2016 Volume 56 Issue 2 Pages 326-334
The heating rate effect on texture evolution of a 1.09 wt.% Si non-oriented electrical steel was studied by using two heating rates, 0.5°C/s (slow heating) and 15°C/s (fast heating), in the final annealing. The comparison of textures was made on similar microstructural basis, i.e., recrystallized fraction in the recrystallization stage and grain size in the grain growth stage. Change of heating rate caused little influence on the texture evolution in the recrystallization stage, implying that the texture at complete recrystallization is mainly determined by the cold-rolling structure. In the growth stage, the heating rate caused little influence on the evolution of {111}<110>, {111}<112>, and Cube components, however, made significant influence on the evolution of Goss component. For both heating rates, the intensity of Goss component increased continuously during recrystallization, but decreased rapidly as the grains grew. Comparing to the fast heating, the slow heating caused more drastic decrease in Goss component as the grains grew. Microstructure observations indicated that the average size of Goss grains was similar to that of the average recrystallized grains in the recrystallization stage, but became smaller than that of the average grains in the growth stage. It was attributed to an orientation pinning effect due to the preferential nucleation of Goss grains at shear bands. Rapid heating could make the Goss grains more dispersed in the recrystallized structure, which reduced the probability of orientation pinning encountered by these Goss grains, and delayed the decrease of growth rate of Goss grains in grain growth.
Nonoriented electrical steel (NOES) sheets have been widely used in rotary machines such as generators and motors. The electromagnetic characteristics of NOES sheets are influenced by their alloy chemistry, microstructure and texture. It is well known that ferrite grains having their <100> direction parallel to the external magnetic field can be easily magnetized, whereas high electric fields are needed to magnetize grains in <110> and <111> directions. Accordingly, the close relation between anisotropic magnetic properties and texture of the electrical steels has long been recognized.1,2,3) Because the NOES sheets are typically applied in rotary machine, in-plane isotropic magnetic properties are important. Thus, a texture of strong <100>//ND and weak <111>//ND is favorable.
Texture can be described by the orientation distribution function (ODF) expressed in an Eulerian space. For low carbon steels, the Euler angles of the major components composing the cold-rolling (CR) texture and the recrystallization (RX) texture are summarized in Table 1. The CR texture is mainly composed of a strong α-fiber (<110>//RD) and a moderate γ-fiber (<111>//ND), in which strong {100}<011> (rotated cube, R) and {112}<011> (D) components are formed in the α-fiber, and the {111}<110> (E) component is more strongly developed than the {111}<112> (F) component in the γ-fiber texture. In order to improve the magnetic properties of the NOES sheets, a proper control of the final annealing is necessary to optimize the grain size as well as the RX texture, that is to enhance <100>//ND and to reduce <111>//ND fibers.
| Euler angle | phi 1 | phi | phi 2 |
|---|---|---|---|
| α-fiber: RD//<110> | 0 | 0–90 | 45 |
| γ-fiber: ND//<111> | 0–90 | 54.7 | 45 |
| η-fiber: RD//<100> | 0 | 0–90 | 0 |
| θ-fiber: ND//<001> | 0 | 0–90 | 45 |
| C: {001}<100> Cube | 45 | 0 | 45 |
| R: {001}<110> Rotated cube | 0 | 0 | 45 |
| D: {112}<110> | 0 | 35 | 45 |
| E: {111}<110> | 0, 60 | 54.7 | 45 |
| F: {111}<112> | 30, 90 | 54.7 | 45 |
| G: {011}<100> Goss | 90 | 90 | 45 |
The development of RX texture is mainly determined by the microstructure and texture of the cold-rolled state, however, processing parameters of annealing also play an important role. There are a number of processing parameters, such as heating rate, annealing temperature, holding time and cooling rate, which can be varied to tailor the RX texture. During annealing, recovery is in competition with the recrystallization process, because both processes are driven by the stored energy accumulated in the cold-rolled state. Heating rate is considered to be a direct factor which influences the relative contribution of recovery involved in the recrystallization process. It is generally accepted that increasing heating rate can reduce the extent of recovery and keep more stored energy to drive the formation of RX nuclei. Based on this consideration, it was suggested that a higher heating rate would increase RX nucleation rate which consequently resulted in finer grains.4,5) However, different findings on the effect of heating rate on RX grain size also appeared in the literatures. Park et al.6) showed that the average size of the recrystallized grains was similar for samples annealed at different heating rates. Conversely, Fang et al.7) indicated that the average grain size was rapidly increased when the heating rate was increased from 3 to 25°C/s but decreased when the heating rate was greater than 25°C/s.7)
