MICROSTRUCTURE CHARACTERIZATION OF GOES AFTER HOT ROLLING AND COLD ROLLING + DECARBURIZATION ANNEALING

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1 MICROSTRUCTURE CHARACTERIZATION OF GOES AFTER HOT ROLLING AND COLD ROLLING + DECARBURIZATION ANNEALING Vlastimil VODÁREK 1, Jan HOLEŠINSKÝ 1, Anastasia MASLOVA 1, František FILUŠ 1, Šárka MIKLUŠOVÁ 2, Ondřej ŽÁČEK 2 1 VŠB TU Ostrava, 17. listopadu 15/2172, Ostrava - Poruba, Czech Republic, vlastimil.vodarek@vsb.cz 2 ArcelorMittal Frýdek-Místek, Křižíkova 1377, Frýdek-Místek, Czech Republic Abstract The sharpness of the final Goss texture in GOES is significantly affected by structural inheritance during the manufacturing process. This paper deals with results of detailed microstructural investigations on strips of GOES after the first two steps of the production process: hot rolling and cold rolling + decarburization annealing. Microstructural characterization of samples was carried out using light metallography, scanning electron microscopy combined with electron backscattered diffraction (EBSD) and transmission electron microscopy. The attention has been paid to texture evaluation and minor phase evolution. In the hot rolled state the ferrite grain size reached several tens of micrometres, some grains with the orientation close to the Goss orientation were scattered across the thickness of the strips. Cold rolling + decarburization annealing resulted in a significant refinement of ferritic grains across the strip thickness. EBSD results manifested the effect of structural inheritance during cold rolling. Intensive precipitation processes took place during decarburization annealing. Most precipitates were nitrides (mainly AlN). A small addition of copper in the GOES investigated affected stability and distribution of sulphides during processing of strips. The typical size of AlN particles was several tens of nanometres. Keywords: GOES, precipitation processes, texture, EBSD 1. INTRODUCTION Magnetic properties (easy magnetisation, low hysteresis loss and low eddy current loss) of grain oriented electrical steel (GOES) depend strongly on the sharpness of the Goss texture ( ). It is agreed that the perfection of the final texture is significantly affected by structural inheritance during a complex processing procedure of GOES 1. It has been proved that the desired orientation first appears during the initial hot rolling as a friction induced shear texture at and near the surface. It is believed that Goss orientated grains are not completely lost during cold rolling they can partly survive in transition bands separating the texture components formed during cold rolling 1,2. The necessary conditions for the growth of grains with the Goss orientation are provided by microstructural control. Particles of inhibition phases (MnS or AlN, depending on a processing route) keep the ferrite grain size small during the early stages of the final high temperature annealing. Coarsening and dissolution of these particles makes the secondary recrystallization possible and the desired Goss-orientated grains grow and dominate the microstructure. Furthermore, abnormal grain growth is also promoted by the presence of a sharp texture 1. It has been suggested that the strong texture develops from near-surface recrystallized grains and although the final high temperature annealing is a critical factor in the processing, there is still no generally accepted explanation of the scientific basis of this phenomenon. Factors that are considered to be important include the size of the initial Goss grains, their orientation with respect to the other grains and the role of particles of inhibition phases during processing 3. Experimental studies are complicated by the fact that one grain in the final microstructure consumed about a million initial grains during its growth 4.

