# Texture Control During the Manufacturing of  Nonoriented Electrical Steels

Authors: Leo Kestens; Sigrid Jacobs
Journal: Texture, Stress, and Microstructure
 (2008)

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## Abstract

Methods of modern quantitative texture analysis are applied in order to characterize the crystallographic texture of various non-oriented electrical steel grades in view of their relation with the magnetic properties of the steel sheet. A texture parameter is defined which quantifies the density of〈100〉 easy magnetic directions in the sheet planes. An extensive correlation study revealed the relation of this parameter with the hysteresis losses, determined at an induction of 1.5 T, and with the induction measured at an applied external field of 25 A/cm. It is shown that the latter magnetic property is the more texture dependent, whereas the former one is more sensitive to the grain size of the steel. Also various strategies for texture control are critically reviewed. It is shown that the conventional manufacturing process only provides poor tools for optimizing the texture of the final product. In order to obtain a quantum-leap improvement of the magnetic quality of the texture, in combination with other important microstructural features, nonstandard processing strategies are required, such as cross-rolling, two-stage cold rolling, or surface annealing.


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## Body

## 1. INTRODUCTION

As finished products, industrially
manufactured nonoriented electrical steels are single-phase ferrite
polycrystalline materials. A macroscopic volume of such a material contains an
aggregate of a large number of microscopically small granular crystallites. Each of the individual grains in the
polycrystalline aggregate displays a specific orien tation with respect to an
external reference frame. In general, these grains are not randomly oriented,
but a preference for certain orientations may be present. In this case, the
material exhibits a specific crystallographic texture. On the other hand, when
the grains are oriented in an arbitrary manner, the material is textureless or
displays a random texture.Each individual crystal orientation is
characterized by the orientation relation between the sample and the crystal
reference frame. The sample reference frame is a right-handed orthonormal (xs,ys,zs) coordinate system which coincides with the RD, TD,
and ND directions of the rolled sheet (rolling, transverse, and normal
direction, resp.). The crystal reference frame is also a right-handed
orthonormal coordinate system which is attached to the 〈100〉 directions
of the cubic crystal lattice. The orientation relation between crystal and
sample reference system is uniquely determined by the three Euler angles φ1, Φ, φ2.
These angles define three consecutive rotations which must be carried out
according to the schedule of Figure 1 in which the Bunge convention is
adopted [1]. By executing
these three rotations, the sample reference system is brought to coincidence
with the crystal reference system.Figure 1
Definition of the Euler
angles according to the Bunge notations. The set of three Euler angles (φ1, Φ, φ2)
uniquely de-termines the correspondence between the sample reference (xs, ys, zs)
and the crystal reference frame (xc, yc, zc).For each of the grains present in the
polycrystalline material, its orientation can be uniquely specified by means of
these three Euler angles. Each set of three Euler angles (φ1, Φ, φ2)
determines the coordinate of a point in the ori-entation space
which is defined in the usual Euclidean way by the three orthogonal axes
corresponding to the three Euler angles, (cf., Figure 2). Imagine that for each volume element
of a material, it would be possible to determine precisely its crystallographic
orientation. Then, each volume element would be represented by a specific point
in Euler space (cf., Figure 2). In case that the material displays a certain
texture, these representation points exhibit a specific distribution which
characterizes the orientation distribution function (ODF):Figure 2
Representation of individual orientations in Euler space jointly representing a specific distribution which characterizes the crystallographic texture of the material.(1)f(g)dg=dVV
with dV/V = the volume fraction of material represented by an
infinitesimal orientation element dg around an arbitrary orientation g.
This ODF is a three-dimensional distribution function which contains the entire
texture information of the material under consideration.Due to intrinsic symmetry properties of rolled sheet material such
as nonoriented (NO) electrical steels, the domain of the ODF can be resitricted
to the following subspace0<(φ1, Φ, φ2)<90 deg. Moreover, due to a for-tunate coincidence, almost all texture
components which are observed in cold rolled or annealed sheet steel are
represented in the φ2=45 deg
section of Euler space. There-fore, the ODF of a low-carbon steel sheet is most
commonly depicted by this section only. Figure 3 shows the characteristic texture
components which are represented in the φ2=45 deg
section. They are mainly the cube ({001}〈001〉), the rotated cube ({001}〈110〉),
and the Goss ({110}〈001〉) components which are of importance
for magnetic applications. Apart from individual texture components also fibre
components are displayed. These fibre components represent a group of
orientations for which one crystal direction corresponds to a specific sample
direction. The most important fibres are the α, γ, and θfibres
for which 〈110〉//RD, 〈111〉//ND, and 〈001〉//ND,
respectively.Figure 3
Characteristicφ2=45 deg section of Euler
space which represents the most common rolling and recrystallization comp-onents
of low-carbon steels. The α, γ, and θfibres
represent groups of orientations for which 〈110〉//RD, 〈111〉//ND, and 〈001〉//ND, respectively.
C = cube {001}〈100〉, H = rotated cube {001}〈110〉, G = Goss
component {110}〈001〉.


