Evolution of Power Losses in Bending Rolled Fully Finished NO Electrical Steel Treated Under Unconventional Annealing Conditions

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Article Evolution of Power Losses in Bending Rolled Fully Finished NO Electrical Steel Treated under Unconventional Annealing Conditions

Ivan Petryshynets 1,*, František Kováˇc 1,Ján Füzer 2, Ladislav Falat 1, Viktor Puchý 1 and Peter Kollár 2

1 Institute of Materials Research, Slovak Academy of Sciences, Watsonova 47, 04001 Košice, Slovakia 2 Institute of Physics, Faculty of Science, Pavol Jozef Safarik University, Park Angelinum 9, 041 54 Košice, Slovakia * Correspondence: [email protected]; Tel.: +421-55-792-2442

Received: 19 June 2019; Accepted: 5 July 2019; Published: 8 July 2019

Abstract: Currently, the non-oriented (NO) iron-silicon steels are extensively used as the core materials in various electrical devises due to excellent combination of their mechanical and soft magnetic properties. The present study introduces a fairly innovative technological approach applicable for fully finished NO electrical steel before punching the laminations. It is based on specific mechanical processing by bending and rolling in combination with subsequent annealing under dynamic heating conditions. It has been revealed that the proposed unconventional treatment clearly led to effective improvement of the steel magnetic properties thanks to its beneficial effects involving additional grain growth with appropriate crystallographic orientation and residual stress relief. The philosophy of the proposed processing was based on employing the phenomena of selective grain growth by strain-induced grain boundary migration and a steep temperature gradient through the cross-section of heat treated specimens at dynamic heating conditions. The stored deformation energy necessary for the grain growth was provided by plastic deformation induced within the studied specimens during the bending and rolling process. The magnetic measurements clearly show that the specimens treated according to our approach exhibited more than 17% decrease in watt losses in comparison with the specimens treated by conventional heat treatment leading only to stress relief without additional grain growth.

Keywords: electrical steel; crystallographic texture; magnetic losses; bending deformation

1. Introduction Increasing demands of modern society on the efficient exploitation of various energy sources as well as protection of global environment invoke permanent pressure on continuing development of new technologies and materials. In recent years, humankind has actively begun to use various sources of renewable and clean energy. At present, the only way that guarantees the easy and effective use of these energy sources is related to their transformation to electric power and its subsequent distribution to final consumers. The technologies that are pursued for increasing the efficiency of distribution, transformation, and end-use of electricity depend on the research and development of materials that are used as core segments in electrical motors, transformers, and generators. These materials are represented by electrical silicon steels, which are generally classified into two categories, namely the grain-oriented (GO) and non-oriented (NO) electrical steels [1–3]. The NO electrical steels belong to the group of soft magnetic materials, which are easily magnetized and demagnetized [4]. These steels are mainly used in conditions of alternating current (AC)

Materials 2019, 12, 2200; doi:10.3390/ma12132200 www.mdpi.com/journal/materials Materials 2019, 12, 2200 2 of 17 electromagnetic field at a frequency of 50 Hz (or 60 Hz) and represent the base materials for industrial production of the cores for asynchronous motors, large electric rotating machines, small generators, and other electrical motors, which transform electrical energy into mechanical or vice versa [5]. Generally required electromagnetic properties of electrical steels are high magnetic induction, high magnetic permeability, and low core loss [6]. The core loss of NO electrical steel can be separated into its partial contributions, such as hysteresis loss, classical eddy-current loss, and excess loss. Magnetic properties of electrical steel, especially the hysteresis loss and excess loss, are known to be affected by the structure and behavior of magnetic domains under external fields [7–9] and related to the microstructure and texture of the steel. The microstructural parameters that have a strong influence on the magnetic properties are grain size, inclusions, internal stresses, and surface defects. These microstructural parameters determine the domain wall-pinning influencing the coercive force of the material and domain wall motion, which are responsible for the magnetizing behavior at low and medium applied external magnetic fields and the hysteresis losses [10]. The crystallographic texture of silicon steel influences its permeability and magnetic anisotropy. In the case of base-centered cubic (BCC) lattice, the easy axis of magnetization corresponds with <100> crystal orientation, i.e., when a crystal is magnetized along one of the <100> equivalent directions, the highest permeability is obtained as compared to any other magnetization directions {hkl}. Thus, the ideal crystallographic texture of NO electrical sheet is represented by a so-called Cube texture {100} consisting of grains with the plane {001} parallel to the sheet surface and the <100> crystal directions uniformly distributed in the sheet plane [11–14]. Previous scientific works [15,16] have been focused on gradual improvement of microstructural and textural parameters of electro-technical steels during their hot rolling and final annealing by means of their chemical composition modification or introduction of innovative cold rolling and heat treatment processes. It should be noted that NO electrical steel sheets are generally divided into two categories, namely the fully processed (i.e., fully finished) and semi-processed (i.e., semi-finished) grades. The semi-processed electrical steels are finished to their final thickness by the steel producer and then their final magnetic properties are achieved during the final heat treatment at the end-consumers after different cutting processes [17]. The fully finished NO steels are characterized by their final sheet thickness, microstructure, magneto-crystalline texture, and specific magnetic properties, which have been adjusted through hot band annealing, cold rolling, and final annealing of thin steel strip. The above-mentioned structural properties determine the steel’s final magnetic properties. The fully finished NO steels can be readily used for the final assembly by the equipment manufacturer [18]. In the most of electrical rotating devices, their core (i.e., rotor and stator) segments are commonly constructed from fully finished electrical steels. The segments of electrical motors often have complicated shapes, which are achieved by cutting of fully finished sheet. It is well-known that the preparation of laminated magnetic cores is associated with introduction of some residual (e.g., mechanical and/or thermal) stresses leading to the material final magnetic properties degradation. Thus, the performance of fully processed NO electrical steel laminations can, in particular, be seriously impaired by the cutting operations required to form the slotted stator core of rotating electrical machines [19–21]. Therefore, such segments are conventionally subjected to so-called stress relief heat treatment, which assures significant recovery and improvement of magnetic properties of NO silicon steel laminations. The changes in magnetic properties due to the annealing are closely related to thermally induced changes of residual stress, magnetic domain structure, microstructure, and crystallographic texture in the cutting edge zone [22]. Some previous scientific works [23–25] have confirmed a positive effect of conventional stress relief heat treatment on the magnetic properties improvement of punched fully finished silicon steels. Such an annealing process, which is normally used by end consumers in industrial conditions [24], includes the heating at very slow heating rate of 100 ◦C/1 h, followed by soaking for 1 h in the temperate range 700 ◦C–800 ◦C. The cooling of the materials is also realized at a very slow cooling rate. The whole conventional process (i.e., heating, annealing, and cooling) lasts more than 12 h. It is important Materials 2019, 12, 2200 3 of 17 to say that this conventional annealing process does not change the grain size of the samples because the grain growth kinetics is much too slow at 700 ◦C. During the process, only microstructural recovery takes place [23]. In other words, the initial steel microstructure and its texture do not change after such conventional heat treatment. It is because the fact that the material ate of fully finished steel does not possess any additional driving force, which would activate the grain growth under conventional annealing conditions. Some of our previous works [26–29] have clearly shown that substantial improvement of microstructural and textural parameters of electrical steels was possible by using the mechanism of strain-induced selective grain growth. The driving force of this mechanism is directly related to stored deformation energy of electrical steel subjected to temper rolling process. It is important to note that the obtained coarse-grained microstructure in our temper rolled samples was achieved as a result of secondary recrystallization annealing treatment using an extraordinarily high heating rate. Furthermore, the obtained experimental findings indicated that the annealing temperature has to be higher than 900 ◦C. After such dynamic heat treatments, the achieved microstructure and texture of the treated materials were found to have clearly positive effect on final magnetic properties, manifested by substantial decreases in core losses and coercivity. Even more, the proposed approach allowed not only improving the final magnetic parameters of NO steels, but it also enabled the reduction of the final production costs of end customers. In the present work, the fully finished grade of NO silicon steel before the punching was subjected to bending rolling process in order to achieve a small plastic deformation. Subsequent heat treatment of the deformed strips was individually carried out in either conventional (slow heating rate) or unconventional (high heating rate) conditions. The main objectives of present study were focused on mutual comparison and discussion of the results obtained from microstructure, texture, and magnetic properties analyses of fully finished NO silicon steel sheets after initial introduction of fine plastic deformation and subsequent heat treatment in either conventional (long-term) or unconventional (dynamic) annealing conditions.

