1. INFLUENCE OF FINISHING ROLLING TEMPERATURE ON RECRYSTALLIZATION OF ELECTRICAL STEELS
C. Capdevila, J.P. Ferrer and C. García de Andrés
Solid-Solid Phase Transformations Group (MATERALIA)
Department of Physical Metallurgy
Centro Nacional de Investigaciones Metalúrgicas (CENIM)
Consejo Superior de Investigaciones Científicas (CSIC)
Avda. Gregorio del Amo, 8
E Madrid, Spain

2. What is a electrical steel?
Electrical steels (Fe-Si) belong to the group of soft magnetic materials, which are generally used to increase and to conduct magnetic flux inside electromagnetic components.
There are two basic types of electrical steels: non grain- and grain-oriented electrical steels.
The grain-oriented electrical steel (Goss texture) present a high anisotropy of the magnetic properties. For that reason the grain-oriented electrical steels are basically used in magnetic cores of transformers, where the magnetic flux is mainly following one direction.
The non grain-oriented electrical steels (Cube texture) may be used in any application with varying direction of the magnetic flux like motors, generators, etc.
In this work we are dealing with non grain-oriented electrical steel
Solid-Solid Phase Transformation Group (MATERALIA) – CENIM (CSIC)
30th of May, PTM 2005 – Phoenix (USA)

3. Processing route (thickness  0.7 mm)
Electrical steels
Main aim of the work: Improve texture of the final product by controlling hot rolling
Processing route (thickness  0.7 mm)
CT  800 – 950 ºC
CT  800 – 950 ºC
700 – 800 ºC
Ar3
10 ºC/s
FRT
FRT
50 ºC/s
760 ºC
Cold rolling  40 – 80 %
Solid-Solid Phase Transformation Group (MATERALIA) – CENIM (CSIC)
30th of May, PTM 2005 – Phoenix (USA)

4. Processing route to be study (thickness  0.7 mm)
Electrical steels
Processing route to be study (thickness  0.7 mm)
Nuleation at grain boundaries decreases after cold rolling + annealing (high energy nucelation)
Coarse grains obtained during coiling
Formation of deformation bands during cold rolling is enhanced
Nucleation sites for desirable grains (low energy nucleation)
ReX grains with {111} texture
ReX grains with {001} texture
Solid-Solid Phase Transformation Group (MATERALIA) – CENIM (CSIC)
30th of May, PTM 2005 – Phoenix (USA)

5. Table I. Silicon levels and finishes rolling temperatures.
Electrical steels
Table I. Silicon levels and finishes rolling temperatures.
Si, wt.- %
Finish Rolling Temperature (FRT), ºC
Entry
Exit
ES1B
0.3
899
850
ES1C
839
777
Solid-Solid Phase Transformation Group (MATERALIA) – CENIM (CSIC)
30th of May, PTM 2005 – Phoenix (USA)

6. Starting microstructure before coiling
Electrical steels
Starting microstructure before coiling
FRT=840 ºC
20 mm
FRT=900 ºC
Deformation texture is more intense in material with low FRT
Solid-Solid Phase Transformation Group (MATERALIA) – CENIM (CSIC)
30th of May, PTM 2005 – Phoenix (USA)

7. Recrystallisation during coiling
Electrical steels
Recrystallisation during coiling
High FRT induce coarser ReX grains after coiling
ReX grain size decreases as CT is increased
Solid-Solid Phase Transformation Group (MATERALIA) – CENIM (CSIC)
30th of May, PTM 2005 – Phoenix (USA)

8. Recrystallisation during coiling: why these differences in grain size?
Electrical steels
AlN
Recrystallisation during coiling: why these differences in grain size?
The more likely nucleation mechanims is rotation of subgrains.
AlN particles pin subgrain boundaries difficulting nucleation of ReX.
ES1-B
FRT=900 ºC
Annealed at 850 ºC
t=1.5 h
Solid-Solid Phase Transformation Group (MATERALIA) – CENIM (CSIC)
30th of May, PTM 2005 – Phoenix (USA)

9. Recrystalisation Mechanism:
Electrical steels
Recrystalisation Mechanism:
Low FRT  Nucleation + Growth ReX
High FRT  Continuous ReX
Alfa (green) and gamma (red) fibres
Grain Boundaries 2º 5º 10º 15º
Low angle boundaries reveal the existence of subgrains
Solid-Solid Phase Transformation Group (MATERALIA) – CENIM (CSIC)
30th of May, PTM 2005 – Phoenix (USA)

10. 30th of May, PTM 2005 – Phoenix (USA)
Electrical steels
ES1B
(high FRT)
t=10 s
CT=850 ºC
t=100 s
t=3000 s
ES1C
(low FRT)
Solid-Solid Phase Transformation Group (MATERALIA) – CENIM (CSIC)
30th of May, PTM 2005 – Phoenix (USA)

11. At high FRT, ReX and AlN precipitation takes place simultenously
Electrical steels
Evolution of ReX and AlN precipitation
At low FRT, the material ReX first, and then AlN precipitation takes place
At high FRT, ReX and AlN precipitation takes place simultenously
Solid-Solid Phase Transformation Group (MATERALIA) – CENIM (CSIC)
30th of May, PTM 2005 – Phoenix (USA)

12. Conclusions
Because of the interaction between recovery and AlN precipitation on subgrain boundaries, material with high FRT recrystallizes by continuous recrystallization. The final microstructure presents a large grain size. However, in material with low FRT, continuous recrystallization is replaced by ‘discontinuous recrystallization’. The final grain size is lower in this case.
Solid-Solid Phase Transformation Group (MATERALIA) – CENIM (CSIC)
30th of May, PTM 2005 – Phoenix (USA)