J. Barros, Advances in the workability of high silicon electrical steel
Advances in the workability of high silicon electrical steel
J. Barros1, P.R. Calvillo1, T. Ros -Yáñez1, R. Colás1, 2 and Y. Houbaert1
1
2
Ghent University, Department of Metallurgy and Materials Science, Technologiepark 903
B-9052 Gent, Belgium
Universidad Autónoma de Nuevo León, Facultad Ingeniería Mecánica y Eléctrica, AP 149-F
San Nicolás de los Garza 66451, N.L., Mexico
Keywords: workability, high silicon steel, electrical steel, power losses
Abstract. Hot torsion and compression tests at room temperature on four Fe-Si alloys (3.3 to 5.4 Si wt%)
supplied information about the parameters influencing the rolling procedure: Si content, reheating
temperature, number of passes in hot and cold rolling, reduction in each pass, finishing temperature after
hot rolling, cooling rate after hot rolling and time delay prior to cold rolling. The results were verified by
rolling experiments. Finishing temperature of hot rolling not lower than 950°C, a fast cooling rate after
hot rolling and a short time delay prior to cold rolling are the clues for succeeding in the
thermomechanical processing of high silicon steel. Cold rolling without any cracking was possible only
for samples with fast cooling after hot rolling. For these same samples magnetic properties were
measured.
Introduction
The importance of Fe-Si alloys in the electrical industry arises from their excellent electrical and
magnetic properties which are optimized when the Si content reaches 6.5wt%. When the Si is increased,
the electrical resistance is also increased, the eddy current loss is reduced and the magnetostriction
becomes almost zero. However, by increasing the Si concentration the alloy becomes brittle and therefore
difficult to be cold rolled into thin sheet. The limited ductility is mainly due to ordering phenomena
involving two ordered phases known as B2 (FeSi) and D03 (Fe3Si) [1]. For this reason mass production of
Si steel over 3.5 wt% is not common by hot-cold roll traditional procedures and several methods have
been developed to obtain high Si steel without cold rolling, among them CVD (already a real industrial
production in Japan [2]) and hot dipping and diffusion annealing process, a promising alternative under
development [3].
It is known that the workability of high silicon steel can be improved by working in non-equilibrium
conditions, which means to avoid the ordering [4]. Based on this fact rolling procedures can be defined
and optimized allowing the production of high silicon steel by selection of the suitable thermomechanical
parameters. The most important parameters influencing the properties during processing seem to be:
reheating temperature or temperature before hot rolling, number of passes in hot and cold rolling,
reduction in each pass, finishing temperature after hot rolling, cooling rate after hot rolling and time delay
prior to cold rolling. [5].
The mechanisms responsible for the problems during the rolling of high Si-steel are further investigated
in this work: a more fundamental approach is used to investigate and understand the deformation
behaviour of high silicon steel. Hot torsion and compression experiments are used to determine the flow
stresses of the material during deformation and the effect of time delay (this is an “aging” phenomenon) is
quantified and linked to ordering in the alloys. Finally, rolling experiments are described and their results
given. The deformation stresses during hot rolling were measured and compared with the results obtained
using hot torsion and finally magnetic properties of the obtained materials are given.
J. Barros, Advances in the workability of high silicon electrical steel
Experimental procedure
Four Fe-Si alloys with different Si content were produced on a laboratory scale in order to investigate the
effect of increasing the Si over 3% on processing and properties. The composition range for each alloy is
shown in Table 1.
Table 1. Chemical composition of the produced alloys
Alloy
C wt%
Al wt%
Si wt%
Si at%
A
0.003
0.004
3.30
6.39
B
0.009
0.003
4.20
8.05
C
0.002
0.006
4.97
9.42
D
0.006
0.002
5.40
10.25
Hot torsion experiments were performed on alloys B and D in order to study the effect of the silicon
content on the MFS during hot rolling and subsequently the ordering phenomena related to this type of
alloys. The obtained results were used to design the adequate hot rolling schemes for these alloys.
Cylindrical specimens, with diameters and gauge lengths of 10 and 100 mm and 6 and 25 mm for the
deformed zones, were machined from the casting ingots in the rolling direction. The samples were
reheated at 1150°C at 4°C/sec and soaking during 5 minutes at that temperature, lower temperatures than
1200°C were used in order to prevent a high degree of oxidation [5]. After the first deformation the
specimens were cooled down at constant cooling rate given by the difference of temperatures between
deformations divided by the interpass time. Difference of temperatures and interpass time were varied
from 20 to 40°C and 5 to 20 seconds respectively and low strain rate of 0.1 s-1 was selected for the tests.
