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Effect of stress-relief annealing on microstructure, texture and hysteresis curve of mechanically cut non-oriented Fe-Si steel
Accepted Manuscript Effect of stress-relief annealing on microstructure, texture and hysteresis curve of mechanically cut non-oriented Fe-Si steel
Xuesong Xiong, Shubing Hu, Ningyuan Dang, Ke Hu PII: DOI: Reference:
S1044-5803(17)30744-1 doi: 10.1016/j.matchar.2017.06.035 MTL 8736
To appear in:
Materials Characterization
Received date: Revised date: Accepted date:
12 March 2017 30 May 2017 29 June 2017
Please cite this article as: Xuesong Xiong, Shubing Hu, Ningyuan Dang, Ke Hu , Effect of stress-relief annealing on microstructure, texture and hysteresis curve of mechanically cut non-oriented Fe-Si steel, Materials Characterization (2017), doi: 10.1016/ j.matchar.2017.06.035
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ACCEPTED MANUSCRIPT Effect of stress-relief annealing on microstructure, texture and hysteresis curve of mechanically cut non-oriented Fe-Si steel Xuesong Xionga, Shubing Hua, Ningyuan Dangb，Ke Hua aState
Key Laboratory of Material Processing and Die Mould Technology, Huazhong University of
and Development Center of Wuhan Iron and Steel (group) Corporation, Wuhan, 430080,
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bResearch
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Science and Technology, Wuhan 430074, China
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China
Abstract
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The effect of stress-relief annealing at 780℃ for 2 h on microstructure and texture in the cut edge zone as well as hysteresis curve of a mechanically cut
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non-oriented Fe-Si steel with 0.5mm thickness and medium silicon content was investigated. The results from electron backscatter diffraction (EBSD) analysis
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revealed that the annealing resulted in a significant decrease in dislocation densities
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and low-angle boundary (LAGB, 2°≤θ≤15°) percentages, and refined the grain sizes from both the upper surface and the lower surface as result of the occurrence of
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grain recrystallization. The annealing also caused a drastic increase in volume fraction
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of λ fiber ({001}) component, and led to an obvious decrease in that of γ fiber ({001}) component. Microstructure and texture evolution from the upper surface is evidently different from that from the lower surface. The single sheet testing results showed that the annealing improved the hysteresis loops dramatically regardless of the variations of the cutting length per mass. The possible main reasons for the microstructure, texture, and hysteresis loop evolution due to the annealing
Corresponding author. Tel: (+86) 2787540057; fax: (+86) 2787540057. E-mail address: [email protected].
ACCEPTED MANUSCRIPT were discussed in detailed.
1 Introduction With excellent soft magnetic properties Fe-Si steels are widely used as core material for electrical machines such as motors, transformers and generators [1-2]. It
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is well known that core laminations are commonly prepared by the method of
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mechanical cutting such as punching or shearing, for this cutting method may be
laser
cutting
or
wire
electric
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low-cost and more time-saving in comparison with other cutting methods such as discharge
machining.
It has
now
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been definitely established that mechanical cutting deteriorates the hysteresis curves and therefore results in the degraded magnetic properties of Fe-Si steels, such as, the
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increase in iron loss and the decrease in flux density and permeability [3-4]. Induced mechanical stress in the cut edge zone may be responsible for the change in hysteresis
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curves and magnetic properties including iron loss, flux density and permeability, and
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the main reasons are given as follows: firstly, the stress left in a Fe-Si steel is known as residual stress which can induce a uniaxial anisotropy with easy axis perpendicular
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to the field, thus the magnetization may shift to higher field, and the magnetizability and permeability may be decreased [5-6]. The magnetic anisotropy is proportional to
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residual stress according to literature [3]. Secondly, when a Fe-Si steel sheet affected by residual stress is magnetized the stress can prevent the magnetic domains from being aligned in the direction of the external magnetic field [7]. Compared with the magnetic domains aligned in the direction perpendicular to external magnetic field, the magnetic domains aligned in the direction of the external magnetic field contributes to better magnetic conductivity. Therefore, the residual stress in steels is detrimental to magnetic properties such as flux density and permeability. The extent
ACCEPTED MANUSCRIPT of this effect is determined by the magnitude of the residual stress [8-9]. Thirdly, due to the stress caused by mechanical cutting the texture of Fe-Si steels can be deteriorated based on literature [4], which can be regarded as one of main reasons for the decrease in iron loss and flux density after mechanical cutting. The degraded magnetic properties due to the stress caused by mechanical cutting
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processing, compared to those obtained by standardized metrological measurements
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carried out on the cold-rolled product, directly affect the efficiency and performance
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of electrical machines. The improvement of the magnetic properties of Fe-Si steels with mechanical cutting treatment implies the improvement of the efficiency and
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performance of electrical machines. The magnetic properties of mechanically cut Fe-Si steels can be enhanced by the optimized cutting process, such as, the use of
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sharp cutting tools [10] and the decrease in cutting clearance [11-12] which is the distance between the two cutting elements [11]. Though the optimization of cutting
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process is feasible for magnetic property improvement it inevitably leads to a
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considerable deterioration in magnetic properties [10-12]. The reason may be that the optimization of cutting process can narrow the region affected by residual stress or
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reduce the residual stress in the cut edge zone, but this effect is very limited. Paolinelli et al. found that stress-relief by annealing treatment at high
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temperature contributes to a significant improvement in magnetic properties of non-oriented Fe-Si steel with mechanical cutting [13]. The change in magnetic properties due to annealing may be related to the change in residual stress, magnetic domain structure, microstructure and crystallographic texture in the cut edge zone. Magnetic domain structure evolution has been widely reported [7, 14-15]. However, microstructure and texture evolution has not been reported yet. Furthermore, there has no detailed report about the effect of stress-relief annealing on hysteresis curves of a
ACCEPTED MANUSCRIPT mechanically cut Fe-Si steel. Literature [16] shows that annealing treatment at 800 and 850 °C alters the microstructure and texture of cold-rolling low-carbon steel (X70 pipeline steel). Literatures [17-18] show that annealing treatment at high temperature results in the evident improvement in magnetic properties and texture of non-oriented Fe-Si steel with residual stress. Therefore, this paper focuses on the effect of
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stress-relief annealing treatment at 780℃ on microstructure and texture as well as
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hysteresis curve of a 0.5mm thick non-oriented Fe-Si steel with mechanical cutting.
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We select 780℃ as the stress-relief annealing temperature, and the reason is that 780℃ would be a ideal annealing temperature for magnetic property improvement in
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accordance with the literature [13]. We have characterized the microstructures and
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textures from both the upper surface and the lower surface of the investigated Fe-Si steel, as shown in Fig.1. The results and conclusions obtained by us should be
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significant to the applications of Fe-Si steels.
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The cut edge
Cutting direction
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The upper surface
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The lower surface
Fig. 1 A three-dimensional model for Fe-Si steel with mechanical cutting treatment
2. Experimental The experimental steel obtained from Wuhan Iron and Steel (Group) Corp. was non-oriented (NO) fully processed Fe-Si steel with 0.5 mm thickness and medium silicon content. The chemical composition of the steel is shown in Table 1. Some experimental samples with 300 mm length and 60 mm width were prepared by
ACCEPTED MANUSCRIPT mechanical cutting and subsequent annealing treatment at 780 ℃ for 2 h in a nitrogen atmosphere. Then, the prepared samples were cut into narrow strips with width of 30 mm and 20 mm as well as 15 mm along rolling direction (length direction) using a mechanical shear having a clearance of 4% of the thickness and a rake angle
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of 1°, as shown in Fig.2. Finally, the cut strips were annealed at 780 ℃ for 2 h under the protection of nitrogen to eliminate the stress in the cut edge zone. The magnetic
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properties of the cut and the cut-and-annealed samples with total width of 60 mm
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were measured at 50 Hz using a single sheet tester in rolling direction of the samples. Four metallographic specimens were prepared to investigate the microstructure and
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texture evolutions of both the upper surface and the lower surface due to the annealing treatment. Microstructure and texture analysis was performed by a FEI/Quanta 200
diffraction
(EBSD)/TexSEM
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scanning electron microscope (SEM) equipped with an electron backscatter Laboratories
(TSL)
system.