2. Materials and Methods The material investigated in the present work was a vacuum degassed fully finished NO steel (U.S. Steel Košice, Košice, Slovakia) of M530-50A grade with the following chemical composition in wt.%: Fe = 97.90, C = 0.004, Si = 1.32, Mn = 0.289, P = 0.039, Al = 0.346, other elements ~0.102%. A strip of 0.5 mm in thickness was taken from industrial line after final continuous annealing and marked as R1 samples. Then, these fully finished steel samples in the form of strips were variously thermo-mechanically treated under laboratory conditions. The description of individual material states of experimental samples is presented in Table1.

Table 1. Description of individual material states of experimental samples.

Samples Treatment Conditions R1 The steel taken from the industrial line in fully ﬁnished state. R2 The R1 samples treated according to conventional stress-relief annealing. R3 The R1 samples heat treated at dynamic conditions with high heating rate R4 The R1 samples which were subjected to the bending deformation. R5 The R4 samples treated according to the conventional stress-relief annealing. R6 The R4 samples heat treated at dynamic conditions with high heating rate.

A soft plastic deformation has been introduced into experimental material by employing our in-house-designed three-roller system. A corresponding scheme of the bending deformation process is presented in Figure1. Such a rolling system was used in order to achieve continuous bending of a plate under each straightening roll during the straightening. The diameter of each roller was 25 mm. Materials 2019, 12, 2200 4 of 17

The mechanical treatment used aimed at increasing the storage deformation energy of experimental samples without changing their thickness. Both conventional (long-term) and unconventional (dynamic) heat treatments were individually Materials 2019, 12, x FOR PEER REVIEW 4 of 17 applied to our experimental samples to study the grain growth phenomena in fully ﬁnished NO silicon steel. For individual analyses (i.e., microstructure, texture, and magnetic measurements analyses), analyses), experimental samples were heat treated in laboratory conditions using the electric experimental samples were heat treated in laboratory conditions using the electric resistance furnace resistance furnace Nabertherm RS 120/1000/13 (Nabertherm GmbH, Lilienthal, Germany). The long- Nabertherm RS 120/1000/13 (Nabertherm GmbH, Lilienthal, Germany). The long-term annealing term annealing treatment was carried out according to the EN 10 106 standard [30]. The samples were treatment was carried out according to the EN 10 106 standard [30]. The samples were slowly heated up slowly heated up to 790 °C at a heating rate of about 100 °C/h. Subsequently, they were held at the to 790 C at a heating rate of about 100 C/h. Subsequently, they were held at the temperature of 790 C temperature◦ of 790 °C for 1 h and finally◦ cooled at a rate of 200 °C/h. During the dynamic heat◦ for 1 h and ﬁnally cooled at a rate of 200 C/h. During the dynamic heat treatment, the samples were treatment, the samples were heated up to◦ 950 °C at a heating rate of about 15–20 °C/s and then they heated up to 950 C at a heating rate of about 15–20 C/s and then they were kept at this temperature for were kept at this◦ temperature for 5 min. Afterwards,◦ the cooling process was realized at a rate of 5 5 min. Afterwards, the cooling process was realized at a rate of 5 C/s. The annealing atmosphere for °C/s. The annealing atmosphere for both the heat treatments was◦ pure hydrogen with a dew point of both the heat treatments was pure hydrogen with a dew point of 25 C. The heating and cooling rates 25 °C. The heating and cooling rates of the conventional long-term annealing◦ process were controlled of the conventional long-term annealing process were controlled by internal thermocouples operated by internal thermocouples operated by digital controller of the furnace. In the case of dynamic heat by digital controller of the furnace. In the case of dynamic heat treatment, the heating and cooling rates treatment, the heating and cooling rates were calculated from the measured data of external were calculated from the measured data of external thermocouple, which were joined to the annealed thermocouple, which were joined to the annealed sample. In order to achieve such a high heating sample. In order to achieve such a high heating rate, the sample together with thermocouple was very rate, the sample together with thermocouple was very quickly put into the furnace heated to 950 °C. quickly put into the furnace heated to 950 ◦C.

Figure 1. Three-roller bending system for mechanical treatment of experimental samples. Figure 1. Three-roller bending system for mechanical treatment of experimental samples. Microstructural analyses of selected representative samples corresponding to individual material Microstructural analyses of selected representative samples corresponding to individual states were performed using light optical microscope (LOM) OLYMPUS GX71 (OLYMPUS Europa material states were performed using light optical microscope (LOM) OLYMPUS GX71 (OLYMPUS Holding GmbH, Hamburg, Germany). In order to obtain statistic distribution of grain size, the selected Europa Holding GmbH, Hamburg, Germany). In order to obtain statistic distribution of grain size, microstructural states were processed by using free metallographic analysis software ImageJ (version the selected microstructural states were processed by using free metallographic analysis software J2). It enabled us to estimate the average grain size of the samples after individual heat treatments. ImageJ (version J2). It enabled us to estimate the average grain size of the samples after individual The selected representative samples were also used for texture analyses. The texture analyses heat treatments. were carried out by means of electron back-scattered diﬀraction (EBSD) method in the normal direction The selected representative samples were also used for texture analyses. The texture analyses plane for each sample of 25 mm 10 mm in size. The scanning electron microscope (SEM) JEOL JSM were carried out by means of× electron back-scattered diffraction (EBSD) method in the normal 7000F FEG (Jeol Ltd., Tokyo, Japan) with the EBSD detector Nordlys-I (HKL technology A/S, Hobro, direction plane for each sample of 25 mm × 10 mm in size. The scanning electron microscope (SEM) Denmark) were used to perform the texture analysis. The obtained EBSD data were processed by the JEOL JSM 7000F FEG (Jeol Ltd., Tokyo, Japan) with the EBSD detector Nordlys-I (HKL technology CHANNEL-5,A/S, Hobro, Denmark) HKL software were package used to (Serviceperform pack the 7).texture analysis. The obtained EBSD data were processedThe nano-hardness by the CHANNEL-5, testing wasHKL carriedsoftware out package using Nano(Service Indenter pack 7). G200 (Agilent Technologies, Inc., Chandler,The nano-hardness AZ, USA). testing These was measurements carried out wereusing performed Nano Indenter using G200 Berkovich (Agilent indenter Technologies, on the surfaceInc., Chandler, of longitudinal AZ, USA). cross-sections These measurements of experimental were samples performed in both using fully ﬁnishedBerkovich and indenter bending on rolled the materialsurface of states. longitudinal The cross-section cross-sections along theof experimental length of tested samples sample in was both divided fully intofinished 9 imaginary and bending lines. Therolled nano-hardness material states. measurements The cross-section were performed along th alonge length these of individual tested sample lines mainlywas divided in the grains’into 9 imaginary lines. The nano-hardness measurements were performed along these individual lines mainly in the grains’ interiors at the distance minimally 5–10 µm from grains boundaries. The distance between individual indents was about 50 µm. The obtained values from around 10 indentations were averaged in order to evaluate the general hardness in each linear series parallel to the surface.

Materials 2019, 12, 2200 5 of 17 interiors at the distance minimally 5–10 µm from grains boundaries. The distance between individual indents was about 50 µm. The obtained values from around 10 indentations were averaged in order to evaluate the general hardness in each linear series parallel to the surface. The magnetic properties of experimental fully ﬁnished NO steels subjected to bending deformation and then heat treated according to both individual annealing schemes were investigated. These measurements were recorded on the toroid-shaped samples with outer diameter of 25mm and inner diameter of 15mm. The alternating current (AC) hysteresis loops were measured within the frequency range from 10Hz to 50Hz and at maximum induction of 1.5T by an AC/DC (direct current) Permeameter AMH-1K-S (Laboratorio Elettroﬁsico, Milan, Italy). The strip samples with planar dimensions of 80 mm 30 mm (the longest dimension parallel to the rolling direction) were used for measurement of × coercivity by Foerster Koerzimat 1.097 HCJ (Foerster, Reutlingen, Germany). Both types of the samples were prepared by electrical discharge machining using spark erosion machine EIR-EMO 2N (Emotek s.r.o., Nové Mesto nad Váhom, Slovak Republic).

