The effect of heat treatments and Si contents on B2 ordering reaction in high-silicon steels

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1 Materials Science and Engineering A307 (2001) The effect of heat treatments and Si contents on B2 ordering reaction in high-silicon steels J.H. Yu a, J.S. Shin a, J.S. Bae a, Z.-H. Lee a, T.D. Lee a, H.M. Lee a, *, E.J. Lavernia b a Department of Materials Science and Engineering, Korea Ad anced Institute of Science and Technology, Kusung-Dong 373-1, Yusung-Gu, Taejon, South Korea b Department of Chemical and Biochemical Engineering and Materials Science, Uni ersity of California at Ir ine, Ir ine, CA , USA Received 22 June 2000; received in revised form 17 November 2000 Abstract The silicon content was increased up to 6.5% (by weight, unless specified otherwise) to reduce the power loss of the silicon steels. These steels were prepared by the conventional casting method or by spray forming and were investigated with the aid of light optical microscopy (LOM) and transmission electron microscopy (TEM). The difference in the casting method did not result in any difference in suppressing the B2 ordering. The D0 3 phase was observed only in the as-cast 6.5%Si steel. It was almost impossible to suppress the B2 ordered phase keeping the silicon level as high as 6.5% even after the heat treatment at 1000 C for 24 h or after hot rolling. It was necessary to change the Si level and control the cooling rate to suppress the ordering reaction, especially, in cooling after heat treatment. The silicon level of 5.87% was observed to be a critical value in suppressing the B2 ordering reaction Elsevier Science B.V. All rights reserved. Keywords: B2 ordering reaction; Heat treatments; High-silicon steels; Si contents 1. Introduction The silicon steels are mainly used in transformers, power generators and motors that are composed of iron cores and coils. While the silicon steels with well-oriented grains are used in transformers, those with equiaxed grains are used in motors and generators. For the 3%Si steel that has been used, especially, as the iron core, an electric power loss occurs due to eddy current and hysteresis. Recently, the power loss in the United States is estimated to be about 45 billion kwh and amounts to 3 billion dollars every year [1]. The power loss by eddy current is proportional to the square of the frequency. Thus, it is obvious that there exists a large amount of electric power loss in transformers used in high frequency. To overcome this hurdle, 6.5%Si steel has been suggested as the material of choice [2]. The resistivity of steel sheets has been reported to increase largely from cm for 3%Si to cm for 6.5%Si with * Corresponding author. Tel.: ; fax: a significant reduction in the power loss and the induced current. Moreover, it has been known that the loss by eddy current in high frequency is remarkably reduced compared with earlier 3%Si steels, and the noise of transformers decreases from 60 to 38 db because the magnetostriction is almost zero. The power loss can be treated by increasing the level of Si, but another problem of brittleness arises; thus, the production of steel sheets by cold rolling becomes almost impossible, and manufacture even by hot rolling becomes difficult if the Si content exceeds 4%. It has been verified that the B2 long-range ordering is the main cause for material brittleness since it induces the formation of dissociated superlattice dislocations and the correlated high intragranular stress concentration [3]. It may be predicted that the ordering reaction from A2 (disordered bcc) to B2 is suppressed in the temperature range above about 800 C in the 6.5%Si steel [4,5]. Below 600 C at this composition, a two-phase field of B2+D0 3 [5] occurs. Therefore, the effect of heat treatments on the existence of ordered phases will be investigated by LOM and TEM. Also, the effects of composition changes and production methods such as /01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S (00)