Simple axisymetric compression tests at constant true strain rate were carried out in a servo-hydraulic
computer driven machine on the four alloys produced. A polytetrafluorethylene (PTFE) tape of 0.1mm
thickness was used as lubricant for all the test specimens in order to minimize frictional effects at
specimen-tooling interface. Raw data, in terms of the load and displacement were acquired and corrected
before computing the true stress strain curves. Different annealing treatments (expected to produce some
degree of ordering) at temperatures of 700, 800, 900 and 1000°C for 15 min followed by cooling in
different environments: water, air and in the furnace were performed on the four alloys. Because the
workability of high Si-alloys is believed to be influenced by ordering reactions, the annealing
temperatures were chosen higher and lower than the critical ordering temperatures for each alloy [1, 6].
The different cooling rates should highlight the possible influence of the cooling rate on the
transformation behaviour [5, 6]. The compression tests were performed after the annealing treatments
with different amounts of delay: immediately after annealing or after 2, 6 or 24 hours.
The rolling experiments are centred on establishing a hot and cold rolling procedure for the experimental
alloys, which permits the processing avoiding the brittleness problems. A wide range of
thermomechanical combinations, following the results obtained by the hot torsion and compression tests,
were attempted where temperatures, number of passes during hot rolling, percent of reduction and cooling
after hot rolling were the variable parameters. All the processing steps were physically simulated in the
laboratory using a reversible two-high laboratory mill with a maximum rolling force of 3000kN and roll
diameter of 320 and 345mm for hot and cold rolling respectively (typical sheet width is 250mm).
Samples were reheated for 1h in the temperature range of 1150°C-1250°C. It appeared that reheating
temperatures of 1150°C offer better initial conditions for the hot rolling because of the extremely high
degree of oxidation at 1250°C. The hot rolling experiments were carried out in 2, 4, 6 and 12 passes with
reduction of 15, 30, 50 and 80% in each pass. The influence of increasing the Si content and the
deformation temperature on the Mean Flow Stress (MFS) during rolling was determined. Also the
finishing hot rolling temperature was recorded.
The influence of the cooling rate after hot rolling on the cold workability of these alloys was studied
using three cooling media: water, at the air and a coil-cooling simulation in a furnace from 800°C down to
room temperature. The cold rolling was performed in several passes in the deformed state without
additional treatment. A final annealing was done at 950°C during 2 hours in a wet HNx-atmosphere
(N2+5%H2).
Magnetic measurements were done on samples of the alloys B and D using a single strip tester. The
magnetic data were calculated using the density δ = 7.55g/cm3 for alloy B and δ = 7.50g/cm3 for the alloy
D and are comparable with those from Epstein samples.
J. Barros, Advances in the workability of high silicon electrical steel
Results and discussion
Hot torsion tests
The measured torque, T, and twist, θ, were converted to von Mises effective stress σ and strain ε using
the formulas given by [7]. Strains from 0.15 to 0.7 per step of deformation were kept constant. The
specimens were tested at least twice to ensure reproducibility of the results and once the calculations of σ
and ε were done it was possible to obtain the MFS to each pass from the flow using the same procedure
described by J.J. Jonas [8].
I
II
III
(a)
I
II
III
(b)
Figure 1. Curves of MFS vs.1000/T(K) obtained at strain of 0.15 for step of deformation and inter pass
time of 5 sec. (a) alloy B and (b) alloy D.
The MFS obtained for each step were plotted using curves of MFS vs. 1000/T (K) applying a linear
regression in order to fit it to a linear behaviour and Fig. 1 shows the results for alloys B and D. Two
changes of slope are clearly noticed, although only one change in the slope, corresponding to the nonrecrystallization temperature, is expected because these alloys are ferritic in the whole range of
temperatures. The two slope changes in the curve can be interpreted as two transition temperatures: the
order-disorder and the non-recrystallization transition temperatures. The curves can be divided in 3
regions:
• Region I of high-temperature deformation where full static and dynamic recovery and/or
recrystallization took place and there was not stress accumulation so the MFS remained almost
constant.
• Region II there was strain accumulation, at starting temperature around 1050°C, from pass to
pass, being the dynamic recrystallization the main softening mechanism during deformation, so
that the MFS increase with decreasing temperature.
• Region III the MFS increased more rapidly than in Region II and according to the equilibrium
diagram could be due to the appearance of an ordered structure, but also could be the Tnr related
by previous experiments done on the same alloys [5]. For a better understanding of this change
on the MFS additional experiments using calorimetry and dilatometry should be performed.
