The present study was made to investigate secondary recrystallization in grain-oriented steels annealed at short time temperature exposures with application of dynamical heating. The investigated GO steels for experiments were taken from one industrial line after final cold rolling reduction and subsequent box annealing. It was shown that application of short time heat treatment conditions could lead to complete abnormal grain growth in the investigated GO steel. The texture and microstructure obtained in the laboratory treated material is similar to that observed in the same GO steel taken after industrial final box-annealing. However, some “parasitic” grains were observed in the secondary recrystallized matrix of the laboratory treated GO steel. These “parasitic” grains possess the unwanted from magnetic properties point of view {111} orientation components.Ova studija istražuje sekundarnu rekristalizaciju u čelicima s orijentiranim zrnima odžarenim kratkotrajno, a pod utjecajem dinamičkog zagrijavanja. Istraživanja ZO čelika za eksperimente uzeti su iz jedne industrijske linije nakon konačnog hladnog redukcijskog valjanja i naknadnog žarenja u komori. Pokazano je, primjena uvjeta kratkotrajne toplinske obradbe mogu dovesti do potpuno nenormalnog rasta zrna u istraživanom ZO čeliku. Tekstura i dobivena mikrostruktura u laboratorijski obrađivanim materijalima, slični su onim primjećenih kod nekih ZO čelika uzetih nakon konačnog industrijskog žarenja u komori. Međutim, neka „parazitna“ zrna primjećena su tijekom sekundarne rekristalizacije laboratorijski tretiranih ZO čelika. Ova „parazitna“ zrna posjeduju nepoželjne (111) orijentirane komponente s točke gledišta magnetnih svojstava