## 2. EVALUATION OF THE MAGNETIC QUALITY  OF A TEXTURE

When one wants to investigate the
effect of texture on the magnetic properties of electrical steel, reliable
quantitative parameters are required to characterize both the dependent and the
independent variables, that is, magnetic properties and texture, respectively. With regard to the magnetic prop-erties, the usual quantitative features can be employed such as the core losses, the permeability, the coercive field, the remnant
induction, and other characteristics of the hysteresis curve. With regard to the texture, one must take into account the physical fact that the〈100〉
directions are the directions of easy magnetization of a Fe-single crystal [2]. The Fe-single
crystal magnetization curves show that optimum soft magnetic properties are
obtained when the external field is applied in the 〈100〉 direction.
Therefore, the ideal texture of a soft iron core is the one that maximizes the
density of 〈100〉 crystal directions parallel to the flux lines of magnetic
induction. Because in rotating applications the flux lines are nearly isotropically
distributed in the laminated sheets, the ideal texture is the one that maximizes
the incidence of 〈100〉 directions in the plane of the sheet. This is not
the well-known Goss component ({110}〈001〉) which is usually observed in
grain-oriented steels [3], but rather the
orientations of the cube fibre for which the {001} planes are parallel to the
rolling plane. The latter texture components display two coplanar 〈100〉
directions in the plane of the sheet, whereas the former component only
delivers one 〈100〉 direction to the lamellar plane.In order to quantify the magnetic quality of an arbitrary NO steel texture, a precise parameter is defined which can  be derived from the ODF. For every arbitrary crystal ori-entation g, the
angle A(g) can be determined which is the minimum angle between the direction
of the magnetization vectorM and the closest 〈100〉 direction
(out of 3 symmetric equivalent 〈100〉 directions), compared to (cf., Figure 4).The texture parameter Aθ is defined as the orientation-averaged
value of A(g), taking into account the volume fraction of each texture component, for an arbitrary magnetic field direction θ. This orientation weighted average value can be very
elegantly expressed by the convolution integral of Aθ(g)
and the ODF f(g),
(2)Aθ=∫f(g)Aθ(g)dg.Figure 4
definition of theAθ(g) parameter as the minimum angle between
the direction of the magnetization vector M and the nearest 〈100〉 direction of the crystal.The value ofAθ is confined to the range (0−54.7deg). The lower
limit corresponds to the case of a single component texture in which one of
the 〈100〉 directions of the crystal lattice corresponds to the magnetization
direction, whereas the upper limit is observed for a single component texture
in which the direction of magnetization is aligned with the 〈111〉 lattice
direction.In rotating electrical machine applications, the directions of the magnetizing
field will be equally distributed in the lamellar plane of the rolled sheet.
Consequently, a direction averaged A parameter must be considered as(3)A=∫Aθdθ,
in which the integral expands over all
possible planar directions of the rolled sheet. For a sample with orthorhombic
sample symmetry, as usually observed in cold-rolled sheet steel, the integral
of (3) can be readily approximated by the following expression:
(4)A=A0+2A45+A904,
in which A0, A45, and
A90 represent the A values (according to (2)) for an applied
field which encloses an angle of 0, 45, and 90 deg, respectively, with the
direction of rolling.A low value of theA parameter corresponds to a high magnetic
quality of the texture because it implies that, on average, the 〈100〉
directions are closely aligned with the flux lines which are randomly
distributed in the plane of the sheet. Table 1
presents the direction averaged (according to (4)) A parameters for a number of ideal texture components. It can be demonstrated that the theoretically lowest value of the A parameter is 22.5 deg. According to the data
of Table 1, this value is displayed by all individual components of the θfibre, and also by the θfibre itself. Therefore, the above presumption is vindicated, that is, the cube fibre (θfibre) is truly the ideal texture for
NO electrical steels. This conclusion was reported as well by Rollett et al. [4].Table 1
Direction-averagedA values (in deg) of a number of texture components and fibres which are present
in the typical texture of NO electrical steels (random refers to a random
texture).
{001}〈110〉θfibreαfibreγfibre22.522.530.138.7{110}〈001〉{001}〈001〉{112}〈110〉random33.822.536.131.9


