Grain oriented silicon steel sheet with a ceramic film characterized by ultra-low iron loss

Vacuum/volume

Pergamon PII:

SOO42-207X(96)00082-6

47lnumbers 6-8/pages 857 to 86211996 Copyright 0 1996 Elsevier Science Ltd Printed in Ereat Britain. All rights reserved 0042-207X/96 $15.00+.00

Grain oriented silicon steel sheet with a ceramic film characterized by ultra-low iron loss Y Imkuti,

Research

Laboratories,

Kawasaki

Steel Corporation,

Japan

An improvement in the iron loss of grain oriented silicon steel sheet was investigated when using a ceramic coating such as TiN, CrN or Tic produced by ion plating on polished silicon steel sheet. iron loss W,,,50fw/kg) of silicon steel sheets with a ceramic film showed a dramatic improvement of about 40% in the final magnetic properties concerning magnetic domain refinement. These ceramic-coated silicon steel sheets had the superior properties of high magnetic flux density, low magnetostriction and a high lamination factor. The influence of different PVD coating methods on the magnetic properties was also investigated. The degree of improvement of W,,,,, (w/kg) by the HCD method was about twice that by the EB+RF method. Silicon steel sheet TiN-coated by the HCD method had more marked compressive stress sensitivity than that coated by the EB+RF method. In order to clarify the mechanism responsible of the ultra-low iron loss of TiN-coated silicon steel sheet, a direct observation was made of the domain structure obtained by coating with a thin TiN film. The domain structure of the chemically polished area showed an original main domain with large black and white 180” domain wall spacings, while that of the TiN-coated area showed marked domain refinement resulting from the strong tensile stress induced by the TiN film. Copyright 0 1996 Elsevier Science Ltd. Key words: Grain oriented tensile stress.

silicon steel sheet, ultra-low

Introduction oriented silicon steel sheet, in which secondary recrystallized grains are highly aligned in (110) [OOl] orientation, namely, Goss orientation, are used mainly in a core material for transformers and other electrical machinery and equipment. Grain oriented silicon steel sheet has usually been manufactured through many complicated steps, leading to such silicon steel products being designated as an “artistic product of iron”. Energy conservation has made it important to produce grain oriented silicon steel sheet with a low core loss. In a recent innovation,‘.’ an ultra-low iron loss was obtained by applying II ceramic coating such as TIN, CrN or TIC by physical vapor deposition (PVD) or chemical vapor deposition (CVD) to polished silicon steel sheets. These ceramic film-coated silicon steel sheets, which showed no deterioration in magnetic properties after strain-relief annealing, had superior compressive stress dependence on magnetostriction, and good lamination performance. The attainment of the ultra-low iron loss was caused by an improvement in the hysteresis loss obtained by using a polished steel surface that was clean and free from impurities, and to the reduced eddy current loss arising from the strong tension of the ceramic film. Grain

iron loss, TIN films, hollow

cathode

discharge,

strong

This article presents typical experimental results for TiNcoated silicon steel sheets, which have given good magnetic properties (See Figure 3) and adhesion in polished silicon steel coated with various ceramic films.’ the influence of different PVD coating methods on these magnetic properties, a comparison of the magnetostriction in gram oriented silicon steel and TiNcoated silicon steel. and the mechanism of the domain refinement that is responsible for the ultra-low iron loss of TiNcoated grain oriented silicon steel. The PVD methods studied here were HCD (Hollow Cathode Discharge) and EB (Electron Beam) -t RF (Radio Frequency) methods that were each used for applying a TIN coating with PVD apparatus.

Experimental procedure The starting materials were two types of grain oriented silicon steel sheets of 0.23 mm in thickness, the chemical composition of the two raw materilas being, in mass%, 1) C: 0.043, Si: 3.31, Mn: 0.072. Se: 0.019. Sb: 0.023, and MO: 0.012, and 2) C: 0.040, Si: 3.28, Mn: 0.070, and S: 0.017. These silicon steel products were cut down to a sample size of 0.23 x 30 x 300 mm, and strain-relief annealing was carried out for 7.1 ks at 1073 K. This specimen size was determined by the frame size of the single sheet tester 857

Y Inakuti:

Grain oriented

silicon steel sheet

HCD

method

EB+RF

method

Figure 1. Schematic diagram of the ion plating PVD apparatus by the HCD and EB + RF methods.

