Note: Descriptions are shown in the official language in which they were submitted.
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Method for thermally treating a flat steel product, thermally treated flat
steel
product and use thereof
The invention relates to a method for thermally treating a flat steel product.
Furthermore, the invention relates to a thermally treated flat steel product
and use
thereof.
The use of steels with high levels of hardness and strength is known for the
ballistic
protection of civil as well as military vehicles, which are referred to as
safety steels. Such
steels generally have high levels of thickness in order to be able to
completely break
down the energy of an impacting object (projectile, fragment) and/or a
pressure wave
and to primarily prevent a penetration of the material. In order to reduce the
weight and
to optimize the characteristics profile, in particular across the thickness of
the steels
used for ballistic purposes, multilayer steels are deemed suitable, which are
composed
of at least two steel layers and comprise at least one combination of a hard
and a second
steel with a high level of ductility, wherein the hard steel destroys the
impacting object
and the more ductile steel should absorb the arising energy of the impact. In
the prior
art, different methods to manufacture multilayer steels according to this
class are
known which are cladded (e.g. DE 21 42 360 A), for example, by means of
explosive
cladding (e.g. US 6 360 936, DE 692 02 131 T2), which is very cost-intensive,
by means
of hot-rolled cladding (e.g. EP 2 123 447 Al) or by means of roll cladding
with the use of
a heat source (e.g. DE 44 29 913 Cl). All cladding methods have in common that
the
semi-finished products to be cladded must be processed in an elaborate, and
therefore,
cost-intensive manner at their connecting surfaces (e.g. see DE 43 44 879 C2,
DE 10
2005 006 606 B3) in order to ensure a secure connection of the individual
layers to one
another so that no failure can occur between the layers in the event of a
stress load,
which would lead to insufficient ballistic protection. Even in the case of
further
processing, problems between the layers of the different steels may occur,
which, for
example, can become evident in the form of tension cracking, in particular
when
subjected to a stress load that could result due to the volatile changes in
characteristics
on the boundary/contact surfaces between the layers. However, diffusion
processes, in
particular, of interstitially solute atoms, can also prove to be problematic,
such as carbon
or nitrogen for example, which can penetrate into adjacent metal layers during
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successive thermal treatment steps, for example, in the case of hot-rolled
cladding, and
can influence the substance characteristics of these layers in a negative
manner. The
cladding methods mentioned as an example are also suitable for the
manufacturing of
other components, which are not designed for ballistic purposes, but, for
example, for
components in areas that are subject to highly abrasive factors and therefore
require
high wear-protection characteristics (cf. DE 10 2005 006 606 B3 as an
example). Further
potential for improvement exists with reference to the prior art.
The underlying object of the invention is to provide a method for thermally
treating a
flat steel product, which overcomes the disadvantages of the known prior art,
is able to
be implemented in an economic and easy manner, in particular, with comparable
results, as well as to indicate a thermally treated flat product and a related
use thereof.
In accordance with a first aspect of the invention, the object is thereby
achieved in that a
method for thermally treating a flat steel product according to the invention
is indicated,
which comprises the following steps:
- providing a flat steel product with a structure with a first hardness,
- heating the flat product at least in sections to an austenitizing
temperature,
- cooling the flat product heated at least in sections so that a structure
with a second
hardness is configured within the flat product at least in sections, which has
a higher
level of hardness in comparison to the structure with the first hardness,
wherein the heating and the cooling down of the flat product are coordinated
with each
other in such a way that the structure with the second hardness is formed
across the
thickness of the flat product in sections and, at least in one section, the
structure with
the first hardness remains constant across the thickness of the flat product.
The inventors have found that a specific thermal treatment of a monolithic
flat steel
product can configure different characteristics across the thickness of the
flat product,
which could only be provided up until this point by means of a multilayer
construction.
By providing the method according to the invention, also, no problems with
regard to
undesired diffusion processes can occur and elaborate and cost-intensive pre-
treatment
and method steps can be omitted. By means of the invention, in particular, a
variable
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hardness profile across the thickness of a monolithic steel substance can be
specifically
configured.