## 3. CORRELATION BETWEEN TEXTURE ANDMAGNETIC PROPERTIES

Once a reliable procedure has been
established to quantify both the texture and the magnetic properties, a
relation be-tween them can be investigated. Such a correlation study can only be
carried out successfully on condition that the independent variable (i.e., the
crystallographic texture char-acterized by itsA value) can be varied over a
sufficiently wide range in order to gauge the effect on the dependent variable.
In order to achieve this goal a dedicated data set was produced composed of a
number of samples for which the Aθ parameters varied between 27.7
and 36.3 deg. For each of these samples belonging to the data set, the
hysteresis losses were measured for a magnetic induction of 1.5 T and the
permeability was assessed by measuring the magnetic induction for an applied
magnetic field of 25 A/cm. All these magnetic properties were measured at
0, 45, and 90 deg with respect to the rolling direction and correlated with A0,
A45, and A90, respectively.The metallurgical methods employed to
produce samples with varying textures had also introduced a considerable spread of grain size in the dataset. Because grain size may also have an important effect on the magnetic properties, this parameter had to be included in the correlation study as well. The linear regression equations which correlate theA value and the average grain diameter D with the magnetic properties (Ph at
1.5 T and B25) reveal the following cor-respondences:
(5)Ph=(−3.9±0.8)D+(0.15±0.04)A−0.869,withr2=0.632,(6)B25=(−0.016±0.04)A+2.164,withr2=0.848.
In the above equations, the hysteresis loss is expressed in W/kg, the induction B25 in T, the A parameter in hex-adecimal degrees and the grain size D in mm. Because the grain size only had a minor effect on the induction, the
parameter D could be ignored in the regression equation (6).In order to estimate the texture effect, the following comparison can be considered. If the texture could be controlled in the most ideal way such that the theoretically optimumA parameter could be achieved of 22.5 deg, it would imply a drop of approximately 10 deg compared to the A value of 32 deg which is regularly observed on industrially manufactured NO steels (cf., infra). The above regression equations (5) and (6) show that such a 10 deg drop
of the A value will produce a decrease of 1.5 W/kg in hysteresis losses and
an increase of induction (B25) with 160 mT. These values must be compared with the changes that can be produced by other independent state variables of the microstructure such as grain size, precipitation density, or residual stresses. When such a critical analysis is carried out, it can be concluded that the texture has its most significant effect on the in-duction B25, which in this study is used as a measure for the permeability. With regard to the hysteresis Ph losses at 1.5 T, the grain size is much more
decisive than the texture. The relative lack of correlation with a regression coefficient of r2=0.632 indicates that also other microstructural
features which were not taken into account here such as, for example, residual stresses or presence of precipitates may affect the hysteresis losses.On the theoretical level, it could be argued that the induction
B25 measured at an applied field of 25 A/cm is not the most sensitive magnetic parameter to reflect the influence of texture neither. In the magnetization
curve, two different parts can be distinguished: the lower part (below the characteristic deflection point) which is controlled by magnetic domain wall displacement and the upper part (above the characteristic deflection point)
which is controlled by rotational magnetization. It is the latter part which is the most dependent on the texture. This part of the magnetization curve can be appropriately characterized by the difference in saturation inductionBsat and remnant induction Brem. Therefore, in spite of the practical
problems in determining the exact Bsat value experimentally, in
order to further examine the effect of texture on the magnetic behavior of NO electrical steels, it is suggested to use this difference (Bsat-Brem) as the reference magnetic parameter.


