Sn Influence on the Recrystallization of Non-Oriented Electrical Sheet
Vpliv Sn na rekristalizacijo neorientirane elektro pločevine
M. Godec1, M. Jenko, IMT Ljubljana, Slovenija
R. Mast, Max-Planck-lnstitut fur Eisenforschung, Dusseldorf, Germany F. Vodopivec, IMT Ljubljana, Slovenija
H. J. Grabke, H. Viefhaus, Max-Planck-lnstitut fur Eisenforschung, Dusseldorf, Germany
Prejem rokopisa - received: 1996-10-01; sprejem za objavo - accepted for publication: 1996-11-04
During the recrystailization microalioyed tin in non-oriented silicon steel segregates to the surface and grain boundary and as a surface active element selectively decreases the surface energy of grains, planeš of vvhich (100) lie parallel to the surface sheet. This phenomenon can be used to achieve non-oriented electrical steel vvith improved electromagnetic properties. Auger electron spectroscopy was used to measure the grain boundary and surface segregation of tin in non-oriented electrical steels. The grain boundary segregation of the specimens. vvhich vvere previously aged at 550°C for different times and vvere fractured in UHV conditions, vvas measured. The segregation temperature dependence and its kinetics vvere follovved in polycrystalline specimens in the temperature range from 400"C to 900"C on the grains of knovvn orientations: (100), (111) and (110). In spite of fact that the grain boundary segregation is much smaller compared vvith surface segregation, both might have an influence on recrystallization and on texture development in electrical steel. The textures of electrical steels vvere measured by X-ray texture goniometer. The results vvere presented as orientation distribution functions. The selective grain growth can be achieved by controlled surface segregation by vvhich the electrical properties of non-oriented electrical steel are improved. The best results vvere obtained by alloying it vvith 0.05 wt. % Sn.
Key vvords: non-oriented silicon steel, tin, surface and grain boundary segregation, recrystallization, texture
Kositer, mikrolegiran v neorientirani elektro pločevini, pri rekristalizaciji segregira na površino in meje zrn in kot površinsko aktivni element selektivno zmanjša površinsko energijo zrn, katerih ravnine (100) ležijo vzporedno s površino pločevine. Ta pojav lahko izkoristimo za izdelavo neorientirane elektro pločevine z izboljšanimi elektromagnetnimi lastnostmi. S spektroskopijo Augerjevih elektronov smo zasledovali segregacijo po mejah zrn in na površini neorientirane elektropločevine. Segregacijo po mejah zrn smo merili na vzorcih, ki so bili predhodno starani na temperaturi 550°C različno dolgo in prelomljeni v pogojih UVV. Tudi temperaturno odvisnost segregacije in njeno kinetiko smo zasledovali na polikristalnih vzorcih, v temperaturnem območju od 400°C do 900°C, na zrnih znanih orientacij: (100), (111) in (110). Kljub temu, da je segregacija po mejah zrn veliko manjša od segregacije na površini, pa imata verjetno obe vpliv na rekristalizacijo in tako na razvoj teksture elektro pločevine. Teksturiranost elektro pločevin smo določili z rentgenskim goniometrom. Rezultati so predstavljeni z orientacijskimi porazdelitvenimi funkcijami. S kontrolirano površinsko segregacijo dosežemo selektivno rast zrn, kar izboljša električne lastnosti neorientirane elektro pločevine. Najboljše rezultate smo dosegli pri legiranju z 0.05 mas.% Sn.
Ključne besede: neorientirana elektro pločevina, kositer, površinska segregacija in segregacija po mejah zrn, rekristalizacija, tekstura
1 Introduction
Recrystallization, corrosion, adsorption, catalysis, surface diffusion, adhesion, sintering and some other processes are decisively depended of chemical composi-tion and structure of surface. On the other hand the mechanical properties and the corrosion resistivity of metals and alloys are greatly influenced by the atomic composi-tion of grain boundaries and interfaces1.
The chemical composition of surfaces and grain boundaries are drastically changed during heat treatment of steels due to the well-known phenomenon called segregation. Some of the alloying elements and also some of the tramp elements in ppm level from IV A to VI A group enrich surface and grain boundaries. Equilibrium segregation is reached by the interaction of free bonds on the surface vvith the segregating elements. This decreases the surface energy and releases the elastic energy of the lattice2.