The experimental results indicate that the transition temperature between the regions II and III in the
diagram increases with the interpass time, while the transition temperature between the regions I and II
increases with the percent of silicon and decreases with the interpass time. For strains of 0.35 or higher it
was not possible to achieve the transition temperature between the regions II and III because of the
appearance of cracks.
Compression tests
The compression tests were carried out in order to study the deformation behavior at room temperature in
function of a previous thermal treatment: reheating temperature, cooling rate after reheating, and effect of
delay time between the end of the cooling down to room temperature and the beginning of the cold
deformation.
J. Barros, Advances in the workability of high silicon electrical steel
Reproducible stress-strain curves were obtained from the load-displacement data. Each steel exhibits a
different behavior for yield and strain-hardening as a function of the silicon content: as the Si content
increases strengthening is observed, the yield stress and the level of the stress-strain curves as a whole are
raised. Results are in good agreement and they are comparable with previous work performed on similar
alloys [6].
1400
1300
1300
Stress (MPa)
1500
1400
Stress (MPa)
1500
1200
1100
1000
1200
1100
1000
900
900
800
800
700
700
5.5
0
t (h
4.0
15
rs)
20
3.5
25
5.0
5
4.5
10
5.5
0
5.0
5
4.5
10
15
t (h
rs)
%
wt
Si
3.0
4.0
20
3.5
25
%
wt
Si
3.0
furnace
water
……… air
------- --
(a)
(b)
Figure 2. Stress at a strain of 0.6, in function of the Si content and the delay after annealing for samples
with different cooling rates (annealing temperatures: (a) 700°C, (b) 900°C).
In Fig. 2 the compressive stresses for all the samples at 0.6 strain are shown in function of the cooling
conditions. The higher stress values for the samples cooled in the furnace are clearly visible, especially
for the higher Si-contents. It is also visible that the strengthening obtained in the water quenched samples
during the delay is more pronounced (150MPa after 24 hours) than in the rest of the samples (10 to 30
MPa after 24 hours). The combined effect of the cooling rate after annealing and the delay between
annealing and testing (0, 2, 6 and 24 hours) on the deformability of the four alloys shows that for lower
silicon contents (alloy A), neither the cooling rate, nor the delay between the annealing and the further
deformation, seem to affect too much the stress values. However, for higher Si contents (≥4.2%Si) there
is a clear time effect, which was called “aging” in literature [5, 6]. There is also an effect of the cooling
rate, being higher after an annealing at 900 or 1000°C.
Alloy D (with 5.4%Si) quenched in water after annealing at 900°C exhibits a lower compression stress if
the material is deformed immediately after the cooling, whereas only 6 hours later the compression stress
increases with 150 MPa. The same is observed for alloy C (4.97 % Si), which demonstrates that
effectively a “time effect” or “aging” phenomenon exists in these alloys, probably due to the reestablishment of short range ordering in the material, even at room temperature.
Table 2. Compression stresses at 0.6 true strain for alloy D (5.4%Si) after annealing at 900°C and without
annealing, different cooling rates
Stress (MPa)
Cooling media
Immediately after annealing
24 hours after the annealing
Quenched in water
897
1045
Cooled at the air
1120
1131
Slowly cooled in the furnace
1404
1436
As cast (without any annealing)
1114
Table 2 shows the differences between the compressive stresses at 0.6 strain in samples with 5.4 %Si for
different cooling rates and time delays: the stresses are 500MPa lower for water quenched material in
comparison with furnace cooling. Samples cooled at the air show similar values as samples without
J. Barros, Advances in the workability of high silicon electrical steel
additional annealing. After a 24 hours delay, the differences becomes lower because of the “aging“ of the
samples, but the samples quenched in water always exhibit lower compressive stress values than the
samples cooled in the furnace or at the air and with the samples deformed in as cast condition.
Hot and cold rolling experiments
Fig. 3 shows the evolution of the MFS values as a function of the temperature (measured before each
rolling pass) for hot rolling in 4 passes for all experimental alloys rolled with 55% of reduction per pass.
When the temperatures are higher than 1000°C the MSF during are not strongly affected by the increasing
of the Si content. Only when the temperatures are lower than 1000°C the MFS increases proportional with
the increasing of silicon as was observed before in the hot torsion experiments. Obviously, increasing the
percent of reduction and/or reducing the number of passes will increase the MFS.
Figure 3. Evolution of the mean flow stress with the temperatures during hot rolling in 4 passes (80-8040-35% of reduction per pass) in samples reheated at 1150°C (TM2 scheme) for the four steels.