Nanoindentation
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measurements using the metallographic specimens were carried out by use of nanoindentor with the displacement and load accuracy of 0.2 nm and 3 nN, respectively. Depth control was used to keep the maximum displacement constant as
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300 nm. In the region inside the annealed sample three random points were selected to
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test and the average of the contact areas of the three points was used as reference.
C
Si
Mn
P
S
Al
Fe
<0.003
2.1
0.26
<0.015
<0.003
0.24
BaL
Table 1 Chemical compositions of the NO Fe-Si steel in wt.%.
Steel NO Fe-Si
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Rolling direction
0.86
1.71
2.57
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0
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300 mm
Cutting length per mass (m/kg):
Fig. 2. The detailed cutting processes for the non-oriented Fe-Si steel
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3. Results and analysis
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3.1 Residual stress
The measurement of residual stresses is based on the calculation of the difference between the indentation contact areas of stressed and stress-free samples in
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nanoindentation experiment, and the detailed method can be found in literature [15]. Fig. 3 presents the residual stress distributions in the cut edge zone for both the upper surface and the lower surface of the referenced steel with different treatments. From
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Fig. 3, it can be seen that the residual stress tends to decrease with increasing distance
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from the cut edge. It is notable that the upper surface displays compression residual stress, whereas the lower surface exhibits tensile residual stress. For any of the surfaces, the residual stress of the cut steel is obviously higher than that of the cut-and-annealed steel, which indicates that the annealing resulted in a significant decrease in residual stress. According to literature [4], for a Fe-Si steel with mechanical cutting treatment its dislocation density increase means the increase in residual stress. The annealing treatment led to a clear decrease in dislocation density as shown in Fig. 10 and therefore reduced the residual stress in the cut edge zone.
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The upper surface The lower surfac The mechanically cut steel The cut-and-annealed steel
300
100
0
-100
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Residual stress (Mpa)
200
-200
0
200
400
600
800
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-300
1000
1200
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Distance from the cut edge (m)
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Fig. 3 Residual stress in the cut edge zone as a function of the distance from the cut edge for both the upper surface and the lower surface of the referenced steel with different treatments
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3.2 Microstructure
Fig.4 shows the orientation maps in the edge zone of the experimental steel with
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different treatments. From Fig.4 (a) and (b), it is clear that for the cut steel, the hues of
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some grains adjacent to the cut edge are not uniform, whereas these hues were changed into uniform hues after the annealing, which indicates that the annealing
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caused an evident change in grain orientations in the cut edge zone. In addition, from Fig.4, it is clear that the annealing treatment resulted in the formation of
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inhomogeneous microstructure in the both surfaces. Most of grains in the cut edge zone were refined by the annealing treatment except several grains near the cut edge. Mean grain size in the cut edge zone was estimated in terms of intercept length measurement, starting at the cut edge including all grains (about 500 grains) for a distance of 500 μm from the cut edge. For the cut-and-annealed steel, the mean grain sizes in the cut edge zone are 25.6 μm and 30.7 μm for the upper surface and the lower surface, respectively. Mean grain size of the experimental steel before the
ACCEPTED MANUSCRIPT annealing is 31.9 μm. Thus, the annealing reduced the mean grain sizes of the both surfaces of the cut steel. In addition, the annealing resulted in larger refinement of mean grain size for the upper surface as compared with the lower surface. The cut edge
The cut edge
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b
The cut edge
The cut edge
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d
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a
Fig. 4 EBSD orientation image maps in the cut edge zone for: (a, c) the upper surface and (b, d) the lower surface; (a, b) the mechanically cut steel; (c, d) the cut-and-annealed steel.
Fig. 5 and 6 presents misorientation distributions in the edge zone for the upper surface and the lower surface, respectively. As we can see that for any of the surfaces,
ACCEPTED MANUSCRIPT the misorientation distributions of the cut steel are clearly different from those of the cut-and-annealed steel, which indicates that the annealing has altered misorientation distributions. Fig. 7 and 8 illustrates a summary of the low-angle boundary (LAGB, 2°≤θ≤15°) percentages and average boundary misorientations, respectively. From Fig 7, it can be found that for any of the surfaces, the cut steel displays higher LAGB
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percentage as compared to the cut-and-annealed steel regardless of the variations of
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the distance from the cut edge, which reveals that the annealing caused a significant
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decrease in LAGB percentages in the cut edge zone. The decrease in LAGB percentages due to the annealing may be responsible for the increase in average
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boundary misorientations, as shown in Fig. 8. Fig. 9 presents the kernel average misorientation (KAM) distributions with θ≤2º in the edge zone with different
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distance from the cut edge for both the upper surface and the lower surface of the experimental steel with different treatments. Due to the annealing, the KAM
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distribution curves for the both surfaces of the cut steel shift toward to the left and become more steep, and the max values of the curves increase, thus, the mean values of the KAMs decrease, as shown in Fig. 10.The decrease in the mean values of KAMs
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implies the decrease in mean dislocation densities which is illustrated in Fig. 11. The
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detailed method for dislocation density estimation using the mean values of KAMs can be found everywhere [4, 19-20].
ACCEPTED MANUSCRIPT 35 The distance from cut edge: 50 um 100 um 200 um 300 um 400 um 500 um
(a)
25 20 15 10 5 0
2-5
5-10 10-15 15-20 20-25 25-30 30-35 35-40 40-45 45-50 50-55 55-60 60-65
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Misorientation angle (degree) 35
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The distance from cut edge: 50 um 100 um 200 um 300 um 400 um 500 um
(b)
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25 20 15 10
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Number percentage(%)
30
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Number percentage(%)
30
5 0
5-10 10-15 15-20 20-25 25-30 30-35 35-40 40-45 45-50 50-55 55-60 60-65
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2-5
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Misorientation angle (degree)
Fig. 5 Misorientation distributions in the edge zone (the upper surface) with different distance
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from the cut edge for: (a) the mechanically cut steel and (b) the cut-and-annealed steel.
ACCEPTED MANUSCRIPT 35 The distance from cut edge: 50 um 100 um 200 um 300 um 400 um 500 um
(a)
25 20 15 10 5
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Number percentage(%)
30
0
2-5
5-10 10-15 15-20 20-25 25-30 30-35 35-40 40-45 45-50 50-55 55-60 60-65
35
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The distance from cut edge: 50 um 100 um 200 um 300 um 400 um 500 um
(b)
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25 20 15 10
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Number percentage(%)
30
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Misorientation angle (degree)
5 0
5-10 10-15 15-20 20-25 25-30 30-35 35-40 40-45 45-50 50-55 55-60 60-65
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2-5
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Misorientation angle (degree)
Fig. 6 Misorientation distributions in the edge zone (the lower surface) with different distance
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from the cut edge for: (a) the mechanically cut steel and (b) the cut-and-annealed steel.
