materials Article
Effect of Cold Rolling Process on Microstructure, Texture and Properties of Strip Cast Fe-2.6%Si Steel Yunbo Xu 1 , Haitao Jiao 1, * 1 2
*
ID
, Wenzheng Qiu 1 , Raja Devesh Kumar Misra 2 and Jianping Li 1
State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China;
[email protected] (Y.X.);
[email protected] (W.Q.);
[email protected] (J.L.) Laboratory for Excellence in Advanced steel Research, Department of Metallurgical, Materials, and Biomedical Engineering, University of Texas at El Paso, El Paso, TX 79968, USA;
[email protected] Correspondence:
[email protected]; Tel.: +86-024-8368-6642
Received: 6 June 2018; Accepted: 5 July 2018; Published: 8 July 2018
Abstract: The use of twin-roll strip casting for the preparation of non-oriented silicon steel has attracted widespread attention in recent years, but related reports are limited. In this study, both one- and two-stage cold rolling with three intermediate annealing temperatures were employed to produce strip cast non-oriented silicon steel. The evolution of the microstructure and texture through the processing routes and its effect on magnetic properties were studied. Compared with one-stage rolling, two-stage rolling increased the in-grain shear bands and the retention of Cube texture in the cold rolled sheets, thereby promoting the nucleation of favorable Goss and Cube grains and restraining the nucleation of harmful {111} grains. With the increase in intermediate annealing temperature, the η-fiber texture in annealed sheets was gradually enhanced, and the average grain size was increased, leading to significant improvement of magnetic properties. Keywords: cold rolling; strip casting; non-oriented silicon steel; texture; magnetic properties
1. Introduction Non-oriented silicon steels are commonly used soft magnetic materials in electrical machines which assist in the conversion between electrical and mechanical energy [1,2]. After experiencing the transition from hot-rolled materials to cold-rolled materials, the manufacturing process of non-oriented silicon steels is now well developed [3,4]. The magnetic properties of silicon steels are mainly related to the crystallographic texture and grain size of the final annealed sheets [5,6]. These microstructural characteristics of materials depend on the whole processing history, which involves casting, rolling, and recrystallization annealing. Extensive research on optimizing the processing parameters, such as hot rolling temperature and cold rolling direction and annealing rate, has been carried out to obtain favorable cube and Goss textures, and to improve magnetic properties [7–9]. In recent years, twin-roll strip casting technology is considered as a promising alternative for the fabrication of silicon steels with high magnetic induction [10–12]. Strip casting significantly simplifies the process by directly supplying the thin strip from molten steel, and also gives rise to different microstructures and texture evolution compared with conventional processes. The present study of strip cast non-oriented silicon steel mainly focuses on the initial microstructure of as-cast strip and the evolution of several special orientations. Park et al. [13] and Liu et al. [14] reported that the initial microstructure and texture of as-cast Fe-Si alloy strips was sensitive to casting parameters, while the high superheat promoted the formation of columnar grains. Jiao et al. [15] found that the coarse-grained strip with strong {100} components contributed to high magnetic induction and low core loss compared to the fine-grained strip with strong {110} (Goss) texture. It is known that the recrystallization texture is developed from the deformation microstructure, Materials 2018, 11, 1161; doi:10.3390/ma11071161
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{110} (Goss) texture. It is known that the recrystallization texture is developed from the deformation which is highly dependent on rolling history, as well as the initial Materials 2018, 11,microstructure, 1161 2 of 12 microstructure [16]. However, there are few studies on the relationship between microstructural evolution and the magnetic properties of strip cast non-oriented silicon under different deformation which is highly dependent on rolling history, as well as the initial microstructure [16]. However, there processes. are few on thework, relationship betweenas-cast microstructural evolution the magnetic of In studies the present a Fe-2.6%Si strip produced byand twin-roll strip properties casting was strip cast non-oriented under rolling different deformation processes. respectively processed silicon by one-stage and two-stage rolling methods in an attempt to optimize In the present work, a Fe-2.6%Si as-cast strip produced by twin-roll strip casting respectively the texture and magnetic properties. During two-stage rolling, a moderate rollingwas reduction was processed by one-stage rolling and two-stage rolling methods in an attempt to optimize the texture selected for every stage to obtain shear bands microstructures that promoted the formation of and magnetic properties. DuringIntwo-stage rolling, a moderate rolling reduction selected for every favorable //RD texture. addition, different intermediate annealingwas temperatures were stage to obtain shearthe bands microstructures that promoted the formation favorable //RD designed because intermediate annealing temperature affected ofthe final deformation texture. In addition, different intermediate annealing temperatures were influence designed on because the microstructure and recrystallization texture. These characteristics had great magnetic intermediate annealing temperature affected the final deformation microstructure and recrystallization properties, e.g., large grain size decreased the hysteresis loss and strong {100} texture increased the texture. These characteristics great influence ontexture magnetic properties, e.g., large grain size decreased magnetic induction [5,6]. Thehad microstructure and evolution along the entire processing route the loss in anddetail. strongThe {100} texture the magnetic induction [5,6].deformation The microstructure washysteresis investigated focus wasincreased on elucidating the effects of cold and an and texture evolution along the entire processing routemicrostructure, was investigated in detail. The focus was on intermediate annealing process on through process texture evolution, and final elucidating the effects of cold deformation and an intermediate annealing process on through process magnetic properties of strip cast non-oriented silicon steel. microstructure, texture evolution, and final magnetic properties of strip cast non-oriented silicon steel. 