MICROSTRUCTURE CHARACTERIZATION OF GOES...

15. - 17. 5. 2013, Brno, Czech Republic, EU

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,

[email protected]

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%.


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


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.


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


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.


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

1 HUMPHREYS F. J, HATHERLY M.: Recrystallization and Related Phenomena, Elsevier, Amsterdam 2004.

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.

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