3. Results and Discussion

3.1. Nano-Indentation Measurements During the bending of the thin steel sheet with a small radius, weak mechanical strains through the material thickness are generated and can be used for selective grain growth during the subsequent heat treatment. To investigate the accumulation of stored deformation energy and its dependence on the distance of the surface in small volumes in bending rolled samples, the nano-hardness tests were carried out. It is well-known that steel sheets after plastic deformation possess accumulated deformation energy, which is stored in dislocation structures. It means that the steel subjected to mechanical treatment is characterized by increased dislocation density causing a certain material hardening, which can be detected via hardness measurements [31]. Based on this assumption, the nano-hardness measurements across the sample thickness were performed for R1 samples without any mechanical treatment (red line) and R4 samples processed by bending deformation (blue line) (see Figure2). It is obvious from Figure2 that the R1 sample without bending deformation exhibits uniform (i.e., almost unchanging) distribution of nano-hardness values through the whole steel sheet thickness. The average nano-hardness value was calculated to be 2.11 GPa. This value was considered the reference hardness of investigated fully finished steel defined by its chemical composition. In the case of the R4 sample after the bending rolling treatment, the corresponding nano-hardness dependence shows a significant hardness gradient from the sample surface to its central part (see blue line in Figure2). The obtained dependencies clearly indicated significant increase in nano-hardness values in surface areas of plastically deformed R4 samples. The observed hardening in these areas reached more than 4%, compared to the central region. However, the central part of bending rolled samples was also characterized by some weak hardening, compared to the undeformed R1 samples. The obtained results can be related to inhomogeneous distribution of stress fields throughout the samples’ thickness during the bending deformation. Wang et al. [32] showed that thin plates subjected to bending deformation induced by roller systems with a small radius are characterized by plastic and elastic deformation regions throughout their cross-section. They have determined that the plastic deformation zones were localized near the plate surfaces and the elastic ones in its central part. It means that the mechanical stresses induced in the steel plate have bilinear distribution with maximal values in both subsurface regions and minimal value in the middle part. Qualitatively similar behavior was clearly confirmed by our nano-hardness measurements (Figure2). It can be concluded, that the bending deformation leads to formation of certain gradient of stored deformation energy, without significant influence of initial thickness of the steel plate. The stored deformation energy is a necessary pre-condition for strain-induced grain boundary migration phenomena, which may take place after their thermal activation during subsequent secondary recrystallization heat treatment. Materials 2019, 12, 2200 6 of 17 MaterialsMaterials 2019 2019, ,12 12, ,x x FOR FOR PEERPEER REVIEW 6 of 617 of 17

FigureFigure 2. 2.Cross-sectional Cross-sectional nano-hardnessnano-hardness dependence dependence of of expe experimentalrimental steel steel in inits itsinitial initial fully fully finished finished Figure 2. Cross-sectional nano-hardness dependence of experimental steel in its initial fully finished statestate (R1) (R1) and and after after subsequent subsequent bendingbending deformation (R4). (R4). state (R1) and after subsequent bending deformation (R4). 3.2.3.2. Microstructure Microstructure Evolution Evolution 3.2. Microstructure Evolution TheThe microstructure microstructure evolutionevolution of experimental steel steel treated treated at at different different thermo-mechanical thermo-mechanical conditionsconditionsThe microstructure is is presented presented in in FigureevolutionFigure3 3.. TheThe of light-opticalexperimental observations observations steel treated were were at performed performeddifferent on thermo-mechanical on metallographic metallographic cross-sectionsconditionscross-sections is presented ofof individuallyindividually in Figure processed processed 3. The light-opticalsamples samples with with observations their their longitudinal longitudinal were partperformed oriented part oriented on parallel metallographic parallel to the to thecross-sectionsrolling rolling direction. direction. of individually The Theobservations observations processed show samples showthat all that sampleswith all their samples are longitudinal characterized are characterized part by oriented monophasic by parallel monophasic ferrite to the ferriterollingmicrostructure microstructure direction. with The various withobservations various distribution distributionshow of that grains all of .samples grains.The initial Theare microstructure initialcharacterized microstructure ofby industrially monophasic of industrially fully ferrite fullymicrostructurefinished finished NO steel NO with (sample steel various (sample R1) distributionis presented R1) is presented inof Figure grains 3a. in. The FigureIt can initial be3a. seen microstructure It that can the be sample seen of that is industrially characterized the sample fully is characterizedfinishedby relatively NO steel byfine-grained relatively (sample andR1) fine-grained homogeneousis presented and in microstructure homogeneous Figure 3a. It can with microstructure be average seen that grain the with sizesample average of about is characterized grain 67.6 µm size of aboutby± 3.5relatively 67.6µm. µ m fine-grained3.5 µm. and homogeneous microstructure with average grain size of about 67.6 µm ± ± 3.5 µm.

(a) (b)

(e) (f)

FigureFigure 3. 3.Microstructural Microstructural variationvariation of individual experimental experimental samples samples of ofinvestigated investigated non-oriented non-oriented (NO)(NO) electrical electrical steel steel corresponding corresponding(e) toto didifferentﬀerent material states: states: R1 R1 (a ();a );R2 R2 (b ();b );R3 R3 ((cf); ()c R4); R4 (d); (d R5); R5(e); (ande); and R6 (f). R6Figure (f). 3. Microstructural variation of individual experimental samples of investigated non-oriented (NO) electrical steel corresponding to different material states: R1 (a); R2 (b); R3 (c); R4 (d); R5 (e); and MicrostructuralMicrostructural characteristics characteristics of fully finished ﬁnished steel steel after after conventional conventional long-term long-term heat heat treatment treatment R6 (f). (sample(sample R2) R2) and and after after unconventionalunconventional dynamic dynamic annealing annealing (sample (sample R3) R3) are are presented presented in Figure in Figure 3b,c,3b,c, respectively. Microstructural The microstructure characteristics of of sample fully finished R2 after steel conventional after conventional annealing long-term does not heat exhibit treatment any (sample R2) and after unconventional dynamic annealing (sample R3) are presented in Figure 3b,c,