2. Materials and Methods 2. Materials and Methods The as-cast Fe-Si alloy strip with a thickness of 2.4 mm was prepared by a laboratorial twin-roll The as-cast alloy strip with a thickness 2.4 mmcomposition was prepared by amaterial laboratorial strip caster with Fe-Si a melt superheat of ~50 °C. The of chemical of the was ◦ C. The chemical composition of the material twin-roll strip caster with a melt superheat of ~50 Fe-0.005C-2.6Si-0.2Mn-0.4Al (in wt %). The calculation by Thermo-Calc indicated that the was Fe-0.005C-2.6Si-0.2Mn-0.4Al (in wt %). at The calculation by Thermo-Calc indicated the microstructure of this material was ferrite any temperature below liquidus, i.e., nothat phase microstructure this material ferrite at any temperature below phase transformation. of Samples of size was 90 mm (length) × 120 mm (width) wereliquidus, cut fromi.e., the no strip and transformation. Samples of size 90 mm (length) × 120 mm (width) were cut from the strip and pickled pickled in a hydrochloric acid solution to remove the oxide scale. Subsequently, four different in a hydrochloric acid solution to remove therolling oxide scale. four differentat processes processes were adopted: (1) one-stage cold to 0.35Subsequently, mm, with final annealing 1000 °C were for 6 ◦ adopted: (1) one-stage cold rolling to 0.35 mm, final annealing at 1000 C for min, referred min, referred as OCR process; (2) two-stage cold with rolling to 0.35 mm, with the first cold6 rolling to 0.90 as OCR (2) two-stage cold rolling 0.35 with thefinal firstannealing cold rolling 0.90°C mm the mm andprocess; the intermediate annealing at 900to°C formm, 6 min and at to 1000 forand 6 min, ◦ ◦ intermediate annealing at 900 C for 6 min and final to annealing 1000theCfirst for 6cold min,rolling referred as TCR9 referred as TCR9 process; (3) two-stage cold rolling 0.35 mm,atwith to 0.90 mm process; (3) two-stage cold rolling to 0.35 mm, with the first cold rolling to 0.90 mm and the intermediate and the intermediate annealing at 1000 °C for 6 min and final annealing at 1000 °C for 6 min, referred ◦ C for 6 min, referred as TCR10 process; (4) annealing at 1000 ◦(4) C for 6 min and final annealing 1000with as TCR10 process; two-stage cold rolling to 0.35atmm, the first cold rolling to 0.90 mm and the two-stage cold rolling to 0.35 mm, with the first cold rolling to 0.90 and the annealing intermediate annealing at 1100 °C for 6 min and final annealingmm at 1000 °C intermediate for 6 min, referred as ◦ C for 6 min and final annealing at 1000 ◦ C for 6 min, referred as TCR11 process. Here, the at 1100 TCR11 process. Here, the intermediate annealing temperature of 900–1100 °C was above the ◦ C was above the recrystallization temperature of intermediate annealing temperature 900–1100 The recrystallization temperature of thisofmaterial. intermediate annealing schedule, including this material. The annealing to schedule, temperature and time, was designed to temperature and intermediate time, was designed obtain including a fully recrystallized microstructure, thereby obtain a fully recrystallized microstructure, thereby eliminating the hereditary detrimental deformation eliminating the hereditary detrimental deformation texture of the first reduction. The annealing texture theconducted first reduction. annealingof process was conducted in an atmosphere of pure N2 . and The processofwas in anThe atmosphere pure N 2. The schematic diagram of strip casting schematic diagram of strip casting and different processing routes is shown in Figure 1. different processing routes is shown in Figure 1.
Figure 1. Schematic diagram of different processing routes for the as-cast Fe-Si alloy strip. Figure 1. Schematic diagram of different processing routes for the as-cast Fe-Si alloy strip.
The microstructure of all samples was observed using a Leica Q550IW optical microscope (OM， microstructure of allGermany). samples was a Leica optical microscope (OM, LeicaThe Camera AG, Wetzlar, IPPobserved softwareusing was used to Q550IW analyze the distribution of grain Leica Camera AG, Wetzlar, Germany). IPP software was used to analyze the distribution of grain size. Samples for OM observations were mechanically polished and etched with 5% nital solution. To
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size. Samples for OM observations were mechanically polished and etched with 5% nital solution. To analyze macro-textures, incomplete pole figures {200}, {110}, and{211} {211}were weremeasured measuredonona analyze thethe macro-textures, thethe incomplete pole figures {200}, {110}, and aBruker Bruker D8 Discover X-ray diffraction (XRD, Bruker, Billerica, MA, USA) with polar α D8 Discover X-ray diffraction (XRD, Bruker, Billerica, MA, USA) with polar angle α angle ranging ranging from to 75°. The ODFs (orientation distribution were calculated based on three from 0◦ to 75◦ .0° The ODFs (orientation distribution functions)functions) were calculated based on three incomplete incomplete pole figures by the series expansion method (I max = 22). Micro-textures of partially pole figures by the series expansion method (Imax = 22). Micro-textures of partially recrystallized recrystallized wereby determined by a EBSD (electron diffraction) backscatter diffraction) system samples weresamples determined a EBSD (electron backscatter system attached toattached a ZEISS to a ZEISS ULTRA 55 field emission scanning electron microscope (ZEISS, Oberkochen, Germany). ULTRA 55 field emission scanning electron microscope (ZEISS, Oberkochen, Germany). Samples for Samples XRD and were EBSDelectropolished analysis werebeforehand electropolished beforehand 11% perchloric XRD and for EBSD analysis with 11% perchloric with acid/alcohol solution. acid/alcohol solution. In addition, a single sheet tester was used to measure the magnetic properties In addition, a single sheet tester was used to measure the magnetic properties of annealed sheets, of annealed sheets, both in rolling direction and transverse direction. The magnetic induction 50 and both in rolling direction and transverse direction. The magnetic induction B50 and core loss PB 15/50 at core loss P 15/50 at frequency of 50 Hz was determined at a field strength of 5000 A/m and a magnetic frequency of 50 Hz was determined at a field strength of 5000 A/m and a magnetic flux density of flux of 1.5 T, respectively. 1.5 T,density respectively. 3. 3. Results Results and and Discussion Discussion
