Effect of normalization on the microstructure and texture evolution during primary and secondary recrystallization of Hi-B electrical steel

Transcription

1 Indian Journal of Engineering & Materials Sciences Vol. 23, April & June 2016, pp Effect of normalization on the microstructure and texture evolution during primary and secondary recrystallization of Hi-B electrical steel Zhao-Yang Cheng a, Jing Liu a *, Jia-Xin Yang b, Jia-Chen Zhu a, Shuai Liu a & Zhi-Dong Xiang a a The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan, , P R China b National Engineering Research Center for Silicon Steel, Wuhan, , P.R. China Received 20 March 2015; accepted 11 January 2016 Normalization, as an important process step in the production of Hi-B electrical steels, influences the microstructure and texture evolution during the subsequent primary and secondary recrystallization annealing. The effects are investigated by TEM, EBSD and XRD for a Hi-B electrical steel. The results show that large numbers of small and dispersed AlN precipitated after normalizing treatment. The precipitated AlN inhibitors promoted the formation of CSL boundaries of Σ = 3, 11 and 13b, and boundaries with misorientation angles between 25-45º during the primary recrystallization, which are believed to promote the selective growth of the Goss grains during secondary recrystallization. After primary recrystallization, the normalized specimen showed the increase in the intensity of {111}<112> component and the decrease of the α*-fiber intensity. As a result, the growth of Goss component was promoted during the secondary recrystallization. Keywords: Hi-B electrical steel, Normalizing; Inhibitors, CSL boundary, Misorientation angle, Texture Grain-oriented silicon steels are a type of soft magnetic materials that is mainly used in the core of transformers 1,2. The magnetic properties of this type of steels are predominantly determined by the orientational sharpness of the {110}<001> texture component, i.e., Goss texture. High permeability electrical steels with magnetic inductance value B 8 higher than 1.88 T are also called Hi-B electrical steels 3,4. Normalizing is a critical step in the manufacture of Hi-B electrical steel sheets 5-7. Chen et al. 8 reported that the normalizing conditions, including temperature and cooling rate, have great effects on the microstructure, the distribution of inhibiting particles and the magnetic properties of the final product. Similar effects have also been observed by others researchers The main purpose of normalization is to promote the precipitation of fine particles that can effectively hinder primary recrystallization and facilitate the growth of Goss texture during secondary recrystallization. Primary recrystallized microstructure and texture influence the secondary recrystallized microstructure and texture. Since a significant amount of Goss texture forms during secondary recrystallization, the mechanism or origin of its formation is of great practical and scientific interests. Shimizu et al. 12 and *Corresponding author ( liujing@wust.edu.cn) Homma et al. 13 suggested in their coincidence site lattice (CSL) boundary theory that the selective growth of the secondary recrystallized Goss grains could be explained by the preferential migration of CSL boundaries. The CSL boundaries are supposed to have lower grain boundary energy, and hence having higher mobility than other types of boundaries with less pinning effect by the inhibitors. However, it is uncertain as to which kind of CSL boundaries are particularly important for the selective growth of Goss grains. Schimizu et al. 12 and Homma et al. 13 argued that the Σ = 9 boundary has lower energy and higher mobility than other grain boundaries. However, Hayakawa et al. 14 reported that the calculated boundary energy of Σ=9 boundary is relatively high and other boundaries such as low angle grain boundaries or high angle grain boundaries with misorientation angle over 45, such as Σ = 11 boundary, have much lower boundary energy than Σ = 9 boundary. Chang 9 suggested that the selective growth of Goss grains during secondary recrystallization was more likely related to CSL boundaries of Σ = 1, 3 and 15. This, however, was in sharp contrast to the predictions of the high energy boundary (HE boundary) model developed by Hayakawa and Szpunar 15, which indicated that high mobility boundaries were not CSL boundaries but the boundaries with misorientation angles of 20-45º. This

