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Journal of Magnetism and Magnetic Materials 281 (2004) 135–139

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+86278787

E-mail

0304-8853/

doi:10.1016

Magnetic properties of high silicon iron sheet fabricatedby direct powder rolling

Ran Lia,*, Qiang Shena, Lianmeng Zhanga, Tao Zhangb

aState Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology,

Wuhan 430070, ChinabSchool of Materials Science and Engineering, Beihang University, Beijing 100083, China

Received 9 December 2003; received in revised form 3 April 2004

Available online 11 May 2004

Abstract

In this research, the high silicon–iron strips (near to 6.5%Si) were produced by the direct powder rolling (DPR)

technique. The continuous strips could be compacted by rolling from silicon and iron powders and sintered by the

following heat treatments. The chemical formula, Fe+Si-Fe (Si)+Fe3Si (Si), controls the reaction of silicon and iron

powders. Optimized mechanical properties of the strip due to the composite phase between Fe3Si (Si) and Fe (Si) make

the further rolling possible. The soft magnetic properties of final samples were measured in the frequency from 50Hz to

20 kHz and the results show that the core loss is relatively lower above 1 kHz.

r 2004 Elsevier B.V. All rights reserved.

PACS: 75.50.Bb; 81.20.Ev; 81.30.Hd; 81.40.Rs

Keywords: High silicon–iron strip; Direct powder rolling; Powder reaction; Heat treatment

1. Introduction

It is well known that high silicon–iron alloyscontaining 6–6.5wt% Si possess excellent softmagnetic properties [1–3], such as high saturationmagnetization, near zero magnetostriction and lowiron loss in high frequencies. However, it isimpossible for the alloys to be rolled into a thinsheet at low temperature because of their poor

onding author. Tel.: +862787217002; fax:

9468.

address: leerun@sina.com (R. Li).

$ - see front matter r 2004 Elsevier B.V. All rights reserve

/j.jmmm.2004.04.098

ductility and therefore their commercial applica-tion is limited. Many researchers have attemp-ted to fabricate the alloy sheets with large scaleand high soft magnetic properties by variousmethods.

Arai and Tsuya reported the preparation ofribbon-form silicon–iron alloy containing around6.5% silicon by the rapid quenching technique. Inthis technique the ribbons were fabricated throughthe molten alloy cooled immediately on the surfaceof a rapid rotating roll. The size was 20 to 150 mmin thickness and 2 to 25mm in width [4]. Althoughthe ribbons could be easily cold rolled, the

d.

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R. Li et al. / Journal of Magnetism and Magnetic Materials 281 (2004) 135–139136

thickness and width were limited and the techniquecould not satisfy the commercial production.

Yamaji and Abe et al. reported that thechemical vapor deposition (CVD) technique wassuccessfully applied to produce the Fe-6.5%Sisheet by the NKK Corp. The thickness ofcommercial productions ranged from 0.1–0.3mmand the maximum width was 600mm. In thistechnique the cold-rolled 3% Si steel sheet wasused as the basic material, on which the chemicalreaction, SiCl4+5Fe=Fe3Si+2FeCl2, took placeby CVD. The following high temperature diffusionmade it homogeneous [5]. However the manufac-turing capacity is limited by the CVD siliconizingprocess and the pollution may be induced sinceSiCl4 is used.

The direct powder rolling (DPR) technique hasbeen studied for a century, which can be easilyrealized in the commercial production for itssimple instrument and mature technique. Not onlysome conventional alloys, such as brass, stainlesssteel, nickel-iron alloys, but some brittle alloys,such as permanent-magnet alloys and cobalt–ironmagnetic alloys, can be fabricated into sheetsthrough this technique followed by the compac-tion sintering at different temperatures [6]. Haja-ligol et al. reported FeAl alloy intermetallic sheet,which has a low ductility at the room temperature,could be fabricated from the water atomizedpowders by powder rolling [7]. Therefore, thetechnique of the direct rolling with iron and siliconpowders can give us a hope to overcome theshortcoming of the poor ductility of the highsilicon–iron alloy. Wang reported that the ironpowder and FeSi17 powder were consolidated intoFe-3%Si strips by directly rolling, which were0.8mm in thickness and 50mm in width [8]. Baset al. gave a review about the properties andapplications of soft magnetic materials sintered byPowder Metallurgy, which only included Fe-3%Sialloy [9]. Whereas so far there is few report aboutfabrication of high silicon–iron sheets containingSi near 6.5wt% by direct powder rolling. Recently,we have succeeded in the fabrication of highsilicon–iron strips containing 6–6.5wt.% Si byDPR. In this paper we present the sinteringprocess, component and magnetic properties ofthe strips.

2. Experimental procedures

To ensure good soft magnetic properties of thefinal product, atomized iron powder with a highpurity and a size of about 150 mm was used. Siliconpowder was of 99.9% purity with particle sizeo60 mm. The admixture containing 93.5wt.% ironpowder and 6.5wt.% silicon powder were blendedby the light milling for 3 h. The mixed powder wasfed into the gap between a set of vertical rotatingrolls and thus was compacted into a continuousstrip with a width of 65mm.