Microstructure and and Texture of As-Cast Strip 3.1. Microstructure The microstructure microstructure and and texture texture of of the as-cast strip are given in Figure microstructure The Figure 2. 2. The microstructure of of columnar grains andand some equiaxed grains with grain through the the thickness thicknesswas wasmainly mainlycomposed composed columnar grains some equiaxed grains with diameters ranging from ~60 ~770 average grain grain size was 2a). The grain diameters ranging fromto~60 to µm; ~770the μm; the average size~330 was µm ~330(Figure μm (Figure 2a).small The grains grains were mainly observed at the surface strip. The textureThe wastexture characterized by pronounced small were mainly observed at theofsurface of strip. was characterized by λ-fiber (//ND) texture with strong {001} orientation (Figure (Figure 2b). In 2b). addition, a few pronounced λ-fiber (//ND) texture with strong {001} orientation In addition, components, e.g., e.g., {114} andand {110}, were also adeformation few deformation components, {114} {110}, were alsonoted. noted.This This kind kind of as-cast structure was was similar similar to to Liu’s Liu’s observation observation [14], [14], but but different different to the result of Park [13]. During During strip strip structure casting, the the heterogeneous heterogeneous nucleation nucleation with with high high nucleation nucleation rate rate occurred occurred under under large large supercooling casting, when molten molten steel steelmet metcasting castingrollers, rollers,leading leadingtotothe theformation formation solidified shells with fine grains when ofof solidified shells with fine grains at at the surface of strip. Subsequently, supercooling at the solidification front decreased, resulting the surface of strip. Subsequently, thethe supercooling at the solidification front decreased, resulting in a reduction nucleation rate.Moreover, Moreover, highsuperheat superheatininthis thisstudy study contributed contributed aa large large ainreduction of of nucleation rate. thethehigh temperature gradient gradient along along the the normal normal direction direction or or heat heat extraction extraction direction, direction, which which promoted promoted the the temperature growth of of columnar columnardendritic dendriticgrains grainsand andthe theformation formation {100} texture [15]. Before left growth of of {100} texture [15]. Before the the stripstrip left the the casting rollers, the original solidification microstructure experienced plastic deformation, casting rollers, the original solidification microstructure experienced slightslight plastic deformation, and and thus, the deformation texture developed. thus, the deformation texture was was developed.
Figure 2. (a) metallographic structure of the as-cast strip and (b) macro-texture (φ2 = 45°◦ section of Figure 2. (a) metallographic structure of the as-cast strip and (b) macro-texture (ϕ2 = 45 section of ODFs) of the as-cast strip. ODFs) of the as-cast strip.
3.2. Effect of Cold Rolling Process on Microstructure 3.2. Effect of Cold Rolling Process on Microstructure Typical optical micrographs of cold rolled and annealed sheets processed by one stage rolling Typical optical micrographs of cold rolled and annealed sheets processed by one stage rolling are shown in Figure 3. Three kinds of deformed microstructure corresponding to various etching are shown in Figure 3. Three kinds of deformed microstructure corresponding to various etching degree were observed (Figure 3a): rough and darkly etched grains with high dislocation density, degree were observed (Figure 3a): rough and darkly etched grains with high dislocation density, such such as region A; moderately etched grains with in-grain shear bands, such as region B; and smooth as region A; moderately etched grains with in-grain shear bands, such as region B; and smooth and and lightly etched grains, such as region C. As reported by Sanjari et al. [17], the smooth grains had lightly etched grains, such as region C. As reported by Sanjari et al. [17], the smooth grains had low low stored energy and were generally oriented by //ND or //RD textures. In contrast, the
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stored energy and were generally oriented by //ND or //RD textures. In contrast, the rough stored energy andand werewere usually characterized by //ND texture with rough grains grainsexhibited exhibitedhigh high stored energy usually characterized by //ND texture high factor. factor. It can be thatseen grains thoselike in regions and C dominated the cold-rolled with Taylor high Taylor It seen can be thatlike grains those inA regions A and C dominated the microstructure of OCR sample. Figuresample. 3b displays the 3b annealed microstructure; statistical analysis cold-rolled microstructure of OCR Figure displays the annealedthe microstructure; the of grain diameters measured from about 200 random grains using IPP. Here, d is average grain size, statistical analysis of grain diameters measured from about 200 random grainsa using IPP. Here, da is and SD isgrain standard reflect the degree oftograin size dispersion, uniformity. The sizei.e., of average size, deviation and SD istostandard deviation reflect the degree of i.e., grain size dispersion, grains was mainly in the range of 10–60 µm; the average value was ~33 µm with a standard deviation uniformity. The size of grains was mainly in the range of 10–60 μm; the average value was ~33 μm of ~15a µm, revealing a relatively with standard deviation of ~15 homogeneous μm, revealing microstructure. a relatively homogeneous microstructure.
Figure 3. 3. Optical Opticalmicrostructure microstructureofofsheets sheets processed one-stage rolling. (a) cold-rolled Figure processed byby one-stage coldcold rolling. (a) cold-rolled sheetsheet and and (b) final annealed sheet. (b) final annealed sheet.