MaterialsMaterials2019 2019, 12, ,12 2200, x FOR PEER REVIEW 7 of7 17 of 17

respectively. The microstructure of sample R2 after conventional annealing does not exhibit any specificspecific microstructural microstructural changes. changes. Its Its average average graingrain size about 74 74 µmµm ± 3.83.8 µm.µm. On On the the other other hand, hand, the the ± samplesample R3, R3, which which was was subjected subjected to to dynamicdynamic annealingannealing treatment, treatment, demonstrates demonstrates somewhat somewhat bimodal bimodal microstructuremicrostructure with with average average grain grain size size of 80 ofµ m80 µm3.2 ±µ m.3.2 Theµm. microstructureThe microstructure of experimental of experimental samples ± subjectedsamples tosubjected the bending to the deformation bending isdeformation presented inis Figurepresented3d. Obviously,in Figure the3d. microstructureObviously, theof samplemicrostructure R4 is similar of sample to the R4 microstructure is similar to the of micr sampleostructure R1. Itof cansample be concludedR1. It can be that concluded our proposed that our proposed mechanical treatment of fully finished steel does not affect its microstructural mechanical treatment of fully finished steel does not affect its microstructural characteristics. In the characteristics. In the case of bending rolled samples, R5 and R6, the grain growth behavior was also case of bending rolled samples, R5 and R6, the grain growth behavior was also studied after their studied after their heat treatments in conventional (Figure 3e) and dynamic (Figure 3f) conditions, heat treatments in conventional (Figure3e) and dynamic (Figure3f) conditions, respectively. It can be respectively. It can be clearly seen that the microstructures of these samples are quite different clearly seen that the microstructures of these samples are quite different compared to the previous ones. compared to the previous ones. In the case of sample R5 (Figure 3e), the microstructure exhibits Innotable the case ofimprovement sample R5 (Figure(i.e., increase)3e), the microstructure of average grain exhibits size notable in comparison improvement with (i.e., the increase) initial of averagemicrostructure grain size (Figure in comparison 3a). Moreover, with thein the initial case microstructure of sample R6 (Figure (Figure 3f),3a). the Moreover, grain size in increase the case is of sampleeven more R6 (Figure significant.3f), the grain size increase is even more significant. InIn order order to to quantify quantify microstructural microstructural featuresfeatures of individual individual mi microstructures,crostructures, their their grain grain size size distributiondistribution characteristics characteristics werewere evaluatedevaluated (see Figure 44).). From From Figure Figure 44 itit can can be be seen seen that that the the R1 R1 samplesample microstructure microstructure (Figure (Figure4a) 4a) is characterizedis characterized by by the the grain grain size size variation variation in in the the range range from from 10 10µ m upµm to 200up toµm. 200 However, µm. However, about about 65% of 65% R1 sampleof R1 sample microstructure microstructure is formed is formed of grains of grains with theirwith sizestheir in thesizes range in fromthe range 60 µ mfrom to 14060 µmµm to and 140 about µm and 18% about of microstructure 18% of microstructure is occupied is occupied by the grains by the with grains their sizeswith up their to 60 sizesµm. up The to microstructure 60 µm. The microstructure of the sample of R2 the is sample characterized R2 is characterized by uniform distributionby uniform of ferritedistribution grains withof ferrite average grains grain with size average about 74grainµm size3.8 aboutµm. Also, 74 µm 72% ± of3.8 the µm. microstructure Also, 72% of areathe is ± coveredmicrostructure by grains area with is thecovered size fromby grains 80 µ mwith to 140the µsizem (seefrom Figure 80 µm4b). to The140 µm grain (see size Figure distribution 4b). The of R3grain sample size microstructure distribution of R3 (Figure sample3c) microstructure shows that 32% (Figure of microstructure 3c) shows that is 32% covered of microstructure by grains in is the rangecovered from by 160 grainsµm upin the to 220rangeµ m.from These 160 µm large up grains to 220 areµm. mostly These large located grains incentral are mostly part located of the steelin sheetcentral cross-section. part of the Thesteel sub-surfacesheet cross-section. region isThe formed sub-surface of grains region that is are formed similar of tograins those that occurring are similar in R2 to those occurring in R2 sample microstructure. Figure 4d shows the distribution of grain sizes of the sample microstructure. Figure4d shows the distribution of grain sizes of the bending rolled sample R4. bending rolled sample R4. It can be clearly seen that its grain size distribution is very similar to It can be clearly seen that its grain size distribution is very similar to sample R1 (Figure4a). Both these sample R1 (Figure 4a). Both these material states are characterized by same average grain size. It is material states are characterized by same average grain size. It is possible to note that this kind of possible to note that this kind of mechanical treatment does not influence the grain matrix mechanical treatment does not influence the grain matrix parameters. The grain size distribution of R5 parameters. The grain size distribution of R5 sample (Figure 4e) shows very similar characteristics as samplethat for (Figure the 4R2e) showssamplevery (Figure similar 4b).characteristics Thus, it can be as thatconcluded for the that R2sample conventional (Figure long-term4b). Thus, heat it can betreatment concluded of that bending conventional rolledlong-term steel (R5 heat sample) treatment practically of bending does rolled not steelaffect (R5 its sample) microstructure practically doesdevelopment. not affect its In microstructure the case of R6 development. sample proces Insed the by case bending of R6 sample rolling processed and subsequent by bending dynamic rolling andannealing subsequent treatment, dynamic the annealing situation is treatment, quite different. the situation The microstructure is quite different. of R6 sample The microstructure is characterized of R6 sample is characterized by bimodal grain size distribution with average grain size of 110 µm 4.6 µm by bimodal grain size distribution with average grain size of 110 µm ± 4.6 µm (see Figures 3f and± 4f). (seeTwo Figures microstructure3f and4f). Tworegions microstructure can be clearly regions distingu canished. be clearly The coarse-grained distinguished. area The with coarse-grained grain sizes arearanging with grainfrom sizes180 µm ranging to 320 from µm covers 180 µm mostly to 320 theµm middle covers part mostly of the the sample middle and part it of occupies the sample more and it occupiesthan 50% more of the than sample 50% ofcross-section. the sample cross-section.The fine-grained The microstructure fine-grained microstructure mostly occurs mostlyin the sub- occurs insurface the sub-surface regions of regions the sample. of the sample.This part This of microstructure part of microstructure is characterized is characterized by the grain by thesize grain in the size inrange the range from from 60 µm 60 toµ m140 to µm. 140 µm.

(a) (b)

Figure 4. Cont. Materials 2019, 12, 2200 8 of 17 Materials 2019, 12, x FOR PEER REVIEW 8 of 17

(e) (f)

FigureFigure 4. 4.Grain Grain size size distributiondistribution characteristicscharacteristics of of in individualdividual microstructures microstructures of ofinvestigated investigated NO NO electricalelectrical steel steel corresponding corresponding to to di differentfferent material material states: states: R1 R1 ( a();a); R2 R2 ( b(b);); R3 R3 ( c(c);); R4 R4 ( d(d);); R5 R5 ( e(e);); and andR6 R6 (f). (f). The comparison of average grain sizes of investigated samples in their individual material states is presentedThe in comparison Figure5. It of clearly average demonstrates grain sizes of the investigated e ffects of samples individual in their applied individual procedures material (Table states1) on microstructureis presented in improvements Figure 5. It clearly of fully demonstrates finished electrical the effects steel. of individual It can be concluded applied procedures that our proposed(Table unconventional1) on microstructure treatment improvements of the investigated of fully finished steel, i.e.,electrical the bending steel. It rollingcan be concluded in combination that our with subsequentproposed unconventional dynamic annealing treatment at extraordinarily of the investigat highed steel, heating i.e., the rate, bending leads to rolling significant in combination increase in itswith average subsequent grain size dynamic (sample annealing R6) in comparisonat extraordinarily with high the initialheating industrially rate, leads to treated significant microstructure increase in its average grain size (sample R6) in comparison with the initial industrially treated microstructure (sample R1). From Figure5, it is apparent that the observed microstructural di fferences of individual (sample R1). From Figure 5, it is apparent that the observed microstructural differences of individual experimental samples can be directly related to individual processing treatments, which were mutually experimental samples can be directly related to individual processing treatments, which were different from each other. Conventional annealing of fully finished electrical steels is used in industrial mutually different from each other. Conventional annealing of fully finished electrical steels is used conditions in order to remove residual stresses induced in material during cutting operations. The in industrial conditions in order to remove residual stresses induced in material during cutting stressoperations. relief heat The treatmentstress relief is heat realized treatment at slow is realized heating at ratesslow heating and maximum rates and annealing maximum temperatureannealing oftemperature about 800 ◦C. of Theabout scheme 800 °C. of The long-term scheme heatof long-ter treatmentm heat is treatment very similar is very to the similar process to the of secondaryprocess recrystallizationof secondary recrystallization heat treatment, heat which treatment, is commonly which is usedcommonly for the used semi-finished for the semi-finished electrical electrical steels [17 ]. However,steels [17]. in contrastHowever, to in semi-finished contrast to semi-finished steel, which issteel, characterized which is bycharacterized pronounced by microstructural pronounced evolution,microstructural the microstructure evolution, the of micr fullyostructure finished steelof fully does finished not exhibit steel does any significantnot exhibit changesany significant after the heatchanges treatment after (seethe heat Figure treatment3b). This (see observation Figure 3b). can This be observation easily explained can be byeasily the explained fact that fully by the finished fact steelthat does fully not finished possess steel the does necessary not possess storage the nece deformationssary storage energy, deformation which represents energy, which the mainrepresents driving forcethe formain strain-induced driving force grainfor strain-induced boundary migration grain boundary (SIBM) [migration33]. In our (SIBM) previous [33]. works In our [26 previous,27], it has beenworks shown [26,27], that it pronounced has been shown selective that grainpronounced growth selective could be grain achieved growth in temper-rolledcould be achieved NO in steel subjectedtemper-rolled to small NO plastic steel deformation subjected to (about small 2%–6%)plastic deformation and subsequent (about dynamic 2%–6%) heat and treatment subsequent using a verydynamic high heatingheat treatment rate. The using light a very optical high images heating in ra Figurete. The3 lighta–chave optical clearly images shown in Figure that 3a–c in the have case ofclearly fully finished shown that steels in the that case were of fu subjectedlly finished to steels the heatthat were treatment subjected in eitherto the heat conventional treatment in long-term either (sampleconventional R2) or unconventionallong-term (sample short-term R2) or unconventi dynamic annealingonal short-term conditions dynamic (sample annealing R3), the conditions occurrence of(sample grain growth R3), the was occurrence rather limited. of grain This growth behavior was rather fairly limited. agrees withThis behavior the well-known fairly agrees hypothesis with the that somewell-known minimum hypothesis level of storedthat some deformation minimum energy level isof neededstored deformation for activation energy of grain is needed growth. for The

