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 MetallurgyCentro Nacional de Investigaciones Metalúrgicas (CENIM)

Consejo Superior de Investigaciones Científicas (CSIC)Avda. Gregorio del Amo, 8

E-28040 Madrid, Spain

www.cenim.csic.es

What is a electrical steel?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)

Processing route (thickness 0.7 mm)

Ar3

FRT

760 ºC

50 ºC/s

CT 800 – 950 ºC

Cold rolling 40 – 80 %

10 ºC/s

700 – 800 ºC

Solid-Solid Phase Transformation Group (MATERALIA) – CENIM (CSIC)

30th of May, PTM 2005 – Phoenix (USA)

Main aim of the work: Improve texture of the final product by controlling hot rolling

FRT

CT 800 – 950 ºC

Electrical steelsElectrical steels

Electrical steelsElectrical steels

Coarse grains obtained during coiling

Processing route to be study (thickness 0.7 mm)

Nuleation at grain boundaries decreases after cold rolling + annealing (high energy nucelation)

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)

Table I. Silicon levels and finishes rolling temperatures.

Si, wt.- %Finish Rolling Temperature

(FRT), ºC

Entry Exit

ES1B 0.3 899 850

ES1C 0.3 839 777

Electrical steelsElectrical steels

Solid-Solid Phase Transformation Group (MATERALIA) – CENIM (CSIC)

30th of May, PTM 2005 – Phoenix (USA)

Electrical steelsElectrical steels

20 m

Starting microstructure before coiling

FRT=840 ºC

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)

0

20

40

60

80

100

120

140

160

750 800 850 900 950T (ºC)

Gra

in S

ize

(mm

)

ES1C

ES1B

Electrical steelsElectrical steels

Recrystallisation during coiling

High FRT induce coarser ReX grains after coilingReX grain size decreases as CT is increased

Solid-Solid Phase Transformation Group (MATERALIA) – CENIM (CSIC)

30th of May, PTM 2005 – Phoenix (USA)

Electrical steelsElectrical steels

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.

AlN

ES1-BFRT=900 ºCAnnealed at 850 ºCt=1.5 h

Solid-Solid Phase Transformation Group (MATERALIA) – CENIM (CSIC)

30th of May, PTM 2005 – Phoenix (USA)

Electrical steelsElectrical steels

Recrystalisation Mechanism:

High FRT Continuous ReXLow FRT Nucleation + Growth ReX

Alfa (green) and gamma (red) fibres

Grain Boundaries 2º 5º 10º 15º

Solid-Solid Phase Transformation Group (MATERALIA) – CENIM (CSIC)

30th of May, PTM 2005 – Phoenix (USA)

Low angle boundaries reveal the existence of subgrains

Electrical steelsElectrical steels

t=10 st=100 s

t=3000 s

ES1B(high FRT)

ES1C(low FRT)

CT=850 ºC

Solid-Solid Phase Transformation Group (MATERALIA) – CENIM (CSIC)

30th of May, PTM 2005 – Phoenix (USA)

Evolution of ReX and AlN precipitation

At low FRT, the material ReX first, and then AlN precipitation takes placeAt high FRT, ReX and AlN precipitation takes place simultenously

Electrical steelsElectrical steels

Solid-Solid Phase Transformation Group (MATERALIA) – CENIM (CSIC)

30th of May, PTM 2005 – Phoenix (USA)

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)