Figures 4 and 5 illustrate the microstructural evolution in samples processed by two-stage cold Figures 4 and 5 illustrate the microstructural evolution in samples processed by two-stage rolling with a different intermediate annealing temperature. Compared to the deformation cold rolling with a different intermediate annealing temperature. Compared to the deformation microstructure in the OCR process, the first cold-rolled sheet with a reduction of ~62.5% exhibited microstructure in the OCR process, the first cold-rolled sheet with a reduction of ~62.5% exhibited more rough grains, with a number of in-grain shear bands (Figure 4a). The microstructure was more rough grains, with a number of in-grain shear bands (Figure 4a). The microstructure was completely recrystallized after intermediate annealing. Several abnormally large grains were completely recrystallized after intermediate annealing. Several abnormally large grains were observed observed in 1000–1100 °C annealed sheets, and the average grain size increased from ~85 μm to in 1000–1100 ◦ C annealed sheets, and the average grain size increased from ~85 µm to ~120 µm ~120 μm with increase in annealing temperature. After second cold rolling, the majority of grains with increase in annealing temperature. After second cold rolling, the majority of grains through the through the thickness of TCR9 sample were smooth and moderately elongated, except for the thickness of TCR9 sample were smooth and moderately elongated, except for the locally observed locally observed grains with high density of dislocations (Figure 5a). For TCR10 and TCR11 grains with high density of dislocations (Figure 5a). For TCR10 and TCR11 samples, the smooth samples, the smooth deformation grains gradually decreased, while the fraction of shear bands deformation grains gradually decreased, while the fraction of shear bands gradually increased gradually increased (Figure 5b,c). Furthermore, the length of shear bands was also increased. After (Figure 5b,c). Furthermore, the length of shear bands was also increased. After final annealing, final annealing, the average size of corresponding recrystallized grains increased from ~37 to ~64 the average size of corresponding recrystallized grains increased from ~37 to ~64 µm, together with μm, together with the deterioration of microstructural homogeneity. the deterioration of microstructural homogeneity. It is known that dislocation slip is the main mechanism during plastic deformation, which leads to the formation of dislocation cells, dislocation walls, or microbands, depending on the imposed strain and initial orientation [18,19]. In addition, shear bands occur as a specific manifestation of local plastic instability at medium to large strains [20]. In the case of one-stage cold rolling, the shear bands that formed at moderate reduction, such as the case in Figure 4a, were destroyed during the further reduction, resulting in severely fragmented microstructure with a high density of dislocations (such as region A in Figure 3a). This kind of microstructure had high stored energy and provided a number of nucleation sites, leading to small grain size after recrystallization (Figure 3b). In the case of two-stage cold rolling, the grain size of intermediately annealed sheets and the strain during second cold rolling were the deciding factors affecting the final deformation microstructure. According to the results
Figures 4 and 5 illustrate the microstructural evolution in samples processed by two-stage cold rolling with a different intermediate annealing temperature. Compared to the deformation microstructure in the OCR process, the first cold-rolled sheet with a reduction of ~62.5% exhibited more rough grains, with a number of in-grain shear bands (Figure 4a). The microstructure was completely recrystallized after intermediate annealing. Several abnormally large grains were Materials 2018, 11, 1161 5 of 12 observed in 1000–1100 °C annealed sheets, and the average grain size increased from ~85 μm to ~120 μm with increase in annealing temperature. After second cold rolling, the majority of grains reported the elsewhere [20–22], shear bands readily developed at a reduction of 61.2%except duringfor second through thickness of TCR9 samplewere were smooth and moderately elongated, the rolling in this study. Nevertheless, thedensity small grains in intermediately of TCR9 locally observed grains with high of dislocations (Figureannealed 5a). Forsheets TCR10 and sample TCR11 restricted the shear banding because of grains the existence of a large number of boundaries. Asofforshear TCR10 and samples, smooth deformation gradually decreased, while the fraction bands TCR11 samples, the increased grain size in intermediate annealed sheets promoted localization gradually increased (Figure 5b,c). Furthermore, the length of shear bands was alsoshear increased. After because of weakthe strain coordination in grains. On the other hand,grains the increased to final annealing, average size of corresponding recrystallized increasedgrain fromsize ~37prior to ~64 deformation the nucleation sites, and thereby increased the size of recrystallized grains. μm, together also withdecreased the deterioration of microstructural homogeneity.
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Figure Microstructureofofsheets sheetsprocessed processedbybytwo-stage two-stage cold rolling. First cold-rolled sheet; Figure 4. 4. Microstructure cold rolling. (a) (a) First cold-rolled sheet; (b) ◦ ◦ (b) C intermediate-annealed sheet; 1000°C Cintermediate-annealed intermediate-annealedsheet sheetand and (d) (d) 1100 1100 ◦°C C 900900 °C intermediate-annealed sheet; (c)(c) 1000 intermediate-annealed intermediate-annealed sheet. sheet.
Figure 5. 5. Microstructural Microstructural evolution evolution during during second second cold cold rolling rolling and and final final annealing annealing corresponding corresponding to to Figure (a) TCR9; (b) TCR10 and (c) TCR11 processes. (a) TCR9; (b) TCR10 and (c) TCR11 processes.
It is of known that dislocation slip is the main mechanism during plastic deformation, which 3.3. Effect Cold Rolling Process on Texture leads to the formation of dislocation cells, dislocation walls, or microbands, depending on the The evolution of macro-texture during[18,19]. differentInprocessing characterized and imposed strain and initial orientation addition,routes shearwas bands occur asbyaXRD specific ODF, where the expressed by Euler (ϕ1 , Φ,[20]. ϕ2 ). In Inthe thecase case of of one-stage OCR, the cold manifestation oforientations local plasticwere instability at medium toangles large strains cold rolling texture was characterized by strong γ-fiber (//ND) texture and α-fiber (//RD) rolling, the shear bands that formed at moderate reduction, such as the case in Figure 4a, were texture composed (Figurein6a). This belongs to the microstructure typical rolling texture destroyed during of the{001}–{111} further reduction, resulting severely fragmented with a in low carbon steel sheets [23]. In addition, some {001} component was observed along high density of dislocations (such as region A in Figure 3a). This kind of microstructure had with high stored energy and provided a number of nucleation sites, leading to small grain size after recrystallization (Figure 3b). In the case of two-stage cold rolling, the grain size of intermediately annealed sheets and the strain during second cold rolling were the deciding factors affecting the final deformation microstructure. According to the results reported elsewhere [20–22], shear bands were readily developed at a reduction of 61.2% during second rolling in this study. Nevertheless,
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the //ND orientation (λ-fiber) line, indicating the retention of initial {100} texture in as-cast 6 of 12 components was ~2.15. The maximum of f (g) = 11.7 was presented ◦ ◦ ◦ at (0 , 5 , 45 ) close to {100} orientation. After annealing, α deformation texture was rarely intensity (85°, 60°, 45°). The recrystallization texture consisted of predominant observed,dropped except fortoa ~4.7 smallatamount of {110} at (0◦ , 10◦ , 45◦ ). The maximum orientation intensity 1 ◦ , 60◦ , 45 ◦ ). {001} {111} deviation and components, together with of weak α*-fiber ({11h}) dropped towith ~4.7 5° at (85 The recrystallization texture consisted predominant {111} Materials 2018, 11, x FOR PEER REVIEW strip. The volume fraction of {100}
ℎ
1 texture. kind of annealing texture in the OCR sample with differed from the commonly in with 5◦ This deviation and {001} components, together weak α*-fiber ({11h}) texture. strip cast non-oriented silicon steels 0.50sample mm, which displayed strong Cube and Goss texture This kind of annealing texture in theof OCR differed from the commonly observed in strip and cast weak γ-fiber silicon texturesteels [15,24,25]. can bedisplayed attributedstrong to the change in recrystallization behavior non-oriented of 0.50 This mm, which Cube and Goss texture and weak γ-fiber due to the increaseThis of severely fragmented grains andin the decrease of shear bandsdue in the cold rolled texture [15,24,25]. can be attributed to the change recrystallization behavior to the increase sheets. of severely fragmented grains and the decrease of shear bands in the cold rolled sheets.
