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Acta Metall Sin  2017, Vol. 53 Issue (1): 19-30    DOI: 10.11900/0412.1961.2016.00213
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Phenomena of Σ3 and Orientation Gradients in an ElectricalSteel Appliedα→γ→α Transformation
Louwen ZHANG,Ping YANG(),Weimin MAO
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
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Abstract  

At present, the quality of commercial non-oriented electrical steels is improved mainly by optimizing deformation and recrystallization textures, but the most desirable {100} texture for the magnetic properties of sheets is normally no more than 20% in volume fraction. Throughα→γ→α transformation, however, the percentage of {100} texture can be up to 50%, even as high as 80% or more. The characteristics of transformation microstructure in ultra-low carbon non-oriented electrical steel are basically revealed in this work, and the formation mechanisms are analyzed and discussed. The cold-rolled sheets of electrical steels are heated inγ single phase region,α→γ→α transformation occurs in hydrogen and nitrogen atmosphere, respectively. The results indicate that strong {100} texture with monolayer pancake grains is developed in hydrogen, and the size of {100} oriented grains reaches more than 1 mm; whereas near {100} and {110} textured columnar grains are formed at the surface layer of the sheets in nitrogen, and the equal-axed grains with {111} and {114} textures in the center layer are obtained finally. Σ3 grain boundaries generally appear in the transformation microstructure where grain orientations are preferred, and its formation mechanism is closely related to K-S relationship which is followed during variant selection induced by surface-effect. There is an approximate linear orientation gradient in the columnar grains at the surface of the sheet annealed in nitrogen, and this phenomenon should be resulted from the accumulation of transformation strain induced by the suppression of the growth of surface grains withγ→α transformation along the normal direction.

Key words:  electrical steel      α→γ→α transformation      Σ 3 grain boundary      K-S relationship      orientation gradient     
Received:  31 May 2016     
Fund: Supported by National Natural Science Foundation of China (No.51271028)

Cite this article: 

Louwen ZHANG,Ping YANG,Weimin MAO. Phenomena of Σ3 and Orientation Gradients in an ElectricalSteel Appliedα→γ→α Transformation. Acta Metall Sin, 2017, 53(1): 19-30.

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https://www.ams.org.cn/EN/10.11900/0412.1961.2016.00213     OR     https://www.ams.org.cn/EN/Y2017/V53/I1/19

Fig.1  Orientation map and orientation distribution function (ODF) of electron backscattered diffraction (EBSD) data displayed atφ2=45° section for through thickness cross-section of sample 1 after Q1 (φ1,Φ andφ2 are the Euler angles, which form a three-dimensional orientation space; RD, TD and ND represent rolling direction, transverse direction and normal direction of the sheets, respectively)(a) ODF(b) {111} pole figure of white frame in Fig.1c(c) orientation map shown in IPF-Z of Fig.1a (IPF-Z—projection of the grain orientations that are parallel to ND in the crystal coordinate system)(d) Kikuchi band quality map with Σ3 grain boundaries in Fig.1c(e) rotation axis distribution(f) misorientation angle distribution
Fig.2  Orientation map and ODF of EBSD data displayed atφ2=45° section for rolling plane at the surface of sample 1 after Q1(a) ODF(b) {111} pole figure of {100} large-grain interior referred by the arrow in Fig.2c (c) orientation map shown in IPF-Z of Fig.2a(d) Kikuchi band quality map with Σ3 grain boundaries in Fig.2c (e) rotation axis distribution(f) misorientation angle distribution
Fig.3  Misorientation profile measured by the black lines in Figs.1c and 2c
Fig.4  Orientation map and ODF of EBSD data displayed atφ2=45° section for through thickness cross-section of sample 2 after Q2(a) ODF(b) {111} pole figure of the large-grain interior referred by the arrow in Fig.4c(c) orientation map shown in IPF-Z of Fig.4a(d) Kikuchi band quality map with Σ3 grain boundaries in Fig.4b
Fig.5  Orientation map and ODF of EBSD data displayed atφ2=45° section for the surface of sample 2 after Q2(a) ODF(b) {111} pole figure of near {110} grain interior referred by the arrow in Fig.5c(c) orientation map shown in IPF-Z of Fig.5a(d) Kikuchi band quality map with Σ3 grain boundaries in Fig.5c(e) rotation axis distribution(f) misorientation angle distribution
Fig.6  OM image of rolling plane at the surface of sample 2 after Q2 (Arrow denotes stream-like morphology)
Fig.7  Orientation gradient within the grains measured along the lines shown in Figs.4c and 5c
Fig.8  Orientation map and ODF of EBSD data displayed atφ2=45° section for rolling plane in the bulk of sample 2 after Q2(a) ODF (b) orientation map shown in IPF-Z of Fig.8a(c) Kikuchi band quality map with Σ3 grain boundaries in Fig.8b(d) rotation axis distribution(e) misorientation angle distribution
Fig.9  Orientation map and ODF of EBSD data displayed atφ2=45° section for rolling plane of sample 2 after recrystallization annealing at 700℃(a) ODF(b) orientation map shown in IPF-Z of Fig.9a(c) Kikuchi band quality map with Σ3 grain boundaries in Fig.9b(d) rotation axis distribution(e) misorientation angle distribution
Fig.10  Schematic of forming process of {100} oriented large-grain and Σ3 grain boundaries duringγ→α transformation (γ12 andγ3 represent three adjacentγ grains;V1,V2 andV3 represent three variants of K-S relation between the new phase and the parent phase;α12 andα3 are the smallα grains formed at the special locations during the growth process of {100} oriented large-grain)(a) initial stage(b) intermediate stage(c) final stage
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