Mag. Matjaž GODEC. dipl.inž.met. Institut za kovinske materiale in tehnologije Ljubljana. Lepi pot tt, Slovenija "
By alloying non-oriented electrical steels vvith small additions of surface active elements such as Sn, Sb, Te and Se, the texture can be significantly improved3"9. Tin, vvhen added in the range of a 0.02-0.1 wt%, can improve magnetic properties, though it is not desirable in steel10. During the recrystallization process, tin segregates at the grain boundaries and on the surface. The thickness and structure of the segregated layer depend on the crystal-lographic orientation1'.Thus, by segregation, the surface energy decreases selectively, and so the difference in the total energy of the grain, vvhich is the driving force for its grovvth during recrystallization. It is logical to expect a selective effect on grain grovvth vvith a different space orientation.
The aim of the present vvork vvas to find out the cor-relation of segregation and the texture development. Surface and grain boundary segregation of tin in non-ori-ented electrical steel alloyed vvith 2 wt.% Si and 1 wt.% Al and different contents of tin (0.025, 0.05 and 0.1.wt.%) vvere determined. The temperature dependence and the kinetics of surface segregation vvere studied vvith the emphasis on orientation dependence. The correlation
betvveen tin segregation and texture development was as-certained. The segregation of tin during the recrystalliza-tion increased the grain growth of (100) grains lying in the plane of the sheet and in the same time decreased the growth of (111) grains.
2 Experimental
Four experimental non-oriented electrical sheets were produced from the same basic material. The composi-tions of vacuum melted and čast steels are listed in Table
Table 1: Chemical composition of steels in wt.% Tabela 1: Kemijska sestava preiskovanih jekel v mas.%
Steel	C	Mn	Si	S	Al	Sn
A	0.0015	0.24	2.2	0.0005	1.10	0.000
B	0.0025	0.26	2.01	0.0028	1.10	0.027
C	0.0015	0.23	2.02	0.0005	0.95	0.048
D	0.0015	0.23	2.08	0.0004	0.95	0.097
The resulting ingots of about 15 kg weights vvere hot rolled, at a starting temperature of 1200°C, to the final strip thicknesses of 6 mm and 2.5 mm. The strips vvere descaled and decarburized in a wet hydrogen (devv point 25°C) for two hours at 840°C.
Segregation vvas studied "in situ" using Auger Electron Spectroscopy - AES. The tin enrichment on the surface vvas determined by follovving the peak height rado (PHR) of amplitudes betvveen the dominant Sn(MjN45N45) and the Fe(L?M23M54) Auger transitions, located at the 430 and 651 eV kinetic electron energies.
For grain boundary segregation the notched cylindri-cal specimens of 3.7 mm and 5 mm in diameter and of 3 mm length, vvere prepared from a 6 mm thick hot rolled strip. The specimens vvere encapsulated in quartz tubes and vvere evacuated to 10"6 mbar. After they had been normalised for 24 hours at 1000°C, they vvere aged from 5 to 1000 hours at 550°C. The cylindrical notched specimens vvere introduced into a UHV chamber of the spec-trometer, being cooled to about -120°C, the specimens vvere fractured by impact. The newly-formed surface vvas imaged by a scanning electron microscope (SEM). The Auger spectra vvere taken from as many intergranular fractures as possible and the results vvere averaged12"13.
The specimens for surface segregation vvere prepared from a hot rolled strip of 2.5 mm, descaled, decarburized and, after intermediate annealing (900°C, 1 hour, dry hy-drogen), cold rolled to the final thickness of 0.5, 0.2, and 0.1 mm vvith a cold deformation of 60%. Specimens vvere secondary recrystallized in-situ during AES meas-urements in UHV (10-10 mbar), as vvell as in a tube fur-nace in an argon atmosphere.
The grain orientation vvas determined by the etch pit-ting method14,15. The specimens vvith knovvn orientation vvere heated to 900°C for 10 minutes and cooled dovvn to room temperature. These vvere then sputter cleaned and
annealed in a temperature range of 450°C to 1000°C. The temperature vvas increased in steps of 50°C every 15 minutes and the AES spectra vvere recorded in-situ every 3.5 minutes. For the kinetics studies, the specimens vvere heated to a certain temperature, sputtered to a clean surface and exposed to the same temperature for different periods of time.