Figure 6. Macro-texture of OCR sample after (a) cold rolling and after (b) final annealing, and (c) Figure 6. Macro-texture of OCR sample after (a) cold rolling and after (b) final annealing, and (c) major major orientations and fibers on φ 2 = 0° and φ2 = 45° sections of ODFs. orientations and fibers on ϕ2 = 0◦ and ϕ2 = 45◦ sections of ODFs.
In order to further understand the development of recrystallization texture, the In order to further understand the development of recrystallization the micro-orientation micro-orientation in partially recrystallized OCR sample was measured.texture, The corresponding IPF and in partially recrystallized OCR sample was measured. The corresponding IPF and OIM in Figure 7 show OIM in Figure 7 show that a large number of grains with {111} orientation preferentially that a large number of grains with {111} orientation preferentially nucleated in the {111} nucleated in the {111} and {111} oriented deformation matrix, which can be explained by and oriented {111} oriented deformation matrix, which cantheory be explained by the oriented theory the growth theory and oriented nucleation [26]. Furthermore, the growth recrystallized and oriented nucleation theory [26]. Furthermore, the recrystallized grains generally {111} grains generally possessed larger size than other grains.{111} Many {001} oriented possessed larger size than other grains. Many {001} oriented grains were also observed be grains were also observed to be present in the microstructure. They were inherited fromtothe present in the microstructure. They were inherited from the characteristics of initial columnar grains characteristics of initial columnar grains with {100} texture during rolling and annealing [27]. with {100} the texture during {110} rolling andgrains annealing However, the deformed {110} grainsstored were However, deformed were[27]. hard to recrystallize because of their lower hard to recrystallize because of their lower stored energy, and were gradually consumed by new grains. energy, and were gradually consumed by new grains. It can be inferred that {111} and It can be inferred that {111} and {001} grains dominated subsequent grainand growth {001} grains dominated the subsequent grain growth process by the virtue of nucleation size process by virtue of nucleation and size advantage, thereby leading to the recrystallization texture advantage, thereby leading to the recrystallization texture pattern in OCR sample. pattern in OCR sample. In the case of two-stage cold rolling, the texture after first cold rolling was composed of primary α-fiber and minor λ-fiber texture (Figure 8a), where the maximum f (g) = 11.3 was presented at (15◦ , 0◦ , 45◦ ) or {001}. Compared with the texture of cold-rolled OCR sample (Figure 6a), relatively small deformation significantly decreased the γ-fiber texture and enhanced the retention of the initial {100} texture in first cold rolled sheets. After intermediate annealing at 900–1100 ◦ C for 6 min, a similar texture pattern, i.e., pronounced η-fiber with peaks at Goss and Cube orientation, was developed in the three annealed samples (Figure 8b–d). However, the texture components (especially
nucleated in the {111} and {111} oriented deformation matrix, which can be explained by the oriented growth theory and oriented nucleation theory [26]. Furthermore, the recrystallized {111} grains generally possessed larger size than other grains. Many {001} oriented grains were also observed to be present in the microstructure. They were inherited from the characteristics of initial columnar grains with {100} texture during rolling and annealing 7 [27]. Materials 2018, 11, 1161 of 12 However, the deformed {110} grains were hard to recrystallize because of their lower stored energy, and were gradually consumed by new grains. It can be inferred that {111} and the Goss orientation) on η-fiberthe and the intensity were decreased withbyanvirtue increase in the intermediate {001} grains dominated subsequent grain growth process of nucleation and size annealing temperature. advantage, thereby leading to the recrystallization texture pattern in OCR sample.
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Figure 7. Inverse pole figure map (IPF) and relevant orientation image maps (OIM) of several main components in the partially recrystallized OCR sample.
In the case of two-stage cold rolling, the texture after first cold rolling was composed of primary α-fiber and minor λ-fiber texture (Figure 8a), where the maximum f(g) = 11.3 was presented at (15°, 0°, 45°) or {001}. Compared with the texture of cold-rolled OCR sample (Figure 6a), relatively small deformation significantly decreased the γ-fiber texture and enhanced the retention of the initial {100} texture in first cold rolled sheets. After intermediate annealing at 900–1100 °C for 6 min, a similar texture pattern, i.e., pronounced η-fiber with peaks at Goss and CubeFigure orientation, waspole developed in (IPF) the three annealed samplesimage (Figure 8b–d). However, texture 7. Inverse figure map and relevant orientation maps (OIM) of severalthe main components (especially the Goss orientation) η-fiber and the intensity were decreased with an components in the partially recrystallized OCRon sample. increase in the intermediate annealing temperature.
