Regular Article
Recrystallization of Isolated Deformed Grains in High Purity Iron
2015 Volume 55 Issue 4 Pages 877-883
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2015 Volume 55 Issue 4 Pages 877-883
Early stage of recrystallization behavior was investigated in high purity deformed iron with isolated crystal grains to elucidate the mechanism of recrystallization. Recrystallization of the isolated deformed grains was greater than that of the bi-crystal matrix. The recrystallized grains were observed in the RD×ND section at the tip portion of the deformed isolated grains extending in the rolling direction. Recrystallized grains were also observed inside the deformed isolated grains. The crystal orientations of the recrystallized nucleus were positioned where they were in the perimeter portion of the orientation spread of the deformed matrix, or there may have been the intermediate orientation between the matrix and the isolated grains.
Recrystallization is a process by which new grains with very few dislocations are nucleated and grown from a deformed matrix with a high dislocation density by deformation such as that induced by cold rolling. Many studies have been done to clarify the recrystallization mechanism in steels, such as the orientation relationship between the matrix and the nucleus. Broadly speaking, recrystallization mechanisms can be explained by two theories:1) the oriented growth theory2,3,4,5,6,7,8,9,10,11) and the selective growth theory.12,13,14,15,16) According to the former, the recrystallized nuclei are already present even in the cold-rolled states and the preferred orientation of some special nuclei determines the final recrystallization texture. According to the latter theory, the recrystallized texture is related to CSL boundary, in the case of steel, the Σ19a CSL boundary (about 26.5° rotation of the <110> axes).16) Since the main texture component {112}<110> in cold rolling of steel has the Σ19a CSL boundary relationship with the typical recrystallization main orientation {554}<225>, the selective growth theory has been widely accepted in recrystallization in cold-rolled steels. Lee17) proposed a recrystallization mechanism where the maximum stress direction would become the minimum elastic modulus on recrystallization.
Basic research on recrystallization has been conducted using both bi-crystals and single crystals.9,10,11) Practically, from an engineering point of view, recrystallization should be treated in polycrystalline materials, but since the phenomenon is very complicated to study due to the orientation relationship between grains across the grain boundary, the shape of the grain boundary, and local strain due to cold-rolling, it is very difficult to examine the recrystallization mechanism in detail using polycrystalline materials. Inoko et al.18) have attempted to elucidate the recrystallization mechanism using bi-crystals with the orientation relationship between the two crystals being varied systematically. Tsuzaki et al.10) have also pointed out the importance of the bi-crystal boundary direction to obtain the <111>//ND recrystallized texture in steel, which is favorable for drawability.
Incidentally, it was well known that inclusions or second phases stimulate recrystallization but that a random texture can be formed from the circumference of inclusions at the interface boundaries.19) The effect of the second phase on the surrounding local deformation was simulated using FEM.20)
It is believed that to clarify the recrystallization mechanism, it is preferable to use polycrystalline, which has the same phase as the parent phase with a grain boundary curvature close to that of polycrystalline.
In this paper, focusing on an isolated island-like grain left behind inside the coarse grains obtained by a pre-strain and annealing process, recrystallization behavior of the isolated rolling grain was examined, a previous preliminary investigation having been done.21) Observation of the periphery of deformed island-shaped grains revealed the formation of a deformation zone, in which the crystal orientation gradually changed, and had a series of orientation change. Although the deformation band had orientation dispersion, the recrystallization nuclei had orientations on the rotation sequence. In the former study, however, the orientation of the deformed island particles was not clear, and the positions of the recrystallized grains around the deformed grain were unclear in the observations of the TD×RD section.
In this study, the initial stage of recrystallization process was precisely investigated with additional observation of the RD×TD section.
A sheet bar from vacuum-melted steel containing electrolytic iron was prepared in this study. The initial material having coarse grains with dispersed fine isolated grains was prepared as in.22) Table 1 shows the chemical composition of the sample used. High purity with single-ppm or less together with C, N was obtained. As shown in Fig. 1, the specimen was cut so that the grain boundary of bi-crystal grains was approximately parallel to the rolling direction. The resulting material is termed initial material hereafter.
| (mass%) | ||||||
| C | Si | Mn | P | S | Al | N |
| <0.0005 | <0.0008 | <0.002 | <0.002 | 0.001 | <0.001 | <0.0005 |
Initial material before cold rolling showing grain boundary of bi-crystal grains parallel to rolling direction.