2 166 INDIAN J. ENG. MATER. SCI., APRIL & JUNE 2016 model suggests that these HE boundaries have high grain-boundary diffusivity and hence high mobility. In both the above two modes, the explanation for the selective Goss AGG in relation to such precipitates is that Goss grains have a large fraction of high-mobility boundaries, which are less pinned by such precipitates. However, other researchers indicated that grain boundary migration was driven by the grain boundary curvature, which was determined by the force balance at the triple junction. Thus in order for the abnormal grain growth (AGG) of Goss grains to occur, the high mobility of the grain boundary is not sufficient but the high mobility of the triple junction is required, which is the solid-state wetting mode. In this mode, the role of precipitates is somewhat different. Precipitates inhibit not only grain boundary migration but also solid-state wetting along a triple junction if they lie on the triple junction line. Therefore, solid-state wetting along a triple junction can occur only when precipitates on triple junctions are dissolved. The precipitates, by inhibiting grain growth, preserve the high energy grain boundaries, which are energetically favorable for being penetrated by solid-state wetting. If grain growth is not inhibited by precipitates, most high-energy boundaries would disappear during appreciable grain growth, leaving mostly low-energy boundaries behind. Then the frequency of the high-energy boundaries to be penetrated is so low that solid-state wetting would hardly occur. In this study, the effects of normalization on the microstructure and texture from primary to secondary recrystallization was investigated for two series of hot-rolled electrical steels, one was subjected to normalizing treatment and the other was not. The research is focused on the effects of normalization on the inhibitor precipitation and the mechanism of the selective growth of Goss grains during the annealing treatment. Experimental Procedure The Hi-B electrical steel used in this work was refined and cast into ingot in a vacuum induction melt furnace, yielding the following composition (in mass%): 3.11 Si, Mn, Cu, Al, C, N, S, Fe (balance). The as-cast ingot was forged at about 1150 C, and then hot rolled to 2.46 mm thick at 1350 C. The hot-rolled sheets were normalized at 1120 C for 40 s and then at 900 C for 90 s, followed by cooling in boiling water. After normalizing treatment, both the normalized and non-normalized sheets were cold rolled to 0.28 mm thickness. The specimens were then heated up to 840 C and soaked for 3 min in a wet H2-N2 atmosphere with a dew point at 25 C for decarburization. Secondary recrystallization annealing was carried out in a 25% N2 and 75% H2 mixed atmosphere with a heating rate of 12 C/h up to 1200 C and soaked for 18 h under a 100% H2 atmosphere for purification. Specimens were polished and etched with a solution of 4% nitric acid in alcohol at room temperature. The microstructure of the specimens was examined by optical microscopy and electron backscattered diffraction (EBSD). EBSD analysis was also performed to investigate the primary recrystallization texture. The orientation distribution functions (ODFs) of the secondary recrystallized specimens were measured by X-ray diffraction (XRD) (D/MAX-2500PC) with Mo K α radiation. The precipitates were analyzed by transmission electron microscopy (TEM) (JEM-2100F), and the composition of the inhibitors was analyzed by energy dispersive X-ray spectroscopy (EDS). Results and Discussion Inhibitors The morphology and composition of the inhibitors in the non-normalized and normalized specimens are shown in Fig. 1. The non-normalized specimen contained many secondary phase particles distributed as ribbons, while the normalized specimen showed a large number of precipitates homogeneously distributed across the grains. According to the EDS compositional analysis, the large sized precipitates (about 200~300 nm) were MnS + CuS + AlN (seen spectrum 1); the medium sized ones (about 70~200 nm) were mainly CuS + MnS (spectrum 2); and the small sized ones (about 15~80 nm) are CuS (spectrum 3) and AlN (spectrum 4). Therefore, the majority of inhibitors in the non-normalized specimen were medium sized CuS + MnS, while the normalized specimen mainly contained small sized AlN. There were also a number of large precipitates (MnS + CuS + AlN) with little inhabitation function in both specimens. These types of large particles were formed at the early stage of the precipitation, and coarsening up quickly during the hot rolling process. The medium sized MnS and CuS particles were mainly precipitated during the hot-rolling process due to the decreased solid solubility of S in α phase. AlN phases precipitated mainly during the normalization

3 CHENG et al.: RECRYSTALLIZATION OF Hi-B ELECTRICAL STEEL 167 Fig. 1 Morphology of precipitates and corresponding EDS spectra: (a) non-normalized specimen and (b) normalized specimen process, as the solid solubility of N in γ phase is ten times as much as that of in α phase. The normalizing treatment was conducted at 1120 C for 40 s and then at 900 C for 90 s. The solid solubility equation of AlN in γ phase can be expressed as: lg{[al] [N]} = /T where [Al] is the weight percentage of the dissolved Al, [N] is the weight percentage of the dissolved Al, T is the heat temperature. When the temperature is at 1120 C (T=1393 K), the dissolved [Al][N] is about ; when the temperature is at 900 C (T=1173 K), the dissolved [Al][N] is about Therefore, a large amount of AlN would precipitate out during the normalizing treatment, as observed in this work. According to Zener equation, Z 3 = 4 fσ r where Z is the Zener factor, f is the volume fraction of precipitates, r is the size of precipitates, σ is the surface tension. Thus, the smaller the size of the precipitates (r) and the larger of the volume fraction of the precipitates (f) are, the stronger the inhibition will be. When the size Fig. 2 EBSD microstructure after primary recrystallization: (a) non-normalized and (b) normalized of precipitates is larger than about 200~300 nm, the inhibition ability is near zero 13. According to the differences in sizes, AlN inhibitors can generally be divided into three types, i.e., A type: smaller than 20 nm; B type: 20~50 nm; C type: 100~300 nm.the B type AlN precipitates are the main inhibitors 14. Sakai 14 indicated that the AlN precipitates in the hot-rolled specimen were mainly C type together with a small number of B type and C type, while A type AlN precipitates were dissolved during the normalization process. Therefore in this experiment, the small numbers of AlN precipitates in the non-normalized specimen with large size were the complex of the three types of AlN. After normalization, a large number of B type AlN particles precipitated homogeneously from the matrix, which would play an important inhibition role during the secondary recrystallization process. Microstructure Figure 2 shows the primary recrystallized microstructure as revealed by EBSD after normalizing

