In this study the correlation between the titanium content of steel and the core loss of non-oriented electrical steel sheets was determined. The core loss and titanium content of steel have a weak, but positive, correlation. The core loss was found to increase with an increasing titanium content. The study included a statistical analysis of an industrial data set and a metallographic analysis of the titanium inclusions. The analyzed titanium inclusions in the electrical steel sheets containing 0,006 mas.% Ti and 0,008 mas.% Ti were complex oxycarbonitrides, complex TiC and complex Ti(C,N).U radu se opisuje korelacija između sadržaja titana u čeliku i elektromagnetskih svojstava neorijentiranih elektrolimova. Postoji relativno niski stupanj pozitivne korelacije između magnetskih gubitaka i sadržajem titana. Magnetski gubici rastu sa sadržajem titana u čeliku. Izvedena je statistička analiza podataka za limove industrijske proizvodnje i metalografska analiza titanovih uključaka. Analizirani uključci u elektrolimovima sa 0,006 mas.% Ti i 0,008 mas.% Ti bili su kompleksni oksikarbonitridi, kompleksni TiC i kompleksni Ti(C,N)

A. Jaklič

A. Čop

D. Steiner Petrovič

M. Jenko

English

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D. STEINER PETROVI^, M. JENKO, A. JAKLI^, A. ^OPCORRELATION OF TITANIUM CONTENTAND CORE LOSS IN NON-ORIENTED ELECTRICAL STEEL SHEETSReceived – Prispjelo: 2008-06-22Accepted – Prihva}eno: 2009-03-15Original Scientific Paper – Izvorni znanstveni radINTRODUCTIONThe core loss of electrical steel is defined as the dissi-pation of electrical energy in the form of heat during mag-netization by an alternating current. Many efforts havebeen made to reduce the core loss. Numerous parameters,including the chemical composition and the metallurgicaltechnology, influence the properties of the steel.Electrical steel sheets are mainly produced fromFe-Si-C alloys. Non-oriented electrical steels contain, inaddition to the important alloying elements Si, Al andMn, other elements as well. Non-oriented electricalsteels are produced from scrap steel, and therefore im-purity elements (Ti, Cu, Sn, etc.) can be found in thesteel. These trace elements can influence the character-istics of electrical steels 1-4.In this study the influence of titanium on the coreloss of non-oriented electrical steel sheet was investi-gated. The study included a statistical analysis of thedata set for the cold-rolled steel samples with differentamounts of titanium. SEM and ESD were used in themetallographic analyses of the titanium inclusions.STATISTICAL ANALYSISWe analyzed a data set of 409 coils of non-orientedelectrical steel sheet with the core loss measured at 1,5T. The core loss ranged from 2,918 to 4,442 W/kg.The average chemical composition of the non-orien-ted electrical steel with respect to the most important al-loying elements was Fe (2,18 %) Si (0,99 %) Al (mas.%).Using a statistical analysis we determined numericaldescriptive measures for the titanium content (range,mean, median, mode, standard deviation, confidence in-terval, coefficient of variation). We also determined thecorrelation coefficient between the titanium content ofthe steel and the core loss of the steel sheets, calculatedthe coefficient of determination and plotted the regres-sion curve for the data set. The correlation coefficientand the coefficient of determination are measures of therelationship between the two variables 5.The core loss and titanium content of the steel have aweak, positive, linear correlation.The relationship between core loss and titanium con-tent could also be explained by using a regression analy-sis.By using the linear regression model, the magnitudeof the change in the core loss (W/kg) due to a change inthe titanium content is shown in Figure 1/a.As depicted in Figure 1/a, as the titanium content in-creases the core loss slowly increases.Between 0,012 and 0,014 mas.