until recently, little attention has been paid on the understanding of the textures formation after hot-rolling, which are produced by phase transformation, although it is recognised that they have an effect on the development of the texture in the further process (cold rolling and annealing). Indeed, one of the main difficulties consists in the measurement of texture above ambient temperature, in the austenite range. In the present work, EBSD technique is employed on a low-c steel and a method is proposed to determine local austenite orientation thanks to martensitic one, even if there is no residual austenite in the steel. The orientation relationships between the austenite phase and each of its product phases, here martensite and polygonal ferrite, are analysed and compared. Common Kurdjumov Sachs variants are detected for both phases. Variations in the intensities of these variants are also detected and could be due to the different phase transformation mechanisms, diffusion or shear.. INTRODUCTION The deep-drawability properties of steel sheets are mainly controlled by their crystallographic textures. In the past years, the interest has been mainly focused on the deformation and recrystallisation texture. However the texture of the hot-rolled sheet has an effect on further development of the texture, particularly if low cold-rolling levels are applied. Moreover, development of new kind of steels like TRIP steels, which microstructure is formed during phase transformation, makes it particularly important to understand how these textures form and how we can control them. The aim of this work is to contribute to the understanding of the textures resulting from a transformation from the austenite to the ferrite phase. A brief survey of the literature concerning the orientation relationships between these phases is given in a first part of the paper. Then a method is proposed to determine the previous austenite grain orientation in a low-carbon steel containing no residual austenite. Finally, the orientation relationships between austenite and polygonal ferrite or martensite are analysed and discussed.. ORIENTATION RELATIONSHIPS IN STEEL When a phase transformation takes place in steel, an orientation relationship exists between the lattices of the parent and the produced crystals. The relationships the most frequently cited are Bain [], Kurdjumov-Sachs (K-S) [], Nishiyama [3] and Wassermann [4] (N-W). Their principal characteristics are given in table, in terms of coincidence between plane and direction of austenite () and ferrite phases ().

2 38 JOURNAL DE PHYSIQUE IV Table. Orientation relationship between and Orientation relationship Expression Numbers of variants Corresponding rotation Bain {00} // {00} 3 45 around <00> <00> // <0> Kurdjumov-Sachs {} // {0} 4 90 around <> <0> // <> Nishiyama-Wassermann {} // {0} <> // <0> (ou <0> // <00> ) 95,3 around <h, k, l>* * h 3, k 3, l.. Bain relationship When transforms into according to the Bain relationship, the austenite lattice is compressed along the [00] axis of the austenite and elongated along the other two axes, as illustrated in figure. a [00] a / a a a Figure. Schematic of the Bain transformation. The crystal orientation of after 45 rotation around the common [00] axis corresponds to that of. As it is also possible to choose between two other axis ([00] or [00]), the Bain relationship involves 3 variants. The K-S and N-W relationships are less simple as they lead to 4 or variants respectively... Kurdjumov-Sachs and Nishiyama-Wassermann relationships For both the K-S and N-W crystallographic relationships, it is assumed that a close-packed plane {} of the austenite is parallel to a close-packed plane {0} of ferrite, as illustrated on figure. Figure. Parallelism between () and (0).

3 ICTPMCS 39 Kurdjumov-Sachs relationship : A close-packed plane {} of the austenite is parallel to a closepacked plane {0} of ferrite. There are 4 possibilities of {} plane. Among each of these planes, a close-packed direction of austenite <0> (3 possibilities of <0> direction in a {}plane) is parallel to a close-packed direction of ferrite <> ( directions <> in a {0} plane). Hence, as it is showed in table, there are 6 variants of K-S for each {} plane. Nishiyama-Wassermann relationship: As in the KS-OR, there are 4 possibilities in the choice of the {} plane. There are 3 possibilities of <> directions in each {} plane but contrary to K-S relationship, there is only one possibility of ferrite <0> direction lying in a {0} ferritic plane (illustrated table ). This explains that there are twice less N-W variants than in K-S case. Table. Variants of K-S and N-W relationships. K-S N-W [ 0 ] [ 0 ] [ 0 ] [ ] [ ] [ ] [ ] [ 0 ] [ ] [ ] [ 0 ] [ 0 ] [ ] [ ] [ ] So, the K-S and N-W relations involve the same parallelism between planes and there is a difference of only 5,3 between the parallel directions. A schematic {00} pole figure of the bcc variants formed from a (00)[00] oriented fcc crystal for the 3 relationships is presented in figure 3. (00) (00) (00) (00) Variants Bain variants de Bain - austenite o Variants Kurdjumov-Sachs de Kurjdumov-Sachs variants Figure 3. Representation of the variants on a {00} pole figure. Variants Bain de Bain variants - austenite O Variants de Nishiyama-Wassermann variants

