The influence of cooling rate on microstructure and magnetic properties of cast Fe-6.5wt%Si steel

Fe-6.5wt%Si steel samples of different thicknesses (1 mm, 2 mm, and 5 mm) were prepared by arc-melting and suction-casting with sub-rapid solidification rates. The microstructures and magnetic properties of the samples were studied, and the effect of solidification rates were discussed. Results show that columnar crystals are generated with a <100> fiber texture after casting. Ordered phases B2 and D03 are inhibited by sub-rapid solidification, and such an effect is clear in the 1 mm thick sample with the largest cooling rate. With decreasing cooling rate, the grain size increases, with the mean values being 66.6±30.8 μm, 93.0±42.8 μm, and 149.7±66.8 μm, and the intensity of the <100> texture increases, with the values being 3.14 mrd, 3.56 mrd, and 5.85 mrd, respectively. The 5 mm thick sample with the lowest cooling rate exhibits the best magnetic properties (B8=1.30 T, B50=1.55 T, P1/50=2.19 W/kg, P1/400=37.85 W/kg).


Introduction
Due to high magnetic flux density and low energy loss, silicon steel plays an important role in the generation, transmission and conversion of electric energy, which is widely used in transformers, electric machines and inductors [1,2].Robert Hadfield et al. found that the energy loss can be reduced by adding Si element to pure iron [3].Fe-3.0%Si steel performs satisfactorily in low frequency application, but exhibits great energy loss at high frequencies.This problem can be solved by increasing the silicon content.When the silicon content reaches 6.5%, the material displays the best soft magnetic properties.The only disadvantage of Fe-6.5%Si steel is poor workability due to the formation of B2 and D03 ordered phases [4].
The effect of poor workability can be avoided by inhibiting the formation of ordered phases by rapid solidification.Planar flow casting [5][6][7][8] is a rapid solidification technique to prepare Fe-6.5%Si ribbons with a thickness of 10~100 m.Liang et al. [9] fabricated continuous Fe-6.5%Si ribbons of 35 m in thickness and 50~60 mm in width.The as-cast ribbons exhibited strong <100> fiber texture to improve the magnetic property and excellent room temperature (RT) ductility.Ouyang et al. [10] studied the effect of wheel speed on microstructures and properties.The twin-roll strip casting process (TRSC) was also applied to fabricate Fe-6.5%Si steel with characteristics of sub-rapid solidification rates, and samples were subsequently rolled to obtain Fe-6.5%Si sheets.For this technique, the cooling rate affects the microstructure of cast strips, such as grain size, texture, ordered phase, which determines the performance of the final product.It has been reported that microstructure of reheated cast strips can be controlled by different cooling mediums [11][12][13][14].However, cooling rate of TRSC controlled by process parameters is not consistent.Cooling condition and microstructure of arc-melting and suction-casting (AMSC) are similar to those of TRSC, and cooling rates can be controlled by sample thicknesses.In this work, Fe-6.5%Si steels of different thicknesses were prepared by AMSC with different cooling rates.The effect of cooling rate on the microstructures and magnetic properties was studied.

Material and experimental methods
An ingot of Fe-6.5%Si was prepared using medium-frequency induction melting of high-purity metal chunks of Fe, Si (＞99.99 wt.%) and gravity casting.Small blocks with sizes of 10 mm × 10 mm × 10 mm were cut off from the ingot for remelting.These blocks were placed in a water-through copper crucible connected with the copper mold.The blocks in the crucible were heated and melted completely by tungsten electrode before being filled into the mold by a mechanical pump.High purity argon was used as the protective gas in the remelting process.Fe-6.5%Si samples with thicknesses of 1 mm, 2 mm, and 5 mm were prepared and labelled as H1, H2, and H5, respectively, hereafter.Microstructural characterization was carried out by optical microscopy (OM) and electron backscattered diffraction (EBSD) on the cross-profiles and normal surfaces of samples.OM samples were polished using standard metallographic techniques and etched with a solution of 5% nitric acid + 95% alcohol, while EBSD samples were electrolytically polished with a solution of 10% perchloric acid + 90% alcohol at a voltage of 20 V. The phase analysis was measured by using a Bruker D8 diffractometer with Cu-K radiation.The test angle (2) range was 20°90° (scanning speed of 4 °/min, step size of 0.02°) and 28°34° (scanning speed of 0.3 °/min, step size of 0.01°) , respectively.The closed-loop magnetic properties, including magnetic induction intensity and iron loss, were measured by using an AC Magnetic Tester (TD8535) equipped with a monolithic permeameter.