2 30 J.H. Yu et al. / Materials Science and Engineering A307 (2001) Table 1 Heat-treatment conditions of the 6.5 and 5%Si steels a Composition Forming Heat treatment Miscellaneous (wt%) method Fe 6.5Si Fe 5Si Casting Casting a WQ: water quenching. None (As-cast) 1000 C/8 h and 24 h/wq None (As-sprayed) 1000 C/0.5 h and 5 h/wq None 1000 C/24 h /WQ Hot rolling Hot rolling Table 2 Heat-treatment conditions of steels with a different Si level a Si content (wt%) Heat treatment 5.5, 5.62, 5.75, 5.87, 6.0 None (As-cast) Casting 5.5, 5.62, 5.75, 5.87, C/24 h /OQ 4.5, 5.0, 5.5, C/24 h /WQ a OQ: oil quenching. Forming method spray forming and casting on the ordering reaction will be systematically explored. 2. Experimental procedures The experiments are performed in two parts. Firstly, the effect of forming methods and heat treatment conditions on the ordered phases is investigated. Secondly, the effect of the silicon content is studied. The details of heat treatments and processes of the specimens tested in this work are listed in Tables 1 and 2, and the processing parameters of the spray forming are shown in Table 3. Hot rolling is an indispensable process to produce the Si steel sheets. The optimum condition for reducing iron loss was reported as the silicon content of 6.5% with a slab thickness of about 0.1 mm [3]. Specimens for hot rolling are heated at 1200 C for 20 min and rolled from an initial 8 mm thickness to 2 mm with a total reduction of 75%. Before the final step of hot rolling, they are reheated to C for 5 min. The identification of the ordered phases, B2 and D0 3, is conducted by investigating the diffraction patterns using Philips CM20 TEM operated at 200kV. Thin foils for TEM are prepared by twin jet-polishing using a solution of 24% nital cooled to 30 C at 8 V. Specimens for LOM are polished and etched with a solution consisting of 30 g ammonium persulfate in 300 ml of distilled water. 3. Results The diffraction pattern of the as-cast 6.5%Si steel in the 011 zone axis and its schematic illustration are shown in Fig. 1(a) and (b), respectively. According to the diffraction pattern of Fig. 1(a), the B2 phase is identified from the weak spots and the D0 3 ordered phase from weaker reflections. As may be expected from the binary Fe-Si phase diagram [4], both phases are stable at this composition where a two-phase field is located, and their presence has been verified. This observation is consistent with the results of previous works [6,7]. The reflections coming from the D0 3 phase are so weak that the dark-field image is not obtained in this case. For the B2 phase, this will be shown in the next figure. The as-sprayed (not as-cast) specimen of the same Si content, 6.5%, has been observed. The diffraction pattern in the 011 zone axis and the schematic drawing are shown in Fig. 2(a) and (b), respectively. The diffraction spots of the B2 phase were identified as in Fig. 1, but not D0 3. It seems that the relatively fast cooling in spray formation prevented the D0 3 phase from forming [3]. A thin-foil TEM dark-field image containing antiphase boundaries is shown in Fig. 2(c) with a magnification. The 100 reflection was used to obtain the dark-field image. In this material, the antiphase boundary excess free energy is isotropic, so they are smoothly curved. The smoothly curved boundary has a displacement vector of the type (a/2) 111. The domain size of the antiphase domains is relatively small, namely, smaller than 100 nm because the specimens were not subject to additional heat treatments. The same specimen was investigated by LOM in the assprayed state, and this is shown in Fig. 3. The grain size of the as-sprayed state is in the order of several hundred micrometers, i.e. very coarse. Because the materials already had a coarse grain size through spray Table 3 Processing parameters of the spray forming in this study Composition (wt%) Atomization gas type Atomization pressure (psi) Pouring temperature ( C) Deposition distance (m) Fe 6.5Si Ar Fe 5Si Ar

3 J.H. Yu et al. / Materials Science and Engineering A307 (2001) Fig. 1. TEM micrographs of the as-cast 6.5%Si steel: (a) diffraction pattern in the 011 zone; (b) schematic diagram. The solid circle represents A2, B2 and D0 3, the dashed circle represents B2 and D0 3, and the dotted circle represents only D0 3. forming, this did not fulfill the original purpose that the better workability of materials might have been obtained by making the grain size fine. Thus, the effect of heat treatments on the ordering reaction was investigated, keeping the same 6.5%Si level. Cast or spray-formed samples were held at 1000 C from 0.5 up to 24 h. Spray-formed specimens were held for 0.5 and 5 h while cast samples were held for 8 and 24 h. According to the equilibrium phase diagram [4,5], it was predicted that the ordered phase is unstable at this temperature, and only the disordered A2 matrix phase is in equilibrium. None the less, the B2 Fig. 2. TEM micrographs of the as-sprayed 6.5%Si steel: (a) diffraction pattern in the 011 zone; (b) schematic diagram; (c) dark-field image using a 100 reflection of (b). Only B2, not D0 3, is identified in this condition.