## 4. CONVENTIONAL TEXTURE CONTROL IN NO ELECTRICAL STEELS

On the basis of an extensive testing
program on industrially manufactured NO steel grades, it was concluded that
even the widest variation of conventional processing parameters applied in
state-of-the-art steel manufacturing would only produce limited variations in
the magnetic quality of the textures. In these experiments, the following
processing pa-rameters were considered: slab reheating temperature (SRT), finish
rolling temperature (FRT), hot band coiling temperature (CT), hot band
annealing (yes/no), cold rolling reduction (CR), continuous annealing
temperature, line speed of continuous annealing, and skin pass reduction (SKP).
On top of this, also the chemical composition of these steel was thoroughly investigated. The applied windows for all these processing and chemical parameters are listed in Table2. Despite these wide scale variations,
the A0 parameter only varied between 25 and 31 deg, whereas theoretically this value can change between 0 and 55 deg.Table 2
Variation range of
the following steel manufacturing processing parameters: SRT = slab reheating
temperature, FRT = finish rolling temperature, CT = hot band coiling
temperature, line speed and annealing temperature of the continuous annealing
line, and skin pass reduction. Also the minimum and maximum levels (in mass %)
of various alloying elements are listed.
Process
parameterMin.Max.ElementMin.Max.SRT.
(°C)10701250C0.0210.055FRT
(°C)790920Si0.0002.153CT
(°C)580780P0.0060.032Hot
band annealingnoyesS0.0080.033Line
speed (m/min)150250N20.00170.0224Annealing
temp. (°C)700800Almet0.0111.060Skin
pass red. (%)28Sb0.0000.160Figure5 displays the typical ODFs which are
observed in the subsequent stages of the manufacturing process of NO electrical
steels. The textures reported in Figure 5 are measured in the middle of
the sheet and thus represent the bulk textures. The single most important
processing factor for texture control is the finish rolling temperature
(FRT). Whereas for structural steels the
conventional FRT is in the full austenite region [5], for NO electrical steels the
op-timum textures are obtained with finish rolling in the two-phase region or
even the full ferrite phase. Figure 5(a) displays the hot band texture which is observed
after such a two-phase rolling. It is characterized by a very strong maximum on
the rotated cube component (with an intensity maximum of >30x) and an
extension along the αfibre toward the {112}〈110〉 component.
After a typical cold rolling reduction of 70%, the intensity is more
homogeneously spread along the αfibre, but a maximum of >20x still remains
on the rotated cube component (cf., Figure 5(b)). The ro-tation flow as a result of cold deformation can be entirely understood on the basis of the classical full constraints Taylor theory and its relaxed constraints derivatives [6].Figure 5
Typical texture evolution during the subsequent stages of the conventional
manufacturing process of a fully processed NO electrical steel: (a) hot band
texture, (b) cold rolling texture, and (c) annealing texture.After
primary recrystallization, the texture is significantly weakened as compared to
the hot band and the cold rolling textures (cf., Figure5(c)). Although the intensity maximum remains on the
rotated cube component ({001}〈110〉), it has decreased from >20x to
~5x. The metallurgical mechanisms that controlled the recrystallization of the
cold-rolled sheet did not allow the magnetically favorable {001} orientations,
which dominated the hot and cold band textures, to be preserved in the
annealing texture. Conversely, as shown in Figure 5(c), the recrystallization of a 70% cold-rolled
product will rather favor the {111} orientations at the expense of the {001}
orientations [7]. In principle,
for the type of low Si grade NO steels presently under consideration, the
quality of both the hot band microstructure and texture is more favorable with
regard to the magnetic properties than the quality of the cold-rolled and
annealed product. In many instances, also the grain size of hot-rolled sheet
and additional microstructural features such as density or morphology of 2nd
phase particles is commonly appropriate in terms of magnetic quality. Unfortunately,
the required gauges of 0.65 mm or less cannot be obtained on current
state-of-the-art hot rolling mills and thus, a subsequent cold rolling and
annealing stage is unavoidable.A substantial part of the volume of NO
electrical steels is manufactured as semiprocessed sheet. These semiprocessed
products are submitted to a skin pass reduction of 4 to 8% and additionally an
annealing treatment is carried out on the punched laminations which are ready
to be processed in soft-iron cores of electrical devices. This additional heat
treatment, which is commonly carried out not by the steel manufacturer but by
the manufacturer of iron cores, serves multiple purposes such as decarburization,
grain growth, or relieving of internal stresses. This treatment definitely also
affects the texture of the final product as it is exhibited in Figure6. The ODF after this final annealing treatment displays two characteristic maxima:
one on the αfibre in the vicinity of the {113}〈110〉
component and one on the TD fibre (〈110〉//TD) some 10 deg away from the Goss component. Although a marked textural change has oc-curred, the magnetic quality of the texture in terms of its average A value was hardly affected at
all by the additional heat treatment on the stamped lamella. It was shown that
the texture which was produced by the annealing could be explained by
considering the stored energy of plastic deformation after skin pass rolling [8].Figure 6
Texture evolution as a result of skin pass annealing (with decarburization)
on a semiprocessed NO electrical steel: ODF (a) before and (b) after the
annealing treatment.