Figure 8. Macro-textures Macro-texturesofof cold rolled afterreduction; first reduction; 900 °C intermediately Figure 8. (a)(a) cold rolled sheetsheet after first (b) 900 ◦(b) C intermediately annealed annealed sheet; (c) 1000 °C intermediately annealed sheet and (d) 1100 °C intermediately annealed ◦ ◦ sheet; (c) 1000 C intermediately annealed sheet and (d) 1100 C intermediately annealed sheet during sheet during two-stage rolling process. two-stage rolling process.
Figure 9 shows the texture after second rolling for the two-stage processed sample. The texture Figure 9 shows the texture after second rolling for the two-stage processed sample. The texture of of the TCR9 sample was mainly characterized by α-fiber and γ-fiber texture with a peak at near the TCR9 sample was mainly characterized by α-fiber and γ-fiber texture with a peak at near {111} {111} orientation. In the case of TCR10 and TCR11, α-fiber was increased while γ-fiber was orientation. In the case of TCR10 and TCR11, α-fiber was increased while γ-fiber was weakened. weakened. Meanwhile, the maximum intensity of texture was gradually increased, and the Meanwhile, the maximum intensity of texture was gradually increased, and the corresponding corresponding orientation shifted from {111} to {114}, and then to {118}, which was orientation shifted from {111} to {114}, and then to {118}, which was 10◦ away 10° away from {100}. In addition, more {100} components were observed in the latter two from {100}. In addition, more {100} components were observed in the latter two samples. samples. Compared to one-stage cold rolling (Figure 6), α and γ texture in the cold rolled sheets Compared to one-stage cold rolling (Figure 6), α and γ texture in the cold rolled sheets processed by processed by two-stage rolling was weakened, and the retention of {100} texture was enhanced two-stage rolling was weakened, and the retention of {100} texture was enhanced because of smaller because of smaller crystal rotation. crystal rotation.
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Figure 9. Second cold rolling textures of (a) TCR9 sample; (b) TCR10 sample and (c) TCR11 sample. Figure 9. Second cold rolling textures of (a) TCR9 sample; (b) TCR10 sample and (c) TCR11 sample. sample.
Figure 10 shows the recrystallization texture of final annealed sheets produced by two-stage Figure 10 10 shows shows the the recrystallization recrystallization texture texture of of final final annealed annealed sheets sheets produced produced by two-stage two-stage Figure rolling. All samples featured very weak γ-fiber texture and pronounced η-fiber texture by with distinct rolling. All samples featured very weak γ-fiber texture and pronounced η-fiber texture with distinct rolling. AllCube samples featured very weakwas γ-fiber texture and pronounced texture distinct Goss and components, which clearly different from the η-fiber {111} andwith {001} Goss and andCube Cube components, which was clearly different from the {111} and {001} Goss components, which was clearly from the {111} and {001} annealing annealing texture in the OCR sample (Figuredifferent 6). Moreover, η-fiber texture was further enhanced annealing texture in the OCR sample (Figure 6). Moreover, η-fiber texture was further enhanced texture in the OCR sample (Figure 6). Moreover, η-fiber texture was further enhanced with increased with increased intermediate annealing temperature. This was related to the change of deformation with increased intermediate annealingThis temperature. This waschange relatedoftodeformation the change of deformation intermediate annealing temperature. was related to the microstructure microstructure and texture. microstructure and texture. and texture.
Figure 10. Final annealing textures of (a) TCR9 sample; (b) TCR10 sample and (c) TCR11 sample. Figure 10. Final annealing textures of (a) TCR9 sample; (b) TCR10 sample and (c) TCR11 sample. Figure 10. Final annealing textures of (a) TCR9 sample; (b) TCR10 sample and (c) TCR11 sample.
The development of the recrystallization texture in two-stage rolling processed samples was The development of the recrystallization texture in two-stage rolling processed samples was related the change ofofdeformation microstructure texture. Figure shows thesamples orientation Thetodevelopment the recrystallization textureand in two-stage rolling11processed was related to the change of deformation microstructure and texture. Figure 11 shows the orientation image partially and TCR11 with one-stage rolling, relatedmaps to theofchange ofrecrystallized deformation TCR10 microstructure andsamples. texture. Compared Figure 11 shows the orientation image maps of partially recrystallized TCR10 and TCR11 samples. Compared with one-stage rolling, the two-stage cold rolling processTCR10 significantly changed theCompared recrystallization behavior of image maps of partially recrystallized and TCR11 samples. with one-stage rolling, the two-stage cold rolling process significantly changed the recrystallization behavior of deformation The {111}-oriented deformation matrix showedbehavior slow recrystallization, the two-stagemicrostructure. cold rolling process significantly changed the recrystallization of deformation deformation microstructure. The {111}-oriented deformation matrix showed slow recrystallization, whereas the in-grain shear bands within these grains recrystallized rapidly. Furthermore, the grains microstructure. The {111}-oriented deformation matrix showed slow recrystallization, whereas the whereas the in-grain shear bands within these grains recrystallized rapidly. Furthermore, the grains nucleated in the {111}within deformation matrix had major Goss orientation and minor Cube nucleated orientation, in-grain shear bands these grains recrystallized rapidly. Furthermore, the grains in nucleated in the {111} deformation matrix had major Goss orientation and minor Cube orientation, in with other experimental resultsand [15,21,28]. Neworientation, Goss and in Cube grains theaccordance {111} deformation matrixrelated had major Goss orientation minor Cube accordance in accordance with other related experimental results [15,21,28]. New Goss and Cube grains gradually and dominated results the microstructure by Goss consuming the grains surrounding deformation with other grew related experimental [15,21,28]. New and Cube gradually grew and gradually grew and dominated the microstructure by consuming the surrounding deformation matrix. It was noted that the density of Gossdeformation grains in the TCR11 sample dominated the microstructure by nucleation consuming the surrounding matrix. It was noted was that matrix. It was noted that the nucleation density of Goss grains in the TCR11 sample was significantly higher than that of the TCR10 sample. The Goss grains were known to originate from significantly higher than that of the TCR10 sample. The Goss grains were known to originate from the retention of initial Goss orientation between microbands and the newly formed Goss orientation the retention of initial Goss orientation between microbands and the newly formed Goss orientation