The upper surface The lower surface The cut steel The cut-and-annealed steel
40
30
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20
10 0
100
200
300
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Percentage low-angle boundaries (%)
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400
500
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Distance from cut edge (m)
Fig. 7 Percentage low-angle boundaries in the edge zone of the experimental steel with different
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The upper surface The lower surface The cut steel The cut-and-annealed steel
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40
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35
30
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Average boundary misorientation (degree)
treatments as a function of the distance from the cut edge.
25
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0
100
200
300
400
500
Distance from cut edge (m)
Fig. 8 Average boundary misorientations in the edge zone of the experimental steel with different treatments as a function of the distance from the cut edge.
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25
50m
300m
100m
400 m
200 m
500 m
20 15 10
20
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15 10
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5
0.5
1.0
1.5
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0
Kernel average Misorientation (degree)
2.0
100m 200 m
400 m 500 m
15 10
1.0
1.5
30
(d)
Distance from the cut edge:
25
50m
300m
100m 200 m
400 m 500 m
20 15 10
5 0 0.0
0.5
1.0
1.5
Kernel average misorientation (degree)
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Fig. 9 The KAM distributions in the edge zone with different distance from the cut edge for: (a, c) the upper surface and (b, d) the lower surface; (a, b) the mechanically cut steel; (c, d) the cut-and-annealed steel
2.0
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0.5
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300m 400 m 500 m
Number percentage (%)
50m
300m
Kernel average misorientation (degree)
Distance from the cut edge: 100m 200 m
50m
20
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(c)
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Number percentage (%)
25
35
25
0.0
Distance from the cut edge:
0 0.0
0.5 1.0 1.5 2.0 Kernel average misorientation (degree)
35 30
(b)
5
5 0 0.0
30
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Distance from the cut edge:
(a)
Number percentage (%)
Number percentage (%)
30
35
2.0
The upper surface The lower surface The cut steel The cut-and-annealed steel
0.8
0.7
0.6
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0.5
0.4 0
100
200
300
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Mean values of the KAMs (degree)
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400
500
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Distance from the cut edge (m)
Fig. 10 Mean values of the KAMs in the cut edge zone of the experimental steel with different
4.5x10
13
4.0x10
13
3.5x10
13
3.0x10
13
2.5x10
13
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The upper surface The lower surface The cut steel The cut-and-annealed steel
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5.0x10
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-2
Dislocation density (m )
treatments as a function of the distance from the cut edge.
2.0x10
13
100
200
300
400
500
Distance from cut edge (m)
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0
Fig. 11 Mean dislocation densities in the cut edge zone of the experimental steel with different treatments as a function of the distance from the cut edge.
3.3 Texture Fig. 12 demonstrates the texture evolution in the cut edge zone of the mechanically cut Fe-Si steel due to the annealing treatment by making use of the φ2=45° sections of orientation distribution functions (ODFs) obtained by EBSD. The specimens as shown in Fig. 4 were measured and ODFs were calculated starting at the
ACCEPTED MANUSCRIPT cutting edge including all grains for a distance of 500 μm from the cut edge. The area for texture analysis includes about 500 grains. Fig.13 illustrates the volume fraction variations of both λ fiber ({001}) and γ fiber ({111}) components in the edge zone with the distance of 50μm from the cut edge due to the annealing treatment. Texture volume fractions were obtained from EBSD data with 15º tolerance angle.
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From Fig. 12 and 13, we can find that the texture of the cut-and-annealed steel is
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clearly different form that of the cut steel, which reveals that the annealing led to an
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evident change in texture of the cut experimental steel. The change in material texture may be attributed to the change in its grain orientations as shown in Fig.4. From Fig.
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13, it is obvious that for the both surfaces, the annealing caused a drastic increase in volume fraction of λ fiber component, and resulted in an obvious decrease in that of γ
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fiber component. Moreover, we can find that before the annealing, volume fraction difference between the λ fiber and γ fiber is very obvious, but after annealing, volume
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fraction of γ fiber is very approximate to that of λ fiber. From Fig.12-13, due to the
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annealing, texture evolution of the upper surface is evidently different form that of the lower surface. We know that deformed texture strongly affects annealed texture when
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a certain annealing process is adopted for adjustment of material texture. The deformed texture of the upper surface is clearly different from that of the lower
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surface, which may be the main reason insulting in the different texture evolutions in the two surface during the annealing, as shown in Fig. 12.
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c
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b
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d
Fig.12 φ2=45° sections of ODFs obtained by EBSD in the cut edge zone of the referenced steel
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with the distance of 500 μm from the cut edge for: (a, b) the cut steel; (c, d) the cut-and-annealed
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steel; (a, c) the upper surface; (b, d) the lower surface. (intensity level:1,2,3…).
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fiber (the cut steel) fiber (the cut steel) fiber (the cut-and-annealed steel) fiber (the cut-and-annealed steel)
20
10
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Volume fraction (%)
30
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0
The upper surface
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The lower surface
Fig. 13 Volume fractions of λ and γ fibers in the cut edge zone with the distance of 500 μm from
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the cut edge
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4 Hysteresis curve
Fig. 14 presents the hysteresis loops for both the mechanically cut steel and the cut-and-annealed steel. From Fig.14, as we can find that after the annealing treatment,
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the hysteresis loops of the cut steel become more steep, which indicates that the annealing resulted in an evident improvement in hysteresis loops. The improvement in hysteresis loops means the improvement in magnetic properties such as iron loss, flux
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density, remanence (Br) and permeability (μ). Furthermore, the hysteresis loops of the
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cut-and-annealed steel (see Fig. 145 (b)) are almost the same as that of the uncut steel (the red curve in Fig. 14 (a)) regardless of the variations of the cutting length per mass. This evidence indicates that almost all the lost magnetic properties due to mechanical cutting can be fully recovered by the annealing.
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2000
2000
1000 Cutting length per mass： 0 0.86 m/kg 1.71 m/kg 2.57 m/kg
500
1500
1000
0
Cutting length per mass： 500
0 0
400
800
0.86 m/kg 1.71 m/kg 2.57 m/kg
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1500
0
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Magnetic Induction B(mT)
(b)
400
Magnetic field H (A/m)
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Magnetic field H (A/m)
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Fig. 14 Hysteresis loops for: (a) the mechanically cut specimens with the cutting length per mass from 0 to 2.57 m/kg and (b) the cut-and-annealed specimens with the cutting length per mass from
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0.86 to 2.57 m/kg.