Materials 2019, 12, 2200 9 of 17 Materials 2019, 12, x FOR PEER REVIEW 9 of 17 eactivationffect of dynamic of grain annealing growth. The conditions effect of on dynamic the columnar annealing grain conditions growth was on the investigated columnar bygrain Sidor growth and co-workerswas investigated [34]. Theyby Sidor showed and co-workers that rapid [34]. heating They is showed responsible that forrapid the heating occurrence is responsible of temperature for the gradientoccurrence along of temperature the normal of gradient the sheet along thickness the normal and this of gradientthe sheet might thickness represent and this additional gradient driving might forcerepresent for the additional grain growth. driving Other force research for the worksgrain gr [35owth.,36] haveOther clearly research indicated works a[35,36] beneficial have e ffclearlyect of rapidindicated annealing a beneficial on the effect increase of rapid in average annealing grain on size the and increase significant in average improvement grain size of crystallographic and significant textureimprovement of electrical of crystallographic steel after strip-casting texture of and electrical cold rolling steel after process. strip-casting and cold rolling process.

FigureFigure 5.5. Average grain sizesize ofof studiedstudied fullyfully ﬁnishedfinished electricalelectrical steelsteel inin individualindividual materialmaterial statesstates accordingaccording toto TableTable1 .1.

InIn thethe casecase ofof fullyfully finishedfinished steelsteel thatthat waswas subjectedsubjected toto plasticplastic deformationdeformation viavia bendingbending rollingrolling process,process, thethe microstructuralmicrostructural evolutionevolution duringduring subsequentsubsequent heatheat treatmenttreatment waswas variedvaried independentindependent ofof thethe usedused annealingannealing conditions.conditions. InIn thethe casecase ofof conventionalconventional (long-term)(long-term) annealingannealing processprocess ofof bendingbending rolledrolled fullyfully finishedfinished steelsteel (sample(sample R5,R5, FigureFigure3 3e),e), the the microstructure microstructure did did not not exhibit exhibit any any significant significant changeschanges comparedcompared to to conventionally conventionally annealed annealed microstructure microstructure without without prior prior bending bending rolling rolling (sample (sample R2, FigureR2, Figure3b). A 3b). significant A significant microstructural microstructural evolution occurredevolution only occurred in the bendingonly in rolled the bending microstructure rolled subjectedmicrostructure to dynamic subjected annealing to dynamic treatment annealing (sample treatment R6, Figure (sample3f). The R6, observed Figure 3f). grain The growth observed has beengrain obtainedgrowth has by been means obtained of combination by means of of two combination driving forces. of two One driving of them forces. is relatedOne of tothem the is gradient related ofto storedthe gradient deformation of stored energy, deformation which wasenergy, confirmed which was by nano-indentation confirmed by nano-ind measurementsentation measurements (see Figure2) and(see theFigure second 2) and one the to second the strong one gradientto the strong of heat gradient flow, of which heat hasflow, been which achieved has been thanks achieved to the thanks high heatingto the high rate heating during rate the dynamicduring the heat dynamic treatment. heat treatment. 3.3. Texture 3.3. Texture In order to investigate the texture evolution of selected samples with most significant improvement In order to investigate the texture evolution of selected samples with most significant of microstructure, detailed EBSD analyses were performed. The crystallographic orientations of grain improvement of microstructure, detailed EBSD analyses were performed. The crystallographic structures of the investigated samples in original fully finished state and those subjected to both orientations of grain structures of the investigated samples in original fully finished state and those bending deformation and dynamic heat treatment are presented by Inverse Pole Figure (IPF) maps in subjected to both bending deformation and dynamic heat treatment are presented by Inverse Pole Figure6a,b, respectively. Figure (IPF) maps in Figure 6a,b, respectively.

Materials 2019, 12, 2200 10 of 17 Materials 2019, 12, x FOR PEER REVIEW 10 of 17

Materials 2019, 12, x FOR PEER REVIEW 10 of 17

(a) (b) (c)

FigureFigure 6. 6.IPF IPF representation representation ofof graingrain crystallographiccrystallographic orie orientationsntations of of investigated investigated NO NO electrical electrical steel steel inin initial initial fully fully finished finished(a) state—sample state—sample R1R1 (a) and after after bending bending(b) rolling rolling and and subsequent subsequent dynamic dynamic(c) heat heat treatment—sampletreatment—sampleFigure 6. IPF representation R6 R6 ( b(b).). TheThe of keykey grain forfor crystallographic the identification identification orie of ofntations crys crystallographictallographic of investigated orientation orientation NO electrical is located is located steel on on thethe rightin right initial (c ().c ).fully finished state—sample R1 (a) and after bending rolling and subsequent dynamic heat treatment—sample R6 (b). The key for the identification of crystallographic orientation is located on TheThethe coloured coloured right (c). maps maps in in Figure Figure6 6 characterize characterize crystallographiccrystallographic orientations orientations of of all all individual individual grains grains locatedlocated in in cross-sectional cross-sectional plane plane ofof thethe steelsteel sheet.sheet. It It can can be be clearly clearly seen seen that that the the individual individual samples samples diffdifferer from fromThe each colouredeach otherother maps not inonly only Figure by by their 6 theircharacterize microstructural microstructural crystallographic parameters parameters orientations (i.e., grain (i.e., of grainsize) all individual but size) also but by grains alsotheir by theircrystallographiclocated crystallographic in cross-sectional texture. texture. In planethe In casethe of the caseof samplesteel of samplesheet. R1, Itthe R1,can dominant thebe clearly dominant crystallographic seen crystallographic that the individual orientation orientation samples is so- is calleddiffer deformation from each other texture not which only by is characterisedtheir microstructural by the leastparameters effective (i.e., magnetization grain size) but direction also by <111>their so-called deformation texture which is characterised by the least effective magnetization direction parallelcrystallographic to the normal texture. of the In sheetthe case plane of sample (the blue-c R1, theoloured dominant grains). crystallographic The presence orientation of this texture is so- is <111> parallel to the normal of the sheet plane (the blue-coloured grains). The presence of this rathercalled undesirable deformation in texture soft magnetic which is materials characterised becaus bye the it deterioratesleast effective their magnetization final magnetic direction properties. <111> texture is rather undesirable in soft magnetic materials because it deteriorates their final magnetic Onparallel the other to thehand, normal the sample of the sheetR6 is characterizedplane (the blue-c by olouredpronounced grains). cube The crys presencetallographic of this texture texture (the is properties. On the other hand, the sample R6 is characterized by pronounced cube crystallographic red-colouredrather undesirable grains) inwhich soft magneticis characterized materials by becaus the easye it magnetizationdeteriorates their direction final magnetic <100>. Inproperties. addition, textureFigureOn (thethe 6b other clearly red-coloured hand, shows the grains)samplethat the R6 whichrotated is characterized is cube characterized crystallographic by pronounced by the orientation easy cube magnetization crys tallographicis mostly directionrelated texture to (the< the100 >. Ingrains addition,red-coloured which Figure were grains)6b formed clearly which after shows is characterized the that dynamic the rotated byheat th e cubetr easyeatment crystallographicmagnetization and are localised direction orientation in <100>. the iscentral mostlyIn addition, part related of tothe theFigure cross-section. grains 6b which clearly These were shows formedresults that theof after selective rotated the dynamic cubegrain crystallographic growth heat treatment correspond orientation and well are with localised is mostly the results in related the presented central to the part ofin the grainsFigure cross-section. which 3e. were These formed results after of the selective dynamic grain heat growthtreatment correspond and are localised well with in the central results part presented of in FiguretheThe cross-section.3e. crystallographic These resultstexture of of selective investigated grain samplesgrowth correspondcan be interpreted well with by the orientation results presented fibres in theinThe Euler Figure crystallographic space. 3e. The most texture relevant of fibres investigated for the NO samples electrical can steels be interpreted are θ-fibre by (<100>//ND), orientation γ fibres-fibre in The crystallographic texture of investigated samples can be interpreted by orientation fibres in the(<111>//ND), Euler space. and The α-fibre most (<110>//RD) relevant fibres [10] for which the NOare presented electrical steelsby means are θof-fibre orientation (<100>// distributionND), γ-fibre the Euler space. The most relevant fibres for the NO electrical steels are θ-fibre (<100>//ND), γ-fibre (//ND), (ODF). and Theα-fibre ODF ( //RD) taken [10 at] φ which2 = 45° arefor presentedthe investigated by means samples of orientation R1 and R6 are distribution shown (<111>//ND), and α-fibre (<110>//RD) [10] which are presented by means of orientation distribution functionin Figure (ODF). 7a,b, Therespectively. ODF sections taken at ϕ2 = 45◦ for the investigated samples R1 and R6 are shown function (ODF). The ODF sections taken at φ2 = 45° for the investigated samples R1 and R6 are shown in Figure7a,b, respectively. in Figure 7a,b, respectively.