B. Julius

F. Kovac

V. Stoyka

English

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V. STOYKA, F. KOVAC, B. JULIUSSTUDY OF SECONDARYRECRYSTALLIZATION IN GRAIN-ORIENTED STEELTREATED UNDER DYNAMICAL HEAT TREATMENT CONDITIONSReceived – Prispjelo: 2008-06-22Accepted – Prihva}eno: 2008-09-20Preliminary Note – Prethodno priop}enjeINTRODUCTIONGrain oriented silicon steel (GO) is mainly used as thecore material in transformers, and it is the only productmanufactured in the steel industry that applies the sec-ondary recrystallization phenomenon. Its magnetic prop-erties are closely related to the secondary recrystallizationtexture, i.e., the sharpness of 110(001), Goss texture1. This sharp texture develops due to a discontinuous orabnormal grain growth of Goss grains during high tem-perature annealing at the end of industrial production pro-cess. This heat treatment performed for secondaryrecrystallization and subsequent purification from the in-hibitor elements. This high temperature annealing is con-ducting at low heating rate ( 25 °C/hour) up to the puri-fication annealing temperature (1200 °C) 2. This istime and power consuming process as the duration of thewhole process is about several tens of hours. That is why,now nearly all producers are targeting on more compactand less expensive production routes of the GO steels.Although it is a matter of intensive basic and appliedresearch since more than 50 years, there is no generalagreement on the origin of preferred growth of the Gossgrains during final box annealing. There are two similarbut competing views frequently repeated in the litera-ture of the subject. The first one originates in the sugges-tion that the coincidence site lattice (CSL) misorien-tation relationships are a factor in the secondaryrecrystallization in silicon steel 3. Briefly, accordingto the model, the boundaries of Goss oriented grains aremore frequently of the CSL-type than boundaries ofother grains 3. Moreover, in the presence of precipi-tates, the CSL boundaries are assumed to be more mo-bile than general boundaries. Thus, the Goss grains havethe opportunity to grow. The second view is based onthe assumption that the high mobility is a feature of theso called “high energy” (HE) boundaries defined thereas boundaries between grains misoriented by the angleof 20 to 45° 4.Again, the advantage of the Goss grains would comefrom the fact that they are bounded by the HE bound-aries more frequently than other grains.METALURGIJA 48 (2009) 2, 99-102 99The present study was made to investigate secondary recrystallization in grain-oriented steels annealed at shorttime temperature exposures with application of dynamical heating. The investigated GO steels for experimentswere taken from one industrial line after final cold rolling reduction and subsequent box annealing. It was shownthat application of short time heat treatment conditions could lead to complete abnormal grain growth in the in-vestigated GO steel. The texture and microstructure obtained in the laboratory treated material is similar to thatobserved in the same GO steel taken after industrial final box-annealing. However, some “parasitic” grains wereobserved in the secondary recrystallized matrix of the laboratory treated GO steel. These “parasitic” grains pos-sess the unwanted from magnetic properties point of view 111 orientation components.Key words: abnormal grain growth, Goss texture, grain oriented electrical steel, coercive forceProu~avanje sekundarne rekristalizacije u ~eliku s orijentiranim zrnima pri dinami~kim uvjetimatoplinske obradbe. Ova studija istra`uje sekundarnu rekristalizaciju u ~elicima s orijentiranim zrnima od`are-nim kratkotrajno, a pod utjecajem dinami~kog zagrijavanja. Istra`ivanja ZO ~elika za eksperimente uzeti su iz jed-ne industrijske linije nakon kona~nog hladnog redukcijskog valjanja i naknadnog `arenja u komori. Pokazano je,primjena uvjeta kratkotrajne toplinske obradbe mogu dovesti do potpuno nenormalnog rasta zrna u istra`iva-nom ZO ~eliku. Tekstura i dobivena mikrostruktura u laboratorijski obra|ivanim materijalima, sli~ni su onim pri-mje}enih kod nekih ZO ~elika uzetih nakon kona~nog industrijskog `arenja u komori. Me|utim, neka „parazitna“zrna primje}ena su tijekom sekundarne rekristalizacije laboratorijski tretiranih ZO ~elika. Ova „parazitna“ zrna po-sjeduju nepo`eljne (111) orijentirane komponente s to~ke gledi{ta magnetnih svojstava.Klju~ne rije~i: nenormalni rast zrna, ZO teksture, zrnato orijentirani elektro ~elici, koericitivna silaISSN 0543-5846METABK 48(2) 99-102 (2009)UDC – UDK 669.14.018.583=111V. Stoyka, F. Kovac, Institute of Materials Research, Kosice;B. Julius, Technical University of Kosice, Kosice, Slovak RepublicThe main objective of the current work is to analyzethe application of dynamical heat treatment conditionsto GO steels. The second purpose of the paper is to dis-cuss secondary recrystallization during short time an-nealing on the base of the aforementioned hypotheses.EXPERIMENTAL PROCEDUREGrain oriented steels were used in the experimentalprocedure. The investigated G and FG steels were takenfrom one industrial line after cold rolling and subse-quent final box annealing respectively. The cold rollingprocess was conducted with total reductions in thicknessof 92 %. The chemical composition of the investigatedsteel is presented in Table 1.Usually, to obtain desired magnetic properties, coldrolled GO electrical steels are subjected to decarbu-rization annealing with subsequent long-time box an-nealing. Such industrial treatment process was realizedon FG material. During the industrial decarburizationthe carbon level was reduced up to 0,006 % in wt. The fi-nal texture and microstructure state of the FP materialwas obtained after industrial box annealing.The heat treatment of cold rolled sample G was real-ized by furnace “Nabertherm” with an electronic controlsystem C19/S19. The dynamical heat treatment condi-tions were applied in order to investigate microstructureand texture development under such conditions. Thespecimens of the G steel were treated in wet decar-burizing atmosphere (d.p.