4 32 J.H. Yu et al. / Materials Science and Engineering A307 (2001) It is notable that oil quenching is used in the actual manufacturing process instead of water quenching, so as to avoid thermal shock. Thus, oil quenching was used after heat treatment rather than water quenching. The 5.5 and 6%Si steels with a rather higher silicon content and a higher capability of ordering than 4.5 and 5%Si steels were oil-quenched after heat treatment at 1000 C for 24 h. The diffraction pattern of the 6%Si steel in the as-cast condition is shown in Fig. 6(a) and (b) is true for the condition of heat-treating. Ordering was observed in both cases, while it was not observed in water quenching after heat treatment. For the 5.5%Si steel, the ordering reaction was suppressed either by oil Fig. 3. Microstructure of the as-sprayed 6.5%Si steel observed by LOM. The grain size is in the order of several hundred micrometers. phase was identified in these specimens, although their diffraction patterns were not given here. Additional heat treatment at 1000 C, way above the B2 ordering temperature, did not revert the ordering reaction. Therefore, the effect of hot rolling on ordering was studied because it is a mandatory process in making silicon steels, and this temperature belongs to the stable disordered phase region. The diffraction pattern and its schematic diagram of the spray formed 6.5%Si steel followed by hot rolling are shown in Fig. 4(a) and (b) with a 001 zone axis. The ordered phase is still present, and it has been proved that even the hot rolling is not sufficient to suppress the B2 phase. One of the final goals of this study is to find the minimum heat treatment after which the ordered structures disappear. Now, it seems practically impossible to make the ordered phase suppress or disappear, keeping the silicon content as high as 6.5%, which is consistent with previous reports [8,9]. Thus, the lower 5%Si content was tried as a test. The diffraction pattern of the spray-formed 5%Si steel is shown in Fig. 5 when heattreated at 1000 C for 24 h after hot rolling. The zone axis was 011, and the B2 phase was not observed. In considering the effect of earlier extended heat treatment or hot rolling applied to the 6.5%Si steel, the reduced silicon level was regarded as a major factor in suppressing the ordered phase rather than the combined heat treatments. It was decided to reduce and vary the silicon content systematically. Four different compositions were chosen, 4.5, 5, 5.5 and 6%, and the cast specimens with the new silicon levels were heat-treated at 1000 C for 24 h followed by water quenching. According to the binary Fe Si phase diagram [5], the phase boundary between A2 and B2 is extrapolated to 5%Si at room temperatures. Fortunately, the B2 phase was suppressed and not identified in all the reduced Si contents below 6%. Reduced silicon levels seem to be very effective in suppressing the ordering reaction. Fig. 4. TEM micrographs of the spray-formed 6.5%Si steel after hot rolling: (a) diffraction pattern in the 001 zone; (b) schematic diagram. B2 is identified.

5 J.H. Yu et al. / Materials Science and Engineering A307 (2001) outcome was unsatisfactory because of the coarser assprayed grain size than expected. Thus, the specimens made by the conventional casting were mostly used in this study instead of the specimens by the spray-forming method. Even the difference between casting and spray forming did not exert a noticeable effect on the ordering reaction itself. The extended heat treatment at 1000 C up to 24 h and/or hot rolling did not prove to be effective in suppressing the ordering reaction, either, as long as the Fig. 5. Diffraction pattern of the spray-formed 5%Si steel heat-treated at 1000 C for 24 h after hot rolling, in the 011 zone. No ordered phase is identified. quenching or by water quenching after heat treatment, as shown in Fig. 7. Keeping in mind that the 5.5%Si specimen did show the presence of the B2 phase in the as-cast condition, the B2 ordered phase disappeared after heat treatment of the same specimen. By now, it has been realized that the B2 ordering reaction is suppressed through heat treatment followed by oil quenching with silicon levels of between 5.5 and 6%, even if it occurs in casting. Thereby, three more compositions of 5.62, 5.75 and 5.87%Si steels were selected, heat-treated at 1000 C for 24 h and oilquenched. As a result, the ordered phase was not identified, and it must have been suppressed in all the specimens. It was concluded that the ordering reaction could be suppressed up to the 5.87%Si level of the electrical steel when they were heat-treated and quenched in oil. 4. Discussion There have been several attempts to make thin steel sheets directly by melt spinning [10] or by depositing Si on the 3%Si steel sheet through the chemical vapor deposition (CVD) followed by diffusion [11], however, difficulties in mass production still remain, and even the production costs are very high. Lately, a large amount of research has been carried out to produce Si steel by the spray-forming method [12,13]. The spray-forming method is reported to bring about enhanced mechanical properties due to the fine grain size and little macrosegregation in the final materials, and it takes little time for homogenization afterwards. As a result, the sprayforming process was applied to this work. However, the Fig. 6. Diffraction patterns of the 6%Si steel: (a) in the as-cast condition; (b) after heat treatment at 1000 C for 24 h, in the 011 zone. Oil quenching is used in cooling, and the B2 phase is identified in both conditions.