4. Discussion
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4.1 Microstructure and texture evolution due to the annealing
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In the cut edge zone, the microstructures from both the upper surface and the lower surface of the cut-and-annealed steel are extremely inhomogeneous, as shown
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in Fig. 4. This phenomenon can be easily found in rolled-and-annealed electrical steel [1, 21-23]. The reason for the inhomogeneous microstructure should be attributed to inhomogeneous stored deformation energy in the steel before the annealing. This is
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Magnetic Induction B(mT)
(a)
because stored energy introduced into a material during deformation provides the driving force for the subsequent annealing [24], and how the microstructure evolves due to the annealing depends, to a great extent, on the amount of stored deformation energy of material. From Fig. 10 and 11, we know that before the annealing, the grains near the cut edge have higher stored deformation energy as compared to the rest of the grains in the cut edge zone. Higher stored deformation energy contributes
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ACCEPTED MANUSCRIPT to earlier recrystallization nucleation and more crystal nucleus growth during the annealing. As a consequence, after the annealing the grains near the cut edge commonly display larger grain size while the rest grains exhibit relatively smaller grain size. Recrystallization mechanism may be that discontinuous static recrystallization took place after annealing at the vicinity of primary grain boundaries
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or within deformed grains. In the early stages of crystallization, the nuclei were
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outlined by low-angle boundaries, the misorientations of which gradually increased
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until the attained values typical of high-angle boundaries. This mechanism can reasonably explain why low-angle boundary percentages decrease, whereas average
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boundary misorientations increase after the annealing, as shown in Fig. 7 and 8. Fig. 4 also shows that in the cut edge zone of the cut-and-annealed steel, the
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microstructure of the upper surface is distinctly different from that of the lower surface, and the lower surface exhibits larger mean grain size in comparison with the
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lower surface. Literatures [22, 25] show that with increasing the deformation level,
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average grain size of the deformed-and-annealed steel tends to decrease gradually. Thus, the reason for the microstructure difference between the two surfaces may be
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due to the difference in their deformation levels before the annealing. From literature [4] and Fig. 6-10 in this paper, as we can see that the upper surface has undergone
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larger plastic deformation than the lower surface before the annealing, which may be responsible for its more refined microstructure after the annealing. Why plastic deformation affects annealed microstructure? The reason may be that larger plastic deformation gives more low-angle boundaries which can act as nucleation sites for grain recrystallization during annealing. Therefore, more new grains should develop after the annealing, which results in smaller mean grain size. Fig. 12-13 indicates that in the cut edge zone for both the upper surface and the
ACCEPTED MANUSCRIPT lower surface, the mechanically cut steel obviously exhibits stronger γ fiber component than λ fiber component. Fig.14 reveals that before the annealing, the texture volume fraction in γ fiber is much higher than that in λ fiber for the both surfaces. The deformed γ-grains having high stored energy usually tend to form new recrystallized γ-grains in grain interiors and boundary regions during the annealing
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according to literatures [26-27]. The formed new γ-grains commonly have an
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advantage in size and number over the other oriented recrystallized grains such as {100} grains. Thus, after the annealing, the texture volume fraction of γ fiber
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component should be much larger than that of the other components including λ fiber.
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But experimental result shows after the annealing, the volume fraction of γ fiber component is very approximate to that of λ fiber (see Fig.13). Literature [1] reveals
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that {001} recrystallized nuclei can be preferentially form mainly within the boundary regions of deformed {111} and {110} grains. The preferred crystallographic
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orientation <100> has a fast-growing rate. If the dislocation density near the grain
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boundaries is insufficient to facilitate the recrystallization nucleation, a selective growth mechanism will operate. By the jointing mechanism of competitive grain
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growth and preferred nucleation, more {100} nuclei grow into {100} grains at expense of the other grain growth such as λ grains. Thus, after the annealing the
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volume fraction of λ fiber increases whereas the volume fraction of γ fiber component increases. The volume fraction difference between the λ fiber and γ fiber components after the annealing is not as obvious as that before the annealing, as shown in Fig.14. 4.1 Hysteresis loop evolution due to the annealing Undoubtedly, the annealing resulted in a dramatic improvement in hysteresis loops of the cut experimental steel regardless of the variations of cutting length per mass, as shown in Fig. 14. The improvement in hysteresis loops is beneficial to the
ACCEPTED MANUSCRIPT improvement in magnetic properties such as iron loss, flux density, remanence (Br) and permeability (μ). Stress relief by the annealing treatment may be a main reason that causes the improvement of hysteresis loops, because it contributes to elimination of magnetic anisotropy which can deteriorate magnetization of the steel. Furthermore, as the annealing has a significant effect on microstructural characteristics of the cut
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Fe-Si steel, the change in hysteresis loops can also be attributed to the change in
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microstructure and texture. Fig.4 reveals that in the cut edge zone, the microstructures
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form both the upper surface and the lower surface of the cut steel are refined by the annealing. The reason for the microstructure refinement can be attributed to the
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occurrence of grain recrystallization or phase transformation. Refined microstructure is not helpful to the improvement of the hysteresis loop, and the reason is that the
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refinement of microstructure can induce grain boundary area increase, restricting the movement and rotation of magnetic domains during magnetization [28], thus, flux
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density and permeability decrease, and iron increases. Fig.4 also indicates that the
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microstructures from the both surfaces of the cut-and-annealed steel are extremely inhomogeneous, which is detrimental to magnetic properties and thus is not a reason
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for the improvement of hysteresis loop. Fig. 11 shows that the annealing caused a drastic decrease in dislocation density in the cut edge zone of the cut steel. literature
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[29] shows that dislocations are correlated with domain wall motion during magnetization process, and dislocation tangles can promote pining of domain wall. Thus, the decrease in dislocation densities in the cut edge zone due to the annealing may be one of reasons for the improvement of hysteresis loops. Fig. 13 presents that in the edge zone, because of the annealing, the intensity of λ fiber component from both the upper surface and the lower surface of the cut steel increases obviously while γ fiber component decreases. As we know that magnetization of Fe-Si steel in outer
ACCEPTED MANUSCRIPT magnetic field is dependent on its crystal orientation, and <001> and <111> directions are easy and hard magnetization directions, respectively. When the direction of outer magnetic field is parallel to <001> direction, the magnetization will increase to saturation rapidly. For this reason, λ fiber, with two easy magnetization <001> directions in the rolling plane, is most favorable, whereas γ fiber, without <001>
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directions in the rolling plane, is the most deleterious. The texture of the cut
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experimental steel is evidently improved by the annealing, which is beneficial to its
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magnetization and therefore can be considered to be one of reasons for the
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improvement of hysteresis loops.
5. Conclusion:
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The effect of the stress-relief annealing on microstructure and texture in the edge zone as well as hysteresis loop of a mechanically cut non-oriented Fe-Si steel was
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investigated. The main conclusions were drawn as follows:
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1. The annealing resulted in a significant decrease in dislocation densities and low-angle boundary (LAGB, 2°≤θ≤15°) percentages, and refined the grain
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sizes of both the upper surface and the lower surface. As the upper surface has undergone larger plastic deformation before the annealing, the annealing caused
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more refinement in grain size for the upper surface in comparison with the lower surface.
2. The annealing also caused a drastic increase in volume fraction of λ component, and led to an obvious decrease in that of γ fiber component. The formation of λ fiber texture may be attributed to the jointing mechanism of preferred nucleation and competitive grain growth. Furthermore, the texture evolution from the upper surface is obviously different from that of the lower surface, and the reason may
ACCEPTED MANUSCRIPT be due to their different deformed textures before the annealing. 3. The annealing dramatically improved the hysteresis loops regardless of the variations of the cutting length per mass. Almost all the lost magnetic properties due to mechanical cutting can be fully recovered by the annealing. The reason for the hysteresis loop improvement may be attributed to the decrease in residual
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stress and dislocation density, and the improvement of material texture.
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Acknowledgments
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This work was supported by the National Basic Research Program of China
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(Grant No. 2014CB046704) and the National Natural Science Foundation of china (51375005). The Analytical and Testing Center of HUST has provided useful
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characterizations for the work.
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References
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[1] Li HZ, et al. Characterization of microstructure, texture and magnetic properties in twin-roll casting high silicon non-oriented electrical steel. Mater Charact 2014; 8:81-6. [2] Kurosaki Y, et al. Importance of punching and workability in non-oriented electrical steel
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[4] Xiong XS, et al. Texture and magnetic property evolution of non-oriented Fe-Si steel due to mechanical cutting. J Magn Magn Mater 2016; 401:982-90.
[5] Pervertov O, et al. Influence of applied compressive stress on the hysteresis curves and magnetic domain structure of grain-oriented transerse Fe-3%Si steel. J Phys D: Appl Phys 2012; 45:135001. [6]. Ulutan D, et al. Analytical modelling of residual stresses in machining. J Mat Process Technol 2007; 183:77-87.