2 This paper deals with results of detailed microstructural investigations on three strips of GOES after the first two steps of the production process: hot rolling and cold rolling + decarburization annealing. 2. EXPERIMENTAL MATERIALS AND PROCEDURES Chemical compositions of three hot rolled coils, manufactured by different steelmaking and hot rolling procedures, are shown in Table 1. All coils correspond to an AlN + Cu production variant, where the first cold rolling is followed by the decarburization annealing, then the second cold rolling is applied and finally the high temperature annealing creates the desired Goss texture 3. Table 1 Chemical compositions of hot rolled coils, wt.% Sample C Mn Si S Cu Altot. N2 Ti+V+Nb A B C The thickness of hot rolled strips A, B and C was 2.30, 2.50 and 2.50 mm, respectively. After pickling cold rolling to mid-thickness of mm was applied and it was followed by decarburization annealing at temperatures of C in the atmosphere containing %H2. Carbon contents of strips after decarburization annealing were about wt.%. Samples for microstructural investigations were taken from similar positions in strips. Cross sections parallel to the rolling direction (RD) and sections parallel to the strip surface (10 and 50% under the surface) were studied. Microstructural characterization of samples was carried out using light metallography, scanning electron microscopy combined with electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). The attention has been paid to texture evaluation and minor phase evolution across the thickness of strips. The software OIM DC and OIM Analysis (both EDAX/TSL, version 6.1.3) were used for indexing of Kikuchi diffraction patterns and for the evaluation of the orientation data. Grain size was evaluated using EBSD data. Precipitation processes were studied on carbon extraction replicas in TEM. Electron diffraction and EDX techniques were applied for identification of minor phases. 3. RESULTS 3.1 EBSD studies Hot rolling of GOES slabs is carried out in two phase + region. Trans-crystallization processes affect the deformation texture and redistribution of solutes between coexisting phases results in heterogeneity of precipitation processes during next steps of GOES processing. Fig. 1a shows that microstructure after hot rolling was mostly recrystallized, non-recrystallized grains (misorientation inside grains greater than 2 ) were found mainly in the middle of the strip thickness. A map of IPF in Fig. 1b shows that microstructure consists of ferritic grains which are usually slightly elongated in the rolling direction. Fig. 1c shows grains exhibiting a deviation from the Goss orientation less than 15. A small number of these grains was scattered across the whole thickness of strips investigated. Thin bands of ferritic grains in hot rolled strips probably correspond to areas which were austenitic during hot rolling. Results of grain size evaluation across the thickness (upper, middle and bottom positions) of strips are summarised in Table 2. In all hot rolled specimens the average grain size in the middle thickness was larger than that under upper and bottom surfaces. Differences in the grain size among strips reached up to 30%.

3 Table 2 Results of the grain size evaluation using EBSD data, µm Sample Hot rolled Cold rolled + Decarburized upper middle bottom upper middle bottom A B C Note: Average diameter standard deviation a. b. c. d. Fig. 1 A section through the strip thickness parallel to the rolling direction, sample A1, a. recrystallized and deformed (red) fractions of microstructure, b. IPF map for RD, c. grains close to the Goss orientation + high angle boundaries ( 15 ) in image quality (IQ) map, d. the legend for Fig. 1c

4 , Brno, Czech Republic, EU Figs. 2a and 2b show the 100 pole figures for the hot rolled state (sample A1) and for the cold rolling + decarburization annealing (sample A2), respectively. It is known that the rolling texture of low carbon steels is largely independent of composition and processing variables and even such major microstructural inhomogeneities as shear bands have a little effect 1. Deformation textures in Figs. 2a and 2b are similar. It manifests the effect of structural inheritance during cold rolling of GOES. Fig. 3a shows IPF map for the RD in sample A2 after cold rolling + decarburization annealing. Grains are very fine, differences in the grain size under both surfaces and in the middle of the thickness are small. Results of the grain size evaluation in samples after cold rolling + decarburization annealing are summarised in Table 2. All samples exhibit very similar grain size across the thickness. Also differences among the strips investigated are negligible, Table 2. a. b. Fig. 2 The 100 pole figures for: a. sample A1, hot rolled state, b. sample A2, cold rolled + decarburization annealed state a. b.

5 c. Fig. 3 A section through the strip thickness parallel to the rolling direction, sample A2, a. IPF map for RD, b. grains close to the Goss orientation + high angle boundaries ( 15 ) in IQ map, c. the legend for Fig. 3b Fig. 3b shows grains exhibiting a deviation from the Goss orientation less than 15. These grains are scattered across the whole thickness of the strip, some of them have a very small deviation from the exact Goss orientation, see Fig. 3c. 3.2 TEM investigations Interactions of minor phase particles with defects of crystal lattice affect recovery and recrystallisation processes during the production of GOES. In the case of the AlN + Cu variant, AlN is regarded as the most important inhibition phase 3. The role of a 0.5 wt.%cu addition to GOES has not been fully understood yet. The following mechanisms have been proposed: - precipitation of -Cu not conclusively proved yet, - dissolution and re-precipitation of copper sulphides or complex sulphides of manganese and copper, - segregation of copper at grain boundaries. Ferrite grain boundaries in hot rolled strips were decorated by fine films of iron-rich carbide, cementite, see Figs. 4a and 4b. Precipitation of cementite took place during cooling of coils. Inside ferritic grains a very low number density of particles was observed. The following minor phases were identified: TiX, where X can be N and/or C, sulphides of manganese and complex sulphides of manganese and copper, rod-like AlN particles. The size of these precipitates reached up to several hundreds of nanometres. Copper additions in strips investigated affected significantly stability of sulphides. Solution temperature of MnS is about 1300 C, but copper sulphides dissolve at temperatures close to 950 C 3. Solubility limits of complex sulphides of manganese and copper lie between the above temperatures. Based on solubility limits of AlN phase in Fe - 3 wt.%si steels at the soaking temperature all aluminium should be in solid solution 3. However, at the finish rolling temperature and during cooling of strips a part of aluminium could precipitate in the form of AlN particles. a. b. Fig. 4 Precipitation in sample A1, a. cementite particles at the grain boundary, b. EDX spectrum of cementite

