D
J. Chem. Chem. Eng. 8 (2014) 191-194
DAVID
PUBLISHING
Effect of Magnesia Blending on the Magnetic Properties
of Grain Oriented Silicon Steel
Carolina Cesconetto Silveira* and Sebastião da Costa Paolinelli
Research Center, Aperam South America, Minas Gerais 35180-018, Brazil
Received: October 29, 2013 / Accepted: November 14, 2013 / Published: February 25, 2014.
Abstract: Decarburized samples of grain oriented silicon steel were coated with alone and blended magnesias and submitted to
the high temperature annealing. The magnesias and their blendings were characterized using granulometry measurements, ignition
loss and reactivity tests. After high temperature annealing, forsterite film morphology, magnetic properties and Goss deviation
were also analyzed. Better magnetic properties and sharper Goss orientation were found in samples which had used blended
magnesias. These results are explained by the magnesias particle size distributions, forsterite film formation and rate of inhibitors
release.
Key words: Forsterite, glass film, grain oriented, magnesia, silicon steel.
1. Introduction
GO (grain oriented) silicon steel is mainly used in
transformers cores. This steel is made up of around 3%
(wt) of Si and it is produced in order to induce a strong
{110} <001> crystallographic texture, known as Goss
texture [1].
During the processing of GO steel, there is a process
production step called decarburization, where the
carbon amount is reduced in order to prevent magnetic
ageing [2]. This decarburization is conducted in a
controlled wet atmosphere of H2 and N2, and it is also
formed an oxidation layer mainly composed by fayalite
(Fe2SiO4) and silica (SiO2) [3]. Afterwards, the steel is
coated with magnesia (MgO) slurry and it is submitted
to the HTA (high temperature annealing). During this
process, secondary recrystallization occurs and the
reaction among fayalite, silica and magnesia turns in to
a continuous oxidation at the surface mainly
constituted of forsterite (Mg2SiO4), commonly called
glass film [4].
Some functions of the glass film are to electrically
*
Corresponding author: Carolina Cesconetto Silveira,
M.Sc., research fields: physical metallurgy and electrical steels.
E-mail: carolina.silveira@aperam.com.
insulate the steel in order to diminish eddy currents,
apply tensile stress to decrease hysteresis losses and
help purifying the steel during HTA [5].
In order to provide stabilization of the secondary
recrystallization process and also a stable formation of
forsterite, the choice of the right magnesia should take
into account its raw properties, such as CAA (citric
acid activity), viscosity, chemical composition,
granulometry, porosity and surface area [6-9].
Mainly, all the desirable properties are not present in
only one magnesia. Therefore, blending magnesias
with different properties is interesting in order to take
advantage of each one best characteristic, and
consequently, resulting on superior GO silicon steel
magnetic properties and surface quality.
The objective of this paper is to evaluate a GO steel
by submitting it to a coating of alone and blended
magnesias with different levels of CAA, granulometry
and chemical composition.
2. Experiments
The samples were in the dimension of 305 × 100 ×
0.27 mm3 obtained from a 3% (wt.) Si steel
decarburized coil. The preparation of the slurries
Effect of Magnesia Blending on the Magnetic Properties of Grain Oriented Silicon Steel
included 1,200 g of deionized cold water (lower than
5 °C), 200 g of magnesia plus additives. Two
magnesias were used in this work, which were applied
alone and blended (Table 1). The steel samples were
coated using rolls and dried by heating to an enough
temperature to evaporate the water. Afterwards, all the
samples were submitted to the same HTA at 1,200 °C
for 15 h under controlled atmosphere.
The granulometry of the magnesias and their
blendings were characterized using a laser diffraction
particle sizer, Marlvern, Master Sizer S. The ignition
loss (LOI), which is a measure of the degree of
hydration of MgO, was obtained by heating the sample
until 1,000 °C with soaking time of 1 h, by weighing
the sample before and after the heat treatment. CAA is
a measure of the magnesia hydration rate and was
measured by the necessary time for which a given
weight of magnesia provides sufficient hydroxyl ions
to neutralize a given weight of citric acid.