ACCEPTED MANUSCRIPT [7] Takezawa M, et al. Investigated effect of Strain by Mechanical Punching on Nonoriented Si-Fe electrical sheets for a nine-slot motor core. IEEE Trans Magn 2006; 42:2790-92. [8] LoBue M, et al. Power losses and magnetization process in Fe–Si non-oriented steels under tensile and compressive stress. J Magn Magn Mater 2000; 215-216:124-126. [9] Moses AJ, et al. Magnetostriction in non-oriented electrical steels. J Magn Magn Mater 2000; 215-216:669-672.
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[12] Schmidt KH. Influence of punching on the magnetic properties of electric steel with 1% silicon. J Magn Magn Mater 1975; 1-3:136-50.
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[13] Paolinelli SC. Effect of stress relief annealing temperature and atmosphere on the magnetic properties of silicon steel. J Magn Magn Mater 2006; 304:e599-e601. [14] Senda K, et al. Influence of shearing process on domain structure and magnetic properties of
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non-oriented electrical steel. J Magn Magn Mater 2006; 304:e513-15.
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[15] Cao HZ, et al. The influence of punching process on residual stress and magnetic domain structure of non-oriented silicon steel. J Magn Magn Mater 2016; 406:42-47.
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[16] Eskandari M, et al. Evolution of the microstructure and texture of X70 pipeline steel during cold-rolling and annealing treatments. Mater Des 2016; 90:618-27.
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(a)
ACCEPTED MANUSCRIPT [21] Chen L, et al. Retaining {100} texture from initial columnar grains in electrical steels, Scr Mater 2012; 67:899-902 [22] Li HZ, Effects of warm temper rolling on microstructure, texture and magnetic properties of strip-casting 6.5wt% Si electrical steel. J Magn Magn Mater 2014; 370:6-12. [23] Park JT. Evolution of recrystallization texture in nonoriented electrical steels, Acta Mater 2003; 51:3037-51
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[25] Pan HJ, et al. Strong <001> crystallization texture component in 6.5wt.% Si electrical steel thin sheets by secondary cold rolling and annealing. J Magn Magn Mater 2016; 419:500-11.
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[28] Park JT, Szpunar JA. Effect of initial grain size on texture evolution and magnetic properties
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in nonoriented electrical steels. J Magn Magn Mater 2009; 321:1928-32. [29] Calcagnotto M, et al. Orientation gradients and geometrically necessary dislocations in ultrafine grained dual-phase steels studied by 2D and 3D EBSD, Mater Sci Eng A. 2010;
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ACCEPTED MANUSCRIPT Highlights 1. Microstructures and textures in the cut edge zone were characterized via EBSD. 2. The effect of annealing on microstructure and texture was investigated.
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3. The effect of the annealing on hysteresis loops was investigated.
Xuesong Xiong, Shubing Hu, Ningyuan Dang, Ke Hu PII: DOI: Reference:
S1044-5803(17)30744-1 doi: 10.1016/j.matchar.2017.06.035 MTL 8736
To appear in:
Materials Characterization
Received date: Revised date: Accepted date:
12 March 2017 30 May 2017 29 June 2017
Please cite this article as: Xuesong Xiong, Shubing Hu, Ningyuan Dang, Ke Hu , Effect of stress-relief annealing on microstructure, texture and hysteresis curve of mechanically cut non-oriented Fe-Si steel, Materials Characterization (2017), doi: 10.1016/ j.matchar.2017.06.035
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ACCEPTED MANUSCRIPT Effect of stress-relief annealing on microstructure, texture and hysteresis curve of mechanically cut non-oriented Fe-Si steel Xuesong Xionga, Shubing Hua, Ningyuan Dangb，Ke Hua aState
Key Laboratory of Material Processing and Die Mould Technology, Huazhong University of
and Development Center of Wuhan Iron and Steel (group) Corporation, Wuhan, 430080,
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bResearch
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Science and Technology, Wuhan 430074, China
SC
China
Abstract
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The effect of stress-relief annealing at 780℃ for 2 h on microstructure and texture in the cut edge zone as well as hysteresis curve of a mechanically cut
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non-oriented Fe-Si steel with 0.5mm thickness and medium silicon content was investigated. The results from electron backscatter diffraction (EBSD) analysis
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revealed that the annealing resulted in a significant decrease in dislocation densities
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and low-angle boundary (LAGB, 2°≤θ≤15°) percentages, and refined the grain sizes from both the upper surface and the lower surface as result of the occurrence of
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grain recrystallization. The annealing also caused a drastic increase in volume fraction
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of λ fiber ({001}
Corresponding author. Tel: (+86) 2787540057; fax: (+86) 2787540057. E-mail address: [email protected].
ACCEPTED MANUSCRIPT were discussed in detailed.
1 Introduction With excellent soft magnetic properties Fe-Si steels are widely used as core material for electrical machines such as motors, transformers and generators [1-2]. It
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is well known that core laminations are commonly prepared by the method of
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mechanical cutting such as punching or shearing, for this cutting method may be
laser
cutting
or
wire
electric
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low-cost and more time-saving in comparison with other cutting methods such as discharge
machining.
It has
now
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been definitely established that mechanical cutting deteriorates the hysteresis curves and therefore results in the degraded magnetic properties of Fe-Si steels, such as, the
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increase in iron loss and the decrease in flux density and permeability [3-4]. Induced mechanical stress in the cut edge zone may be responsible for the change in hysteresis
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curves and magnetic properties including iron loss, flux density and permeability, and
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the main reasons are given as follows: firstly, the stress left in a Fe-Si steel is known as residual stress which can induce a uniaxial anisotropy with easy axis perpendicular
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to the field, thus the magnetization may shift to higher field, and the magnetizability and permeability may be decreased [5-6]. The magnetic anisotropy is proportional to
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residual stress according to literature [3]. Secondly, when a Fe-Si steel sheet affected by residual stress is magnetized the stress can prevent the magnetic domains from being aligned in the direction of the external magnetic field [7]. Compared with the magnetic domains aligned in the direction perpendicular to external magnetic field, the magnetic domains aligned in the direction of the external magnetic field contributes to better magnetic conductivity. Therefore, the residual stress in steels is detrimental to magnetic properties such as flux density and permeability. The extent
ACCEPTED MANUSCRIPT of this effect is determined by the magnitude of the residual stress [8-9]. Thirdly, due to the stress caused by mechanical cutting the texture of Fe-Si steels can be deteriorated based on literature [4], which can be regarded as one of main reasons for the decrease in iron loss and flux density after mechanical cutting. The degraded magnetic properties due to the stress caused by mechanical cutting
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processing, compared to those obtained by standardized metrological measurements
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carried out on the cold-rolled product, directly affect the efficiency and performance
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of electrical machines. The improvement of the magnetic properties of Fe-Si steels with mechanical cutting treatment implies the improvement of the efficiency and
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performance of electrical machines. The magnetic properties of mechanically cut Fe-Si steels can be enhanced by the optimized cutting process, such as, the use of
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sharp cutting tools [10] and the decrease in cutting clearance [11-12] which is the distance between the two cutting elements [11]. Though the optimization of cutting
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process is feasible for magnetic property improvement it inevitably leads to a
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considerable deterioration in magnetic properties [10-12]. The reason may be that the optimization of cutting process can narrow the region affected by residual stress or
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reduce the residual stress in the cut edge zone, but this effect is very limited. Paolinelli et al. found that stress-relief by annealing treatment at high
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temperature contributes to a significant improvement in magnetic properties of non-oriented Fe-Si steel with mechanical cutting [13]. The change in magnetic properties due to annealing may be related to the change in residual stress, magnetic domain structure, microstructure and crystallographic texture in the cut edge zone. Magnetic domain structure evolution has been widely reported [7, 14-15]. However, microstructure and texture evolution has not been reported yet. Furthermore, there has no detailed report about the effect of stress-relief annealing on hysteresis curves of a
ACCEPTED MANUSCRIPT mechanically cut Fe-Si steel. Literature [16] shows that annealing treatment at 800 and 850 °C alters the microstructure and texture of cold-rolling low-carbon steel (X70 pipeline steel). Literatures [17-18] show that annealing treatment at high temperature results in the evident improvement in magnetic properties and texture of non-oriented Fe-Si steel with residual stress. Therefore, this paper focuses on the effect of
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stress-relief annealing treatment at 780℃ on microstructure and texture as well as
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hysteresis curve of a 0.5mm thick non-oriented Fe-Si steel with mechanical cutting.