## 5. ADDITIONAL METHODS OF TEXTURE CONTROL  IN NO ELECTRICAL STEELS

### 5.1. Cross rolling

In order to control the recrystallization texture, one has to bear
in mind that during the annealing of cold-rolled sheet a selection will be made
of orientations which have developed during the cold rolling process. Therefore, it
should be attempted to form as few as possible unfavorable orienta-tions of theαand γfibres as a result of the rolling strain.
Unfortunately, these are the stable end orientations which appear after
conventional rolling reductions of 60 to 80% [5]. By changing the orientations of the rolling-mill
rolls with respect to the sheet, the rotation paths of the orientation flow
during plastic deformation can be affected. In the present study, a laboratory
scale experiment was carried out in which the hot-rolled sheet was 90 deg rotated before cold rolling. In doing so
the RD of hot rolling turns into the TD of cold rolling and vice versa. Hence, the hot band texture, with its characteristic strong αfibre (cf., Figure 5(a)) is transformed into a texture with a strong
〈110〉//TD fibre by simply substituting the hot rolling by the cold
rolling reference frame. Because the latter fibre is highly unstable for cold
rolling, all its components will rotate toward the semistable {001}〈110〉
orientation. This procedure will give rise to an extremely strong rotated cube component ({001}〈110〉) with an extraordinary
intensity of >150x, compared to Figure 7(a), which offers the advantage that
it does not introduce the unfavorable αand γfibre components during cold reduction. This favorable texture is not preserved, though,after the primary recrystallization treatment
which produces a rather weak texture with a maximum in the vicinity of
{112}〈110〉 (cf., Figure 7(b)).ODFs observed after cross rolling: (a) rolling texture, (b) primary
recrystallization texture, and (c) after additional annealing treatment.
(a)(b)(c)When this material is cold rolled to a reduction of 4 to 8%
and submitted to an additional annealing treatment, comparable to the one which
is applied to semiprocessed products, the rotated cube component spectacularly
reappears in the material and even other components of theθfibre are formed. The texture presented in Figure 7(c) is the result of such an additional annealing
treatment. It displays a maximum of >50x on the rotated cube component and
~10x on the cube component. The exceptional magnetic quality of this texture is
reflected in the very low average A value of 24.3 deg which is only 1.8 deg away
from the theoretical minimum value of 22.5 deg. It is obvious that cross rolling cannot be implemented in a continuous line manufacturing process. Whether
or not the proposed process with cross rolling is of industrial relevance is
entirely dependent of economic considerations, that is, added economic value by
superior material properties versus increased manufacturing cost because of
loss of production efficiency.