The initial material was subjected to cold rolling with a reduction of 75%. The finishing thickness was 0.385 mm. The cold-rolled sheets were subjected to heat treatment of 400–600°C for 20 s in a alumina fluidized furnace.
The final cold-rolled material and the final annealed material were subjected to optical microscopy and measurement of crystal orientation in the TD×ND and RD×ND cross sections by electron back scatter diffraction pattern (EBSD) measurements using software of TexSEM laboratories. The step size of EBSD measurement was 0.5 μm in a hexagonal grid.
Figure 2 shows an Image quality (IQ) map in the ND×TD section of the base initial material before cold rolling. The specimen included a grain boundary that divided two large coarse grains and small island grains. The bi-crystal grain boundary extended through the plate thickness although it was not a straight line. The coarse grain boundaries on small island-shaped crystals were scattered on the matrix. The average grain size of the small island-shaped grains was approximately 30 μm.
Image quality (IQ) map (a) and the inverse pole figure (IPF) map (b) of the ND direction of the base initial material before cold rolling in the ND×TD section. Figure 2(c) is a pole figure showing the orientations of two coarse grains.
The crystal orientation of coarse grains A and B measured by EBSD were {φ1, Φ, φ2} = {85.0°, 62.6°, 54.1°} and {23.5°, 113.1°, 288.9°} in Bunge Euler angles representation, respectively. The misorientation between these two coarse grains was 27° with a rotation axis of <15, 11, –3>.
3.2. Recrystallization BehaviorExamples of observations of isolated deformed grains and recrystallized grains in the deformed matrix grain A in the RD×ND cross section are shown in Fig. 3. Grain A which had a crystal orientation of near <111>//ND, had a greater advantage than grain B in recrystallization. This can be explained as being due to accumulated stain with cold-rolling, as in the index such as the Taylor factor. The figures show the image quality (IQ) map and the inverse pole figure (IPF) map of the ND direction. Figures 3(a) and 3(b) are the maps at a heating temperature of 400°C. There were no recrystallized grains shown by the IQ map, but an isolated particle was apparently divided into small sub-grains. The surrounding matrix was also locally deformed. In particular, the IQ value of the lower part of the isolated grain was quite low. On the other hand, Figs. 3(c) and 3(d) are at 600°C. In this case, deformed isolated grains were not observed, but a group of recrystallized grains were observed to slightly extend in the rolling direction. From observations at the low annealing temperature, since isolated deformed grains or bi-crystal grain boundaries were the preferential recrystallization site, recrystallization of isolated deformed grains occurred and the recrystallized grains scavenged the isolated deformed grains. Since this study was concerned with the recrystallization mechanism, analysis of the initial stage of recrystallization at an annealing temperate of 550°C or below is focused on the following sections.
Image quality (IQ) map and the inverse pole figure (IPF) map of the ND direction of isolated deformed grains and recrystallized grains in the deformed matrix grain A in the RD×ND cross section. Figures 3(a) and 3(b) are maps at a heating temperature of 400°C. Figures 3(c) and 3(d) are at 600°C.
Figure 4 shows a microstructure near the grain boundary of bi-crystal grains by heat treatment at 550°C. As seen in Fig. 2, the grain boundary before rolling was slightly curved rather than a straight line, and the bending of the boundary was emphasized after rolling; it became saw-toothed. Focusing on the matrix bi-crystal, the orientation spread in the grain A side was a particularly large compared with that of the grain B side; a deformation band was also observed inside the deformed matrix grain A. The IQ value was quite low at the lower part of the saw tooth in which grain B was protruding into grain A. Recrystallized grain C was observed at a largely deformed part in which the crystal orientation change was large. Figure 5 shows the (200) pole figures. Figure 5(a) is the one from the overall area in Fig. 4. There was an orientation spread along the route from the grain A orientation to the grain B orientation before rolling. Figure 5(b) shows the pole figure from the area D part in Fig. 4. Compared with the initial crystal orientation, the orientation of the two grains became closer to each other as the result of the rolling although the orientation dispersion in the grain A side was particularly large. Figure 6 shows plots of the misorientation distribution on the three lines in the RD direction. In line 1X, the orientation spread of the grain A was relatively small; there was a large misorientation at bi-crystal grain boundary. On the other hand, there was large orientation change at the side of the grain A toward the bi-crystal grain boundary in line 1Y. The orientation in grain A was gradually rotated with local cyclic fluctuations in a fine pitch due to some deformation bands such as intragranular shear bands; misorientation at the bi-crystal grain boundary in line 1Y was also high as in line 1X. A small isolated deformed grain was seen at the grain B side, but recrystallization did not occur around it.
