2017 Volume 58 Issue 5 Pages 838-841
The orientation characteristics of a pure iron tape fabricated by cold rolling and annealing were evaluated using electron backscatter diffraction (EBSD) analysis. The {1 7 1}<4 $\bar{3}$ 17> orientation, which is a near-cube orientation, was strongly formed on the tape surface. For the oriented iron tape, the out-of-plane misorientation of (0 0 1) from the cube orientation is ca. 11°. The areal fraction of cube orientations with an angular deviation ≤20° amounts to 94.7%. The average grain size is approximately 370 μm for the near-cube oriented iron tape.
Control of crystal orientation for soft magnetic materials has been performed in order to make effective use of magnetic property for the materials. Pure irons and silicon steels, which are soft magnetic materials, have anisotropy of magnetization. The <1 0 0> axis is the easiest direction of magnetization.1) The <0 1 1> axis is the intermediate direction of magnetization. The <1 1 1> axis is the hard direction of magnetization. Based on the anisotropy of magnetization, the {0 1 1}<1 0 0> and {0 0 1}<1 0 0> orientations for the silicon steel sheets are suitable orientations for a reduction in iron loss of the sheets. Actually, grain oriented silicon steel (GOSS) sheets with a {0 1 1}<1 0 0> orientation are used for the core materials with a low iron loss in electrical transformers. For the iron-based sheets fabricated by rolling and subsequently annealing, the preferred crystal orientations are the {1 1 1} orientations such as {1 1 1}<1 1 0>, {1 1 1}<1 1 2> and γ-fiber on pure iron and steel sheet surfaces.2,3) It is difficult to fabricate biaxially oriented iron-based sheets, which are the GOSS sheets and cube oriented silicon steel sheets with a {0 0 1}<1 0 0> orientation. For the fabrication of oriented silicon steels without the {1 1 1} orientations, the control techniques of crystal orientation have been developed. The GOSS sheets have been fabricated using secondary recrystallization for grains with {0 1 1}<1 0 0> orientation.4) Especially, the GOSS sheets in which an average deviation of the <1 0 0> axis from the rolling direction (RD) is 3–4°, have been manufactured industrially using processes of rolling and subsequent annealing with inhibitors such as MnS, MnSe and AlN,5–7) which restrict the growth of grains with an orientation other than {0 1 1}<1 0 0>.8,9)
On other hand, the cube oriented silicon steel sheets have been fabricated also using techniques of unique cold rolling and annealing. A cube oriented silicon steel sheet was fabricated using a cross cold rolling technique, which the silicon steel was rolled alternately along two perpendicular directions for the cross rolling process.7) The cube oriented silicon steel sheet with a chemical composition of approximately Fe-2.9%Si-0.7%Mn-0.0007%C has been fabricated using an oxide-separator-induced decarburization technique, whereby approximately 90% of the grains are aligned within 10˚ of the cube orientation.10) The formation of cube orientation is disturbed by the carbon in the silicon steel. A sheet which consists of SiO2 and Al2O, and Ti2O powders were used for the decarburization of silicon steel sheet.
In the previous studies, the cube oriented silicon steel sheets have been reported. However, the manufacture of cube oriented silicon steel sheets has not been carried out. This is related to the difficulty of fabrication of perfectly cube oriented silicon steel sheets with a low iron loss. It is important to develop a strict control technique of cube orientation for the industrial manufacture of perfectly cube oriented silicon steels. In this paper, the orientation characteristics of pure iron with a near-cube orientation fabricated by cold rolling and annealing were investigated using scanning electron microscopy (SEM)-electron backscatter diffraction (EBSD). For the basic orientation control of crystal grains, the pure iron having the same bcc structure as that of the silicon steel was used. The technology of crystal orientation control for the pure iron is expected to be a basic technology of crystal orientation control for the simple production of Fe-Si sheets with a highly cube orientation. In generally, the crystal orientation is affected by impurities and additive elements. The orientation control of crystal grains becomes easy by using the pure iron with little influence of impurities on the crystal orientation. The oriented iron tape was prepared using a two-stage cold-rolling technique. An attention is paid to the thickness of the pure iron tape. The thickness of typical silicon steel sheets is approximately 0.3 mm for the iron cores in electrical transformers.8,10) The pure iron tape having a thickness close to that of the commercial silicon steel sheets was fabricated. Furthermore, in this study, it is also a purpose to investigate a mechanism of near-cube orientation for the pure iron tape. Pure iron tapes with the cube orientation fabricated by cold rolling and annealing have not been reported yet.