% of Ti there is an in-creasing step in the core loss. Therefore, a nonlinear re-gression model was used for a description of the rela-tionship between the core loss and the titanium contentfor this population.METALURGIJA 49 (2010) 1, 37-40 37In this study the correlation between the titanium content of steel and the core loss of non-oriented electricalsteel sheets was determined. The core loss and titanium content of steel have a weak, but positive, correlation.The core loss was found to increase with an increasing titanium content. The study included a statistical anal-ysis of an industrial data set and a metallographic analysis of the titanium inclusions. The analyzed titanium in-clusions in the electrical steel sheets containing 0,006 mas.% Ti and 0,008 mas.% Ti were complexoxycarbonitrides, complex TiC and complex Ti(C,N).Key words: titanium, correlation, core loss, non-oriented electrical steel sheetKorelacija izme|u sadr`aja titana u ~eliku i elektromagnetskih svojstava neorijentiranih elektro-limova. U radu se opisuje korelacija izme|u sadr`aja titana u ~eliku i elektromagnetskih svojstava neorijentira-nih elektrolimova. Postoji relativno niski stupanj pozitivne korelacije izme|u magnetskih gubitaka i sadr`ajemtitana. Magnetski gubici rastu sa sadr`ajem titana u ~eliku. Izvedena je statisti~ka analiza podataka za limoveindustrijske proizvodnje i metalografska analiza titanovih uklju~aka. Analizirani uklju~ci u elektrolimovima sa0,006 mas.% Ti i 0,008 mas.% Ti bili su kompleksni oksikarbonitridi, kompleksni TiC i kompleksni Ti(C,N).Klju~ne rije~i: titan, korelacija, magnetski gubici, neorijentirani (dinamo) elektrolimoviISSN 0543-5846METABK 49(1) 37-40 (2010)UDC – UDK 669.14.018:669.295:621.039(083.5) = 111D. Steiner Petrovi~, M. Jenko, Institute of Metals and Technology,Ljubljana, A. Jakli~, A. ^op, Acroni d.o.o., Jesenice, Slovenia.The fitted regression curve shows an exponentialgrowth and explains 26 % of the variance of the core loss(Figure 1/b). From a nonlinear fit we can see that as the ti-tanium content increases the core loss slightly increasesand then, after a certain point, the core loss is higher forevery subsequent increase in the titanium content.METALLOGRAPHICANALYSIS OF INCLUSIONSTwo samples of industrial non-oriented electricalsteel sheets with an average content of titanium of 0,006and 0,008 mas.% of Ti were used in the metallographicanalysis (mean = 0,00785 mas.% Ti). The samples wereground and polished. Scanning electron microscopy andEDS were performed to analyze the titanium inclusions.The EDS analysis revealed that the major part of thenon-metallic inclusions that contain titanium are verycomplex. The inclusions containing titanium were pre-dominantly small in size (< 1µm). Inclusions with differ-ent contents of titanium can be seen in Figures 2, 3 and 4.Figure 2 represents the complex inclusions in thesteel containing 0,006 mas.% Ti. The results of an EDSanalysis are shown in Table 2.We can see that the inclusions that contain smallamounts of Ti are very complex (Table 2). The inclu-sions contain even more zirconium than titanium. Thehigh oxygen and zirconium contents of the inclusionsreveal the presence of zirconium oxides. Besides this,also magnesium and calcium, which have a strong affin-ity for oxygen, were detected in these complex inclu-sions (Table 2, Spectra 1-4). Spectrum 5 in Figure 2shows the EDS analysis of the steel matrix.38 METALURGIJA 49 (2010) 1, 37-40D. STEINER PETROVI^ et al.: CORRELATION OF TITANIUM CONTENT AND CORE LOSS IN NON-ORIENTED...Table 1. Calculated measures of the central tendency,the dispersions and the correlation coeffi-cient between the titanium content and thecore lossTi / mas.% Core loss / W/kgRangeMax. 0,01400 4,442Min. 0,00500 2,918Mean 0,00785 3,307Median 0,00800 3,275Mode 0,00800 3,279Standard deviation 0,00183 0,188Confidence interval 0,00018 0,018Upper limit (Ul) 0,00803 3,326Lower limit (Ll) 0,00767 3,289Ul - Ll 0,00035 0,036Coefficient of variation 23,3360 5,6762Correlation coefficient 0,26469Figure 1/a. Linear correlation between core loss (W/kg)and titanium content of steelFigure 1/b. Fitted regression curve between core loss(W/kg) and titanium content of steelFigure 2. Complex inclusions in the sample ofnon-oriented electrical steel sheet containing0,006 mas.