The green strips were firstly sintered at 1000�Cfor 3 h in a 5% hydrogen-95% argon atmosphere,then rolled repeatedly. The strips were homoge-nized by the following heat treatment at 1200�Cfor 3 h and nitride boron was used to preventconglutination of the strips. Finally, annealing at800�C for 1 h was adopted after the surfaces of thestrips were burnished for controlling the planenessand removing the surface oxide. The final stripswere 60mm wide and 0.2–0.35mm thick, whichcontained 6.0–6.5% silicon because some siliconpowders may be lost during the fabricated process.

X-ray diffraction data (XRD) over angularrange 20�p2yp120� were collected at a scanningrate of 3�/min. by a Rigaku RTP300 X-raydiffractometer at 40 kV and 150mA with Cu Karadiation. DC hysteresis curves were obtainedusing the vibrating sample magnetometer (VSM).The size of the samples was 8mm� 4mm�0.3mm. Core losses were measured using ring-form specimens with a size of F14mm� 6mm, ina frequency range of 50Hz–20 kHz by a ACBH-100K. All of specimens were annealed at 800�Cfor 1 h after being cut from the strips.

3. Results and discussion

The continuous green strip prepared by theDPR technique can be coiled with a bendingdiameter less than 100mm by a coil bucket, asshown in Fig. 1. Fig. 2 shows the XRD patterns ofthe green strip and the strips at different stateduring the fabrication process. The consolidatedgreen strip by the direct powder rolling isconstituted by silicon and iron powders. After

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Fig. 1. Continuous green strip compacted and coiled from Fe

and Si powders by DPR.

Fig. 2. XRD patterns of the green strip and strips at different

state during the fabrication process.

Fig. 3. DC hysteresis loop of the final strip (0.3mm in

thickness).

Fig. 4. Core losses of the final strip in frequencies 50Hz–

10 kHz (0.2mm).

R. Li et al. / Journal of Magnetism and Magnetic Materials 281 (2004) 135–139 137

sintering at 1000�C for 3 h, the XRD pattern ofstrip exhibits peaks of Fe and Fe3Si. It is suggestedthat the following reaction occurred during thesintering,

Feþ Si-Fe ðSiÞ þ Fe3Si ðSiÞ;

where

Fe3Si ðSiÞ ¼ The Si solution based on Fe3Si;

Fe ðSiÞ ¼ The Si solution based on Fe:

It is also seen that the silicon peaks absolutelydisappeared, indicating that the reaction betweenthe iron and silicon powders was complete. Fe3Si(Si) phase distributed in the Fe (Si) strip can be the

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Table 1

Comparison of magnetic properties among high silicon–iron strips produced by DPR in this work, Fe-6.5%Si sheets produced by CVD

and conventional Si Steels

Thickness (mm) Core loss (W/kg)

W10/50 W10/400 W2/1k W2/10k

High silicon iron sheet (PDR) 0.2 1.78 19.08 1.65 70

Fe-6.5%Si (CVD) [10] 0.1 0.51 5.98 0.96 32

0.3 0.49 10.0 1.80 74

Grain-oriented 3%Si steel [10] 0.30 0.35 10.5 2.70 >150

R. Li et al. / Journal of Magnetism and Magnetic Materials 281 (2004) 135–139138

storage of a part of silicon atoms and preventedthe strip from the formation of the high silicon–iron phase which results in low ductility bydiffusing. Therefore the treated strips could beeasily processed by the following rolling. It is alsoseen in Fig. 2 that the heat treatment at 1200�C for3 h made the whole strip homogeneous. Thepattern of annealing shows that cristobalite andnitride boron on the surface could be eliminatedby burnishing.

The mechanical properties of the high siliconiron sheets produced by DPR technique are sogood that strips can be lathed and punched intorings which were used to attain the AC magneticproperties in this work. The DC hysteresis loop ofthe final strip is shown in Fig. 3. The saturationinduction is close to the theoretical value.

Fig. 4 shows the core loss of the high silicon–iron specimens (0.2mm in thickness) from 50Hz–20 kHz. It is indicated that the alloy produced byDPR has a relatively low core loss especially inhigh frequencies. The comparison of the values ofiron loss among high silicon iron sheets by DPR,Fe-6.5%Si sheets produced by CVD and conven-tional Si steels is shown in Table 1. In low andmiddle frequencies (below 400Hz), the core loss ofour sample is much higher than that of Fe-6.5%Sisheet produced by CVD. The origin is theinhomogeneity of the microstructure of the stripsfabricated by the powder metallurgy. Some partsof grain bounds of powder are enriched by the tinyoxide, which can not be eliminated easily by thehydrogen atmosphere in high temperature. There-fore the homogenization by the heat diffusionamong the different components is not easy. In

high frequencies (above 1 kHz), the value of coreloss was relatively good, which is similar to Fe-6.5%Si sheet productions by CVD. Because theproduction technique of the high silicon–iron strip(near to 6.5%Si) by DPR has many advantages,such as low energy loss and simple productiontechnique, it has a great potential of continuousindustrial production.

4. Conclusions

High silicon–iron strip (near to 6.5%Si) wasfabricated by the direct powder rolling (DPR)technique successfully. The XRD results showedthat the diffusion of Si and Fe atoms was controlled by the reaction, Fe+Si-Fe (Si)+Fe3Si(Si). The composite phase between Fe3Si (Si) andFe (Si) brought the strip good mechanical proper-ties which made the further rolling possible. Thestrip exhibited an excellent magnetic performancein high frequencies of 1–10 kHz. It is indicated thatthe technique has the potential industrial value forproducing high silicon–iron strips.

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