An iron ingot was prepared by arc melting iron metal with 99.99% purity. Table 1 shows the chemical composition of the pure iron used for fabrication of the specimens. A rectangular rod specimen with dimensions of approximately T10 × W11 mm2 was cut into from the iron ingot. The iron tape was fabricated by a two-stage cold-rolling process with intermediate annealing, followed by final annealing. The cold rolling was performed in one direction. After homogenization annealing, the specimen was first cold rolled by multi-passing until a 75% reduction in thickness was achieved, followed by intermediate annealing at 1473 K for 3.6 ks in a vacuum. A specimen with a tape thickness of 0.26 mm was fabricated by the second cold-rolling process, which is a final cold-rolling process performed by multi-passing until an approximately 90% reduction in thickness. A cumulative reduction in thickness during the two-stage cold-rolling is approximately 98%. The iron tape was then annealed at 1073 K for 3.6 ks in a vacuum of less than 10−2 Pa. After each isothermal annealing, the specimen was cooled in a furnace. The reproducibility of recrystallization was investigated also using the final-rolled tape. The surface of the iron tape was electropolished with a solution of perchloric acid and ethanol (= 1:9) for EBSD observation. The orientation of the iron tape was analyzed using an SEM-EBSD system (JEOL JSM-6360 equipped with a TSL MSC-2200 EBSD camera). The step size used to obtain the EBSD images was 10 μm. The orientation characteristics of the iron tape were analyzed using TSL orientation imaging microscopy (OIM) analysis software (version 6.1).
| C | P | S | Si | Mn | Cu | O | N | H | Fe | |
|---|---|---|---|---|---|---|---|---|---|---|
| Pure iron | <0.0020 | <0.0005 | <0.0005 | <0.0005 | <0.0005 | <0.0005 | <0.0050 | <0.0010 | <0.0005 | Bal. |
Figure 1 shows an inverse pole figure (IPF) map and {0 0 1} pole figure for the iron tape (specimen-1) after the second cold rolling and final annealing at 1073 K for 3.6 ks. The orientation of the iron tape is characterized as close to ideal cube orientation. The {0 0 1} pole figure indicates that the pure tape has almost no other orientation components other than an orientation composition of {1 7 1}<4 $\bar{3}$ 17>. These results indicated that the orientation of the IPF shown in Fig. 1(a) was a near-cube orientation of {1 7 1}<4 $\bar{3}$ 17>. The out-of-plane misorientation of (0 0 1) from the cube orientation is approximately 11°. The microstructure shown in Fig. 1(a) is slightly inhomogeneous with regard to the grain size, and the average grain size is approximately 370 μm. The fractions of low-angle grain boundaries (<15°) and high-angle grain boundaries (≥15°) from the IPF map are 73.5% and 26.5%, respectively. Generally, in the grain boundary analysis of EBSD images, the fraction of grain boundary misorientations is affected by the boundaries of fine grains in microstructures with a mixture of coarse and fine grains. Here, the misorientation angle was evaluated using a function of the misorientation angle in the TSL OIM analysis software. The analysis method of grain boundary misorientations for EBSD images has been reported in detail.11) The orientation characteristics of the recrystallized iron tape (specimen-1) were evaluated. The relationship between the relative frequency and the angular deviation from a cube orientation for the out-of-plane misorientation of (0 0 1) is shown in Fig. 2. The maximum frequencies of near-cube orientation peaks are for angles from 10 to 15°. The areal fraction of cube orientations amounts to 94.7% for an angular deviation ≤20° and 98.5% for an angular deviation ≤25°. In contrast, the areal fraction of the ($\bar{1}$ $\bar{1}$ 7) plane with an angular deviation <10° is 78.1%, which indicates that the iron tape has a strong {1 7 1}<4 $\bar{3}$ 17> orientation.
(a) IPF map and (b) {0 0 1} pole figure for iron tape (specimen-1) after the second cold rolling and subsequent annealing at 1073 K for 3.6 ks. The white and black lines in the IPF map denote low-angle grain boundaries with angles <15°, and high-angle grain boundaries with angles ≥15°, respectively.
Relationship between relative frequency and the angular deviation from cube orientation for the out-of-plane (0 0 1) misorientation.
Figure 3 shows a {0 0 1} pole figure for a wide area of the iron tape (specimen-2) recrystallized by final annealing. Specimen-2, which was cut from the final-rolled tape, was also annealed under the same annealing conditions as that for specimen-1. The measured area of the tape surface is approximately 16 × 10 mm2. There is significant recrystallization with an orientation close to the {1 7 1}<4 $\bar{3}$ 17> orientation formed on the tape surface. The near-cube orientation over the wide area resembles that for the local area shown in Fig. 1(b), which indicates the homogeneous formation of near-cube orientation on the tape surface. The reproducibility of recrystallization with the {1 7 1}<4 $\bar{3}$ 17> orientation was confirmed for the pure iron.
{0 0 1} pole figure for wide area of iron tape (specimen-2) recrystallized by final annealing.
