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MICROSTRUCTURE CHARACTERIZATION OF GOES AFTER HOT ROLLING AND COLD
ROLLING + DECARBURIZATION ANNEALING
Vlastimil VODÁREK1, Jan HOLEŠINSKÝ1, Anastasia MASLOVA1, František FILUŠ1, Šárka
MIKLUŠOVÁ2, Ondřej ŽÁČEK2
1 VŠB – TU Ostrava, 17. listopadu 15/2172, 708 33 Ostrava - Poruba, Czech Republic,
2 ArcelorMittal Frýdek-Místek, Křižíkova 1377, 738 01 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 (110001). 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 110 001 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 112110 + 111110 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 110001 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.
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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 0.033 0.24 3.21 0.006 0.50 0.014 0.010 0.005
B 0.03 0.25 3.16 0.004 0.50 0.014 0.009 0.008
C 0.033 0.25 3.11 0.006 0.49 0.015 0.013 0.005
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 0.6 – 0.65 mm was applied and it was followed by decarburization annealing at
temperatures of 820 – 840 °C in the atmosphere containing 15 – 20 %H2. Carbon contents of strips after
decarburization annealing were about 0.003 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%.
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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 36.224.8 41.032.8 32.426.2 9.44.6 10.55.5 9.54.4
B 29.719.1 35.922.4 26.518.7 9.75.2 11.57.8 9.44.5
C 24.714.0 32.729.7 24.413.9 11.86.2 12.33.1 11.15.5
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
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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.
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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
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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 700 – 900 °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.
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ACKNOWLEDGEMENTS
This paper was created in the projects FR-TI3-053 and CZ.1.05/2.1.00/01.0040 "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
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2 DORNER D., ZAEFFERER, S., RAABE, D.: Acta Materialia, vol. 5, 2007, p. 2519.
3 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.