Microstructure and texture characterization
The cross-profile morphologies of all the samples are featured with double-layer columnar crystals, see figure 1.Furthermore, the length and width of columnar crystals both increase with increasing sample thickness.In order to investigate the microstructures and grain orientations at cross profiles, inverse pole figures (IPF) and {100} pole figures (PFs) are shown in figure 2. The IPFs and PFs show a typical <100> fiber texture for all samples, and the texture intensities are 6.7 mrd, 14.8 mrd and 12.1 mrd for H1, H2, and H5, respectively.(Note that all values for texture intensities in this work are given in units of multiples of a random distribution, mrd.)So, H2 and H5 exhibit stronger <100> fiber textures than H1.In IPF maps, almost all columnar crystals in H5 show red colour, indicating <100>//ND grain orientations, while many blue and green/orange columnar crystals appearing in H1 manifest that there are more non-<100>//ND grains in the sample.Therefore, with decreasing sample thickness, the orientations of columnar crystals change in the sequence of <100>, <110>, <111> parallel to ND, respectively.The {001} pole of the texture of H2 is tilted away from the normal direction (ND) of the sample, while that of H5 is slightly deflected.This inclination is more obvious and the texture distribution is more dispersed in H1.It shows that sample thickness also affects the deflection of {100} to ND of columnar crystals and samples with smaller thickness have larger deflections.Figure 3 shows the microstructures of all samples.It is seen that all samples are composed of equiaxed crystals on the normal section, and grains in this section are mainly oriented along <100> direction.The volume fraction of <100> orientated grains with a deviation angle of 15 are 20.3%, 25.0%, and 23.0%, respectively.The range of grain diameter distribution (figure 3a, b, c) increases with the increase in sample thickness, which is 20~180 m, 20~220 m, 20~320 m.There are more small grains ranging from 20 m to 40 m in H1 and H2 while the number of such grains is greatly reducing in H5.The average grain sizes are 66.6±30.8m, 93.0±42.8m, 149.7±66.8m for H1, H2, and H5, respectively.Figure 3 also shows the 2=45° cross-section of the orientation distribution function (ODF).It is shown that the <100> fiber texture changes from dispersive to sharp as the sample thickness increases and the maximum ODF intensity are 3.14 mrd, 3.56 mrd, and 5.85 mrd, respectively.Stronger {001} texture will promote the improvement of magnetic properties.The thin strip prepared by melt spinning obtains excellent magnetic properties due to the {001} fiber texture and B8 is 1.26 T which is almost equal to that of the CVD sheets at 1.25~1.30T [9].

Phase analysis
The X-ray diffraction (XRD) patterns are shown in figure 4. The three main peaks, near 45°, 65°, 83° of 2, are body centre cubic (BCC) fundamental diffraction peaks (figure 4a).Combined with Fe-Si binary phase diagram, it is shown that Fe-6.5%Si is mainly composed of ferrite with BCC structure at room temperature.It can be seen that the diffraction peak intensity of (200) plane increase with increasing sample thickness.The B2 superlattice diffraction peaks are found in the XRD patterns near the 2 angle of 32°, but the peaks are very weak.A fine XRD characterization is performed in the 2 range of 28°34°, see figure 4b.The peaks of ordered phases appear in all samples.With the reduction of sample thickness, the intensity of ordered phases peak decreases.In melt spinning process, the formation of ordered phase is inhibited by faster wheel speed of 30 m/s [1].

Magnetic property
At various applied magnetic fields from 800 A/m to 10000 A/m, magnetic induction intensity (B) of H1, H2, H5 is shown in figure 5a.It is seen that B increases with the increase of thickness but efficiency diminishes at higher magnetic fields.The B-H hysteresis loops at applied magnetic fields of 800 A/m and 5000 A/m are shown in figure 5(b, c).The B8 increases slightly from 1.17 T to 1.21 T as the sample thickness increased from 1 mm to 2 mm while the B8 of H5 is highest, about 1.30 T. H5 exhibits higher B even under a low magnetic field, which corresponds to a higher slope of the curve.The phenomenon means that increasing thickness can improve the permeability.The B50 of H1 and H2 shows no significant difference under the large applied magnetic field, being 1.50 T and 1.51 T, respectively.The B50 of H5 can reach 1.55 T. So, the magnetic properties of H5 are the best among these samples.
Figure 5e shows the iron loss at frequencies (f) of 50 Hz and 400 Hz.The value of P1/50, B=1.0 T and f=50 Hz, increases first and then decreases with increasing sample thickness, which is 2.41 W/kg, 2.74 W/kg, 2.19 W/kg, respectively.As the frequency reaches to 400 Hz, the iron loss decreases with the increase of sample thickness which is 42.30W/kg, 41.48 W/kg, 37.85 W/kg. Figure 5f shows a hysteresis loop corresponding to P1/400 and the area surrounded by it decreases with the increase of sample thickness.