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We select 780℃ as the stress-relief annealing temperature, and the reason is that 780℃ would be a ideal annealing temperature for magnetic property improvement in
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accordance with the literature [13]. We have characterized the microstructures and
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textures from both the upper surface and the lower surface of the investigated Fe-Si steel, as shown in Fig.1. The results and conclusions obtained by us should be
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significant to the applications of Fe-Si steels.
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The cut edge
Cutting direction
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The upper surface
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The lower surface
Fig. 1 A three-dimensional model for Fe-Si steel with mechanical cutting treatment
2. Experimental The experimental steel obtained from Wuhan Iron and Steel (Group) Corp. was non-oriented (NO) fully processed Fe-Si steel with 0.5 mm thickness and medium silicon content. The chemical composition of the steel is shown in Table 1. Some experimental samples with 300 mm length and 60 mm width were prepared by
ACCEPTED MANUSCRIPT mechanical cutting and subsequent annealing treatment at 780 ℃ for 2 h in a nitrogen atmosphere. Then, the prepared samples were cut into narrow strips with width of 30 mm and 20 mm as well as 15 mm along rolling direction (length direction) using a mechanical shear having a clearance of 4% of the thickness and a rake angle
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of 1°, as shown in Fig.2. Finally, the cut strips were annealed at 780 ℃ for 2 h under the protection of nitrogen to eliminate the stress in the cut edge zone. The magnetic
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properties of the cut and the cut-and-annealed samples with total width of 60 mm
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were measured at 50 Hz using a single sheet tester in rolling direction of the samples. Four metallographic specimens were prepared to investigate the microstructure and
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texture evolutions of both the upper surface and the lower surface due to the annealing treatment. Microstructure and texture analysis was performed by a FEI/Quanta 200
diffraction
(EBSD)/TexSEM
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scanning electron microscope (SEM) equipped with an electron backscatter Laboratories
(TSL)
system.
Nanoindentation
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measurements using the metallographic specimens were carried out by use of nanoindentor with the displacement and load accuracy of 0.2 nm and 3 nN, respectively. Depth control was used to keep the maximum displacement constant as
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300 nm. In the region inside the annealed sample three random points were selected to
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test and the average of the contact areas of the three points was used as reference.
C
Si
Mn
P
S
Al
Fe
<0.003
2.1
0.26
<0.015
<0.003
0.24
BaL
Table 1 Chemical compositions of the NO Fe-Si steel in wt.%.
Steel NO Fe-Si
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Rolling direction
0.86
1.71
2.57
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0
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300 mm
Cutting length per mass (m/kg):
Fig. 2. The detailed cutting processes for the non-oriented Fe-Si steel
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3. Results and analysis
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3.1 Residual stress
The measurement of residual stresses is based on the calculation of the difference between the indentation contact areas of stressed and stress-free samples in
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nanoindentation experiment, and the detailed method can be found in literature [15]. Fig. 3 presents the residual stress distributions in the cut edge zone for both the upper surface and the lower surface of the referenced steel with different treatments. From
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Fig. 3, it can be seen that the residual stress tends to decrease with increasing distance
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from the cut edge. It is notable that the upper surface displays compression residual stress, whereas the lower surface exhibits tensile residual stress. For any of the surfaces, the residual stress of the cut steel is obviously higher than that of the cut-and-annealed steel, which indicates that the annealing resulted in a significant decrease in residual stress. According to literature [4], for a Fe-Si steel with mechanical cutting treatment its dislocation density increase means the increase in residual stress. The annealing treatment led to a clear decrease in dislocation density as shown in Fig. 10 and therefore reduced the residual stress in the cut edge zone.
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The upper surface The lower surfac The mechanically cut steel The cut-and-annealed steel
300
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200
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800
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Distance from the cut edge (m)
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Fig. 3 Residual stress in the cut edge zone as a function of the distance from the cut edge for both the upper surface and the lower surface of the referenced steel with different treatments
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3.2 Microstructure
Fig.4 shows the orientation maps in the edge zone of the experimental steel with
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different treatments. From Fig.4 (a) and (b), it is clear that for the cut steel, the hues of
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some grains adjacent to the cut edge are not uniform, whereas these hues were changed into uniform hues after the annealing, which indicates that the annealing
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caused an evident change in grain orientations in the cut edge zone. In addition, from Fig.4, it is clear that the annealing treatment resulted in the formation of
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inhomogeneous microstructure in the both surfaces. Most of grains in the cut edge zone were refined by the annealing treatment except several grains near the cut edge. Mean grain size in the cut edge zone was estimated in terms of intercept length measurement, starting at the cut edge including all grains (about 500 grains) for a distance of 500 μm from the cut edge. For the cut-and-annealed steel, the mean grain sizes in the cut edge zone are 25.6 μm and 30.7 μm for the upper surface and the lower surface, respectively. Mean grain size of the experimental steel before the
ACCEPTED MANUSCRIPT annealing is 31.9 μm. Thus, the annealing reduced the mean grain sizes of the both surfaces of the cut steel. In addition, the annealing resulted in larger refinement of mean grain size for the upper surface as compared with the lower surface. The cut edge
The cut edge
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The cut edge
The cut edge
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Fig. 4 EBSD orientation image maps in the cut edge zone for: (a, c) the upper surface and (b, d) the lower surface; (a, b) the mechanically cut steel; (c, d) the cut-and-annealed steel.
Fig. 5 and 6 presents misorientation distributions in the edge zone for the upper surface and the lower surface, respectively. As we can see that for any of the surfaces,
ACCEPTED MANUSCRIPT the misorientation distributions of the cut steel are clearly different from those of the cut-and-annealed steel, which indicates that the annealing has altered misorientation distributions. Fig. 7 and 8 illustrates a summary of the low-angle boundary (LAGB, 2°≤θ≤15°) percentages and average boundary misorientations, respectively. From Fig 7, it can be found that for any of the surfaces, the cut steel displays higher LAGB
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percentage as compared to the cut-and-annealed steel regardless of the variations of
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the distance from the cut edge, which reveals that the annealing caused a significant
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decrease in LAGB percentages in the cut edge zone. The decrease in LAGB percentages due to the annealing may be responsible for the increase in average
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boundary misorientations, as shown in Fig. 8. Fig. 9 presents the kernel average misorientation (KAM) distributions with θ≤2º in the edge zone with different
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distance from the cut edge for both the upper surface and the lower surface of the experimental steel with different treatments. Due to the annealing, the KAM
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distribution curves for the both surfaces of the cut steel shift toward to the left and become more steep, and the max values of the curves increase, thus, the mean values of the KAMs decrease, as shown in Fig. 10.The decrease in the mean values of KAMs
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implies the decrease in mean dislocation densities which is illustrated in Fig. 11. The
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detailed method for dislocation density estimation using the mean values of KAMs can be found everywhere [4, 19-20].