### 5.2. Two-stage cold rolling

Conventional continuous annealing is applied on a cold-rolled
material with a rolling strain of 70% reduction at a temperature between 700
and 850°C and with a heating rate of 2 to 10°C/s. These large rolling reductions
are neces-sary to obtain the relatively thin gauges of 0.60 mm or less which are generally required for the production of soft iron core lamella. The metallurgical mechanisms, however, which control the texture formation during
recrystallization under these circumstances only favor the nucleation of{111}
orientations which are highly unfavorable for magnetic purposes. Other recrystallization mechanisms which
stimulate the formation of the desired {001} orientations only operate after
limited rolling strains (of the order <10%). Therefore, it is an interesting strategy to
apply the necessary reduction in two stages: at first a substantial reduction
(60 to 70%) in order to obtain the required gauge followed by a second much
lower reduction (5 to 10%) in order to control the texture and the
microstructure. This production path can be associated with the manufacturing
of semiprocessed grades of NO steels which are produced by a similar approach.
It is known that the magnetic quality of the texture may largely benefit from
an annealing treatment carried out on a temper-rolled material [9–12].The magnetic properties, measured on a 0.65 mm gauge sheet, of two
NO steels are compared in Table3. One NO steel was cold manufactured in
a one stage process, whereas the other one was produced in a two-stage process.
The first column pertains to a set of samples that were cold rolled in one
stage with a reduction of 70%, whereas the second column refers to the samples
which were submitted to an intermediate annealing treatment between two
successive rolling operations with a reduction of 70 and 10.4%, respectively.
After the second rolling, a final annealing treatment was applied. It can be noticed that primarily the hysteresis losses are favorably
affected by this process, whereas the B25 and B50 values are hardly affected at
all.Table 3
Comparison of the
magnetic properties of two sets of samples. One set is cold rolled in a single
stage with a reduction of 70% and the other set was intermediately annealed
between cold rolling stages with a reduction of 70 and 10.4%, respectively,
prior to a final annealing treatment.
PropertySampleOne-stageTwo-stagecold rollingcold rollingP(1.5)[W/kg]6.615.14Ph(1.5)[W/kg]3.802.28Pw(1.5)[W/kg]2.812.86P(1)[W/kg]3.092.34B25[T]1.651.65B50[T]1.741.73A-parameter (deg)30.931.8(direction averaged)The reason for this behavior must be explained in terms of the
microstructural and textural evolution. The annealing treatment following a
small rolling strain triggers a process of strain-induced abnormal grain
growth [13] which is
highly beneficial for the hysteresis losses. Although this grain growth process
is also accompanied by a marked texture evolution, the A parameter change (cf.,
Table 3) indicates that the magnetic quality of the texture has not improved.
The reason for this behavior is complex and is not entirely understood as yet,
but it is related with the detailed features of the recrystallization and grain
growth mechanisms which operate after such low rolling reductions.


### 5.3. Surface textures

Yoshinaga et al. [14] have demonstrated that a relatively
strong rotated cube texture can be obtained at the surface of an ultra-low-carbon
steel when a conventionally cold-rolled sheet of this material is submitted to
a short annealing treatment (2 to 3 minutes) in the full austenite phase. The
forward and reverse ferrite to austenite transformation produces textures of
the type presented in Figure 8. It canbe observed that there is a
pronounced difference between the surface and the bulk textures. It is assumed that the surface energy will drastically affect the orientation selection
occurring during the diffusion-controlled phase transformation.Figure 8
Textures observed after a forward and reverse ferrite-austenite
transformation in an ultra-low-carbon steel (a) in the bulk and   (b) at the surface of the sheet. Levels: 2-4-6-8-11-14 [14].Once a favorable surface texture is created, a method must be
established which allows the surface grains to grow into the bulk of the sheet.
Such a method is part of the metallurgical process proposed by Tomida [15]. He performed
a two-stage annealing treatment on a cold rolled and recrystallized NO
electrical steel with a Mn content of 1.0 to 2.5 mass% and a C content of 0.05
to 0.10 mass%. The first stage of this annealing treatment was carried out
under vacuum conditions (P<10−5 hPa) during 12 hours at varying
temperatures between 850 and 1100°C. During this stage of the annealing
treatment, the Mn gradually diffused out of the surface layers of the sheet. As
Mn is a strong austenite stabilizing element, the removal of Mn from the
surface layer locally triggered a ferrite transformation near the surface with
a favorable {001} texture, very much comparable to the one observed by Yoshinaga et al. [14].The second stage of the annealing treatment was carried out under a
decarburizing atmosphere. Under these circumstances, the ferrite surface grains
gradually consumed the austenite bulk of the material, hereby extending the
favorable surface texture over the entire volume of the material and thus
creating a columnar structure. By applying this method, extraordinary sharp
cube textures could be produced concurrently with highly favorable columnar
microstructures displaying an average grain size of more than100μm. This
gave rise to a core loss between 1.4 and 1.7 W/kg and an induction of 1.76 to 1.81 T at an applied magnetic field of 50 A/cm.