The term "structure with a first hardness" is primarily understood as meaning
"the
initial structure of the flat product in the delivery condition prior to the
thermal
treatment according to the invention". "Hardness in the structure with the
first hardness
or the second hardness, etc." is understood as meaning hardness average values
across
the respective sections. "Austenitizing temperature" is understood as meaning
a
temperature of at least Ai (austenite starting temperature), particularly
preferably Ac3
(austenite end temperature). The indicated temperature values Pia, Ac3 and
also the
martensite starting temperature Ms depend on the substance at hand and can be
estimated based on the related alloy composition with a good level of
precision.
In accordance with a first embodiment of the method according to the
invention, the
heating takes place at least on one side with the use of at least one heat
source, wherein
the structure with the second hardness is at least configured within one edge
section of
the flat product. The heat input into the flat product can be specifically
influenced via
one-sided heating. In particular, by means of this, the depth of the thermal
treatment
within the flat product can be controlled in sections. In order to avoid
completely
heating the entire thickness of the flat product or rather a complete
hardening by means
of cooling down after heating, the flat product can be cooled on the side
facing away
from the heat source, for example actively, using an appropriate means in
order to
prevent that at least the edge section of the flat product on the side facing
away from the
heat source is not primarily influenced in a negative manner. By means of
this, a change
in the structure can be primarily suppressed in at least one section across
the thickness
of the flat product and the structure with the first hardness is essentially
maintained.
In accordance with an alternative embodiment of the method according to the
invention,
the heating takes place on both sides with the use of at least one heat source
respectively, wherein the structure with the second hardness is configured in
both edge
sections of the flat steel product. By heating on both sides, the heat input
into the flat
product can be specifically influenced from both sides. In particular, by
means of this,
the depth of the thermal treatment within the flat product can be controlled
in sections,
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wherein a complete heating of the flat product in its entirety, in particular
the core
section, should be prevented. By heating on both sides with subsequent
cooling, hard
edge sections can be configured with a ductile core section. It is favorable
if a symmetric
or asymmetric hardness profile can be configured across the thickness of the
flat
product depending on the depth of the thermal treatment. The hard edge
sections must
not have the same hardness, but can also be configured in a different manner
so that, on
one side of the flat product, an edge section with a structure with a second
hardness is
configured and an edge section with a structure with a third hardness is
configured on
the other side of the flat product, wherein the structure with the third
hardness has a
lower level of hardness than the structure with the second hardness, however,
a higher
level of hardness than the structure with the first hardness.
Preferably, in accordance with another embodiment of the method according to
the
invention, at least one inductor is used as heat source. Inductive heat
sources can be
operated in an economic and simple manner, and can heat workpieces at least in
sections, in particular, the thermal treatment depth can be controlled in a
specific and
relatively simple manner. The inductor is operated, for example, with a
frequency
ranging between 10 Hz and 1 MHz, in particular, ranging between 100 Hz and 400
kHz.
The distance between the inductor and the flat product, for example is between
1 and 10
mm, in particular, between 2 and 5 mm away from the flat product. The so-
called
coupling distance also determines the penetration depth in addition to the
frequency.
The optimal configuration with regard to operating frequency, distance and
thermal
treatment depth depends on the product to be manufactured and can be
determined in a
relatively simple matter by means of a simulation and/or trial-and-error
testing.
In accordance with another embodiment of the method according to the
invention, at
least one edge section of the flat product is heated to at least a temperature
of at least
Ac3 + 20 K during the heating process and is held at this temperature for at
least 1 s up to
a maximum of 60 s. In the case of this temperature and holding time, it can be
ensured
that the area to be hardened within the flat product is also fully
austenitized, meaning
that the structure is fully transformed into austenite at least in the edge
area. Depending
on the heat source, the holding time can be reduced, for example in the case
of using an
inductor, to a maximum of 10s. A temperature of more than 1,100 C should not
be
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exceeded in order to prevent a grain coarsening in the tempered section,
whereby the
characteristic could be influenced in a negative manner.