+30 °C). The investigatedsamples were heated up to elevated temperature with dy-namical heating (heating rate 17 °C/s). Holding time atthe annealing temperature was 5 min. Annealing temper-atures were changed in the range of 850 °C up to 1100 °C.Microstructure of the investigated specimens wasexamined in the longitudinal cross-section and in thenormal direction plane using light microscopy. Crystal-lographic texture study was performed by EBSDmethod. JEOL JSM 7000F FEG scanning electron mi-croscope was employed to perform the texture analysis.The patterns formed from back scattered electrons weredetected by “Nordlys” EBSD detector. The obtainedEBSD data were analyzed and displayed by CHAN-NEL-5, HKL software package.RESULTS AND DISCUTIONGrain growth progress in material G treated in the tem-perature range of 850 °C – 1025 °C is shown in Figure 1.As one can see, the microstructure is fully recrystallized af-ter annealing at 850 °C/10 min in the wet atmosphere ofcracked ammonia (Figure 1a). The increase of temperatureinitiates the abnormal grain growth (AGG) as shown inFigure 1b. Further increase of the temperature provides100 METALURGIJA 48 (2009) 2, 99-102V. STOYKA et al.: STUDY OF SECONDARY RECRYSTALLIZATION IN GRAIN-ORIENTED STEEL TREATED UNDER...Table 1 Chemical composition of the investigated sample / wt.%.Sampl. C Mn Si Cu P S Al NG 0,032 0,23 3,31 0,5 0,011 0,007 0,017 0,008Figure 1. Microstructure of G samples annealed at a) 850 °C and b) 1000 °C c) 1025 °C for 5 min in wet atmosphere ofcracked ammonium (d.p. +30 °C).Figure 2. Microstructure of G sample annealed at 1100 °Cfor 5 min in wet atmosphere of cracked ammo-nium (d.p. +30 °C).coarse-grained microstructure consisting from abnormallygrown ferrite grains (see Figure 1c).Increase of the annealing temperature up to 1100 °Cleads to broken of selectivity conditions for abnormalgrain growth see Figure 2. The broken of selectivityconditions reveals it self in pronounced growth not onlyparticular grains but also the surrounding ones. More-over, the grain growth character of particular grains isnot so pronounced as in the previous case, cf. Figures 1cand 2. From the presented metallographic patterns, it isclearly evident that the application of short time anneal-ing combined with dynamical heating of GO steel at par-ticular temperature (1025 °C) leads to abnormal graingrowth development in the Fe-3 %Si.IPF maps of the G and FG materials are shown inFigure 3a and b respectively. It is evident that in bothcase the developed huge grains possess Goss orienta-tion. However, some parasitic grains are observed in Gmaterial annealed at laboratory conditions. The grainsare “parasitic” because they have got unwanted frommagnetic properties point of view 111 orientation, seeFigure 3a. Moreover these small grains (20 m) areembedded into huge Goss grains. Small “parasitic”grains embedded in to huge Goss grains are not ob-served in the FG material treated under industrial condi-tions, see Figure 3b. The 100 pole figures represent-ing texture state in G and FG materials are shown in Fig-ure 4 and Figure 5 respectively. As one can see thesharpness of the developed huge Goss grains in both ma-terials is similar, cf. Figures 4 and 5. Hence, texture stateof Fe-3 %Si steel developed under dynamical heat treat-ment conditions is very close to that one developed un-der static industrial treatment. Process for elimination ofparasitic grains presence needs further investigation.However the mentioned results obtained under the ap-plied dynamical conditions are a good indication of pos-sible application of continuous annealing for Fe-3Sisteel production. Visions of the possible continuous an-nealing application instead of final box annealing of GOsteels were discussed by Gunter et al. 5.The onset of abnormal grain growth in the sample Goccurs at 1000 °C, see Figure 1b. The map of grainboundaries distribution of the mentioned state is pre-sented in Figure 6a. As one can see, the AGG proceedsin surrounding primary recrystallized matrix. It isclearly evident from the Figure 6a and b that most ofgrain boundaries between growing Goss grain and restsof the surrounding grains are HE boundaries. Moreover,the boundaries between parasitic grains and Goss oneare also boundaries with misorientation in range of40°-55°, see Figure 6a.Concerning mechanism of Goss grains developmentunder the applied dynamical heat treatment conditions,this mechanism arouses a big interest from scientificpoint of view. As was mentioned above, there are twoprincipally different mechanism of Goss texture