6 Typical examples of precipitation in the strip A after cold rolling + decarburization annealing are shown in Figs. 5a and 5b. Grain boundaries are decorated by a network of coarser particles. Two nitrogen-bearing minor phases were identified along grain boundaries: Si3N4 and AlN. In GOES Si3N4 nitride represents a metastable phase which is in the temperature interval of ca C gradually replaced by AlN phase 3. A high number density of fine AlN particles was observed inside ferritic grains, see Fig. 5b. The typical size of intragranular AlN particles reached several tens of nanometres. Intragranular precipitation was inhomogeneous. This variability is probably the consequence of hot rolling in two phase region where some enrichment of austenite in austenite-forming elements (C, N, Al, Cu) takes place. Apart from nitrides, sulphide particles were detected in strips investigated after decarburization annealing. Based on the EDX results, sulphides can be classified as sulphides of manganese, complex sulphides of manganese and copper, sulphides of copper. The size of sulphides was in the interval from several tens to several hundreds of nanometres. Heterogeneous nucleation of AlN particles on the surface of sulphides was often observed. The lowest fraction of precipitates was found in the strip C. It can be related to the fact that its temperature of decarburization annealing was about 20 C lower than that of the strips A and B. a. b. Fig. 5 Precipitation reactions in sample A2, a. AlN and Si3N4 precipitates at grain boundaries and intensive intragranular precipitation of AlN, b. the detail of intragranular precipitation of AlN 4. CONCLUSIONS - Microstructure of hot rolled strips consisted of ferritic grains which were slightly elongated in the rolling direction. A small fraction of structure was not fully recrystallized. The average grain size in all strips reached several tens of micrometres. - Grain boundaries in hot rolled strips were decorated by thin films of cementite, which formed during cooling of coils. Inside ferritic grains a small number density of TiX, complex sulphides of manganese and copper and rod-like AlN particles was found. - Cold rolling + decarburization annealing resulted in a pronounced refinement of the ferrite grain size across the thickness of strips. After decarburization annealing the grain size was almost identical in all strips investigated. - During decarburization annealing an intensive precipitation of Si3N4 and AlN particles took place. Apart from nitrides, sulphide particles with a variable ratio of Mn and Cu precipitated from the ferritic matrix. Intensity of precipitation processes in strips investigated was affected by the temperature of decarburization annealing. The typical size of AlN particles, which formed most precipitates, reached several tens of nanometres. Such particles are very effective in inhibiting grain growth at the beginning of the final high temperature annealing of GOES.

7 ACKNOWLEDGEMENTS This paper was created in the projects FR-TI3-053 and CZ.1.05/2.1.00/ "Regional Materials Science and Technology Centre" within the frame of the operation programme "Research and Development for Innovations" financed by the Structural Funds and from the state budget of the Czech Republic. REFERENCES 1 HUMPHREYS F. J, HATHERLY M.: Recrystallization and Related Phenomena, Elsevier, Amsterdam DORNER D., ZAEFFERER, S., RAABE, D.: Acta Materialia, vol. 5, 2007, p LOBANOV, M.L.: Upravlenije strukturoj i teksturoj eletrotechničeskoj anisotropnoj stali, Abstract of Doctoral Thesis, Jekaterinburg 2010, pp. 48 (in Russian). 4 MORAWIEC, A.: Scripta Materialia, vol. 43, 2000, p. 275.