Core loss at 60 Hz and B8 (magnetic induction at
800 A/m) were measured using MPG 100D Brockhaus
equipment applying the single sheet method (280 × 30
mm). Goss deviation of the crystal direction for <001>
was measured by EBSD (electron backscatter
diffraction).
The samples with glass film were characterized
regarding adherence, by bending at 180o with
calibrated diameter cylinders, and morphology by
measuring the thickness and roughness at an image
analyzer (IA3001).
3. Results and Discussion
activity is too high (low CAA), the glass film is not
totally formed at the steel surface whereas magnesia
with low activity presents thin glass film with poor
adherence [9].
Granulometry results (Fig. 1) for slurry 1, which
corresponds to magnesia 1, show monomodal particle
size distribution and D50 of 3.78 μm. Slurry 2, the
same as magnesia 2, presents a bimodal distribution
with 3.81 μm of D50. The slurries 3 and 4 are the
blending of magnesias and their granulometries
behaved in the same way of their proportion. It can be
noticed that mixing magnesias resulted in lower D50
for slurries 2 and 3, with 3.30 μm and 3.64 μm,
respectively.
Table 1 Parts per weight of the prepared slurries.
Slurry 1
Magnesia 1 1
Magnesia 2 0
Table 2
Slurry 2
Slurry 3
Slurry 4
0
1
2
1
1
2
Citric acid activity and loss on ignition results.
Magnesia 1
Magnesia 2
CAA(s)
LOI(%)
50
130
1.61
1.25
Slurry 1
Distribution
frequency(a.(a.u.)
distribution frequency
u.)
192
Slurry 2
Slurry 3
Slurry 4
3.1 Magnesia and Forsterite Film Characterization
Table 2 shows the results of CAA and LOI for both
magnesias. Ignition loss values are very similar, which
indicates that there are no differences on degree of the
magnesias hydration. The CAA results show that
magnesia 1 is more reactive than magnesia 2; this
indicates that magnesia 1 hydrates faster than magnesia
2. Usually the rate of hydration has greater significance
than the eventual degree of hydration [6, 10]. When the
0,01
0.01
0,1
0.1
10
11
10
Diameter
Diameter ((µm)
m)
100
100
1000
1000
Fig. 1 Particle size distribution.
Table 3
Slurry 1
Slurry 2
Slurry 3
Slurry 4
Glass film characterization.
Thickness average (μm)
1.11 ± 0.41
1.11 ± 0.38
1.14 ± 0.41
1.10 ± 0.45
Relative roughness
1.174 ± 0.029
1.183 ± 0.056
1.193 ± 0.043
1.211 ± 0.056
Effect of Magnesia Blending on the Magnetic Properties of Grain Oriented Silicon Steel
4. Conclusions
Blending magnesias can help on the improvement of
magnetic properties. These results are related with the
Goss deviation, i.e.; the result of the {110}<001>
misorientation direction. Better magnetic properties
were found on samples with lower degree of Goss
deviation.
Lower degree of Goss deviation may be obtained by
1888
1.424
1.42
1885
1.40
1881
1.38
1.371
1880
1.374
1.36
1876
1875
1873
1.349
1.34
1870
Core loss 1.7 T/60 Hz (W/kg)
Fig. 2 shows the core loss and magnetic induction
for each slurry used in this work. It is observed that
magnetic induction is higher and core loss is lower
when blendings of magnesia are applied. In order to
understand this effect, Goss deviation, the result of the
{110} <001> misorientation direction, was evaluated
and shows good correlation with magnetic results (Fig. 3),
i.e., the lower is the Goss deviation to the <001>
direction, better are the magnetic properties.