% TiA higher Ti content in the inclusion could be de-tected in the inclusion presented in Figure 3. The resultsof the EDS analysis are given in Table 3.The analyzed inclusions in Figure 3 are complexTi(C,N) and complex TiC (Figure 4), which also containsmall amounts of other impurity elements like zirconium,magnesium and molybdenum (see Tables 3 and 4).DISCUSSIONThe metallurgy of steels for magnetic applications isvery complex. The silicon steels for non-oriented elec-trical steel sheets are nowadays produced from scrapsteel that contains many impurity elements 1-4. One ofthese impurities is titanium. Impurity elements must becontrolled during melting. It is well known that they canstrongly influence the recrystallization by ''pinning ef-fects'' and precipitation in both grains and grain bound-aries, and therefore can also have an influence on thegrain growth, texture formation and the final magneticproperties of non-oriented electrical steels 6, 7.This study has followed the magnetic properties of arepresentative data set of non-oriented electrical steelsheets from industrial production. Our investigation wasfocused on a determination of the influence of titaniumcontent in Fe-Si-Al alloys on the magnetic properties ofthe non-oriented electrical steel sheets. It was confirmedthat the correlation between the titanium content of thesteel and the core loss is weak, but positive.Linear and nonlinear regression models were used todescribe the relationship between core loss and titaniumcontent for this population. From both fits we can seethat as the titanium content increases the core loss in-creases. Furthermore, there is an increasing step in thecore loss after a certain content of titanium in the steel(0,0120,014 mas.% Ti).The magnitude of the change in the core loss (W/kg)due to a change in the titanium content could be betterdescribed by a nonlinear fit. Using a nonlinear fit thecore loss of the steels containing more than 0,012 mas.%Ti is higher for every subsequent increase in titaniumcontent. The fitted regression curve explains 26 % of thevariance of the core loss and shows an exponentialgrowth.The results of our metallographic analysis revealedthat the titanium in non-oriented electrical steel sheetswas present in the form of very complex inclusions(oxycarbonitrides) and, in addition, in the form of smalland complex carbonitrides and carbides.A disproportionately increased step in the core losswhen the content of 0,012 mas.% Ti was exceeded couldbe the consequence of the increased cumulative (total)value of the impurity elements in the steel. Impuritiescan have a synergy effect on the deterioration of themagnetic properties. Titanium represents just one ofmany impurity elements that can enter the steel throughscrap steel during recycling and through other processesof secondary metallurgy.METALURGIJA 49 (2010) 1, 37-40 39D. STEINER PETROVI^ et al.: CORRELATION OF TITANIUM CONTENT AND CORE LOSS IN NON-ORIENTED...Table 4. EDS analysis of inclusions from Figure 4Spectrum In stats. C O Mg Al Si Ti Cr Fe Zr TotalSpectrum 1 Yes 7,27 0,24 0,63 1,51 14,89 74,05 1,40 100,00Spectrum 2 Yes 4,07 1,31 0,73 1,47 0,59 91,83 100,00Spectrum 3 Yes 3,11 0,58 1,59 0,95 0,50 93,28 100,00Spectrum 4 Yes 2,60 1,01 2,11 94,28 100,00Figure 4. Titanium inclusions in the non-oriented electri-cal steel sheet with 0,008 mas.% TiFigure 3. A Ti(C,N) inclusion in the sample ofnon-oriented electrical steel sheet containing0,008 mas.