(SST) that was usually applied for magnetic measurements. Since the magnetic flux density of the silicon steel products used in these experiments had an influence on the iron loss related to the process treatment, three kinds of specimens with different values of magnetic flux density [B,(T) levels of (A) 1.93T, (B) 1.90T and (C) I .87T] were selected from the silicon steel products. The TIN ceramic coating was applied to the polished silicon steel samples by using the ion plating apparatus shown in Figure 1. This PVD apparatus can be used with either of the two coating methods of HCD and EB + RF in its operation. Figure 2 shows the sample hoiders used in the PVD apparatus, which enabled the tension of 0.5-20 MPa (ordinarily tension: 3.5 MPa) to be applied to the specimen while rotating the specimen holder in one direction to simultaneously deposit a uniform coating on both surfaces. By using these specimen holders in the ion plating apparatus, it was possible to investigate in detail the influence on the magnetic properties by the two PVD coating methods. These specimen holders also prevented the thin silicon steel samples from deformation during coating. Ion plating by the HCD method was done with 20 kW power, 0.093 Pa vacuum, - 100 V bias, 673 K substrate temperature, 150 SCCM reactive Nz gas, 0.1 pm/min deposition rate and 1 .O pm thickness, The conditions for the EB + RF method were

15 kW power, 0.040 Pa vacuum, -500 V bias, 673 K substrate temperature, 500 W RF power, 150 SCCM reactive N2 gas, 0.05 pm/min deposition rate and 1.O pm thickness. As can be seen from Figure 2, it was possible to simultaneously deposit the ceramic coating on five small specimens of 30 x 300 mm, and to two large specimens of 150 x 300 mm. The magnetic properties of each silicon steel sheet sample after TIN coating were measured, and the magnetostriction and variation in magnetic properties caused by strain-relief annealing were measured with some specimens. An observation of the magnetic domain was also done through an electron microscope (H-700).

Experimental results Magnetic flux density B,(T) and iron loss W,,&@/kg) of TiNcoated grain oriented silicon steel by the HCD method have been measured to check the improvement of magnetic properties from silicon steel product to TIN coating. Their values were in good agreement within 5% of the TiN film by use of the similar silicon steel level [samples (B): 1.90 T] of Figure 3.’ Figure 4 shows the variation in B,(T) and W,,,,,(\i~/kg) plotted for each process from the original silicon steel product to the final domain refining treatment, using products with three different values of magnetic

Large specimens

Small specimens Rotation

(a) Figure 2. Schematic 858

diagram

of the tensile holders

Weight for (a) small and (b) large specimens

weight (b)

YInokuti:

Grain oriented

silicon steel sheet

k

P (Product) 0 C (Ceramic film)

0

0.60 -

I I

I

I

t

I

I I

I I

I I

I

I

PC PC PC PC PC PC PC PC Figure 3. Variations in magnetic flux density B,(T) and iron loss m17.S0(w/kg) for the grain oriented silicon steel product (P) and with ceramic coatings (C) of TiN. 0-N. BN, Ti(CN), Tic. SIC. SO, and A&O,‘. {Silicon steel product of samples (B) levels for magnetic flux density B,:l .YOT were used.;

flux density as the starting specimens. Each point plotted in Figure 4 shows the average magnetic properties of five samples TiN-coated by use of specimen holder (a) in Figure 2. It is evident from Figure 4 that B, resulting from chemical polishing, after the 1.96,

1

2

2 0.60$ j

0.70-

5 0.60 -

n

A

CC)

J

EB +RF method 0.50’

I

Product

I

I

I

I

Chemical polishing

Ti N coating

Strain-relief annealing

Domain refining

I

Figure 4. Variations in magnetic flux density B,(T) and iron loss W,, ,JwN:kg) in each process from the original silicon steel product to domain refining for the HCD and EB + RF methods.

insulating coating and Forsterite film had been removed, was increased moderately by 0.004O.011 T.and that W,7,5,, was improved greatly by 0.05-O. 11 w/kg. After the subsequent TiN coating, it can be seen that the degree of improvement in both B, and W,, 5,, by the HCD method was about twice that by the EB+ RF method; namely, the HCD method gave a B8 improvement of 0.00330.007 T, while for EB+RF, B8 was improved by O.OOlLO.003 T. In contrast, the HCD method gave a significant W1,.s0 improvement of 0.094.15 w/kg+ while for EB+RF3, W,, 5,, was improved by about half (0.04-0.08 w/kg) the figure achieved with HCD. After the subsequent strain-relief annealing for 7.2 ks at 1073 K, the magnetic properties by the HCD method were different from those by EB+RF. Both B, and W,, i0 from the HCD method decreased only slightly by 0.001-0.004 T and were thus hardly changed, respectively, while those from the EB+ RF decreased by 0.001~.003 T and 0.0220.04 w/kg, respectively. After the final domain refining, B, of the HCD and EB+RF methods decreased slightly by 0.001LO.003 T, but W,, 5. from the HCD was improved greatly by 0.05-0.09 w/kg, while that from EB + RF was improved by 0.0550.07 w/kg. This improvement in magnetic properties of the silicon steel sheet TiN-coated by the HCD method is in good agreement with the previous results,’ being within an experimental error of 5”/0. The improvement in magnetic properties of the silicon steel sheet TiN-coated by both the HCD and EB + RF methods was greatest for the silicon steel material with a magnetic flux density of I .93 T. For example, with the silicon steel products of B, = 1.93 T and W,, ?,, = 0.88 w/kg, the magnetic properties after domain refinement show an ultra-low iron loss of 0.55 w/kg, which corresponds to an improvement in iron loss of about 40%. Figure 5 shows a comparison between the compressive stress sensitivity of magnetostriction in the original grain oriented silicon steel and TiN-coated materials of samples (B) by the HCD and EB + RF methods. It is apparent that the TiN-coated silicon steels had markedly improved compressive stress sensitivity within the measured range, and that the sensitivity resulting from the HCD method was more marked than that caused by the