One hypothesis is related with the drop rate of the
grain
growth
inhibitor
during
secondary
recrystallization. It is known that inhibitors low drop
rate prevents grain growth, extending the incubation
time before the secondary recrystallization and
promotes the selective grain growth with Goss
orientation [11, 12]. When blending the magnesias, the
distribution particle size changes and probably this
leaded to a more densification of the magnesia which
can form forsterite at lower temperatures. An earlier
forsterite formed film interferes on the secondary
recrystallization by acting as a barrier for the inhibitors
release, and consequently extends the inhibitor
incubation time before the secondary recrystallization
and promotes the selective grain growth with Goss
orientation.
Magnetic induction 800 A/m 60 Hz (mT)
3.2 Magnetic Properties and Goss Deviation
1890
1.32
Slurry 1
Slurry 2
Slurry 3
Slurry 4
Fig. 2 Magnetic induction and core loss.
Magnetic
induction
60Hz
(mT)
Magnetic
induction
800 800A/m
A/m
60A/m
Hz (mT)
Magnetic
induction
800
60 Hz (mT)
Table 3 presents the results of thickness and roughness
of all slurries. As can be noticed, there is no difference
among all conditions, concerning alone and blended
magnesias. Regarding adherence, it was not found any
difference among all slurries.
193
1892
1888
1884
1880
1876
1872
6.0
6.5
7.0
7.5
8.0
8.5
9.0
Goss Deviation (?o
Goss
deviation ( )
Fig. 3 Relation between magnetic induction and Goss
deviation.
extending the time of inhibitors release before secondary
recrystallization. This can be reached by lowering the
glass film temperature formation, which may be due to
the particle size distribution of blended magnesias.
At the same time, forsterite morphology and
adherence did not show any difference concerning
individual and blended samples.
Therefore, blending magnesias can help on the
improvement of the Goss texture and consequently, on
superior magnetic properties of the GO steel, while
glass film properties are preserved.
References
[1]
[2]
[3]
Xia, Z.; Kang, Y.; Wang, Q. J. Magn. Magn. Mater. 2008,
320, 3229-3233.
Eloot, K.; Dilewinjins, J.; Standert, C.; Cooman, B. C. J.
Magn. Magn. Mater. 1994, 133, 223-225.
Yamazaki, T. Transactions ISIJ. 1969, 9, 66-75.
194
[4]
[5]
[6]
[7]
[8]
Effect of Magnesia Blending on the Magnetic Properties of Grain Oriented Silicon Steel
Cunha, M. A.; Cesar, M. G. M. M. IEEE Transactions on
Magnetics 1994, 30(6), 4890.
Steger, J. F. In Morton-Norwich Products, Proceedings of
Magnesium Oxide Steel Coating Composition and Process,
1973.
Cesar, M. G. M. M. D.; Vasconcelos, C. L.; Vasconcelos,
W. L. Journal of Materials Science 2002, 37,
2323-2329.
Sopp, S. W. In Calgon Corporation USA, Proceedings of
Magnesium Oxide Composition for Coating Silicon Steel,
1981.
Yakashiro, K. In Nippon Steel Corporation, Proceedings
of Process for Producing Grain-Oriented Electrical Steel
Sheet Having Excellent Glass Film and Magnetic
Properties, Nov. 16, 1995-Nov. 24, 1998.
Ichida, T. In Kawasaki Steel Corporation, Proceedings of
Method for Forming a Forsterite Insulating Film on the
Surface of a Grain-Oriented Silicon Steel Sheet, Oct. 19,
1979-Feb. 10, 1981.
[10] Haselkorn, M. H. In Armco Inc, Proceedings of Process
of Producing an Electrically Insulative Film, Jul. 17,
1978-Sept. 18, 1979.
[11] Ushigami, Y. Journal of Magnetism and Magnetic
Materials 2003, 254-255, 307-314.
[12] Silveira, C. C. Influence of the Quantity and Morphology
of Fayalite and Silica on the Grain Oriented Electrical
Steel Characteristics. Master Thesis, Federal University
of Minas Gerais, 2010.
[9]