% TiTable 2. EDS analysis of inclusions from Figure 2Spectrum In stats. C N O Mg Al Si Ca Ti Cr Fe Zr TotalSpectrum 1 Yes 6,51 7,61 4,16 2,66 0,32 1,09 3,83 56,49 17,34 100,00Spectrum 2 Yes10,035,81 5,43 2,42 0,59 1,25 3,72 55,67 15,06 100,00Spectrum 3 Yes 8,10 4,52 4,71 2,10 0,51 0,98 4,04 63,68 11,36 100,00Spectru m 4 Yes 5,73 7,31 2,32 1,82 0,43 0,90 0,31 2,48 65,53 13,15 100,00Spectrum 5 Yes 1,90 0,78 2,10 0,44 94,78 100,00Table 3. EDS analysis of inclusion from Figure 3Spectrum In stats. C N Al Si Ti Fe Zr Mo TotalSpectrum 1 Yes 9,36 4,07 0,66 1,33 10,54 72,23 0,94 0,87 100,00Titanium has a strong affinity for carbon and nitro-gen and forms very stable carbonitrides 8. It is re-ported that titanium carbonitrides inhibit the grain coar-sening during the recrystallization of semi-processednon-oriented electrical steel sheets and deteriorated themagnetic induction and core loss 7. On the other hand,very small precipitates of Ti(C,N) (< 30 nm) in the finalproducts of non-oriented electrical steel sheets shouldnot have this harmful effect 9.Using SEM/EDS in this study it was possible to de-termine the presence of titanium in the non-metallic in-clusions. Due to the limitations of this method we couldonly analyze the bigger inclusions and precipitates, withdiameters of several 100 nm.It is assumed, therefore, that the main cause of thepositive correlation between titanium content and thecore loss of the investigated set of non-oriented electri-cal steel sheets may be due to the presence of titaniuminclusions and precipitates. Impurity elements are nor-mally present in steels in the form of inclusions or pre-cipitates and are very often harmful to the magneticproperties. Also, the texture formation might be influ-enced by the presence of those precipitates 6, 7.These precipitates can influence the secondaryrecrystallization and the grain size of the investigatedelectrical steel sheets. The efficiency of the recrysta-llization and grain growth is very important in the pro-duction of non-oriented electrical steels. The contribu-tion of those two production phases to increase or de-crease the final core loss is, nevertheless, of great impor-tance 10 - 14.To ensure improved magnetic properties of thenon-oriented electrical steel sheets the content of the im-purity elements and the inclusions should be rigorouslycontrolled and minimized.CONCLUSIONSThe correlation between the titanium content ofnon-oriented electrical steel sheets and the core loss mea-sured at 1,5 T was determined. The core loss and titaniumcontent of the steel have a weak, positive correlation.From the regression analysis we can see that there is acritical value of titanium content that influences the coreloss of the non-oriented electrical steel sheets. A dispro-portionately increased step in the core loss could be theconsequence of the increased cumulative (total) value ofthe impurity elements in the steel. Impurities can have asynergy effect on the deterioration of the magnetic prop-erties. A higher titanium content also means a larger num-ber of titanium inclusions and precipitates.The study revealed that the major titanium inclusionsin the non-oriented electrical steel sheet containining0,006 mas. % Ti and 0,008 mas. % Ti were complex tita-nium carbides, complex titanium carbonitrides andcomplex oxycarbonitrides, which contained loweramounts of titanium.REFERENCES1 Y. Zhao et al., Mater.Sci. Forum, 204-206 (1996), 641-646.2 D. Steiner Petrovi~, M. Jenko, Vacuum, 71 (2003), 2, 33-40.3 D. Steiner Petrovi~ et al., ISIJ Int., 46 (2006), 10,1452-1457.4 D. Steiner Petrovi~ et al., Metalurgija, 46 (2007), 2, 75-78.5 P.S. Mann, Statistics, John Wiley & Sons, Inc., New York,1995, 657-713.6 T. Nakayama, M. Takahashi, J. Mater. Sci., 30 (1995),5979-5983.7 T. Nakayama, T. Tanaka, J. Mater. Sci., 32 (1997),1055-1059.8 V. Raghavan, J. of Phase Equilibria, 24, 1 (2003), 75-76.9 K. Jenkins, M. Lindenmo, J. Magn. Magn. Mater., 320(2008), 2423-2429.10 G. Abbruzzese, A. Campopiano, J. Magn. Magn. Mater.,133 (1994), 123-135.11 D. Steiner Petrovi~, F. Vodopivec, M. Jenko, Journal dePhysique IV, C7 (1995), 5, 255-258.12 S.D. Paolinelli et al, J. Magn. Magn. Mater., 320 (2008),e641-e644.13 M.A. da Cunha, S.D. Paolinelli, J. Magn. Magn. Mater., 320(2008), 2485-2489.14 F. Kovac, V. Stoyka, I. Petryshynets, J. Magn. Magn. Ma-ter., 320 (2008), e627-e630.Note: The responsible for English language is Paul McGuiness.40 METALURGIJA 49 (2010) 1, 37-40D. STEINER PETROVI^ et al.: CORRELATION OF TITANIUM CONTENT AND CORE LOSS IN NON-ORIENTED...

Korelacija između sadržaja titana u čeliku i elektromagnetskih svojstava neorijentiranih elektrolimova

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