According to Duan et al.,8) increasing heating rate can reduce core loss and enhance permeability of electrical steel, which might be attributed to different RX texture produced by fast heating. However, regarding the effect of heating rate on RX texture evolution, the experimental results reported in the literatures are inconsistent. Some reports9,10) suggested that fast heating was favorable to the development of <111>//ND texture, and others indicated that rapid heating could strengthen the Goss texture.11) On the other hand, Park et al.4) showed that the heating rate effect on the development of Goss and {111}<112> texture components also depended on the grain size prior to the cold rolling. They found that in the coarse-grained specimen, the Goss texture is significantly strengthened as the heating rate increases, while in the fine-grained specimen, the {111}<112> intensity is greatly decreased upon rapid heating. Several mechanisms have been proposed to account for the experimental results of different heating rate on the evolution of annealing texture. Park et al.4) suggested that enhancing recovery by reducing heating rate caused decrement in stored energy, which would reduce the probability of nucleation and growth of Goss grains in recrystallization. Wang et al.5) suggested that high heating rate could reduce the extent of recovery during heating such that those low stored energy regions might have enough driving force for recrystallization nucleation, and consequently the difference in the probability of nucleation of grains with different orientations was reduced. The mechanism proposed by Wang et al.5) could rationalize the findings that an increase in heating rate were unfavorable to the development of <111>//ND texture.9,10)
The aforementioned mechanisms are based on the same argument that fast heating could reduce recovery which can retain enough driving force to make recrystallization nucleation possible in regions with relatively lower stored energy. Consequently, the advantage of the development of recrystallization nuclei in those preferred sites, having high stored energy, might be reduced by increasing heating rate. Both Goss and {111}<112> are preferred orientations for recrystallization nucleation. Based on the aforementioned mechanism, these two components should be reduced by rapid heating, but the experimental observations regarding this point are not consistent, especially for the effect of heating rate on Goss component.4,9)
The recrystallization texture depends upon the deformed structure, the annealing temperature and the heating rate. For different heating rate, samples annealed at the same temperature experience different thermal history. Therefore, the same annealing temperature is not a good comparison basis for studying the effect of heating rate. In the present work, the influence of heating rate on texture evolution were analyzed based on microstructural characteristics, where “recrystallization fraction” and “average grain size” were used as the comparison basis in the recrystallization stage and in the grain growth stage, respectively. Based on this study, we can gain a clearer insight of the effect of heating rate on the texture evolution in recrystallization.
The chemical composition of the NOES used is shown in Table 2. The ingot was hot-rolled in nine passes from a thickness of 200 mm to 4 mm with a finish-rolling temperature of 910°C, and the hot band was then annealed at 850°C for 5 h. The average grain size of the hot band was 48 μm. The hot band was machined on both surfaces to a thickness of 2.3 mm to remove the scale. The hot band was subsequently cold rolled with a 78% reduction to a final thickness of 0.5 mm. A final annealing was carried out in an infrared furnace for a holding time of 60 s to simulate a continuous annealing process. The effect of heating rate was investigated by using two heating rates, 0.5°C/s (slow heating) and 15°C/s (fast heating), in the final annealing.
| Si | Mn | Al | P | C | N | S | Fe |
|---|---|---|---|---|---|---|---|
| 1.09 | 0.31 | 0.22 | 0.01 | 0.0015 | 0.001 | 0.0015 | Bal. |
After mechanical polishing and chemical etching in a 5% nital solution, optical micrographs (OMs) were obtained from the TD (transverse direction) section of the specimens. The texture was measured using X-ray diffraction (XRD) with Co-Kα radiation in a Bruker D8 diffractometer. The orientation distribution functions (ODFs) were calculated from three pole figures, (200), (220) and (211), using the series expansion method. The XRD pole figures were obtained from the mid-plane of the specimen on the ND (normal direction) section. Orientation image mapping was conducted on the TD section using the electron backscattered diffraction (EBSD) technique. EBSD analyses were performed at 20 kV using a Zeiss Supra 55 field-emission scanning electron microscope equipped with an Oxford Nordlys detector. The EBSD data was analyzed using the Channel 5 software. The samples for EBSD analysis were electropolished in a solution consisting of 20% perchloric acid and 80% glacial acetic acid at room temperature.