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(a)
25 20 15 10 5 0
2-5
5-10 10-15 15-20 20-25 25-30 30-35 35-40 40-45 45-50 50-55 55-60 60-65
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Misorientation angle (degree) 35
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The distance from cut edge: 50 um 100 um 200 um 300 um 400 um 500 um
(b)
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25 20 15 10
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Number percentage(%)
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Number percentage(%)
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5-10 10-15 15-20 20-25 25-30 30-35 35-40 40-45 45-50 50-55 55-60 60-65
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2-5
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Misorientation angle (degree)
Fig. 5 Misorientation distributions in the edge zone (the upper surface) with different distance
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The distance from cut edge: 50 um 100 um 200 um 300 um 400 um 500 um
(b)
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Number percentage(%)
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Misorientation angle (degree)
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5-10 10-15 15-20 20-25 25-30 30-35 35-40 40-45 45-50 50-55 55-60 60-65
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Misorientation angle (degree)
Fig. 6 Misorientation distributions in the edge zone (the lower surface) with different distance
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from the cut edge for: (a) the mechanically cut steel and (b) the cut-and-annealed steel.
The upper surface The lower surface The cut steel The cut-and-annealed steel
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500
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Fig. 7 Percentage low-angle boundaries in the edge zone of the experimental steel with different
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The upper surface The lower surface The cut steel The cut-and-annealed steel
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35
30
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Average boundary misorientation (degree)
treatments as a function of the distance from the cut edge.
25
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0
100
200
300
400
500
Distance from cut edge (m)
Fig. 8 Average boundary misorientations in the edge zone of the experimental steel with different treatments as a function of the distance from the cut edge.
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25
50m
300m
100m
400 m
200 m
500 m
20 15 10
20
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Kernel average Misorientation (degree)
2.0
100m 200 m
400 m 500 m
15 10
1.0
1.5
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(d)
Distance from the cut edge:
25
50m
300m
100m 200 m
400 m 500 m
20 15 10
5 0 0.0
0.5
1.0
1.5
Kernel average misorientation (degree)
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Fig. 9 The KAM distributions in the edge zone with different distance from the cut edge for: (a, c) the upper surface and (b, d) the lower surface; (a, b) the mechanically cut steel; (c, d) the cut-and-annealed steel
2.0
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Number percentage (%)
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300m
Kernel average misorientation (degree)
Distance from the cut edge: 100m 200 m
50m
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25
35
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Distance from the cut edge:
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0.5 1.0 1.5 2.0 Kernel average misorientation (degree)
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(b)
5
5 0 0.0
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Distance from the cut edge:
(a)
Number percentage (%)
Number percentage (%)
30
35
2.0
The upper surface The lower surface The cut steel The cut-and-annealed steel
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0.7
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Mean values of the KAMs (degree)
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Distance from the cut edge (m)
Fig. 10 Mean values of the KAMs in the cut edge zone of the experimental steel with different
4.5x10
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4.0x10
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3.5x10
13
3.0x10
13
2.5x10
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The upper surface The lower surface The cut steel The cut-and-annealed steel
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Dislocation density (m )
treatments as a function of the distance from the cut edge.
2.0x10
13
100
200
300
400
500
Distance from cut edge (m)
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Fig. 11 Mean dislocation densities in the cut edge zone of the experimental steel with different treatments as a function of the distance from the cut edge.
3.3 Texture Fig. 12 demonstrates the texture evolution in the cut edge zone of the mechanically cut Fe-Si steel due to the annealing treatment by making use of the φ2=45° sections of orientation distribution functions (ODFs) obtained by EBSD. The specimens as shown in Fig. 4 were measured and ODFs were calculated starting at the
ACCEPTED MANUSCRIPT cutting edge including all grains for a distance of 500 μm from the cut edge. The area for texture analysis includes about 500 grains. Fig.13 illustrates the volume fraction variations of both λ fiber ({001}
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From Fig. 12 and 13, we can find that the texture of the cut-and-annealed steel is
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clearly different form that of the cut steel, which reveals that the annealing led to an
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evident change in texture of the cut experimental steel. The change in material texture may be attributed to the change in its grain orientations as shown in Fig.4. From Fig.
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13, it is obvious that for the both surfaces, the annealing caused a drastic increase in volume fraction of λ fiber component, and resulted in an obvious decrease in that of γ
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fiber component. Moreover, we can find that before the annealing, volume fraction difference between the λ fiber and γ fiber is very obvious, but after annealing, volume
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fraction of γ fiber is very approximate to that of λ fiber. From Fig.12-13, due to the
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annealing, texture evolution of the upper surface is evidently different form that of the lower surface. We know that deformed texture strongly affects annealed texture when
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a certain annealing process is adopted for adjustment of material texture. The deformed texture of the upper surface is clearly different from that of the lower
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surface, which may be the main reason insulting in the different texture evolutions in the two surface during the annealing, as shown in Fig. 12.
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Fig.12 φ2=45° sections of ODFs obtained by EBSD in the cut edge zone of the referenced steel
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with the distance of 500 μm from the cut edge for: (a, b) the cut steel; (c, d) the cut-and-annealed
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steel; (a, c) the upper surface; (b, d) the lower surface. (intensity level:1,2,3…).
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fiber (the cut steel) fiber (the cut steel) fiber (the cut-and-annealed steel) fiber (the cut-and-annealed steel)
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Volume fraction (%)
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The lower surface
Fig. 13 Volume fractions of λ and γ fibers in the cut edge zone with the distance of 500 μm from
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the cut edge
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4 Hysteresis curve
Fig. 14 presents the hysteresis loops for both the mechanically cut steel and the cut-and-annealed steel. From Fig.14, as we can find that after the annealing treatment,
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the hysteresis loops of the cut steel become more steep, which indicates that the annealing resulted in an evident improvement in hysteresis loops. The improvement in hysteresis loops means the improvement in magnetic properties such as iron loss, flux
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density, remanence (Br) and permeability (μ). Furthermore, the hysteresis loops of the
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cut-and-annealed steel (see Fig. 145 (b)) are almost the same as that of the uncut steel (the red curve in Fig. 14 (a)) regardless of the variations of the cutting length per mass. This evidence indicates that almost all the lost magnetic properties due to mechanical cutting can be fully recovered by the annealing.
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400
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0.86 m/kg 1.71 m/kg 2.57 m/kg
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1500
0
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Magnetic Induction B(mT)
(b)
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Magnetic field H (A/m)
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Magnetic field H (A/m)
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Fig. 14 Hysteresis loops for: (a) the mechanically cut specimens with the cutting length per mass from 0 to 2.57 m/kg and (b) the cut-and-annealed specimens with the cutting length per mass from
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0.86 to 2.57 m/kg.