The X-ray diffraction method vvas used for texture measurements. A goniometer using MoKa radiation vvas applied and the (200), (110) and (211) pole figures vvere performed. Additionally, orientation distribution func-tions (ODF) vvere calculated and texture fibres vvere plotted.
3 Results and discussion
Tin added into experimental steels vvas in the range of solubility in a-Fe at ali examined temperatures but it vvas belovv the detection limit of AES. After the specimens vvere exposed to higher temperature tin enriched the surface, grain boundaries and interfaces due to equi-librium segregation and its segregation vvere detectable by AES. Ali AES spectra v/ere normalised to Fe(L3M23M54) Auger transition at the 651 eV kinetic en-ergy".
3.1 Grain boundary segregation
The equilibrium segregation of tin vvas attained after annealing the specimen alloyed vvith 0.1% Sn for 200 hours at 550°C (figure 1). Considering that tin is equal distributed on both fractured sides, it vvas estimated a 7% tin monolayer at grain boundaries. The scattering of results vvas rather large due to the strong dependence of tin segregation to grain boundary orientation12,16. Steel al-loyed vvith 0.05% Sn had much less intergranular facets. Evaluated equilibrium segregation vvas smaller than in steel alloyed vvith 0.1% Sn. Detailed AES analyses of
£■ 0.02
Sn grain boundary segregation of Fe-Si-Sn (0.1%) steel aged at temperature of 550° C
0	100	200	300	400	500
Time (h)
Figure 1: Peak height ratio (PHR) between the dominant Sn(M5N45N45) and the Fe(LjM23M54) Auger transitions at the kinetic electron energy of 430 eV and 651, respectively, in dependence of different ageing time
Slika 1: Razmerje višine vrhov (RVV) med Sn(MsN45N45) in Fe(L3M23M54) Augerjevimi prehodi pri kinetični energiji elektronov 430 eV in 651 eV v odvisnosti od časa staranja
Steel Fe-Si-Sn(0.05%) Grain orientation (111)
ent surface tin segregation on different grains was no-ticed. Different grain orientation provided different sites for segregated tin atoms. By comparing PHRs Sn/Fe among different specimens one should take care of so called channelling effect especially due to the fact that Auger iron signal is very sensitive to the angle of sample surface and analyser axis13.
Figure 3 shows the temperature dependence of surface segregation of alloying and tramp elements of non-oriented electrical steel alloyed vvith 0.05% Sn on different grain orientations - (001) and (111) - respectively. Electrical steel is a multicomponent system and so very complicated to understand the temperature dependence behaviour of surface segregation, therefore the results obtained on binary alloys should be considered17. The re-lations of the surface segregation enthalpies and volume diffusivities are as follows: AH°si < AH°c < AH°p and Dcv » Dsiv > Dpv17.
At lower temperatures (~300°C), C segregated to the surface due to very high diffusion coefficient in compari-son to Si and P, although the bulk concentration was at very low 15 ppm. At higher temperatures, C atoms were displaced by Si atoms'8. The P and S atoms displaced the silicon at higher temperatures17. Their bulk diffusion co-
Steel Fe-Si-Sn(0.05%) Grain orientation (100)
Grain	1	2	3	4	5	7
PHR Sn/Fe	0.23	0.31	0.29	0.40	0.30	0.40
Orientation	(144)	(025)	(118)	(111)	(5913)	(236)
free surfaces betvveen inclusion (A1N, AbCh) and matrix clearly indicated that the considerable tin segregation oc-curs at the interface. The degree of tin segregation at the interface is five times larger than at the grain boundaries.
3.2 Surface segregation
Scanning Auger image (SAM) of non-oriented electrical steel heated to 800°C for 10 minutes was taken. The orientation of individual grains vvas determined by the method described in our previous publication15. Figure 2 shovvs SEM and SAM images of surface. A differ-
Figure 2: a) SEM image of 0.2 mm thick non-oriented electrical steel alloyed with 0.1% Sn. b) a SAM image Sn-MNN transition recorded on a same area, c) table shows a relation between grain orientation and Sn PHR
Slika 2: a) SEM posnetek površine neorientirane elektro pločevine legirane z 0.1% Sn, b) SAM posnetek Sn MNN prehoda posnet na istem mestu, c) tabela podaja zvezo med orientacijami zrn in RVV
T (°C)
Figure 3: Temperature dependence of surface segregation of C, Si, P, S and Sn of electrical steels alloyed vvith 0.05% Sn a) (100) oriented grain and b) (111) oriented grain
Slika 3: Temperaturna odvisnost površinske segregacije C, Si, P, S in Sn za elektro pločevino legirano s 0.05% Sn a) zrno (100) orientacije in b) zrno (111) orientacije
efficient was rather lovv, but their segregation enthalpy vvas very high, so tin started segregating significantly above 600°C. The kinetics study confirmed the orientation dependence of tin surface segregation as well as thickness of segregated layer.