The fraction of recrystallization of annealed specimens was determined by using point-counting method on optical micrographs. Figure 1 shows the variation of recrystallization fraction as a function of annealing temperature. The slow heating specimen exhibits a higher recrystallization fraction at each annealing temperature. The temperatures for full recrystallization are 680°C and 710°C for slow heating and fast heating, respectively. The average grain sizes of fully recrystallized specimens obtained by the two heating rates are quite close, which are 18 μm and 19 μm for fast heating and slow heating, respectively (see Fig. 2). The effect of heating rate on the grain size in the grain growth stage is shown in Fig. 3. It indicates that the slow heating results in large grains, and the difference of grain size subjected to different heating rates is enlarged as the annealing temperature increases.
Effect of heating rate on the recrystallization fraction of the cold-rolled NOES annealed at different temperature. (Online version in color.)
Optical micrographs showing the microstructures of completely recrystallized specimens. (a) specimen annealed at 680°C by slow heating (0.5°C/s), and (b) specimen annealed at 710°C by fast heating (15°C/s). The average grain sizes are 19 μm and 18 μm for specimens annealed at 0.5°C/s and 15°C/s, respectively.
Effect of heating rate on the grain size of specimens annealed at different temperature. (Online version in color.)
Textures of cold-rolled specimen and specimens annealed at 800°C are represented by phi 2=0° and 45° sections of ODF in Fig. 4. The CR texture is mainly composed of α-fiber and γ-fiber (Fig. 4(a)), in which the strongest texture component is {100}<110> (R), and {111}<110> (E) is slightly stronger than {111}<112> (F) in γ-fiber. The textures of 800°C annealed specimens for the two heating rates are displayed in Figs. 4(b) and 4(c). They exhibit similar features consisting of mainly F, C and G components, but the fast heating produces higher intensities in both F and G orientations. Figure 5 summarizes the variation of four fiber intensities with the annealing temperature. The fiber intensity is the summation of the orientation density, f(g), along the fiber. It indicates that heating rate imposes different influence on the intensity of each fiber. The α-fiber intensity decreases significantly with increasing the recrystallization fraction. The intensity of the γ-fiber decreases in early and middle stages of recrystallization (15–80%) and then increases to a maximum value at the end of recrystallization followed by a gradual decrease on grain growth. The fast heating postpones the recrystallization reaction, and therefore delays both the decrease of the α-fiber intensity and the presence of the γ-fiber maximum. The variation of the η- and θ-fibers with the annealing temperature follows the same trend. In addition, the fast heating specimens have a higher intensity of the η-fiber but a lower one of the θ-fiber after annealed at high temperatures (710–900°C).
Textures of (a) cold rolled specimen as well as specimens annealed at 800°C by (b) slow heating (0.5°C/s) and (c) fast heating (15°C/s) represented by phi 2=0° and 45° sections of ODF. (Online version in color.)
Variation of fiber intensities as a function of the annealing temperature. The intensity of each fiber is the summation of f(g) along the fiber. (Online version in color.)
The difference mentioned above in recrystallized microstructure and texture resulted from heating rate could be partly attributed to the long thermal exposure time before reaching the annealing temperature by slow hating. Therefore, it is not proper to compare the annealing structures simply based on the annealing temperature. Instead, the influence of heating rate on texture evolution in annealing will be analyzed on the basis of microstructure in the following text. “Recrystallization fraction” will be used as the comparison basis for the recrystallization stage (before the completion of recrystallization), and “average grain size” will be served as the comparison basis for the grain growth stage.