4 40 JOURNAL DE PHYSIQUE IV However, it is to be noticed that the transformation of austenite to ferrite frequently involves only a restricted set of the K-S (or N-W) variants. This phenomenon is known as variant selection. This is the first explanation to the discrepancies which are usually observed between experimental and simulated transformation textures. Another explanation is the selective growth of certain orientations, which will be reviewed in more detailed in the 4 th section. 3. EXPERIMENTAL RESULTS 3.. Materials The steel with the chemical composition listed in table 3 was reheated, hot rolled and quenched according to the process conditions illustrated by the figure 4. The amount of strain done at 840 C has been varied between 0 and 0.8, to test the influence of deformation on the phase transformation kinetics and microstructures [5]. After the final quenching, the microstructure contains a mixture of polygonal ferrite and martensite. Table 3. Chemical composition of the steel used, wt-%. C Mn Si Al N S P Ar 3 ( C) Ae 3 ( C) C, 5 min 0 C /s 840 C 700 C 50 C /s min Figure 4. Hot rolling schedule. Specimens were then analysed by EBSD to compare the orientation relationship that each ferrite phase has with the initial austenite. EBSD maps were obtained on a Jeol SEM equipped with an automatic EBSD system, by beam scanning and with fine grid steps (between 0, and 0,5m). 3.. Determination of austenite orientation thanks to martensitic one One of the first problems is that, for this chemical composition and this thermomechanical treatment, no initial austenite is retained in the final microstructure, as it could have been the case for higher carbon content TRIP steels. The following procedure shows however how we can deduce the austenite orientation from the orientations of the martensite even though there is no more austenite. The EBSD investigation of the sample, hot rolled at =0,8 is represented in figure 5. The orientation cartography of figure 5-a shows that it contains both polygonal and martensitic ferrite. The two constituants, which have the same bcc crystallographic structure, can however be distinguished in the cartography by their different quality index. Indeed, the sharpness of the diffraction patterns in martensite are lower, probably due to its higher dislocation content and induces then a lower quality index of pattern recognition by the automatic system.

5 ICTPMCS 4 martensite polygonal ferrite 5 m a b c Figure 5. EBSD orientation cartography (a) and corresponding pole figures for martensite (b) and polygonal ferrite (c). Looking at the {00} pole figure (PF), it seems that the transformation from austenite into martensite follows a relationship close to K-S one. This is particularly clear when comparing the experimental PF with the one of Fig.3: we see three circles made by <00> directions, typically found around the previous <00> directions according to KS. All the variants are present although it does not mean that there is no variant selection since the intensity of each of these variants can be different, and that may not be seen on this representation in point mode. The 3 centers of the 3 circles are 90 away from each other and they determine a fourth orientation, underlined by the three crosses in the figure 6. This is the orientation of the austenite grain from which the martensite comes. It is easy to determine it thanks to these three circles on the {00} pole figure. Starting back from this orientation of austenite we can calculate the transformation products by K-S and as we can see the experimental pole figure and the calculated one are very similar. This procedure allows then, thanks to the martensite crystallographic orientations, to determine the orientation of the previous austenite grain, even if there is no residual austenite in the steel. This method was used previously by different authors [6] Comparison between polygonal ferrite and martensite orientations The Fig. 7 shows the same map and the corresponding martensite and polygonal ferrite pole figures in level of intensity. We can notice that the {00} pole figures of martensite and polygonal ferrite are different, but that they have one common orientation (marked by the triangles). This seems to show that when austenite transforms into ferrite or martensite, common texture components are produced although ferrite is formed by diffusion and martensite by a shear mechanism. Figure 6. Experimental pole figures and simulated one.