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the nucleation density of Goss grains in the TCR11 sample was significantly higher than that of the 9 of 12 TCR10 sample. The Goss grains were known to originate from the retention of initial Goss orientation between and newly formed orientation within shear bands {111} within themicrobands shear bands in the {111} crystalsGoss [29,30]. Similar to Goss the orientation, newinCube grains crystals [29,30]. Similar to Goss orientation, new Cube grains nucleated at shear bands and Cube nucleated at shear bands and Cube deformation bands (the retention of Cube orientation) [31], as deformation bands (the retention of Cube orientation) [31], as shown in Figure 11. The volume fraction shown in Figure 11. The volume fraction of Goss orientation in cold-rolled TCR10 and TCR11 of Goss orientation cold-rolled TCR10 and TCR11 samples was ~1.80 ~1.66, samples was ~1.80 in and ~1.66, respectively, while that of Cube were and ~3.53 and respectively, ~3.48, with while small that of Cube were ~3.53 and ~3.48, with small differences between the two samples. Therefore, differences between the two samples. Therefore, more shear bands in the TCR11 sample more were shear bands in the TCR11 wereofmainly for the formation of morein Goss and Cube mainly responsible for thesample formation more responsible Goss and Cube nuclei. The increase intermediate nuclei. The increase inincreased intermediate annealing temperature increased the grain size prior final annealing temperature the grain size prior to final rolling, resulting in more sheartobands rolling, resulting in more shear bands in final cold rolled sheets. Ultimately, the η-fiber texture with in final cold rolled sheets. Ultimately, the η-fiber texture with sharp Goss and Cube components sharp Goss andenhanced. Cube components was gradually enhanced. was gradually Materials 2018, 11, x FOR PEER REVIEW
Figure 11. Inverse pole figure map (IPF) and relevant orientation image maps (OIM) of several main Figure 11. Inverse pole figure map (IPF) and relevant orientation image maps (OIM) of several main components in(a) (a)partially partially recrystallized TCR10 sample andpartially (b) partially recrystallized TCR11 components in recrystallized TCR10 sample and (b) recrystallized TCR11 sample. sample.
3.4. Magnetic Properties under Different Rolling and Annealing Processes 3.4. Magnetic Properties under Different Rolling and Annealing Processes It is known that the recrystallization texture and annealing microstructure has great effect on the It is known that the recrystallization texture and annealing microstructure has great effect on magnetic properties of non-oriented silicon steels. In bcc iron, crystal direction is the easiest the magnetic properties of non-oriented silicon steels. In bcc iron, crystal direction is the magnetization direction because of the lowest magnetocrystalline anisotropy energy, and is the easiest magnetization direction because of the lowest magnetocrystalline anisotropy energy, and hardest magnetization direction [11,12]. Thus, //RD (η-fiber) and //ND (λ-fiber) textures is the hardest magnetization direction [11,12]. Thus, //RD (η-fiber) and //ND are considered to be favorable for magnetic properties, while the //ND (γ-fiber) texture is (λ-fiber) textures are considered to be favorable for magnetic properties, while the //ND harmful. In addition, the total core loss of non-oriented silicon steel consists of hysteresis loss, classical (γ-fiber) texture is harmful. In addition, the total core loss of non-oriented silicon steel consists of eddy current loss, and anomalous loss, while hysteresis loss dominates at low frequencies [11,32]. hysteresis loss, classical eddy current loss, and anomalous loss, while hysteresis loss dominates at Usually, hysteresis loss decreases with an increase in the grain size of the annealed sheets, because of a low frequencies [11,32]. Usually, hysteresis loss decreases with an increase in the grain size of the decrease in area of the domain walls. Figure 12 shows the average magnetic properties of all annealed annealed sheets, because of a decrease in area of the domain walls. Figure 12 shows the average sheets. The values of B and P15/50 as ~1.708 T and ~3.36 W/kg, respectively, were obtained with the magnetic properties of50all annealed sheets. The values of B50 and P15/50 as ~1.708 T and ~3.36 W/kg, DCR sample. Here, the strong {111} recrystallization texture with hardest magnetization respectively, were obtained with the DCR sample. Here, the strong {111} recrystallization direction in the DCR sample was responsible for the low magnetic induction. The high core loss was texture with hardest magnetization direction in the DCR sample was responsible for the low mainly related to the small grain size, even though the detrimental {111} texture also increased magnetic induction. The high core loss was mainly related to the small grain size, even though the the loss. In the case of the TCR9 sample processed by two-stage rolling, the γ-fiber texture was detrimental {111} texture also increased the loss. In the case of the TCR9 sample processed by significantly weakened, while the λ-fiber and η-fiber textures were enhanced (Figure 10a), together two-stage rolling, the γ-fiber texture was significantly weakened, while the λ-fiber and η-fiber with a slight increase in average grain size (Figure 5a). This suggested more easy magnetization textures were enhanced (Figure 10a), together with a slight increase in average grain size (Figure direction in rolling plane, as well as less domain walls. As a result, a slight reduction in P15/50 , 5a). This suggested more easy magnetization direction in rolling plane, as well as less domain together with an increase of B50 by 0.01 T, was present in the TCR9 sample. When higher intermediate walls. As a result, a slight reduction in P15/50, together with an increase of B50 by 0.01 T, was present annealing temperatures were adopted, the magnetic properties were further improved, while the in the TCR9 sample. When higher intermediate annealing temperatures were adopted, the magnetic highest magnetic induction (~1.745 T) and lowest core loss (~2.92 W/kg) were observed in the TCR11 properties were further improved, while the highest magnetic induction (~1.745 T) and lowest core loss (~2.92 W/kg) were observed in the TCR11 sample. This is attributed to further enhancement of λ-fiber and η-fiber textures, as well as increase in grain size. In general, two-stage rolling was an effective method to optimize the microstructure, texture, and magnetic properties of strip cast non-oriented silicon steels with thickness of 0.35 mm or less. The intermediate annealing
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sample. This is attributed to further enhancement of λ-fiber and η-fiber textures, as well as increase in grain2018, size. general, two-stage rolling was an effective method to optimize the microstructure, Materials 11,In x FOR PEER REVIEW 10 of 12 texture, and magnetic properties of strip cast non-oriented silicon steels with thickness of 0.35 mm or temperature played anannealing importanttemperature role in thisplayed process. It should be otherItprocessing less. The intermediate an important rolenoted in thisthat process. should be conditions duringprocessing two-stageconditions rolling, such as the annealing atmosphere, may also affect the final noted that other during two-stage rolling, such as the annealing atmosphere, texture and properties. best suitable set ofthe processing conditions needs to be further may also affect the final Therefore, texture andthe properties. Therefore, best suitable set of processing conditions studied the magnetic performance. needs totobemaximize further studied to maximize the magnetic performance.