## 5.1. Cross rolling

In order to control the recrystallization texture, one has to bear
in mind that during the annealing of cold-rolled sheet a selection will be made
of orientations which have developed during the cold rolling process. Therefore, it
should be attempted to form as few as possible unfavorable orienta-tions of theαand γfibres as a result of the rolling strain.
Unfortunately, these are the stable end orientations which appear after
conventional rolling reductions of 60 to 80% [5]. By changing the orientations of the rolling-mill
rolls with respect to the sheet, the rotation paths of the orientation flow
during plastic deformation can be affected. In the present study, a laboratory
scale experiment was carried out in which the hot-rolled sheet was 90 deg rotated before cold rolling. In doing so
the RD of hot rolling turns into the TD of cold rolling and vice versa. Hence, the hot band texture, with its characteristic strong αfibre (cf., Figure 5(a)) is transformed into a texture with a strong
〈110〉//TD fibre by simply substituting the hot rolling by the cold
rolling reference frame. Because the latter fibre is highly unstable for cold
rolling, all its components will rotate toward the semistable {001}〈110〉
orientation. This procedure will give rise to an extremely strong rotated cube component ({001}〈110〉) with an extraordinary
intensity of >150x, compared to Figure 7(a), which offers the advantage that
it does not introduce the unfavorable αand γfibre components during cold reduction. This favorable texture is not preserved, though,after the primary recrystallization treatment
which produces a rather weak texture with a maximum in the vicinity of
{112}〈110〉 (cf., Figure 7(b)).ODFs observed after cross rolling: (a) rolling texture, (b) primary
recrystallization texture, and (c) after additional annealing treatment.
(a)(b)(c)When this material is cold rolled to a reduction of 4 to 8%
and submitted to an additional annealing treatment, comparable to the one which
is applied to semiprocessed products, the rotated cube component spectacularly
reappears in the material and even other components of theθfibre are formed. The texture presented in Figure 7(c) is the result of such an additional annealing
treatment. It displays a maximum of >50x on the rotated cube component and
~10x on the cube component. The exceptional magnetic quality of this texture is
reflected in the very low average A value of 24.3 deg which is only 1.8 deg away
from the theoretical minimum value of 22.5 deg. It is obvious that cross rolling cannot be implemented in a continuous line manufacturing process. Whether
or not the proposed process with cross rolling is of industrial relevance is
entirely dependent of economic considerations, that is, added economic value by
superior material properties versus increased manufacturing cost because of
loss of production efficiency.


## 5.2. Two-stage cold rolling

Conventional continuous annealing is applied on a cold-rolled
material with a rolling strain of 70% reduction at a temperature between 700
and 850°C and with a heating rate of 2 to 10°C/s. These large rolling reductions
are neces-sary to obtain the relatively thin gauges of 0.60 mm or less which are generally required for the production of soft iron core lamella. The metallurgical mechanisms, however, which control the texture formation during
recrystallization under these circumstances only favor the nucleation of{111}
orientations which are highly unfavorable for magnetic purposes. Other recrystallization mechanisms which
stimulate the formation of the desired {001} orientations only operate after
limited rolling strains (of the order <10%). Therefore, it is an interesting strategy to
apply the necessary reduction in two stages: at first a substantial reduction
(60 to 70%) in order to obtain the required gauge followed by a second much
lower reduction (5 to 10%) in order to control the texture and the
microstructure. This production path can be associated with the manufacturing
of semiprocessed grades of NO steels which are produced by a similar approach.
It is known that the magnetic quality of the texture may largely benefit from
an annealing treatment carried out on a temper-rolled material [9–12].The magnetic properties, measured on a 0.65 mm gauge sheet, of two
NO steels are compared in Table3. One NO steel was cold manufactured in
a one stage process, whereas the other one was produced in a two-stage process.
The first column pertains to a set of samples that were cold rolled in one
stage with a reduction of 70%, whereas the second column refers to the samples
which were submitted to an intermediate annealing treatment between two
successive rolling operations with a reduction of 70 and 10.4%, respectively.
After the second rolling, a final annealing treatment was applied. It can be noticed that primarily the hysteresis losses are favorably
affected by this process, whereas the B25 and B50 values are hardly affected at
all.Table 3
Comparison of the
magnetic properties of two sets of samples. One set is cold rolled in a single
stage with a reduction of 70% and the other set was intermediately annealed
between cold rolling stages with a reduction of 70 and 10.4%, respectively,
prior to a final annealing treatment.
PropertySampleOne-stageTwo-stagecold rollingcold rollingP(1.5)[W/kg]6.615.14Ph(1.5)[W/kg]3.802.28Pw(1.5)[W/kg]2.812.86P(1)[W/kg]3.092.34B25[T]1.651.65B50[T]1.741.73A-parameter (deg)30.931.8(direction averaged)The reason for this behavior must be explained in terms of the
microstructural and textural evolution. The annealing treatment following a
small rolling strain triggers a process of strain-induced abnormal grain
growth [13] which is
highly beneficial for the hysteresis losses. Although this grain growth process
is also accompanied by a marked texture evolution, the A parameter change (cf.,
Table 3) indicates that the magnetic quality of the texture has not improved.
The reason for this behavior is complex and is not entirely understood as yet,
but it is related with the detailed features of the recrystallization and grain
growth mechanisms which operate after such low rolling reductions.