6 4 JOURNAL DE PHYSIQUE IV martensite ferrite polygonale Bain variants o Kurdjumov S h i 5 m Figure 7. EBSD map and corresponding pole figures of martensite and polygonal ferrite. More precisely, we observe that grains of polygonal ferrite have often an orientation similar to one of the orientations of adjacent martensite grains. Fig. 8 represents the EBSD results of the sample hot rolled at = 0,4. On this map we can see polygonal ferrite which has nucleated in previous austenite grain boundary regions and all around martensite laths. Moreover we can especially see that a polygonal ferritic grain which seems to grow into one side of the old austenic grain boundary has one common orientation with the martensite laths on the other side of this boundary. 5 m Figure 8. Common orientation between a ferritic grain and one of the martensitic variants. 4. DISCUSSION An explanation to these common orientations between martensite and polygonal ferrite, based on the transformation temperature is proposed by Savoie and al [7]. As martensitic transformation is nearly instantaneous, martensitic texture is essentially determined by nucleation. On the contrary, polygonal ferrite is formed on a more slowly way by diffusion. After nucleation, the ferritic grains grow by grain

7 ICTPMCS 43 boundary migration at different speed. So, the nucleation texture can evolve by this phenomenon of selective growth and the final texture can be different from the nucleation one. According to this analysis, in the case of polygonal ferrite, if the transformation temperature is high enough, grain growth becomes important and so the texture can evolve during growth. But if ferrite transformation takes place at lower temperature, the ferrite retains its nucleation texture, which is similar to martensite texture. Hutchinson [8] has also observed similarities between bainitic and polygonal ferritic textures in a TRIP steel. His theory is sketched in figure 9. T 3 4 We consider 4 austenite grains,, 3, 4. Nuclei, which develop in the grain boundary regions, are in KSrelationship with one of the two adjacent grains. In this example, say that is in K-S relationship with. 3 4 Growth of polygonal ferrite This nucleus will grow in both adjacent grains: In, the interface is incoherent since the nucleus has no special orientation relationship with this grain. The mobility of this interface is then higher than the mobility of the interface with, with which the nucleus has a KSrelationship. As a consequence the growth of polygonal ferrite will be faster in than in. 3 4 Growth of martensite In the nucleus is coherent with one of the austenite lattice since it has an orientation relationship, namely K- S, with this grain and we can suppose a displacive growth since the temperature decreases. Nevertheless the martensite formed in has the same orientation that the polygonal ferrite formed in. Figure 9. schematisation of phase transformation. Inagaki [9] has also observed on a low carbon Nb steel that in many cases ferritic and adjacent martensitic grains had similar orientations.

8 44 JOURNAL DE PHYSIQUE IV We can synthesise this approach thanks to figure 0: Considering the polygonal ferrite and the martensite contained inside a previous gamma grain ( ): the martensite orientations inside should be in KS relationship with this grain and only this grain whereas polygonal ferrite which nucleated near the grain boundary statistically comes from the neighbouring grains and has no KS-relationship with. The orientations of these polygonal ferrite grains are variants of K-S relationship from, 3, or Figure 0. Synthesis of the approach by nucleation and growth. 5. CONCLUSION The ferrite products resulting from the phase transformation from austenite have been measured by EBSD in a SEM. A method is used to determine the orientation of the previous austenite grain from the martensite orientations, in this low carbon steel where no retained austenite is present at room temperature. The orientation relationships between austenite and polygonal ferrite or martensite are well described by the KS-relationship. It has been found that the polygonal ferrite tends to develop in the austenite grain adjacent to the grain where it nucleated. References [] E.C. Bain: The nature of martensite, Trans. AIME, 70 (94), p. 5. [] G. Kurdjumov and G. Sachs: Z. Phys., 64 (930), p. 35. [3] Z. Nishiyama: X-Ray investigation of the mechanism of the transformation from face-centred cubic lattice to body-centred cubic, Sci. Rep. Tohoku Imp.Univ. 3 (934/935), p [4] G. Wassermann : Arch. Eisenhüttenwes., 6 (933), p [5] S. Lacroix: PhD internal report, oct. 00. [6] M. Humbert and N. Gey: The calculation of a parent grain orientation from inherited variants for approximate (b.c.c.-h.c.p.) orientation relations, J. Appl. Cryst. (00). 35, p [7] J. Savoie, R.K. Ray, P. Butron-Guillen, and J.J. Jonas: Comparison between simulated and experimental transformation textures in a Nb microalloyed steel, Acta Mater., 4 (994), p. 5. [8] B. Hutchinson, L. Ryde, E. Lindh, K. Tagashira: Texture in hot-rolled austenite and resulting transformation products, Material Science and Engineering A57, 998, p [9] H. Inagaki: Nucleation of the proeutectoid ferrite and its role in the formation of the transformation texture in a low carbon steel, Z. Metallkde. 78 (987).