4 168 INDIAN J. ENG. MATER. SCI., APRIL & JUNE 2016 and non-normalizing treatment. After primary recrystallization annealing, all specimens have homogeneous microstructures with fine grains. It is noticed that the microstructure in the normalized specimen is more homogenous than in the specimen without normalizing. EBSD measurements indicated that the specimens without normalization contained about 87.35% recrystallization, while it was about 96.61% recrystallized in the normalized specimens. The grain size in the normalized specimens (about 21.6 µm) was about 1.5 times that of the nonnormalized specimens (about 14.2 µm). As the secondary recrystallized grains are relatively large, not suitable for EBSD technique, an optical microscope has been used to examine the microstructures of the secondary recrystallized specimens with and without normalizing treatment (see Fig. 3). For the specimen without normalization, the secondary recrystallized microstructure was homogenous with an average grain size of about 404 µm. For the normalized specimen, it consisted of a few very large grains together with some relatively small ones. The very large grains with an average size of about 10 mm showed Goss orientation. Thus significant growth of Goss grains occurred during secondary recrystallization in the normalized specimens comparing to the specimens without normalization. Thus, the inhibitors precipitated during the normalization had little effect on the grain growth in the primary recrystallization but had large effect on the grain growth in the secondary recrystallization. In order to study the mechanism of the abnormal grain growth of Goss grains during secondary recrystallization, the CSL grain boundaries and HE boundaries were analyzed. EBSD grain boundary misorientation analysis showed that the CSL boundaries in the non-normalized specimen mainly consisted of Σ= 3, 9 and 11 boundaries, whereas the normalized specimen was dominated with Σ=3, 11 and 13b boundaries (Fig. 4(a)). Σ = 9 boundaries, termed as high mobility grain boundaries by Shimizu et al. 12, were present in both types of specimens thought the fractions of Σ = 9 boundaries in the normalized specimen was slightyly higher than that in the non-normalized one. On the other hand, there were many more Σ =3, 11 and 13b boundaries present Fig. 3 Optical micrographs showing: (a) planar microstructure of the non-normalized specimen after secondary recrystallization and (b) through thickness grain-structure of the normalized specimen after secondary recrystallization Fig. 4 (a) CSL boundaries and (b) misorientation angles after primary recrystallization

5 CHENG et al.: RECRYSTALLIZATION OF Hi-B ELECTRICAL STEEL 169 in the normalized specimen than in the nonnormalized one. It is, therefore, possible that the migration of Σ=3, 11 and 13b boundaries played big role in Goss grain growth during secondary recrystallization. Although this consideration differs from the hypothesis of Schimizu et al. 12 and Homma et al. 13, the results of grain misorientation analysis (Fig. 4(b)) supports the assumptions of Chang 9 and Hayakawa et al. 14 The misorientation angles in the non-normalized specimen are mainly distributed between 35-60º with the number fraction of an individual range below 3.5%. However, the misorientation angles in the normalized specimen spread between 20-60º though the fractions of the misorientation angles between 25-45º are relatively high (between 4% and 7%). Comparing with the specimen without normalization, the fraction of misorientation angles between 25-45º in the normalized specimen was increased. As mentioned previously, these high angle boundaries have high grain-boundary diffusivity, which may contribute to the selective growth of Goss grains. This observation is consistent with the model proposed by Hayakawa and Szpunar 15, in which the boundaries with misorientation angles of 20-45º are of the highest mobility and favorable for secondary recrystallization. Texture Figure 5 shows the ϕ 2 =45 sections of the ODFs from the primarily recrystallized specimens. The texture mainly consisted of γ-fibre with {111} parallel to the rolling plane and α-fibre with <110> parallel to the rolling direction. The intensity of the γ-fiber component from the normalized specimen was higher than that of the specimen without normalization. However, the intensity of the near α-fiber component (α*-fibre component) from the normalized specimen was much lower than that of the specimen without normalization. To illustrate the differences in the orientation density after primary recrystallization between the specimens with and without normalization, the orientation densities along γ-fiber and α-fiber of two specimens were shown in Fig. 6. The γ-fiber showed a peak at {111}<112> component for the normalized specimen, but the peak component of the specimen without normalization deviated from {111}<112>. The intensity of {111}<112> component of the normalized specimen [f(g)=5] was much higher than that of the specimen without normalization [f(g)=4], whereas α*-fiber intensity of the normalized specimen was lower than that of the specimen without normalization. After secondary recrystallization, both the nonnormalized and normalized specimens were highly textured (Fig. 7). Only Goss component was detected in the texture of the normalized specimen, but in the specimen without normalization, both Goss component and α*-fiber were present together with some other strong texture components, such as γ-fiber, which indicates that the selective growth of Goss grains was enhanced in the normalized specimen after secondary recrystallization. Taking into the account of the fact that the intensity of {111}<112> component increased whereas that of α-fiber reduced in the normalized specimen after primary recrystallization, it might suggest that the growth of the Goss component was promoted by the {111}<112> component, which is consistent with the observations of Yan et al. 6 and Cheng et al. 16, where the inhibitors would hinder the growth of Fig. 5 Constant ϕ 2 =45 ODFs of primary recrystallization measured by EBSD: (a) non-normalizing and (b) normalizing