## 5.3. Surface textures

Yoshinaga et al. [14] have demonstrated that a relatively
strong rotated cube texture can be obtained at the surface of an ultra-low-carbon
steel when a conventionally cold-rolled sheet of this material is submitted to
a short annealing treatment (2 to 3 minutes) in the full austenite phase. The
forward and reverse ferrite to austenite transformation produces textures of
the type presented in Figure 8. It canbe observed that there is a
pronounced difference between the surface and the bulk textures. It is assumed that the surface energy will drastically affect the orientation selection
occurring during the diffusion-controlled phase transformation.Figure 8
Textures observed after a forward and reverse ferrite-austenite
transformation in an ultra-low-carbon steel (a) in the bulk and   (b) at the surface of the sheet. Levels: 2-4-6-8-11-14 [14].Once a favorable surface texture is created, a method must be
established which allows the surface grains to grow into the bulk of the sheet.
Such a method is part of the metallurgical process proposed by Tomida [15]. He performed
a two-stage annealing treatment on a cold rolled and recrystallized NO
electrical steel with a Mn content of 1.0 to 2.5 mass% and a C content of 0.05
to 0.10 mass%. The first stage of this annealing treatment was carried out
under vacuum conditions (P<10−5 hPa) during 12 hours at varying
temperatures between 850 and 1100°C. During this stage of the annealing
treatment, the Mn gradually diffused out of the surface layers of the sheet. As
Mn is a strong austenite stabilizing element, the removal of Mn from the
surface layer locally triggered a ferrite transformation near the surface with
a favorable {001} texture, very much comparable to the one observed by Yoshinaga et al. [14].The second stage of the annealing treatment was carried out under a
decarburizing atmosphere. Under these circumstances, the ferrite surface grains
gradually consumed the austenite bulk of the material, hereby extending the
favorable surface texture over the entire volume of the material and thus
creating a columnar structure. By applying this method, extraordinary sharp
cube textures could be produced concurrently with highly favorable columnar
microstructures displaying an average grain size of more than100μm. This
gave rise to a core loss between 1.4 and 1.7 W/kg and an induction of 1.76 to 1.81 T at an applied magnetic field of 50 A/cm.


## 6. SUMMARY AND CONCLUSIONS

In the present paper, the effect of texture on the magnetic properties of nonoriented electrical steels was analyzed. Based on the methods provided by modern quantitative texture analysis, a texture parameter was proposed which accurately characterizes the magnetic quality of any arbitrary steel texture. This parameter was correlated with a number of magnetic properties on an extensive set of steel samples. It was found that the texture has a larger impact on the permeability than on the core losses. The latter property is more dependent on the grain size than on the texture.Various methods of texture control during steel manufacturing were evaluated. An extensive experimental study has revealed that the conventional processing parameters of a modern state-of-the-art steel plant only provide limited tools for an optimum texture control. It was shown that the finish temperature of hot rolling is the one single parameter of key importance for the development of a favorable magnetic texture on the final product after cold rolling and annealing.In general, the comparatively favorable properties of the hot band texture and microstructure cannot be preserved after cold rolling and annealing. Therefore, nonconventional processing strategies were also investigated in the present study. A number of promising alternative techniques were evaluated such as cross rolling, two-stage cold rolling and surface annealing. In laboratory scale experiments, all of these techniques have proven to produce the favorable{001} texture combined with optimum microstructural features favoring highly advantageous magnetic properties through a modification of the hysteresis loss.



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*Source: 173083-2008-04-16.xml*