6 34 J.H. Yu et al. / Materials Science and Engineering A307 (2001) Summary Whether the casting or the spray-forming method was used to make the high-si steel, there was no difference in terms of the ordering reaction to be considered. It was almost impossible to suppress the B2 ordering in the 6.5%Si steel, even after the heat treatment at 1000 C for 24 h. Instead, the silicon level of 5.87% was observed to be a critical value in suppressing the B2 ordered phase. The D0 3 phase was found only in the as-cast 6.5%Si steel. It was necessary to control the cooling rate to suppress the ordering reaction, especially, in cooling after heat treatment. Acknowledgements Fig. 7. Diffraction pattern in the 001 zone axis of the 5.5%Si steel quenched in oil after heat treatment at 1000 C for 24 h, which is essentially the same as Fig. 4(a). B2 is suppressed. 6.5%Si steel was employed. It was verified that the silicon level was a main factor in controlling the B2 ordering reaction. The critical silicon level was found to be 5.87% in this study. The silicon steel with this composition did not show any ordered phase when it was heat-treated and quenched in oil or water. The D0 3 ordered phase was identified only once in this study when the cooling rate was slow enough as in the as-cast condition. Swann et al. [5] identified the D0 3 precipitates in the B2 matrix of the 6.4%Si steel when the samples were held at 600 C for 24 h. Both the strong peaks of B2 and weak peaks of D0 3 ordered phase were observed by a European group in the 6.5%Si steel ribbons produced by planar flow casting [3]. What they reported additionally was that the D0 3 phase became too obscure to be observed on increasing the cooling rate of casting beyond C/min, which is quite consistent with the current results. The cooling rate also must be important in controlling the kinetics of the ordering reaction. The effect of the presence of the ordered phases mainly lies in the material brittleness and workability of the electrical steel. This will be dealt with separately in another publication. This study has been supported in the framework of the international joint research between Korea and USA by the Ministry of Science and Technology. Additional support from the BK21 Project is also appreciated. References [1] A.J. Moses, J. Magn. Magn. Mater. 112 (1992) [2] K.I. Arai, K. Ishiyama, J. Magn. Magn. Mater. 133 (1994) [3] B. Viala, J. Degauque, M. Fagot, M. Baricco, E. Ferrara, F. Fiorillo, Mater. Sci. Eng. A212 (1996) [4] T.B. Massalski, Binary Alloy Phase Diagrams, second ed., ASM International, Materials Park, 1990, p [5] P.R. Swann, L. Granas, B. Lehtinen, Metal Sci. 9 (1975) [6] Y. Takada, M. Abe, S. Masuda, J. Inagaki, J. Appl. Phys. 64 (1988) [7] K. Narita, IEEE Trans. Magn. 16 (1980) [8] I. Ibarrondo, S. Surinach, J. Gonzalez, J. Magn. Magn. Mater. 112 (1992) [9] B.W. Oh, S.P. Hong, Z.H. Lee, J. Kor. Inst. Met. Mater. 30 (1992) [10] G.C. Eadie, J. Magn. Magn. Mater. 112 (1992) [11] H. Haiji, K. Okada, T. Hiratani, M. Abe, M. Ninomiya, J. Magn. Magn. Mater. 160 (1996) [12] P.S. Grant, Prog. Mater. Sci. 39 (1995) [13] K. Narita, M. Enokizono, IEEE Trans. Magn. 15 (1979)