6 170 INDIAN J. ENG. MATER. SCI., APRIL & JUNE 2016 normalization, large numbers of small and dispersed AlN (B type) precipitated. The precipitated AlN inhibitors promoted the formation of CSL boundaries of Σ = 3, 11 and 13b, and boundaries with misorientation angles between 25-45º during the primary recrystallization, which would be responsible for the selective growth of the Goss grains during secondary recrystallization. After the primary recrystallization, the normalized specimen showed the increase in the intensity of {111}<112> component and the decrease of the α*-fibre intensity. As a result, the growth of Goss component is promoted during secondary recrystallization. Acknowledgement This work is supported by the National Basic Research Program (863 Program) of China (Grant No.2012AA03A506) and the Provincial Natural Science Foundation of Hubei, China (Grant No. 2008CDA040). Fig. 6 (a) Orientation densities along γ-fiber and (b) α-fiber of primary recrystallization Fig. 7 ODF (ϕ 2 =45 ) of the surface of the specimens after secondary recrystallization measured by XRD: (a) non-normalizing and (b) normalizing {111}<112> component and promoting the growth of Goss component. Conclusions The present study confirmed that the normalizing treatment has large influence on the inhibitors, as well as on the microstructure and texture evolution from the primary and secondary recrystallization. After References 1 Gheorghies C & Doniga A, J Iron Steel Res Int, 16(4) (2009) Castro N A, de Campos M F & Landgraf F J G, J Magn Magn Mater, 304(2) (2006) e617-e Iwanaga J, Masui H, Harase J, et al, J Mater Eng Perform, 3(2) (1994) Song H Y, Liu H T, Lu H H, An L Z, Zhang B G, Liu W Q, Cao G M, Li C G, Liu Z Y & wang G D, Mater Lett, 137 (2014) Ushigami Y, Kumano T, Haratani T, et al., Mater Sci Forum, (2004) Yan M, Qian H, Yang P, Mao W, Jian Q & Jin W, J Mater Sci Technol, 27(11) (2011) Liu L H, Li L J, Huang J J & Zhai Q, J Magn Magn Mater, 324(14) (2012) Chen L, Li X, Qiu S T & Yong G, J Iron Steel Res Int, 21(7) (2014) Chang S K, Mater Sci Eng A, (2007) Xia Z S, Kang Y L & Wang Q L, J Magn Magn Mater, 320(23) (2008) Yang J X, Liu J & Li S D, Adv Mater Res, (2012) Shimizu R, Harase J, Dingley D J, Acta Metall, 38(6) (1990) Homma H & Hutchinson B, Acta Mater, 51(13) (2003) Hayakawa Y & Kurosawa M, Acta Mater, 50(18) (2002) Hayakawa Y & Szpunar J A, Acta Mater, 45(3) (1997) Cheng Z Y, Liu J, Yang J X, et al., Optoelectron Adv Mater, 8(9-10) (2014) Park H, Kim D Y, Hwang N M,, J Appl Phys, 95(10) (2004) Ko K J, Cha P R, Srolovitz D & Hwang N M, Acta Mater, 57(3) (2009) Lee D K, Ko K J, Lee B J & Hwang N M, Scr Mater, 58(8) (2008)