It was ascertained" that on (100) and (111) faces, the segregation of tin was beyond one monolayer, due to the strong decrease of surface energy. On a surface with a (111) orientation FeSn intermetallic compound of one unit celi thickness was found. Our measurements showed that tin surface coverage dependence on tin bulk concentration and © value approached one for (100) and (111) orientation.
3.3 Texture measurements
The textures of 0.5 mm thick electrical steels were measured on the surface and in the middle plane after the half of the sheet thickness were removed. Taking into ac-count that approximately six crystal grains constitute the 0.5 mm thick cross-section steel sheet and the fact that penetration depths of x-rays vvere less than 0.1 mm one might conclude that there vvere analysed some grains vvhose grovvth vvas not affected by the surface segregated tin. Nevertheless, there vvere not more than 10% of such grains.
{001} <iio>
{110} {111} <iTo> <i?i>
{111} {001} <oil> <100>
{011} o oo>
{001} <1 to>
{110} {111} {m} {001}	{011}
<i7o> <i7i> <0"ii> <ioo>	ooo>
f (g)
t s
h x 0V.Sn	a-fibre
- O 0,0 5 7. Sn
0°	30"	60*	90
Figure 4: Fibre diagram of recrystallized texture for electrical steels measured a) in the middle plane and b) on the surface Slika 4: Diagram vlaken rekristalizacijske teksture za elektro pločevine merjen a) v sredini in b) na površini
The orientation distribution functions (ODF) f (g) vvere calculated from the (200), (110) and (211) pole figures. The textures vvere presented as a, y and r| fibres. Figure 4 shovvs texture fibres in the middle plane (a) and on the surface (b) of electrical steels alloyed vvith and vvithout tin. The volume fraction of grains vvith the (100) planeš measured on the surface and in the middle plane increased to the order of tvvo compared the steel vvithout ■ tin vvith the steel alloyed vvith 0.05% tin. Less hard magnetic orientations vvere found on the surface. Texture development during the recrystallization vvas. Steel alloyed vvith 0.05% Sn, vvhich had previously been aged 25 hours at 550°C, compared to the steel vvithout tin, shovved an increase of (100) planeš parallel to the rolling direction to the order of three.
4	Conclusions
Grain boundary and surface segregation of tin in non-oriented electrical steels vvere determined. Maximum equilibrium segregation on the surface vvere reached at 750°C and approached for majority of orientations one monolayer. One iron atom on the surface corresponds to one segregating tin atom. It vvas proved that thickness of tin segregating layer depended of tin bulk concentration. Tin segregation vvas controlled by bulk diffusion; thus, the equilibrium enrichment of tin on the surface vvas slightly faster for a specimen vvith higher tin contents. The tendency for tin surface segregation vvas much higher compared to grain boundary segregation. At equi-librium grain boundary segregation only 7 and 3% of tin atoms vvere found on a grain boundary for steel alloyed vvith 0.1 and 0.05% Sn, respectively.
Different crystallographic orientations can provide different sites for segregating tin atoms. During the re-crystallization tin atoms segregated on the surface and also at the grain boundary and so decreased the surface energy of crystal grains selectively.
The obtained results confirmed our supposition. Tin segregation took plače during the recrystallization and decreased the surface energy of crystal grains vvith (100) and (110) plains parallel to the sheet surface. Textures represented as sections through three-dimensional orientation distribution space in fixed directions shovved that volume fraction of magnetically soft grains increased for tvvo times compared to steel vvithout tin. Slightly better textures vvere obtained near the surface than in the middle plane of 0.5 mm thick steel sheet. The best results vvere obtained for steel alloyed vvith 0.05% Sn. We sup-pose that only a certain level of segregation promotes de-sired selective grain grovvth.
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