In accordance with another embodiment of the method according to the
invention, the
cooling takes place with the use of appropriate means, preferably the flat
product is
quenched with water in order to transform the previously austenitized area
into a
structure with a higher level of hardness, which, for example, corresponds to
a primarily
martensitic or martensitic/bainitic structure, wherein the quenching above a
critical
cooling speed depending on the substance must take place, for example, at a
cooling
speed of > 30 K/s in order to achieve a high level of hardness.
In accordance with another embodiment of the method according to the
invention, at
least between an edge section with a structure with the second hardness and/or
third
hardness and the section with the structure with the first hardness, an
annealing section
with a structure with a fourth hardness is configured, which, in particular,
has a lower
level of hardness than the section with the structure with the first hardness.
With its
lower level of hardness and higher ductility in comparison to the adjacent
sections, the
annealing section makes an improvement possible with regard to the ballistic
characteristics and further processing. The absorption capacity of an
impacting object
and/or forming suitability for further processing can be furthermore increased
by
means of providing an annealing area.
In order to have a positive effect on crack resistance, in particular, on the
surface of the
thermally treated area of the flat product or rather to reduce this, in
accordance with
another embodiment of the method according to the invention, a decarburized
edge
layer is configured at least in one of the edge sections with a structure with
a second
hardness and/or third hardness. The decarburized edge layer can, for example,
be
configured during heating by means of a related atmosphere, in particular,
moist air
and/or by means of delayed cooling after austenitizing only after the
temperature goes
below 700 C. As an alternative, the crack resistance at the surface can by
configuring an
edge section with a structure with a fifth hardness, which has a lower level
of hardness
in comparison to the edge section with the structure with the second hardness
and/or
third hardness, in particular without edge layer decarburization. This
configuration
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takes place by means of a delayed cooling, wherein a quenching only takes
place after
the temperature at least in the edge layer goes below a temperature which
corresponds
to the Ms temperature.
In accordance with another embodiment of the method according to the
invention, a flat
steel product is used, which is ferromagnetic, which can preferably be
thermally treated
in an inductive manner. In particular, the flat steel product can be provided
in for
production reasons in the as-rolled condition, meaning that no thermal
treatment has
been carried out on the flat product after the last rolling pass, in
particular no
recrystallization annealing has been carried out. Alternatively or
cumulatively, the flat
product can already have a homogenous initial hardness of at least 300 HV10,
for
example, due to the manufacturing process. HV is the Vickers hardness and
hardness
testing is regulated in DIN EN ISO 6507-1:2006-03. Preferably, the flat steel
product
consists of the following alloy components in % by weight:
0.15 <= C <= 0.6,
0.1 <= Si <= 1.2,
0.3 <= Mn <= 1.8,
0.1 <= Cr <= 1.8,
0.05 <= Mo <= 0.6,
0.05 <= Ni <= 3.0,
0.0005 <= B <= 0.01,
Al <=0.15,
Ti <=0.04,
<=0.04,
<= 0.03,
<= 0.03,
The remainder iron and unavoidable impurities. Preferably, the flat product is
a heavy
plate.
In order to generate an end product, which should not be designed to be flat,
according
to another embodiment of the method according to the invention, the flat
product is
formed and/or cut. If required, before and/or after its processing into an end
product,
for example, the flat product can be subjected to another thermal treatment.
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According to a second aspect, the invention relates to a thermally treated
flat steel
product that a structure with a second hardness is formed within the flat
product in
sections across the thickness of the flat product and a structure with a first
hardness is
formed across the thickness of the flat product at least in one section,
wherein the
structure with the second hardness has a higher level of hardness in
comparison with
the structure with the first hardness and is thermally treated. Preferably,
the structure
with the second hardness primarily consists of a martensitic or a
martensitic/bainitic
structure. The section with the structure with the first hardness, however,
consists of an
initial structure which corresponds to the structure in the as-delivered
condition and,
for example, can consist of a ferritic/perlitic or tempered martensitic
structure.