3.2. Texture Development during RecrystallizationFigure 6 shows the evolution of the important texture components in the recrystallization stage, including α-fiber, E: {111}<110>, F: {111}<112>, C: {001}<100>, and G: {011}<100>. Interestingly, the texture variation with RX fraction is quite similar for the two heating rates, and the intensity of the major texture components at complete recrystallization is nearly the same. The important features of the texture variation during the progress of recrystallization can be summarized as follows.
Variation of texture components as a function of recrystallization fraction. (a) α-fiber, (b) E: {111}<110>, (c) F: {111}<112>, (d) C: {001}<100>, and (e) G: {011}<100>. (Online version in color.)
(1) The intensity of the α-fiber remains nearly unchanged until the RX fraction reaching approximately 60% and drops rapidly after then.
(2) The intensity of {111}<110> exhibits a slight increase at the beginning followed by a continuous decrease through 20 to 90% RX.
(3) The intensity of {111}<112> decreases and then increase slightly during the progress of recrystallization.
(4) For both C and G components, the intensity increases continuously as recrystallization proceeds, and reaches the peak value at 100% RX. However, the slow heating results in an earlier saturation of the G component intensity. Accordingly the fast heating specimen indeed has a higher intensity of G component at 100% RX.
Partially recrystallized specimens were examined by using EBSD to reveal the distribution of the RX grains. The RX and deformed grains are differentiated according to the intragranular misorientation angle, θc, after grain reconstruction. A θc value of 1° was used in the present case. As shown in Fig. 7, recrystallized grains are mostly nucleated inside the deformed γ-fiber grains or the existing γ grain boundaries for samples annealed at 590°C. The orientation image maps (OIMs) of partially recrystallized specimens (~60% RX) obtained by annealing at 620°C also indicate that the RX grains are distributed inhomogeneously. It was observed that both heating rates gave similar feature. The deformed γ-fiber grains were nearly completely consumed by new grains, whereas other deformed grains, of mainly {001}<110> and {112}<110> orientations (marked by white circles), were partially replaced by new grains or without any nucleation. The average sizes of RX gains obtained at 620°C are 12.2 μm and 11.8 μm for slow and fast heating, respectively.
Orientation image maps (OIMs) of specimens annealed at (a, b) 590°C and (c, d) 620°C by (a, c) slow heating and (b, d) fast heating. A deviation of 15° is allowed for each texture component in the OIMs. White lines are low angle boundaries (5°<θ<15°) and black lines are high angle boundaries (θ>15°). (Online version in color.)
The main driving force for recrystallization to occur is the stored energy in the deformed matrix. Each deformed grain recrystallizes with its own kinetics which is independent of the other deformed grains. Growth of the recrystallized grains continues until mutual impingement occurs. Theoretically, primary recrystallization finishes when grain impingements occur. However, the distribution of the recrystallized regions is heterogeneous and grain impingement already occurs even for a partially recrystallized state as shown in Fig. 7. Figures 7(a) and 7(b) Recrystallization takes place earlier in deformed grains with higher stored energy. The stored energy introduced into the deformed specimens is in increasing order for the {001}<110>, {112}<110>, {111}<110> and {111}<112> orientations.12) Therefore, the γ-fiber grains are considered more favorable nucleation sites in recrystallization. Deformed γ-fiber grains are consumed first, while deformed {001}<110> grains are consumed last, either by nuclei or by the recrystallized grains which have already nucleated.12)
The present results coincide well the aforementioned phenomenon. Figures 6 and 7 confirm that the intensity of the γ-fiber grains start to decrease at 10–20% RX followed by the decrease of the α-fiber intensity at 60% RX. However, changing the heating rate causes little influence on the texture evolution in the recrystallization stage, and nearly same texture and grain size are resulted at the completion of recrystallization. It implies that the texture at complete recrystallization is mainly determined by the cold-rolling structure.
3.3. Texture Development during Grain GrowthThe variation of four important texture components E, F, G, and C during grain growth are shown in Fig. 8. The intensity of {111}<110> decreases slightly and that of {111}<112> exhibits little change with increasing the grain size for both heating rates. Both C and G components decrease in intensity continuously with the grain size. The heating rate thus causes little influence on the intensity variation of E, F, and C components during grain growth. The only significant influence of the heating rate is on the evolution of the Goss component. Comparing to the fast heating, the slow heating causes more drastic decrease in G component as the grain grows. The influence of heating rate on the evolution of G component was subjected to more detailed investigation.