develop-ment. The both of mechanisms are based on the uniqueproperties of boundaries of Goss grains. The first mecha-nism is coincident site lattice (CSL) mechanism ascribinghigh mobility to CSL boundaries. The second mechanismMETALURGIJA 48 (2009) 2, 99-102 101V. STOYKA et al.: STUDY OF SECONDARY RECRYSTALLIZATION IN GRAIN-ORIENTED STEEL TREATED UNDER...Figure 3. IPF maps representing secondary recrystallized state of a) G steel after laboratory annealing at 1025 °C for 5 minand b) FG steel taken after final industrial box annealing.Figure 4. Pole figure representing texture state in G steelwith abnormal grains annealed at 1025 °C for 5min under laboratory conditions.Figure 5. Pole figure representing texture state in FG steelannealed under industrial conditions.proposed by Szpunar et al. 4 assigns high mobility tohigh energy boundaries that mainly surround Gossgrains. The idea here is that, the HE boundaries are char-acterized by high diffusivity that leads to quick coarsen-ing or dissolution of precipitates during box annealing.The resulting large or missing particles have a lower pin-ning force on the moving boundaries than small particleson the other boundaries. However, none of the theoriescan give the explicit explanation of the AGG phenome-non. Moreover, each of the theories is based on the staticmethod of the annealing for AGG development in the GOsteel (box annealing). Hence, a self evident questionerases from the results presented in the current work:whether the same mechanism of abnormal grain growthtakes place under both static and dynamic conditions?The possible answer could be the following thoughts. Aswas concluded in Ref 6, the Goss nuclei are located inregions with low stored strain energy. Therefore, Gossgrains could be faster recovered from dislocation thanones with other orientations 7. Moreover, Inagaki ob-served fast both recovery and recrystallization in 110deformed grains, while in 100 and 112 deformedgrains these processes proceed very slowly 8. The anal-ogy could be drawn to the AGG development under dy-namical conditions. That is, dislocations or boundaries inGoss grains possess (110) crystallographic planes can es-cape from the retarding pressure of secondary-phase par-ticles first at particular temperature. Moreover, the ob-served high energy grain boundaries around the growingGoss grains are more capable to emit or capture disloca-tions 9. It means that the Goss oriented grains gain anadvantage of grain growth over the other grains and startto growth first. However, the process of secondaryrecrystallization under dynamic conditions is very sensi-tive on temperature. This is because the temperature has adirect impact on the grain boundary energy and mobility10. The elevated temperature has to be sufficient to ini-tiate the growth of the Goss grains while the mobility ofthe rest orientations should approach zero. As was al-ready shown the AGG takes place at particular tempera-ture  1025 °C. The application of somewhat lower orhigher temperatures leads either to incomplete AGG orupset the selectivity conditions for the AGG. Therefore,the appropriative temperature for dynamic heat treatmenthas to be strictly controlled during the secondaryrecrystallization process. Anyway, the nature of abnor-mal grain growth under dynamic conditions is not under-stood enough and need further detailed investigation.CONCLUSIONSThe secondary recrystallization phenomenon was in-vestigated in the GO steel annealed under dynamicalconditions. It was shown that in the investigated GOsteel abnormal grain growth takes place at particulartemperature. The developed under dynamical heat treat-ment conditions Goss grains have similar sharpness asthat ones obtained after final box annealing in industry.The development of the AGG growth under dynamicalheat treatment conditions is a good indication of possi-ble application of continuous annealing for Fe-3 %Sisteel production.REFERENCES1 Goss N.P., “Electrical Sheet and Method and Apparatus forIts Manufacture and Test”, U.S. Patent 1,965,559, (1934).2 Chang S. K. and Hong B. D.: ISIJ International, 44(2004)6,1086-1092.3 Harase J. and Shimizu R., Trans. Jpn. Inst. Met. 29, 388(1988).4 Rajmohan N., Szpunar J.A. and Hayakawa Y., Acta mater.47(1999)10, 2999-3008.5 Günther. K., Abbruzzese. G., Fortunati. S., Ligi. G., SMM16, Düsseldorf 2003 pp. 41-50.6 Dorner D., Zaefferer S., Raabe D.: Acta Mat. 55 (2007)2519-2530.7 Raabe D. and Lücke K., Mater. Sci. Forum. 157-162,597-610 (1994).8 H. Inagaki: Trans. Jpn. Inst. Met., 28 (1987), 251.9 Valiev R.Z., Gertsman V.Yu. and Kainyshev O.A., Phys.Status Solid (a) 97 (1986) p. 11.10 Note: ISIJ International, 37(1997)7, pp.726-728.Acknowledgement – This work was supported by Slo-vak Grant Agency VEGA (2/7195/27).Note: The responsible translator for English language is the author F.Kova~.102 METALURGIJA 48 (2009) 2, 99-102V. STOYKA et al.: STUDY OF SECONDARY RECRYSTALLIZATION IN GRAIN-ORIENTED STEEL TREATED UNDER...Figure 6. Map of grain boundaries distribution (a) and mi-sorientation distribution with appropriative co-lor key for the map (b) in G steel withmicrostructure representing start of abnormalgrain growth.

Proučavanje sekundarne rekristalizacije u čeliku s orijentiranim zrnima pri dinamičkim uvjetima toplinske obradbe

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