Figure Averagemagnetic magnetic properties of final the final produced by different rolling Figure 12. Average properties of the annealanneal sheets sheets produced by different rolling processes. processes.
4. Conclusions 4. Conclusions In this study, a Fe-2.6%Si-0.2%Mn-0.4%Al as-cast strip was processed by one-stage cold rolling and ◦ C. The evolution In thiscold study, a Fe-2.6%Si-0.2%Mn-0.4%Al as-cast was processed by one-stage cold rolling two-stage rolling, with intermediate annealing of strip 900–1100 of microstructure and two-stage cold rolling, with intermediate annealing of 900–1100 °C. The evolution of and texture during different processing routes and its effect on magnetic properties were studied. The microstructure texture during different processing routes and its effect on magnetic properties main results areand listed as follows. were studied. The main results are listed as follows. (1) The cold rolled sheets produced by two-stage showed significantly more in-grain shear bands (1) The cold rolled sheets produced by two-stage showed significantly more in-grain shear bands compared to sheets processed by one-stage rolling. With an increase in intermediate annealing compared to sheets processed by one-stage rolling. With an increase in intermediate annealing temperature, the fraction and length of shear bands in deformed microstructures, and the average temperature, the fraction and length of shear bands in deformed microstructures, and the grain size of final annealed sheets, was gradually increased, whereas the uniformity of the average grain size of final annealed sheets, was gradually increased, whereas the uniformity of microstructure was deteriorated. the microstructure was deteriorated. (2) Two-stage rolling weakened the γ-fiber texture and increased the retention of {100} texture in (2) Two-stage rolling weakened the γ-fiber texture and increased the retention of {100} texture in the cold rolled sheets. The annealed sheets produced by one-stage rolling exhibited a strong the cold rolled sheets. The annealed sheets produced by one-stage rolling exhibited a strong {111} and {001} texture and weak α*-fiber texture, while those produced by two-stage {111} and {001} texture and weak α*-fiber texture, while those produced by rolling displayed very weak γ-fiber texture and pronounced a η-fiber texture with peaks at two-stage rolling displayed very weak γ-fiber texture and pronounced a η-fiber texture with Goss and Cubeand orientation, while the while intensities were gradually an increase in peaks at Goss Cube orientation, the intensities were enhanced gradually with enhanced with an intermediate annealing temperature. increase in intermediate annealing temperature. (3) Two-stage Two-stage cold waswas favorable to improve the magnetic propertiesproperties of strip castofnon-oriented (3) coldrolling rolling favorable to improve the magnetic strip cast silicon steel. The magnetic induction increased and the core loss decreased withdecreased the increase in non-oriented silicon steel. The magnetic induction increased and the core loss with intermediate temperature. best combination B50 and P15/50 T and the increase inannealing intermediate annealingThe temperature. The bestofcombination of as B50~1.745 and P15/50 as ◦ C intermediate annealing was performed. ~2.92 W/kg was obtained when 1100 ~1.745 T and ~2.92 W/kg was obtained when 1100 °C intermediate annealing was performed. Author Contributions: Conceptualization, Y.X. and H.J.; Methodology, Y.X. and H.J.; Validation, Y.X. and H.J.; Author Analysis, Contributions: Conceptualization, Y.X. and H.J.; Methodology, Y.X. and H.J.; Y.X.Curation, and H.J.; Formal Y.X., H.J. and R.D.K.M.; Investigation, W.Q. and J.L.; Resources, W.Q.Validation, and J.L.; Data Formal Analysis, Y.X., H.J. and R.D.K.M.; Investigation, W.Q. and J.L.; Resources, W.Q. and J.L.; Data Curation, W.Q. and J.L.; Writing-Original Draft Preparation, H.J.; Writing-Review & Editing, Y.X., H.J. and R.D.K.M.; W.Q. and J.L.; Writing-Original Draft Preparation, H.J.; Writing-Review & Editing, Y.X., H.J. and R.D.K.M.; Supervision, Supervision, Y.X. Y.X. Funding: The authors authors acknowledge acknowledge support support from from the the National National Natural Natural Science Science Foundation Foundation of of China China Funding: The (Nos.51674080, 51404155 and U1260204), and National Key R&D Program of China (2017YFB0304105). (Nos.51674080, Acknowledgments: R.D.K.M. gratefully acknowledges continued collaboration with the Northeastern University (China) as an Honorary Professor by providing guidance to the students in research. Conflicts of Interest: The authors declare no conflict of interest.
References
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Acknowledgments: R.D.K.M. gratefully acknowledges continued collaboration with the Northeastern University (China) as an Honorary Professor by providing guidance to the students in research. Conflicts of Interest: The authors declare no conflict of interest.
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