Variation of texture components (a) E: {111}<110>, (b) F: {111}<112>, (c) C: {001}<100>, and (d) G: {011}<100> as a function of grain size in the grain growth stage. (Online version in color.)
The XRD results indicate that the intensity of Goss increases continuously during RX (Fig. 6), but decreases rapidly as grains grow (Fig. 8). In addition, the only significant influence of heating rate on the RX texture evolution is on the development of G component. Hence, it is important to understand how Goss grains grow as compared with the growth of other RX grains during the progress of recrystallization.
In a study of the texture evolution in annealing of cold-rolled NOES, Park et al.12) indicated that new Goss grains are mainly nucleated within shear bands in the deformed grains. Because of high local misorientation, shear bands have higher stored energy as compared to the matrix, and hence Goss grains are nucleated early in recrystallization. In addition, Park et al.12) showed that the textures of RX grains are nearly the same for partially recrystallized and fully recrystallized specimens. In the present work, observations of specimen annealed at 620°C indicated that Goss grains do not have size advantage over grains of other orientations during the progress of RX. This finding is consistent with the result reported by Magnusson et al.13) that the grains of different orientations exhibit similar growth rate during RX. It is then suggested that the strong intensity of Goss component in RX texture (Fig. 6) can be attributed to the early nucleation of Goss grains, which is determined by shear bands present in the cold-rolling structure.
After complete recrystallization, the variation of volume fraction, size and number of Goss grains relative to all the RX grains with increasing grain size should provide clue to the mechanism responsible for the variation of Goss texture in the growth stage (Fig. 8(d)). The volume fraction of Goss grains, VG, is calculated from the ODF data with an angular deviation of 15°. The normalized size of Goss grains, αG, is the average size of Goss grains divided by the average size of all RX grains, and the number fraction of Goss grains among all RX grains is denoted as βG. Figure 9 shows the variation of VG, αG and βG as a function of the average size of RX grains. It clear indicates that VG decreases rapidly with increasing annealing temperature from 680°C to 710°C for the slow heated sample and then decreases slowly with further increasing of the annealing temperature. The VG values for the fast heated sample, however, decreases more rapidly in the late stage of grain growth. The values of αG for both heating rates follow the same decreasing trend as the grains grow, and drop to values of less than 0.8 for grain sizes larger than 30 μm. On the other hand, the βG value of slow heated sample is lower than 0.1 for grain size >20 μm, while that of fast heated specimen maintains a value about 0.2 and decreases rapidly as the grains grow beyond 30 μm. The decrease of both αG and βG in grain growth stage indicates that Goss grains grow relatively slower than grains of other orientations do. Explanation for the above mentioned results is given in the following.
The variation of (a) the volume fraction of Goss grains, (b) the normalized size of Goss grains, αG, and (c) the number fraction of Goss grains, βG, with increasing grain size. (Online version in color.)
It is generally accepted that during the growth of a new grain in a heavily deformed matrix in recrystallization, the grain boundary will constantly meet new types of deformation microstructures and new crystallographic orientations, and consequently, its growth conditions will change constantly. A growing RX grain is surrounded, at least partly, by a high angle boundary which is able to migrate through the deformed microstructure. However, a low angle boundary (LAB) may develop whenever a growing grain meets regions of similar orientation and, as the mobility of the LABs is low, such regions act as obstacles to the growth of the grain. Therefore, on the progression of recrystallization, the growth of the new grains depends on both the mean stored energy of the surrounding matrix and on the frequency of new grains re-acquiring a low mobility boundary by meeting similar orientations in the deformed state, i.e., orientation pinning. Figure 10 shows orientation image maps (OIMs) of fully recrystallized specimens obtained by annealing at 710°C, in which grains with orientations within 15° deviation from the major texture components are labeled in different colors. There is considerable population of LAB present in the fully recrystallized specimen. It is noted that recrystallized Goss grains (green color) often exist in large colony, while colonies of Cube (red color) and {111}<112> (blue color) grains are also found but less frequently and smaller in size as compared with that of Goss grain colony. LABs are found frequently in colony of grains of similar orientations. Both heating rates produce similar features as described above. Figure 10(c) shows the misorientation distributions of boundaries surrounding the Goss grains and grains of other orientations for a slow heated sample annealed at 710°C as an example. The length fraction of the LABs surrounding the Goss grains is significantly higher than that of the LABs surrounding grains of other orientations (37% versus 20%).