Yhokuti:

Grain oriented silicon steel sheet

. .-g .-zL x B 4F

6-

/ Grain oriented

4-

silicon steej,,’

Ti N-coated silicon steel / (EB+RF)/TiN-coated silicon steel y-L (HCDI /. -- _A- R-- I I I 8 9 5 6 7 c

2-

1’

o.b -.a.e_-_yz_ o

--

12

/’

,-’ _ ___ _-4 3

/

Compressive stress, Fkure 5. Comuarison of the commessive stress sensitivitv coated by the kCD and EB + RF’methods.

of magnetostriction

EB + RF method. From the results of the compressive stress sensitivity of magnetostriction, it is considered that the strong surface tension added to the silicon steel sheet by the TiN ceramic coating was about 1.5-2 times that added by the insulating coating on grain oriented silicon steel sheet, in which a tension corresponding to about 4 MPa can be added by applying an insulating coating with a low thermal expansion coefficient over the forsterite film. In a similar estimation for the tension level, a tension greater than 8 MPa for HCD method and a tension of about 6.5 MPa for the EB + RF method were added to the TiNcoated silicon steel sheet. Figure 6 shows the domain structure of the chemically polished and TiN-coated areas lying inside the single giant secondary Goss grain. The domain structure of the chemically polished area shows an original 180” main domain with a large black and white wall spacings elongated in the rolling direction, while that of the TiN-coated area showed marked domain refinement caused by the strong tensile stress existing in the TiN film. At the borderline between the chemically polished area and the TiN-coated area, the domain changed drastically, showing six narrow wall spacings in the latter for each wall spacing in the former, and lens-like refined-domains had also been formed at the beginning of the TiN-coated area from the large black and white 180” main domains. On the chemically polished side of this line, longitudinally striped domains had formed distinctively within regions about f45” to the rolling direction because of the local strain arising from the strong surface tension in the TiN film. Discussion Improvement in the magnetic properties of TiN-coated silicon steel sheet. The experimental results shown in Figure 4 clarify that the formation of a thin TiN film on a polished silicon steel sheet improved the magnetic properties dramatically, and was particularly effective for improving the iron loss. In particular, the final magnetic properties after magnetic domain refinement of silicon steel sheets TiN-coated by the HCD method manifested an ultra-low iron loss of 0.55 w/kg, which corresponds to an 860

/’

O/M

for normal

Pa grain oriented

silicon steel sheet and silicon steel sheet TiN-

improvement of about 40%. From the magnetostriction data of Figure 5, it is apparent that the TiN-coated silicon steel sheet by the HCD method added a strong surface tension of more than 8 MPa, while that by the EB + RF method was 6.5 MPa. It should be noted that this strong surface tension added to the silicon steel sheet by the TiN ceramic coating was about 1.5-2 times that added by the insulating coating on grain oriented silicon steel sheet. Also, from observations of the domain structure in Figure 6. it was evident that the domain wall spacing at the borderline between the chemically polished area and the TiN-coated area had changed drastically, showing six narrow wall spacings in the coated area for each wall spacing in the polished area. It is thought the ultra-low iron loss in TiN-coated silicon steel sheet was obtained as a result of reduced eddy current loss arising from the strong surface tension of the TIN coating on the silicon steel sheet, which induced radical domain refinement. Difference in the magnetic properties of TiN-coated silicon steel sheets after applying the HCD and EB+ RF methods. The experimental results shown in Figure 4 clarify that the degree of improvement in iron loss of the silicon steel sheet TiN-coated by the HCD method was about twice that by the EB + RF method. It can also be seen clearly in the magnetostriction data of Figure 5 that the TIN coating by the HCD method could add stronger surface tension than by the EB+RF method. Although this improvement in iron loss of the TiN-coated silicon steel sheet was caused by the reduced eddy current loss arising from the strong surface tension induced in the near-surface of the sheet by the TIN ceramic coating, it should be noted that the degree of this improvement in magnetic properties was influenced greatly by the coating method that was used. In contrast, from measurement8 of the ionization rate, it should be noted that ion plating by the HCD method had a high ionization of 43%, and is known to give a fine and smooth TiN film, whereas ion plating by the EBf RF method had a lower ionization of 6%. Therefore, the fine and smooth TIN film with