(a) (b) (a) (b) Figure 7. ODF sections taken at φ2 = 45° representing the through-thickness textures evolved in

investigatedFigure 7. ODFsamples sections R1 (a taken) and R6at (φb2). = 45° representing the through-thickness textures evolved in Figure 7. ODF sections taken at ϕ2 = 45◦ representing the through-thickness textures evolved in investigatedinvestigated samples samples R1 R1 (a) ( anda) and R6 R6 (b ().b).

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The development of the θ-fibre and γ-fibre of the investigated samples is presented in Figure 8. The texture of the sample R1 has the maximum intensity at the {111}<145> and {111}<112> component which corresponds to the γ-fibre (see Figure 8b). The θ-fibre exhibits the lower intensity of rotated cube with enhanced {100}<110> orientation at φ1 = 25° and 75°, see Figure 8a (blue line). The pronouncedMaterials 2019, 12 evolution, 2200 of rotation cube was detected in sample R6. Here, θ-fibre is expressed11 of by 17 dominant intensity of cube crystallographic orientation with maximum intensity at φ1 = 20° and 65°. The intensity of deformation texture is presented by γ-fibre in Figure 8b (red curve). This figure clearlyThe shows development significant of suppression the θ-fibre and of deformγ-fibreation of the texture investigated component samples {111}<145> is presented and in{111}<112> Figure8. Theof the texture sample of R6 the subjected sample R1 to has the the bending maximum deformation intensity and at the dynamic {111}<145 annealing.> and {111} <112> component whichThe corresponds evaluation to of the textureγ-fibre components (see Figure8 ofb). investig The θ-fibreated exhibitsfully finished the lower steel intensity clearly shows of rotated that cube the withplastic enhanced deformation {100} orientation by mechanical at ϕ1 bending= 25◦ and in 75 combination◦, see Figure with8a (blue dynamic line). annealing The pronounced have a veryevolution positive of rotation effect cubeon the was deve detectedlopment in sampleof desired R6. Here,microstructureθ-fibre is expressed exhibiting by coarse dominant grains intensity with rotatedof cube cube crystallographic crystallographic orientation orientation. with maximumA small deformation intensity at gradientϕ1 = 20 ◦throughand 65 ◦the. The sheet intensity cross- sectionof deformation achieved texture by bending is presented rolling and by γdetected-fibre in by Figure our nano-indentation8b (red curve). This measurements figure clearly (Figure shows 2), providessignificant the suppression necessary stored of deformation deformation texture energy, component serving {111} as primary<145> anddriving {111} force<112 for> of the the selective sample grainR6 subjected migration to the[33]. bending deformation and dynamic annealing.

(a) (b)

Figure 8. TheThe variation variation of of orientation orientation density density along along θ-fibreθ-fibre (a ()a and) and γ-fibreγ-fibre (b ()b in) inNO NO electrical electrical steel steel in fullyin fully finished finished material material state—sample state—sample R1 R1 (blue (blue curve) curve) and and after after bending bending deformation deformation followed followed by dynamic annealing—sample R6 (red curve).

AsThe a evaluationresult, the sample of texture R6 componentsafter bending of rolling investigated and subsequent fully finished dynamic steel clearlyheat treatment shows that shows the plasticsignificant deformation selective inducedgrain growth by mechanical of some specific bending grains in combination of the initial with grain dynamic matrix annealing (Figures have6–8). However,a very positive some eotherffect onworks the [37,38] development reported of that desired the stored microstructure deformation exhibiting energy coarsemay vary grains different with texturerotated cubecomponents crystallographic of polycrystalline orientation. materials A small deformationthat were subjected gradient to through mechanical the sheet deformation. cross-section It hasachieved been found by bending experimentally rolling and that detected the plastic by our deformation nano-indentation which measurements is stored in dislocation (Figure2), structures provides isthe distributed necessary heterogeneously stored deformation between energy, the servinggrains an asd primarydepends drivingon their crystallographic force for the selective orientation, grain atmigration least on orientations [33]. families defined by the crystallographic plane lying parallel to the sheet plane. In vacuumAs a result, degassed the sample ferrite steels, R6 after the bending magnitude rolling of stored and subsequent deformation dynamic energy heat of individual treatment showsgrains orientationssignificant selective can be characterized grain growth as of follows: some specificE{111} > E grains{112} > E of{100} the, where initial the grain lower matrix index (Figuresdescribes6– the8). crystallographicHowever, some otherorientation works of [37 grains.,38] reported It can be that indicated the stored that deformationafter applied energymechanical may treatment, vary different the graintexture structures components with of polycrystalline{111}<112> crystallographi materials thatc component were subjected may to accumulate mechanical deformation.more deformation It has energybeen found than experimentallyrotated cube grains. that theIt can plastic be reason deformationably supposed which that is stored between in dislocation the neighboring structures grains is withdistributed various heterogeneously crystallographic between orientations, the grains a creation and dependsof some small on their gradient crystallographic of stored deformation orientation, energyat least onis related orientations to the families lattice defineddefects bysuch the as crystallographic dislocations, which plane lyingwere parallelinduced to by the the sheet applied plane. mechanicalIn vacuum degassedstress. Moreover, ferrite steels, according the magnitude to the ther of storedmodynamics deformation principle, energy the of gradient individual of stored grains energyorientations representing can be characterized a driving force as for follows: SIBM Eshould{111} > Eenable{112} > promotingE{100}, where the the migration lower index of grains describes with lessthe crystallographicinternal energy [39]. orientation Taking of into grains. account It can all be the indicated mentioned that results after applied and considerations, mechanical treatment, it can be concludedthe grain structures that the fully with finished {111}<112 electrical> crystallographic steels subjected component to bending may deformation accumulate more are characterized deformation byenergy inhomogeneous than rotated distribution cube grains. of It canstored be reasonablydeformation supposed energy between that between the grains, the neighboring which basically grains with various crystallographic orientations, a creation of some small gradient of stored deformation energy is related to the lattice defects such as dislocations, which were induced by the applied mechanical stress. Moreover, according to the thermodynamics principle, the gradient of stored energy representing a driving force for SIBM should enable promoting the migration of grains with less internal energy [39]. Taking into account all the mentioned results and considerations, it can be Materials 2019, 12, 2200 12 of 17 Materials 2019, 12, x FOR PEER REVIEW 12 of 17 concluded that the fully finished electrical steels subjected to bending deformation are characterized depends on their crystallographic orientation and arrangement through the sheet thickness. The by inhomogeneous distribution of stored deformation energy between the grains, which basically stress disparity on some grain boundaries in combination with a high temperature gradient during depends on their crystallographic orientation and arrangement through the sheet thickness. The stress the stress relief annealing leads to the achievement of selective growth of grains with appropriate disparity on some grain boundaries in combination with a high temperature gradient during the rotated cube crystallographic orientation. This fact is directly confirmed by the results obtained in the stress relief annealing leads to the achievement of selective growth of grains with appropriate rotated present investigation showing adequate correlation between microstructure (Figure 3) and texture cube crystallographic orientation. This fact is directly confirmed by the results obtained in the (Figures 6–8) characteristics. The comparison of the microstructural and textural states of different present investigation showing adequate correlation between microstructure (Figure3) and texture samples subjected to stress relief at conventional (long-term) and unconventional (dynamic) (Figures6–8) characteristics. The comparison of the microstructural and textural states of di fferent annealing conditions clearly indicate that the temperature gradient creates appropriate conditions in samples subjected to stress relief at conventional (long-term) and unconventional (dynamic) annealing uniformly bending rolled polycrystalline silicon steels for the evolution of microstructure with conditions clearly indicate that the temperature gradient creates appropriate conditions in uniformly preferential rotated cube crystallographic orientation. bending rolled polycrystalline silicon steels for the evolution of microstructure with preferential rotated cube3.4. Magnetic crystallographic Measurements orientation. 3.4. MagneticThe evolution Measurements of microstructural, sub-structural, and textural characteristics of the investigated steel material states has its direct influence on their final magnetic properties, namely coercivity and The evolution of microstructural, sub-structural, and textural characteristics of the investigated power losses. The magnetic measurements were carried out for all experimental samples in direct steel material states has its direct influence on their final magnetic properties, namely coercivity current (DC) and alternating current (AC) magnetic field. The dependence of coercivity of and power losses. The magnetic measurements were carried out for all experimental samples in experimental samples in individual material states is presented in Figure 9. It clearly indicates that direct current (DC) and alternating current (AC) magnetic field. The dependence of coercivity of the investigated samples differ from each other not only by microstructural, textural, and mechanical experimental samples in individual material states is presented in Figure9. It clearly indicates that the stress characteristics, but also by their coercivity. The initial value of coercivity corresponding to fully investigated samples differ from each other not only by microstructural, textural, and mechanical stress finished sample R1 is about 40.0 A/m ± 0.31 A/m. The samples R2, R3, R5, and R6 show clearly lower characteristics, but also by their coercivity. The initial value of coercivity corresponding to fully finished coercivity values than the sample R1. Very low coercivity values were exhibited by the samples R3 sample R1 is about 40.0 A/m 0.31 A/m. The samples R2, R3, R5, and R6 show clearly lower coercivity and R6, which indicated a crucial± influence of dynamic heat treatment on soft magnetic characteristics values than the sample R1. Very low coercivity values were exhibited by the samples R3 and R6, which improvement. indicated a crucial influence of dynamic heat treatment on soft magnetic characteristics improvement.