Orientation image maps (OIMs) of specimens annealed at 710°C by (a) slow heating (0.5°C/s) and (b) fast heating (15°C/s). A deviation of 15° is allowed for each texture component in the OIM. White lines are boundaries of 2°<θ<5°, yellow lines are boundaries of 5°<θ<15° and black lines are high angle boundaries (θ>15°). The distributions of boundary misorientation in slow heated sample are also shown in (c). (Online version in color.)
It is suggested that the decrease of growth rate of Goss grains during grain grows may be attributed to a high probability of “orientation pinning” encountered by Goss grains. Because RX grains of Goss orientation are nucleated at shear bands, Goss grains exist inhomogeneously in the RX structure, and more likely to form colony during growth, which makes Goss grains meeting grain of similar orientation more frequently and higher probability encountering “orientation pinning”, as revealed in Fig. 10(c). In addition, the slow growing Goss grains become inferior in grain growth and will finally be annihilated by the growth of neighboring grains. Hence, this mechanism can explain the decrease of both αG and βG in the grain growth stage.
The different βG values for the slow and fast heating samples in grain growth might be due to different distribution of the Goss grains being generated by different heating rate. It is suspected that rapid heating makes the Goss grains more dispersed. The probability of these Goss grains to meet with each other is thus reduced, and the decrease of βG is delayed. This can be understood since a reduction in recovery by increasing heating rate could provide extra recrystallization driving force for those regions with relatively low stored energy, and hence the distribution of the Goss nuclei is more dispersed by increasing heating rate.
EBSD analysis can presumably provide evidence for the distribution of the Goss nuclei in samples of different heating rates. An attempt was made to compare the LAB fraction of the Goss grains for the slow and fast heated samples annealed at 710°C. However, the values of both samples are similar, probably due to the small volume fraction and inhomogeneous distribution of Goss grains. A very large sampling size is thus required to reveal the difference accurately. However, a recent result indicated that LAB with misorientation angle less than 1° were detected in RX Al grains by the dark field X-ray microscopy technique,14) indicating that the angular resolution of EBSD is not enough to provide a conclusive evidence for clarifying the problem.
In this study, two heating rates, 0.5°C/s (slow heating) and 15°C/s (fast heating), were applied to the final annealing process of a non-oriented electrical steels. The effect of heating rate on the texture evolution was analyzed based on the microstructural characteristics in different stages of annealing. In the recrystallization stage, the comparison was made according to recrystallization fraction, while the grain size was taken as the basis for comparison in the grain growth stage. The important results are summarized as follows.
(1) At the completion of recrystallization, similar texture and average grain size were obtained by the two heating rates. Moreover, the results of macrotexture analysis reveal that the evolution of recrystallized texture is similar for both of the heating rate conditions. This implies that the heating rate, in the range of 0.5 to 15°C/s, does not affect the evolution of the recrystallized texture, which is mainly determined by the cold-rolled microstructure.
(2) The orientation image mapping of electron backscattered diffraction indicated that most of the RX grains nucleated in the deformed γ-fiber grains.
(3) As the average gain size increases in the grain growth stage, both the relative grain size and the volume fraction of Goss ({011}<100>) grains decrease with grain growth. The decrease of the volume fraction of the Goss grains is faster at slow heating than fast heating. However, the different heating rate causes little influence on the variation of other major RX texture components as the grains grow. It is mainly attributed to an orientation pinning effect due to the preferential nucleation of Goss grains at shear bands.
This work is financially supported by the Ministry of Science and Technology, R.O.C. (MOST-103-2622-E-006-037) and China Steel Corporation (02T1D-RE030).