Yhokuti:

Grain oriented

silicon

steel sheet

ain sNSI areas inside a single giant second&Goss

Figure 6. co&d

ly polished a.nd TiN-

grain in a silicon steel sheet

superior adhesion to the silicon steel substrate by the HCD method made it possible to induce an effective surface tension on the near-surface of the silicon steel sheet. In particular, it should be pointed out that, in the silicon steel sheet TiN-coated by the HCD method, no deterioration in iron loss took place after strain-relief annealing. while, in that by the EB+ RF method, a deterioration in iron loss of 0.02-0.05 w/kg took place. This would have been caused by the diffusion of Ti or N atoms into the silicon steel sheet during strain-relief annealing. In the chemical analysis of silicon steel sheet after stress-relief annealing, Ti or N contents of silicon steel sheet by EB+ RF method were 223 times as high as that by the HCD method. Therefore, it is considered that the deterioration in iron loss will be the result of an increase of Ti or N to silicon steel sheet, and because of due to the tensionloss of TIN film after stress-relief annealing. Further, from a comparison of the magnetic domains3 refinement by the HCD method was about 1.5 times better compared with the EB+RF method. These results indicate that ion plating by the HCD method makes it possible to obtain the lowest iron loss in TiNcoated silicon steel sheet. Also, as deposition parameters of plasma coating and after cooling treatment by the HCD and EB+ RF methods have been done in the same way, the difference in ion plating method on magnetic properties of TiN-coated grain oriented silicon steel sheet is considered due to that of properties of TiN film during plasma coating.

Conclusion

Improvements in the iron loss of TiN-coated grain oriented silicon steel sheet were investigated by ion plating with the HCD or EB + RF method to the polished silicon steel sheet. The following results were obtained: I. The iron loss of TiN ceramic

film-coated

silicon steel sheets

was dramatically improved. For example, in the silicon steel products with B, = 1.93 T, and W,7.q,j = 0.89 m/kg, the final magnetic properties after magnetic domain refinement manifested an ultra-low iron loss of 0.55 w/kg. which corresponds to an improvement in iron loss of about 40%. The degree of improvement in iron loss of the silicon steel sheet TiN-coated by the HCD method was about twice that by the EB+ RF method. This would have been due to higher ionization by the HCD method than that by the EB + RF method. With the HCD method, it was possible to give a strong surface tension to the near-surface of the TiN-coated silicon steel sheet, on which a fine and smooth TiN film with good adhesion had been coated. The silicon steel sheet TiN-coated by the HCD method had more marked compressive stress sensitivity than that coated by the EB+ RF method. It is estimated that a tension of greater than 8 MPa with the HCD method and a tension of about 6.5 MPa with the EB+ RF method were added to the TiN-coated silicon steel sheet, which was about I S- 2 times that added by the forsterite and glass films on grain oriented silicon steel sheet. After making distinct chemically polished and TiN-coated areas inside a single giant secondary Goss grain of a silicon steel sheet, the domain structure of the chemically polished area showed an original main domain with large black and white 180 domain wall spacing elongated in the rolling direction, while that of the TiN-coated area showed marked domain refinement caused by the strong tensile stress induced by the TIN film. 5 It is considered that the ultra-low iron loss in TiN-coated silicon steel sheet was obtained due to the eddy current loss arising from the strong surface tension of the TIN coating on the silicon steel sheet, which induced radical domain refinement. 861

Y Inokuti:

Grain oriented

silicon steel sheet

References ‘Y Inokuti, K Suzuki and Y Kobayashi, Mater Trans JIM, 33, 946 (1992). ‘Y Inokuti, K Suzuki and Y Kobayashi, (1993).

862

Mater Trans JIM,

34, 622

‘Y Inokuti, K Suzuki and Y Kobayashi, Japan Insr Metals, 59, 213 (1995). ‘Y Inbkuti, K Suzuki and 0 Oukubo, Vacuum Society @Japan, 38,633

(1995).