FigureFigure 9. 9.The The measured measured coercivity coercivity of fully of ﬁnishedfully finished electrical elec steeltrical in individualsteel in individual material states material according states toaccording Table1. to Table 1.

The minimum value of coercivity of about 36.33 A/m 0.23 A/m was obtained for the sample R6, The minimum value of coercivity of about 36.33 A/m± ± 0.23 A/m was obtained for the sample whichR6, which was subjected was subjected to the bending to the deformationbending deform with subsequentation with dynamicsubsequent annealing dynamic at extraordinarily annealing at highextraordinarily heating rate. high The heating obtained rate. results The obtained indicate thatresults the indicate heat treatment that the of heat fully treatment ﬁnishedsteels of fully at dynamicfinished heatingsteels atconditions dynamic heating has a much conditions more pronounced has a much emoreﬀect onpronounced their coercivity effect thanon their conventional coercivity stressthan conventional relief annealing stress carried relief out annealing using relatively carried ou slowt using heating relatively rate. The slow highest heating value rate. of The coercivity highest (47.5 A/m 0.38 A/m) was obtained for the sample R4. This result corresponds well with performed value of coercivity± (47.5 A/m ± 0.38 A/m) was obtained for the sample R4. This result corresponds nano-indentationwell with performed measurements nano-indentation and it can measurements be related to notableand it can increase be related in mechanical to notable stresses increase in the in studiedmechanical steel stresses after the in bending the studied deformation. steel after the bending deformation.

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The evolution of power losses of individual experimental samples, prepared in RD, was measuredThe evolution in AC magnetic of power lossesfield at of a individual frequency experimental of 50 Hz. Figure samples, 10 preparedshows an inoverview RD, was measuredof the first inquadrant AC magnetic of recorded ﬁeld at hysteresis a frequency loops of 50of the Hz. studied Figure 10samples. shows an overview of the ﬁrst quadrant of recorded hysteresis loops of the studied samples.

FigureFigure 10. 10.The The B-H B-H hysteresis hysteresis loops loops recorded recorded for for the the studied studied samples samples at at 50Hz 50Hz in in the the ﬁrst first quadrant. quadrant.

TheThe positive positive influence influence of of annealing annealing and and bending bending processes processes on on the the position position of of the the hysteresis hysteresis loops loops kneeknee is is evident evident compared compared with with hysteresis hysteresis loop loop of sampleof sample R1. R1. The The significant significant decrease decrease in the in slopethe slope of the of ascendingthe ascending part ofpart the of hysteresis the hysteresis loop forloop sample for sample R1 is atR1 0.6 is Tat and 0.6 T for and other forsamples other samples at this sameat this point same overpoint 1.0 over T. It 1.0 can T. be It explained can be explained by easier by domain easier wall domain displacement wall displacement in annealed in orannealed bended materialsor bended duematerials to the due generation to the generation of suitable of grain suitable texture. grain Thetexture. external The external applied applied field Hmax fieldfor HBmaxmax for= B1.5max T= is1.5 decreasingT is decreasing with with dynamic dynamic annealing annealing conditions. conditions. The The sample sample R6 R6 acquires acquires Bmax Bmax= 1.5 = 1.5 T T at at the the lowest lowest magneticmagnetic field, field, 770 770 A /A/m.m. The The R6 R6 sample sample material material state state obtained obtained by combination by combination of bending of bending rolling rolling and dynamicand dynamic heat treatmentheat treatment is also is characterized also characterized by the by steepest the steepest hysteresis hysteresis loop, loop, which which is related is related to the to increasethe increase in microstructure in microstructure average average grain grain size andsize and the highthe high intensity intensity of θ -fiberof θ-fiber texture texture components. components. ItIt is is important important to to note note that that the the magnetic magnetic properties properti ofes electrical of electrical steels steels are very are sensitive very sensitive to the valueto the ofvalue the frequency of the frequency of external of external AC magnetic AC magnetic field. It fiel is relatedd. It is torelated the magnetic to the magnetic loss components, loss components, which arewhich called are excess called loss. excess These loss. losses These are losses strongly are dependedstrongly depended on the movement on the movement of magnetic of domainsmagnetic structuresdomains duringstructures the during change the of magnetic change of field magnetic at different field frequency. at different Therefore, frequency. all experimentalTherefore, all samplesexperimental were subjectedsamples were to the subjected magnetic to measurements the magnetic measurements in the AC magnetic in the fieldAC magnetic with the field range with of frequencythe range from of frequency 10 Hz up from to 50 10 Hz. Hz Corresponding up to 50 Hz. Corresponding curves describing curves the describing dependence the of dependence power losses of ofpower the experimental losses of the samples experimental on the magneticsamples on field the frequency magnetic rangefield frequency are presented range in Figureare presented 11. Here, in overallFigure better 11. Here, response overall of thebetter samples response heat of treated the sa aftermples the heat bending treated deformation after the bending at dynamic deformation annealing at conditionsdynamic (Sampleannealing R6—blue conditions line) (Sample is put in R6—blue evidence. Theline) power is put losses in evidence. are decreased The power thanks losses to coarse are grainsdecreased (see Figurethanks3 e),to whichcoarse aregrains mostly (see characterized Figure 3e), which by appropriate are mostly rotated characterized cube crystallographic by appropriate orientationrotated cube (see crystallographic Figure6b). The highestorientation value (see of lossesFigure was 6b). measured The highest for value the sample of losses R4, whichwas measured can be explainedfor the sample by the R4, presence which of can mechanical be explained stresses, by whichthe presence were induced of mechanical into the stresses, steel by thewhich applied were bendinginduced rolling. into the Onsteel the by other the applied hand, thebending dynamic rollin heatg. On treatment the other ofhand, experimental the dynamic samples heat treatment R3 and R6of (i.e.,experimental the samples samples without R3 or withand R6 bending (i.e., deformation),the samples without significantly or with reduces bending their powerdeformation), losses (seesignificantly Figure 11 ).reduces This observation their power can losses be related (see Figure to the 11). increase This observation in average can grain be sizerelated of these to the samples increase afterin average the dynamic grain size heat of treatment. these samples The after reduction the dynamic of power heat losses treatment. of thesamples The reduction R2 and of R5 power subjected losses toof conventional the samples R2 stress and relief R5 subjected annealing to usingconventional slow heating stress raterelief is annealing not as expressive using slow as in heating samples rate R3 is andnot R6.as expressive as in samples R3 and R6.