Image quality (IQ) map (a) and the inverse pole figure (IPF) map (b) near the grain boundary of bi-crystal grains with heat treatment at 550°C. Figure 4(c) is an orientation map of orientation of the recrystallized grain with a tolerance angle of 15°.
(a) is the (200) pole figure from the overall area in Fig. 4. Figure 5(b) shows the pole figure from the area D part in Fig. 4. Figure 5(c) is calculated orientation path assuming rolling. The circle marks in the pole figure corresponds to the initial bi-crystal orientation as shown in Fig. 2.
Plots of misorientation distribution on the three lines in the RD direction.
Line 1Z was a case in which there was a recrystallized grain C. The recrystallization occurred at the grain A side of the bi-crystal grain boundary in which the orientation spread and fluctuation in fine pitch was also observed as in line Y. The misorientation between the matrix grain A and the recrystallized grain was about 20°; such misorientation was also observed in a similar region in line 1Y. In Fig. 4 (c), crystal orientation is shaded by the degree of misorientation from the orientation of the recrystallized grains C. The tolerance angle is 15°. The dense red color means that the crystal orientation is closer to the recrystallized grain C. It can be seen that there were the similar crystal orientation with the recrystallized grain C in the region which was the lower part of the overhanging of grain B, and had a low IQ value.
3.4. Recrystallization of Isolated Deformed GrainsNext, attention was paid to the recrystallization process of isolated grains. Figure 7 shows one example that was observed in the matrix grain A in the RD×ND plane heat treated at 550°C. Several recrystallized grains were observed inthe part of the tip portion of the isolated deformed grain which extended in the rolling direction, or at the grain boundary bent in the lower side the isolated deformed grain. The IQ value inside of the isolated grain was quite low compared with that of the surrounding matrix grain. This heavily deformed region extended to the matrix at the tip of the isolated grain. Crystal orientation of the matrix surrounding the isolated deformed grains greatly varied; it changed to a direction close to (<100>//ND) as shown by red in the IPF map. The deformation bands in the matrix grain were observed more clearly in lower part of the isolated deformed grain and the inclination angle of the deformation bands was approximately 30° to the rolling direction. Shear bands were also observed inside the isolated internal deformed grain. The shear direction inside the isolated grain was, however, opposite that of the inter-granular shear band in the matrix grain.
Image quality (IQ) map (a) and the inverse pole figure (IPF) map (b) of the ND direction in the matrix grain A in RD×ND plane heat treated at 550°C.
Figure 8 shows the distribution of misorientation in each line shown in the Fig. 8(d). The left end point was the basis for calculating the misorientation in the case of the horizontal line; the upper point in a case of the vertical line. Line 2X was intended to be viewed in the longitudinal direction through the isolated grain; it was found that the crystal rotates sharply around the isolated deformed grains. Lines 2Y and 2Z are horizontal lines in the deformed matrix. The misorientation fluctuated in a small pitch, corresponding to the deformation bands. The pitch was finer in the lower part of matrix (Line 2Z) than in the upper part (Line 2Y). Figure 9 shows the orientation relationship between the deformed matrix orientation and recrystallized orientations. In Fig. 9 (a), the matrix grain orientation is colored red. Some recrystallized grains (grain b & c) at the grain boundary had a close orientation with the matrix one with a misorientation lower than 15°. The other colored recrystallized grains in Fig. 9(c) had the following orientations: grain d & e; bisecting orientation between the isolated deformed grain and the matrix, grain a; the perimeter of orientation dispersion of the matrix or isolated deformed grain.
Distribution of misorientation in each line shown in the Fig. 8(d).
