Effect of Ce on the Microstructure, High-Temperature Tensile Properties, and Fracture Mode of Strip Casting Non-Oriented 6.5%Si Electrical Steel
LI Min1,2, LI Haoze1(), WANG Jijie2, MA Yingche1, LIU Kui1
1.Shi -changxu Innovation Center for Advanced Materials, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 2.College of Materials Science and Engineering, Shenyang Areospace University, Shenyang 110136, China
Cite this article:
LI Min, LI Haoze, WANG Jijie, MA Yingche, LIU Kui. Effect of Ce on the Microstructure, High-Temperature Tensile Properties, and Fracture Mode of Strip Casting Non-Oriented 6.5%Si Electrical Steel. Acta Metall Sin, 2022, 58(5): 637-648.
Non-oriented 6.5%Si electrical steel exhibits excellent high-frequency magnetic properties, such as low iron loss and near-zero magnetostriction. Moreover, the high Si content causes poor deformability due to the solution strengthening effect of Si and the resulting ordering transformations, delaying the commercial application. Strip casting is a near-net forming technology that directly produces thin strips from the melt and reduces the required rolling deformation for the fabrication of thin sheets. This technology could be a viable option for industrializing the production of non-oriented 6.5%Si electrical steel. Despite this, even in the strip casting process, this brittle material is prone to edge cracks. Thus, improving the intrinsic plasticity of strip casting non-oriented 6.5%Si electrical steel is necessary through chemical modification to eliminate the deforming defects. The effect of Ce on the ordered phase of the solidification microstructure, high-temperature tensile properties, and fracture mode of strip casting non-oriented 6.5%Si electrical steel was investigated in this work. The results showed that the addition of Ce introduced high-melting Ce2O2.5S and Ce4O4S3 during strip casting, which promoted heterogeneous nucleation and refined the solidification microstructure of the as-cast strip. The presence of Ce did not affect the ordering condition of the as-cast strip. In decreasing order, the tensile temperatures corresponding to the ordered degree of the as-cast strip were 650, 400, and 800oC. With the tensile temperature increasing, the yield and tensile strengths of the as-cast strip decreased, whereas the elongation gradually increased. When the tensile temperature exceeded 500oC, the purification and microstructure refining effects of Ce improved grain-boundary cohesion and prevented intergranular cracking. Moreover, the occurrences of dynamic recovery and recrystallization eventually made the as-cast strip doped with Ce fractured by dimples, leading to a considerably enhanced tensile ductility. According to the findings, the rare-earth treatment should be considered as an effective method for increasing the ductility of strip casting non-oriented 6.5%Si electrical steel.
Table 1 Chemical compositions of the as-cast non-oriented 6.5%Si steel strips
Fig.1 Schematic diagrams of strip casting process (a) and the solidification process (b)
Fig.2 Low (a, c) and high (b, d) magnified OM images of the CS1 (a, b) and CS2 (c, d) as-cast strips (ND—normal direction, RD—rolling direction)
Fig.3 SEM images and the corresponding EPMA elemental maps of the CS1 (a) and CS2 (b, c) as-cast strips
Position
Fe
Si
Ce
O
S
Ce∶O∶S
c-1
38.64
3.37
19.46
27.91
10.62
1.83∶2.63∶1
c-2
30.01
2.78
24.69
23.63
18.89
3.92∶3.75∶3
c-3
7.60
0.33
30.26
35.06
26.75
3.39∶3.93∶3
c-4
12.77
0.86
33.07
29.13
24.16
4.11∶3.65∶3
Table 2 EPMA analysis results of the granular second phases in Fig.3c
Case
[uvw]s
[uvw]n
d[uvw]s / nm
d[uvw]n / nm
θ / (o)
d[uvw]s·cosθ / nm
δ / %
δ-Fe
[]
[] δ-Fe
0.3967
0.4146
0
0.3967
3.9
[]
[] δ-Fe
0.6871
0.7128
0
0.6871
[]
[] δ-Fe
0.3967
0.4146
0
0.3967
δ-Fe
[]
[] δ-Fe
0.3958
0.4146
0
0.3958
4.6
[]
[] δ-Fe
1.0457
1.0970
0
1.0457
[]
[] δ-Fe
0.6851
0.7182
0
0.6851
Table 3 Calculated results of the lattice misfit between Ce2O2.5S, Ce4O4S3, and δ-Fe
Fig.4 Engineering stress-strain curves of the CS1 (a) and CS2 (b) as-cast strips tensile tested from 300oC to 900oC
Fig.5 Comparisons of the yield strength (Rp0.2) (a), tensile strength (Rm) (b), and elongation (c) of the CS1 and CS2 as-cast strips
Fig.6 Dark field TEM images and the corresponding [001] SAED patterns (insets) of the CS1 (a, c, e) and CS2 (b, d, f) as-cast strips tensile tested at 400oC (a, b), 650oC (c, d), and 800oC (e, f) (APB—antiphase boundary)
Fig.7 SEM images showing tensile fracture morphologies of the CS1 (a, c) and CS2 (b, d) as-cast strips tested at 300oC (a, b) and 400oC (c, d)
Fig.8 Longitudinal OM images of the CS1 (a, c) and CS2 (b, d) as-cast strips fractured at 300oC (a, b) and 400oC (c, d)
Fig.9 SEM images showing tensile fracture morphologies of the CS1 (a, c) and CS2 (b, d) as-cast strips tested at 500oC (a, b) and 650oC (c, d)
Fig.10 Longitudinal OM images of the CS1 (a, c) and CS2 (b, d) as-cast strips fractured at 500℃ (a, b) and 650℃ (c, d)
Fig.11 SEM images showing tensile fracture morphologies of the CS1 (a, c, e) and CS2 (b, d, f) as-cast strips tested at 700oC (a, b), 800oC (c, d), and 900oC (e, f)
Fig.12 Longitudinal OM images of the CS1 (a, c, e) and CS2 (b, d, f) as-cast strips fractured at 700oC (a, b), 800oC (c, d), and 900oC (e, f)
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