Materials 2019, 12, 2200 14 of 17 Materials 2019, 12, x FOR PEER REVIEW 14 of 17

FigureFigure 11. 11.Power Power losses losses as as a functiona function of of magnetizing magnetizing frequency frequency in in studied studied samples samples at Batmax Bmax= =1.5 1.5 T. T.

ItIt can can be be clearly clearly seen seen that that the the measurements measurements of of power power losses losses at at varying varying magnetizing magnetizing frequency frequency (Figure(Figure 11 11)) perfectly perfectly correspond correspond with with the the results results of of coercivity coercivity measurements measurements in in the the DC DC magnetic magnetic field field (Figure(Figure9). 9). Moreover, Moreover, the the curves curves presenting presenting the the dependence dependence of power of power losses losses on di onff erentdifferent frequencies frequencies of ACof magneticAC magnetic field field show show that the that bending the bending rolled rolled sample, sample, heat treated heat treated in dynamic in dynamic conditions, conditions, shows moreshows thanmore 17% than decrease 17% decrease in power in losses power at 50 losses Hz in at comparison 50 Hz in withcomparison the samples with treated the samples by conventional treated by heatconventional treatment leadingheat treatment only to stressleading relief only without to stress additional relief without grain growth. additional The divergencegrain growth. of theThe powerdivergence losses of lines the occurspower atlosses higher lines frequencies occurs at high dueer to frequencies excess loss due as a to part excess of the loss dynamic as a part losses of the respondingdynamic losses for eddy responding current lossfor eddy caused current by domain loss caused wall displacement. by domain wall displacement.

4. Summary and Conclusions 4. Summary and Conclusions In present work, the effects of different heat treatment conditions and bending rolling deformation In present work, the effects of different heat treatment conditions and bending rolling were studied with respect to microstructural evolution, crystallographic texture, and magnetic deformation were studied with respect to microstructural evolution, crystallographic texture, and loss characteristics of fully finished non-oriented electrical steel. In order to achieve the effect of magnetic loss characteristics of fully finished non-oriented electrical steel. In order to achieve the strain-induced selective grain boundary motion, the investigated steel was subjected to bending effect of strain-induced selective grain boundary motion, the investigated steel was subjected to rolling prior to heat treatment. The annealing of plastically deformed samples was individually bending rolling prior to heat treatment. The annealing of plastically deformed samples was carried out by either conventional long-term or unconventional dynamic stress relief heat treatment. individually carried out by either conventional long-term or unconventional dynamic stress relief It has been clearly demonstrated that the used bending deformation in combination with subsequent heat treatment. It has been clearly demonstrated that the used bending deformation in combination dynamic heating process results in substantial microstructure improvement characterized by coarsened with subsequent dynamic heating process results in substantial microstructure improvement grains with enhanced intensity rotated cube texture. The main observations and conclusions can be characterized by coarsened grains with enhanced intensity rotated cube texture. The main summarized as follows: observations and conclusions can be summarized as follows: • The nano-indentation measurements have clearly shown that the bending rolled strips are • The nano-indentation measurements have clearly shown that the bending rolled strips are characterizedcharacterized by by bilinear bilinear distribution distribution of nano-hardness of nano-hardness through through the sheet the thickness. sheet thickness. The maximal The nano-hardnessmaximal nano-hardness values were values measured were in bothmeasured sub-surface in both regions, sub-surface whereas regions, the minimal whereas value the inminimal the sheet value cross-section in the sheet middle cross-section part. This observationmiddle part. is This related observation to the high is intensityrelated to of the plastic high deformationintensity of inplastic both thedeformation sub-surface in both regions the andsub-surface only elastic regions stress and in only the middle elastic partstress of in the the samplesmiddle after part theof the bending samples rolling. after Thus, the bending it can be rolling. concluded Thus, that it can the bendingbe concluded deformation that the enabled bending achievingdeformation the gradient enabled ofachieving stored deformation the gradient energy, of stored without deformation changing energy, the initial without thickness changing of the the steelinitial plate. thickness of the steel plate. • TheThe heat heat treatment treatment at at dynamic dynamic annealing annealing conditions conditions offully of fully finished finished NO NO silicon silicon steels steels subjected subjected to • theto mechanical the mechanical bending bending deformation deformation leads to leads apparent to apparent increase increase in average in grainaverage size grain of the size obtained of the microstructure.obtained microstructure. The evolution The ofevolut coarse-grainedion of coarse-grained microstructure microstructure is related tois therelated strain-induced to the strain- graininduced boundary grain migrationboundary mechanismmigration mechanism under the influenceunder theof influence significant of significant temperature temperature gradient throughgradient the through steel sheet the cross-section.steel sheet cross-section. • The texture measurements have clearly shown that the unconventional dynamic annealing of bending rolled experimental samples had a clearly positive impact on the evolution of their

Materials 2019, 12, 2200 15 of 17

The texture measurements have clearly shown that the unconventional dynamic annealing of • bending rolled experimental samples had a clearly positive impact on the evolution of their crystallographic orientation. It has been shown that coarse-grained matrix obtained after the heat treatment with high heating rate was characterized mostly by the grains with desirable rotated cube crystallographic orientations. The magnetic measurements of fully finished samples in DC and AC magnetic field conditions • have clearly indicated that the evolved microstructures and textures of the strips, obtained by application of bending deformation and heat treatment using two different procedures, are directly responsible for their final magnetic characteristics. The power losses data have clearly shown that the investigated steel treated according to our innovative approach exhibited more than 17% decrease in watt losses in comparison with the material treated by conventional stress relief heat treatment without activation of grain growth.

Author Contributions: Conceptualization, I.P. and F.K.; methodology, I.P., F.K., J.F., and P.K.; validation, I.P. and F.K.; formal analysis, I.P., F.K., and L.F.; investigation, I.P., F.K., J.F., L.F., V.P., and P.K.; resources, I.P.; data curation, I.P., J.F., L.F., and P.K.; writing—original draft preparation, I.P. and L.F.; writing—review and editing, I.P. and L.F.; visualization, I.P. and V.P.; supervision, F.K and I.P.; project administration, F.K and I.P.; funding acquisition, F.K. Funding: This research was funded by “The Slovak Research and Development Agency” (contract No. APVV-15-0259). Acknowledgments: This work was carried out within the research project entitled “Unconventional technology development of final processing of isotropic electrical steels”, which is supported by “The Slovak Research and Development Agency” under the contract No. APVV-15-0259 and partly by “The Slovak Research and Development Agency” under the projects VEGA 2/0066/18 and VEGA 2/0073/19. The work was also realized within the frame of the project ITMS 26220220064. Conflicts of Interest: The authors declare no conflict of interest.

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