The near-cube recrystallization process developed for pure iron tape is a secondary recrystallization process. The near-cube orientation of recrystallization grains is related to the initial orientation before final cold-rolling. Figure 4 shows a {0 0 1} pole figure for the iron tape (specimen-2) before final cold-rolling. The formation of a {1 11 6}<2 $\bar{4}$ 7> orientation on the tape surface is observed. The ($\bar{6}$ 1 11) plane is a near-($\bar{1}$ 0 2) plane with a misorientation of approximately 5° from the ($\bar{1}$ 0 2) plane. A {0 1 l} orientation, which is located in the range between the {0 0 1} and {0 1 1} orientations, occurs during annealing at high temperature. This is attributed to the low surface energy of the {0 0 1} and {0 1 1} planes at high temperature, which is approximately 1473 K.12) In the crystal rotation of grains by cold rolling and subsequent annealing, a relationship between the initial orientation before cold rolling and the orientation of the recrystallization grains after annealing has been reported.10,13) For grains in which the (0 0 1) plane lies within 30° from the rolling surface and the [1 0 0] direction close to the RD, the grains approach the cube orientation by crystal rotation due to cold rolling and annealing.13) The empirical rule of crystal rotation is based on experimental data for single crystals of Fe-3%Si.13) Here, it is suggested that an initial orientation with {0 1 2}<1 0 0> results in the cube orientation by crystal rotation for a Fe-3%Si.14) Furthermore, the grains with an initial orientation of near-{0 1 4}<1 0 0> results in the recrystallization with the cube orientation by crystal rotation for the polycrystalline sheet of Fe-3%Si-1%Mn.10) The {0 1 4}<1 0 0>, which lies between {0 1 1}<1 0 0> and {0 0 1}<1 0 0>, is 10° away from {0 1 2}<1 0 0>. These indicate that the initial orientation, which has a [1 0 0] direction parallel to the RD before final rolling, leads to the ideal cube orientation for the recrystallization by crystal rotation. On the other hand, a mechanism of the crystal rotation, which leads to the cube orientation for recrystallization from an initial orientation before cold rolling, has not been clarified theoretically. In this work, the grain with the {1 11 6}<2 $\bar{4}$ 7> orientation has a (0 0 1) plane, which has an out-of-plane misorientation of approximately 30° from the rolling surface and the [1 0 0] direction, which is nearly parallel with the transverse direction (TD). The initial orientation before final rolling is near an orientation with in-plane rotation around approximately the normal direction (ND) from those which result in the cube orientation by crystal rotation for Fe-Si.13,14) The grain with the {1 11 6}<2 $\bar{4}$ 7> orientation for the pure iron changed to recrystallization grain with near-cube orientation by crystal rotation. One of the causes for the formation of the {1 7 1}<4 $\bar{3}$ 17> orientated recrystallization with a misorientation from the cube orientation, is due to the fact that the [1 0 0] direction of grain is away from the RD for the initial orientation of {1 11 6}<2 $\bar{4}$ 7>.
{0 0 1} pole figure for iron tape (specimen-2) before final cold-rolling.
The final annealing in a vacuum also plays an important role for the formation of the near-{0 0 1} orientation in pure iron. The ambient atmosphere during annealing has an effect on the {h k l}-orientation of recrystallization. This is related to the surface energy of the {h k l} plane, which depends on the annealing atmosphere. Annealing in a vacuum, and thus under a low oxygen partial pressure, suppresses the formation of an iron oxide layer on the tape surface. Vacuum annealing is thus effective for the growth of {0 0 1} grains because the surface energy of the {0 0 1} plane is reduced under an appropriate low oxygen partial pressure.15) In this study, near-cube recrystallization for pure iron was developed by the final annealing with an oxygen partial pressure of less than 2 × 10−3 Pa. The {0 0 1} oriented grains of Fe-3%Si-1%Mn grow by vacuum annealing,10) as do those of Fe-Si.12) Furthermore, the additional effect of silicon on the orientation of recrystallization in iron by vacuum annealing has been discussed.12) For Fe-Si with a silicon concentration ≥3%, the growth of {0 1 1} oriented grains is preferable during annealing with recrystallization. A reduction in the silicon concentration of Fe-Si leads to the {0 0 1} orientation during recrystallization.
The primary recrystallization in a tape with a limited thickness is restricted by the thickness effect, whereby the grain size reaches a size of 1–2 times that of the tape thickness.16) The motion of the grain boundaries is pinned on the thermal groove where the grain boundary crosses the tape surface during primary recrystallization. A few grains with a grain size over 520 μm, which is two times the thickness of the pure iron tape, are observed in Fig. 1. The growth of coarse grains with the near-{0 0 1} orientation in the pure iron tape is attributable to the secondary recrystallization. The driving force of grain growth for secondary recrystallization has been described theoretically.17) The growth rate (G) of grains during secondary recrystallization is given by:
| \[ G = M \left( \frac{\gamma_B}{r} + \frac{2\Delta \gamma_s}{t} + C \right), \] | (1) |
The orientation characteristics of iron tape with a near-cube orientation fabricated by cold rolling and annealing were evaluated using EBSD analysis. The recrystallization grains for the pure iron tape were aligned with a {1 7 1}<4 $\bar{3}$ 17> orientation, which is a near-cube orientation. Recrystallization with a near-cube orientation in the iron tape is a secondary recrystallization process. For the oriented iron tape, the out-of-plane misorientation of (0 0 1) from the cube orientation is approximately 11°. The areal fraction of cube orientations with an angular deviation ≤20° amounts to 94.7%. The average grain size is approximately 370 μm for the near-cube oriented iron tape.