4. Discussion
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4.1 Microstructure and texture evolution due to the annealing
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In the cut edge zone, the microstructures from both the upper surface and the lower surface of the cut-and-annealed steel are extremely inhomogeneous, as shown
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in Fig. 4. This phenomenon can be easily found in rolled-and-annealed electrical steel [1, 21-23]. The reason for the inhomogeneous microstructure should be attributed to inhomogeneous stored deformation energy in the steel before the annealing. This is
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Magnetic Induction B(mT)
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because stored energy introduced into a material during deformation provides the driving force for the subsequent annealing [24], and how the microstructure evolves due to the annealing depends, to a great extent, on the amount of stored deformation energy of material. From Fig. 10 and 11, we know that before the annealing, the grains near the cut edge have higher stored deformation energy as compared to the rest of the grains in the cut edge zone. Higher stored deformation energy contributes
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ACCEPTED MANUSCRIPT to earlier recrystallization nucleation and more crystal nucleus growth during the annealing. As a consequence, after the annealing the grains near the cut edge commonly display larger grain size while the rest grains exhibit relatively smaller grain size. Recrystallization mechanism may be that discontinuous static recrystallization took place after annealing at the vicinity of primary grain boundaries
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or within deformed grains. In the early stages of crystallization, the nuclei were
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outlined by low-angle boundaries, the misorientations of which gradually increased
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until the attained values typical of high-angle boundaries. This mechanism can reasonably explain why low-angle boundary percentages decrease, whereas average
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boundary misorientations increase after the annealing, as shown in Fig. 7 and 8. Fig. 4 also shows that in the cut edge zone of the cut-and-annealed steel, the
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microstructure of the upper surface is distinctly different from that of the lower surface, and the lower surface exhibits larger mean grain size in comparison with the
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lower surface. Literatures [22, 25] show that with increasing the deformation level,
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average grain size of the deformed-and-annealed steel tends to decrease gradually. Thus, the reason for the microstructure difference between the two surfaces may be
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due to the difference in their deformation levels before the annealing. From literature [4] and Fig. 6-10 in this paper, as we can see that the upper surface has undergone
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larger plastic deformation than the lower surface before the annealing, which may be responsible for its more refined microstructure after the annealing. Why plastic deformation affects annealed microstructure? The reason may be that larger plastic deformation gives more low-angle boundaries which can act as nucleation sites for grain recrystallization during annealing. Therefore, more new grains should develop after the annealing, which results in smaller mean grain size. Fig. 12-13 indicates that in the cut edge zone for both the upper surface and the
ACCEPTED MANUSCRIPT lower surface, the mechanically cut steel obviously exhibits stronger γ fiber component than λ fiber component. Fig.14 reveals that before the annealing, the texture volume fraction in γ fiber is much higher than that in λ fiber for the both surfaces. The deformed γ-grains having high stored energy usually tend to form new recrystallized γ-grains in grain interiors and boundary regions during the annealing
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according to literatures [26-27]. The formed new γ-grains commonly have an
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advantage in size and number over the other oriented recrystallized grains such as {100} grains. Thus, after the annealing, the texture volume fraction of γ fiber
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component should be much larger than that of the other components including λ fiber.
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But experimental result shows after the annealing, the volume fraction of γ fiber component is very approximate to that of λ fiber (see Fig.13). Literature [1] reveals
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that {001} recrystallized nuclei can be preferentially form mainly within the boundary regions of deformed {111} and {110} grains. The preferred crystallographic
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orientation <100> has a fast-growing rate. If the dislocation density near the grain
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boundaries is insufficient to facilitate the recrystallization nucleation, a selective growth mechanism will operate. By the jointing mechanism of competitive grain
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growth and preferred nucleation, more {100} nuclei grow into {100} grains at expense of the other grain growth such as λ grains. Thus, after the annealing the
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volume fraction of λ fiber increases whereas the volume fraction of γ fiber component increases. The volume fraction difference between the λ fiber and γ fiber components after the annealing is not as obvious as that before the annealing, as shown in Fig.14. 4.1 Hysteresis loop evolution due to the annealing Undoubtedly, the annealing resulted in a dramatic improvement in hysteresis loops of the cut experimental steel regardless of the variations of cutting length per mass, as shown in Fig. 14. The improvement in hysteresis loops is beneficial to the
ACCEPTED MANUSCRIPT improvement in magnetic properties such as iron loss, flux density, remanence (Br) and permeability (μ). Stress relief by the annealing treatment may be a main reason that causes the improvement of hysteresis loops, because it contributes to elimination of magnetic anisotropy which can deteriorate magnetization of the steel. Furthermore, as the annealing has a significant effect on microstructural characteristics of the cut
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Fe-Si steel, the change in hysteresis loops can also be attributed to the change in
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microstructure and texture. Fig.4 reveals that in the cut edge zone, the microstructures
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form both the upper surface and the lower surface of the cut steel are refined by the annealing. The reason for the microstructure refinement can be attributed to the
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occurrence of grain recrystallization or phase transformation. Refined microstructure is not helpful to the improvement of the hysteresis loop, and the reason is that the
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refinement of microstructure can induce grain boundary area increase, restricting the movement and rotation of magnetic domains during magnetization [28], thus, flux
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density and permeability decrease, and iron increases. Fig.4 also indicates that the
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microstructures from the both surfaces of the cut-and-annealed steel are extremely inhomogeneous, which is detrimental to magnetic properties and thus is not a reason
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for the improvement of hysteresis loop. Fig. 11 shows that the annealing caused a drastic decrease in dislocation density in the cut edge zone of the cut steel. literature
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[29] shows that dislocations are correlated with domain wall motion during magnetization process, and dislocation tangles can promote pining of domain wall. Thus, the decrease in dislocation densities in the cut edge zone due to the annealing may be one of reasons for the improvement of hysteresis loops. Fig. 13 presents that in the edge zone, because of the annealing, the intensity of λ fiber component from both the upper surface and the lower surface of the cut steel increases obviously while γ fiber component decreases. As we know that magnetization of Fe-Si steel in outer
ACCEPTED MANUSCRIPT magnetic field is dependent on its crystal orientation, and <001> and <111> directions are easy and hard magnetization directions, respectively. When the direction of outer magnetic field is parallel to <001> direction, the magnetization will increase to saturation rapidly. For this reason, λ fiber, with two easy magnetization <001> directions in the rolling plane, is most favorable, whereas γ fiber, without <001>
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directions in the rolling plane, is the most deleterious. The texture of the cut
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experimental steel is evidently improved by the annealing, which is beneficial to its
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magnetization and therefore can be considered to be one of reasons for the
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improvement of hysteresis loops.
5. Conclusion:
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The effect of the stress-relief annealing on microstructure and texture in the edge zone as well as hysteresis loop of a mechanically cut non-oriented Fe-Si steel was
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investigated. The main conclusions were drawn as follows:
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1. The annealing resulted in a significant decrease in dislocation densities and low-angle boundary (LAGB, 2°≤θ≤15°) percentages, and refined the grain
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sizes of both the upper surface and the lower surface. As the upper surface has undergone larger plastic deformation before the annealing, the annealing caused
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more refinement in grain size for the upper surface in comparison with the lower surface.
2. The annealing also caused a drastic increase in volume fraction of λ component, and led to an obvious decrease in that of γ fiber component. The formation of λ fiber texture may be attributed to the jointing mechanism of preferred nucleation and competitive grain growth. Furthermore, the texture evolution from the upper surface is obviously different from that of the lower surface, and the reason may
ACCEPTED MANUSCRIPT be due to their different deformed textures before the annealing. 3. The annealing dramatically improved the hysteresis loops regardless of the variations of the cutting length per mass. Almost all the lost magnetic properties due to mechanical cutting can be fully recovered by the annealing. The reason for the hysteresis loop improvement may be attributed to the decrease in residual
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stress and dislocation density, and the improvement of material texture.
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Acknowledgments
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This work was supported by the National Basic Research Program of China
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(Grant No. 2014CB046704) and the National Natural Science Foundation of china (51375005). The Analytical and Testing Center of HUST has provided useful
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characterizations for the work.
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ACCEPTED MANUSCRIPT Highlights 1. Microstructures and textures in the cut edge zone were characterized via EBSD. 2. The effect of annealing on microstructure and texture was investigated.
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3. The effect of the annealing on hysteresis loops was investigated.