Orientation relationship between the deformed matrix grain orientation and recrystallized grain orientations. The circle marks in the pole figure corresponds to the initial bi-crystal orientation as shown in Fig. 2, and the orientations of recrystallized grains are marked.
Figure 10 is another example of recrystallization of an isolated deformed grain. Recrystallized grains were generated at the tip of the isolated deformed grain which also extended in the rolling direction. The deformation behavior of the matrix around it was similar to that shown in Fig. 7: that is, the crystal was rotated in the orientation of <100>//ND near the top and bottom of the isolated deformed grains; deformable bands were observed in the matrix. Deformation bands around the isolated grain were clearly observed. Intragranular shear bands inside the isolated grains were not clear, but the shear direction was the same as that of the surrounding matrix, which was different from that shown in Fig. 8. The IQ values inside the isolated deformed grain were varied: they are low in the vicinity of the observed recrystallized grains and there were sub-grain structures. Figures 10(c) and 10(d) show the orientation relationship between the deformed grains and the recrystallized orientation. The colors in Fig. 10(c) correspond to the ones in Fig. 10(d). The texture was divided into two groups. One recrystallized orientation was located in the middle of the orientation between the isolated deformed grains and deformed matrix (grain d). The other recrystallized orientations corresponded to the edge of the orientation spread of the deformed matrix (gain a, b and c).
Image quality (IQ) map (a) and the inverse pole figure (IPF) map (b) of the ND direction of an isolated deformed grain with several recrystallized grains. Figures 10(c) and 10(d) shows the orientation relationship between the deformed grains and recrystallized orientation. The circle marks in the pole figure corresponds to the initial bi-crystal orientation as shown in Fig. 2, and the orientations of recrystallized grains are marked.
Other isolated grains were also observed as shown in Fig. 11. The occurrence of recrystallization was highly probable at the tip position of the isolated deformed grains extending in the rolling direction in either case.
Image quality (IQ) map (a, c) and the inverse pole figure (IPF) map (b, d) of the ND direction in the matrix grain A in RD×ND plane heat-treated at 550°C.
Figure 12 shows an isolated deformed grain in the TD×ND section. Recrystallized grains were generated from the tip or near the isolated grain which was flattened by cold rolling. The near <100>//ND orientation band outside the isolated particles observed in RD×TD extends at a angle close to the vertical direction, forming deformation bands as viewed in the TD×ND section, but the direction was slightly displaced from the ND direction. Figure 12(c) shows the corresponding pole figures. Apparently, the deformed matrix orientations were distributed in the same rotation sequence. Figure 12(d) shows the misorientation distribution (orientation based on the upper tip) along the vertical lines shown in Fig. 12(a). As seen in the RD×ND section, the orientation changed sharply as it approached the isolated deformed grain.
Orientation map of an isolated deformed grain from the TD×ND section. Figure 12(c) is the corresponding (200) pole figures. The circle marks in the pole figure corresponds to the initial bi-crystal orientation as shown in Fig. 2. Figure 12(d) is plots of misorientation distribution on the line in the ND direction.
Thus far, recrystallization had occurred in the region near grain boundaries of the isolated deformed grains, but there was a case where recrystallization occurred inside an isolated deformed grain, as shown in Fig. 13. It was an island-like isolated deformed grain observed in the TD×ND section heat-treated at 550°C. The recrystallized grains were clearly observed inside the isolated deformed grain. Figures 13(b) and 13(c) show the orientation of the island-like deformed grain, the recrystallized grains and the deformed matrix grain. One recrystallized grain in the center had an orientation in the orientation spread of the isolated deformed grain; another recrystallized grain near the grain boundary had the orientation of the edge of the orientation distribution of the matrix. In this way, recrystallization orientations were related to both deformation of the isolated grains and to the surrounding matrix grains.
Orientation map of the island-like isolated deformed grains observed in TD×ND, heat-treated at 550°C. Figures 13(b) and 13(c) show the orientation of an island-like deformed grain, a recrystallized grain and a deformed matrix grain. The circle marks in the pole figure corresponds to the initial bi-crystal orientation as shown in Fig. 2.