This behavior can probably be associated with the ordering phenomena that can occur in the high Si
alloys during cooling. The gradual appearance of order in these alloys with the decrease of temperature
interferes with the workability, being more acute as the Si content increases. According to the phase
diagrams 1000°C should be a temperature too high for the appearance of ordered structures. Nevertheless,
regarding the experimental results previously obtained using hot torsion tests some doubts may exist
concerning the applicability of some literature results, which were obtained under conditions of
equilibrium after very long annealing times. They may not be applicable in industrial circumstances
where cooling and heating rates are extremely important for the position and existence of the
transformation temperatures. Therefore the finishing temperature for hot rolling is an important factor
because when the temperature decreases below 1000°C the MFS during hot rolling increases
considerably.
After water quenching some samples were cold rolled in a large series of passes down to 0.5mm without
experiencing edge cracks. Water quenching is believed to impede the ordering processes in the alloys and
therefore the brittle behavior is not observed. Also samples cooled at the air after hot rolling and
immediately cold rolled (without any time delay) can be cold rolled. However, the water quenched
samples exhibited a higher ductility in cold rolling, while only a smaller reduction could be achieved in
samples air cooled after hot rolling and in some occasions edge cracks appeared. Samples cooled down
very slowly in the furnace were impossible to cold roll because of their brittle behavior.
However, after a relatively short time at room temperature (some hours) these alloys lost their ductility
and the material is impossible to be cold rolled as was shown by the compression tests. Fast cooling after
hot rolling made the material sufficiently ductile for cold rolling, but only during some limited period of
time. On the other hand, slow cooling rates after hot rolling made the alloys too brittle for cold rolling.
Magnetic properties
J. Barros, Advances in the workability of high silicon electrical steel
Curves of the magnetic polarization vs. the magnetic field strength (H) as well as of the total loss (P) vs.
peak magnetic flux density (B) are shown in Fig. 4 for alloys B and D. Some samples of the alloy B and D
showed remarkably large values of magnetic polarization, whereas in other the samples the results were
not like expected and the values of the total power losses some times were larger than expected for such
high Si-contents. The low magnetic polarization values are probably a consequence of the poor surface
quality (high roughness) after annealing and of an inadequate texture, whereas the grain size was
comparable with conventional NO-electrical steels. The texture obtained in both steels after annealing
shows a strong γ-fiber (<111>//ND) with a high intensity at {111}<110>, also a larger deviation of the γfiber is present running to the {001}//ND components. However, in some samples a <001>//ND cube
texture is observed while the γ-fiber components almost disappeared, which can explain better values of
magnetic polarization. Better magnetic properties could be obtained through variations in the
thermomechanical history, especially taking care of the conditions necessary to obtain a good surface
quality.
1.8
1000
400 Hz
200 Hz
10
50 Hz
1
0.1
0.01
0.0
Magnetic Polarization (J)
Specific Total Loss (W/Kg)
1.6
1000 Hz
100
Steel 4
Steel 2
1.4
1.2
1.0
0.8
0.6
Steel 4, 50 Hz
Steel 2, 50 Hz
Steel 4, 400 Hz
Steel 2, 400 Hz
0.4
0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Peak Magnetic Flux Density (T)
1.6
1.8
2.0
100
1000
10000
Peak Magnetic Field Strength (A/m)
(a)
(b)
Figure 4. (a) Curves of total losses vs. peak magnetic flux density and (b) Curves of magnetic polarization
vs. magnetic field strength of the alloys B and D. Thickness of 0.6mm.
Conclusions
Thermomechanical processing of high-silicon steel is possible whenever adequate temperatures and
rolling schedules are used. Differences in Si-content in the studied range affect only slightly the MFS
during hot rolling at high temperatures. Cold rolling remains critical: after a relatively short time (hours)
at room temperature these alloys loss their ductility and the material can not be rolled.
References
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p. 208.
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[3] Ros-Yáñez T., Houbaert Y., De Wulf M., IEEE Transactions on Magnetics 38 (2002) p. 3201.
[4] O. Schneeweiss, Jounal Physic: Condens.Matter 1 (1989) p. 4749.
[5] Ros-Yáñez T., Houbaert Y., Fischer O. and Schneider J., IEEE Transactions on Magnetics, 37(4)
(2001), p. 2321.
[6] Ros-Yáñez T., Barros J., Colas R., Houbaert Y., ISIJ Int. 43 (3) (2003) p. 447.
[7] Dieter G.E., Mechanical Metallurgy (McGraw-Hill Book Company 1988).
[8] T.M. Maccagno, J.J. Jonas et al., ISIJ International vol.34 no. 11(1994) p. 917.