Discussion
The microstructures and magnetic properties of Fe-6.5%Si samples are heavily dependent on the cooling characteristics.A columnar crystal morphology, with its long axis perpendicular to the sample surface, is observed for all samples.With increasing sample thickness, the temperature gradient in the melt decreased.The molten melt in contact with the copper mold begins to solidify at first, and there is a temperature gradient between the core and the surface.Due to the same cooling condition of the copper mold on both sides, columnar crystals form almost simultaneously and grow at the same speed to the middle of the sample, which results in the structure of double-layer columnar crystals (figure 1).A similar structure is found in twin-roll strip casting.Therefore, stronger columnar crystals can be observed in larger thickness samples (figure 2b, c).The preferred growth direction of columnar crystal is <100>, which results in a strong <100> fiber texture of the samples (figure 3).There is also a certain relationship between the thickness and the cooling rate.Based on the Fourier heat flow equation, related to thermal diffusivity and heat capacity, Lin and Johnson [15] obtained a simple equation expressing the cooling rate (CR) with the thickness at the sample core: where the unit of cooling rate is K/s and R is the thickness expressed in mm.
Therefore, the cooling rate of H1, H2, H5 should be 1000 K/s, 250 K/s, 40 K/s, respectively.A decrease in the R from 5 mm to 1 mm increases cooling rate by nearly two orders of magnitude.Different sample thicknesses lead to differences in microstructure and properties, indicating that the cooling rate plays an important role.The slow cooling rate results in full growth of columnar crystals and strengthening of <100> fiber texture (figure 2c).The cooling rate affects not only the grain morphology and texture, but also the formation of ordered phases.According to the Fe-Si binary phase diagram, the B2 and D03 ordered phases are formed at 650~700 C and 850~900 C, respectively.Therefore, rapid cooling becomes an effective way to restrain the ordered phases.A previous study had suggested that different cooling processes after annealing, including air cooling, water quenching, brine quenching and brine ice quenching, will affect the size of the ordered phases [16,17].The faster cooling rate of H1 inhibits the formation of ordered phases in this study.
The difference in magnetic properties is caused by the magnetization, including domain migration and rotation.Therefore, factors that affect the rotation and migration of magnetic domains will ultimately lead to changes in magnetic properties, such as textures and defects [18,19].In the case of 800 A/m, the domain migration determines the magnetization due to the lower intensity of magnetization.When the magnetic field intensity is up to 5000 A/m, it can be magnetized by means of the rotation of the magnetic moment.Due to more <100> oriented grains and fewer grain boundaries in H5, it is easier to perform rotation and migration of magnetic domains.Besides boundaries and textures, ordering degree is also an important factor affecting magnetic induction intensity, weakened by inhibiting the formation of ordered phases [10].The ordered phases in H5 are more than in other samples.Therefore, H5 exhibits the most excellent magnetic properties of B8 and P1/50.However, although magnetization resistance is lower in H2 due to large grain sizes and a strong <100> fiber texture resulting in higher B8, the iron loss is worse than that of H1 at the 50 Hz frequency.According to a previous study, defects in the materials, such as dislocations and antiphase boundaries, can also produce a pinning effect on the domain motion besides grain boundaries [20].The ordered phases formation in H1 is significantly inhibited, resulting in less antiphase boundaries, which is easier to magnetize.The iron loss performance of H2 is better than that of H1 at 400 Hz.This indicates that both grain boundaries and antiphase boundaries affect the magnetization process and the hindrance effect of grain boundaries is dominating.At low frequencies, less iron loss and magnetization barrier exist in materials.Therefore, the iron loss is sensitive to the ordered phases.At high frequencies, the magnetization barrier is large enough to ignore the effect of the ordered phases on iron loss.

Summary
Fe-6.5%Si samples having thicknesses of 1 mm, 2 mm and 5 mm, respectively, were prepared with different cooling rates by sub-rapid solidification method of arc melting and copper suction-casting.The effect of cooling rate on the grain morphologies, textures, ordered phases and magnetic properties of Fe-6.5%Si has been studied.The conclusions are as follows: (1) The cross profile of the samples shows the morphology of double-layer columnar crystal.The length and width of columnar crystals increase with decreasing cooling rate.The sample with the lowest cooling rate (the 5 mm thick sample) exhibits a sharper <100> fiber texture and a small deviation from the ideal texture.
(2) With decreasing cooling rate, the grain diameter on the normal section increases with the mean values being 66.6±30.8m, 93.0±42.8m, 149.7±66.8m, respectively; and the <100> fibre texture increases its intensity with the texture intensity being 3.14 mrd, 3.56 mrd and 5.85 mrd, respectively.The slow-cooled sample shows more <100> oriented grains and less non-<100> oriented grains compared to the fast-cooled sample.
(3) With increasing cooling rate, formation of B2 ordered phases is inhibited, and such an effect is clearer in the sample with the largest cooling rate (the 1 mm thick sample).
(4) The sample with the lowest cooling rate (the 5 mm thick sample) has the best magnetic properties due to its favourite texture and large grain size.Magnetic induction of B8 and B50 are 1.30T and 1.55 T, respectively.P1/50 and P1/400 are 2.19 W/kg and 37.85 W/kg, respectively.