The crystal rotation during cold rolling was simulated by using orientation package popLA.23) The <111> pencil glide was assumed using the Taylor-Bishop-Hill full constraint model. The simulated equivalent strain was 0.5 with a step of 0.025. Figure 5(d) shows the simulated orientation in the (200) pole figure. Some orientation spread by cold rolling was explained that in the experiment by the calculation, but crystal orientation in the calculation was more stable than that in the experimental one. The strain or stress conditions were more complicated in the vicinity of the grain boundary, especially when the grain boundary had some curvature. Moreover, since the matrix grain was quite coarse, the full constraint model seemed to be stricter for this kind of simulation.
3.6. Crystal Orientation of Recrystallized GrainsFrom the observations on the recrystallization in the isolated deformed grain, recrystallized orientations were related both to the isolated deformed grains and to the surrounding matrix grain.
Humpherys19) has reviewed recrystallization stimulated by the existence of the 2nd phase in aluminum. In his study, geometrically necessary dislocations were generated around the 2nd phase particles during deformation, and crystal rotation occurred by complicated mechanisms. Deformation bands were formed in the circumference of the 2nd phase to promote the recrystallization. It can be said that it was similar to the recrystallization in the isolated deformed grains in the present work. The above-mentioned 2nd phase, however, did not deform nor was it divided by cracking due to hardness. On the other hand, in the case of these isolated grains, they had hardness almost equivalent to that of the matrix. Thus, crystal rotation according to the crystal orientation was produced by rolling. The preferred site of the recrystallization was the tip and back end of the isolated grains which were extended in the rolling direction, and that was the place which was heavily deformed locally. The preferred nucleation site was also common also to the recrystallization at bi-crystal grain boundary as in Fig. 4.
Tsuzaki et al.10) investigated the effect of the grain boundary on recrystallization after 70% cold-rolling using a bi-crystal. They reported that the width of the influence of grain boundary on the bi-crystal by cold rolling was about 300 μm. In this work as well, the influence of grain boundary on the bi-crystal was seen in the range of 200 μm, and sharp crystal rotation was especially observed near the grain boundary. Although the effect of grain boundary in the case of the isolated grain was not seen in the above-mentioned range, sharp crystal rotation arose near the interface.
Regarding the inter-grain shear bands inside the deformed isolated grains and the matrix grain, there were two cases: the case where the shear bands were in the same direction (Fig. 8), and the case where they differed (Fig. 7). Although it was thought that such difference was dependent on the initial orientations of isolated grains, the recrystallization nucleation site was the same. As indicated by IQ maps in Fig. 7, the distortion was considered to be concentrated on the inside of an isolated deformed grain in the former case.
As for the crystal orientation of the recrystallized nucleus, in many cases, the perimeter orientation of the orientation was spread in the matrix near the grain boundary, or the bisecting orientation between the matrix and the isolated grain orientation was selected. The latter case was thought to be similar to the Inagaki model6) in which some local crystal rotation occurs in order to complement the inconsistency of the crystal rotation of two grains at grain boundaries by cold rolling and recrystallization occurred in the middle of the crystal rotation. The recrystallized orientations in this work, however, were not always on the shortest distance between two deformed grains. Furthermore, the recrystallization was also observed also inside an isolated grain in this work. Since such local rotation could not explain in the macroscopic strain,24) the active slip system should be carefully analyzed.
The recrystallization behavior at an early-stage of deformed isolated grains in pure iron was investigated, and the following findings were forthcoming.
(1) Recrystallization from deformed isolated grains was observed at an annealing temperature of more than 550°C. The recrystallization was promoted especially in isolated deformed grains rather than the bi-crystal grain boundaries.
(2) Sharp orientation change was observed in the matrix around the isolated grains. Some deformation bands were formed. The IQ value was lower at the tip of the deformed grains extending in the rolling direction, where the local deformation are extremely concentrated.
(3) In the RD×ND section, recrystallized grains were observed in the tip portion of the deformed isolated grains extending in the rolling direction. Recrystallized grains were also observed inside the deformed grains. The bi-crystal grain boundary became saw-toothed after cold rolling, and recrystallized grains were observed in the tip part of the blade similar to the recrystallization in the isolated deformed grains.
(4) The recrystallization orientation was positioned at the edge of the orientation spread of the matrix, or had an orientation of the intermediate matrix and the isolated grains.