In order to avoid repetitions, reference will be made to favorable embodiments
of the
method according to the invention.
In accordance with a first embodiment of the flat product according to the
invention, the
structure with the second hardness is formed within an edge section of the
flat product,
wherein the layer thickness of the edge section can be at least 5% to a
maximum of 80%
of the total thickness of the flat product and the remaining thickness of the
flat product
consists of the section with the structure with the first hardness. By means
of the
asymmetrical hardness profile across the thickness of the flat product, a
"harder" and
,'more ductile" side is made available depending on the application.
In accordance with another embodiment of the flat product according to the
invention,
the structure with the second hardness is formed within both edge sections of
the flat
product or one edge section with a structure with a second hardness is formed
on one
side of the flat product and one edge section with a structure with a third
hardness is
formed on the other side of the flat product, wherein the structure with the
third
hardness has a lower level of hardness than the structure with the second
hardness,
however a higher level of hardness than the structure with the first hardness,
wherein
the layer thickness of the edge section can vary between at least 5% and a
maximum of
45% of the total thickness of the flat product respectively and the remaining
thickness is
formed by the section with the structure with the first hardness. Depending on
the
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application, on an individual basis, an asymmetric or symmetric hardness
profile can be
provided within a flat product with two "hard" or "harder" sides and one
"ductile" or
more ductile" core if both edge sections have the same dimensions.
In accordance with another embodiment of the flat product according to the
invention,
the flat product has a hardness difference of at least 100 HV10, in particular
at least 150
HV10, between the at least one edge section with the structure with the second
hardness
and/or third hardness and the section with the structure with the first
hardness. As a
result, a characteristics profile optimally adapted for any application can be
provided in
a monolithic flat product, which has only been possible by means of a
multilayer
construction up until this point.
In accordance with another embodiment of the flat product according to the
invention,
at least between an edge section with the structure with the second hardness
and/or
third hardness and the section with the structure with the first hardness, the
flat
product comprises an annealing section with a structure with a fourth
hardness, which
has at least a 10 HV10 lower level of hardness, in particular at least a 20
HV10 lower
level of hardness in comparison with the section with the structure with the
first
hardness.
In accordance with another embodiment of the flat product according to the
invention,
the flat product comprises a decarburized edge layer at least in one of the
edge sections
or comprises an edge layer with a structure with a fifth hardness, which has a
lower
level of hardness in comparison to the edge section. The decarburized edge
layer or the
edge layer with the structure with the fifth hardness can be present up to a
thickness of
a maximum of 10%, in particular a maximum of 5% with reference to the total
thickness
of the flat product.
In particular, the flat product has a total thickness between 3 and 80 mm, in
particular,
between 6 and 20 mm. Preferably, the flat steel product is made of a heavy
plate.
In accordance with a third aspect, the invention relates to a use of the flat
product
according to the invention, which can be optionally formed and/or cut into an
end
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product, as a part or component of an armoring or as a part or a component
with special
characteristics, in particular against of the effect of wear influences, such
as abrasive
and/or blast wear.
The invention is explained in detail in the following based on a drawing
showing
exemplary embodiments. Identical parts are referenced with same reference
numbers.
The figures show:
Figure la) a first exemplary embodiment for thermally treating a flat
product
in a schematic view,
Figure lb) an illustration of the hardness progression across the
thickness of
the thermally treated flat product according to the first exemplary
embodiment,
Figure 2a) a second exemplary embodiment for thermally treating a flat
product in a schematic view,
Figure 2b) an illustration of the hardness progression across the
thickness of
the thermally treated flat product according to the second
exemplary embodiment,
Figure 3a) a schematic cross section through a thermally treated flat
product
according to a third exemplary embodiment,
Figure 3b) an illustration of the hardness progression across the
thickness of
the thermally treated flat product in Figure 3a),
Figure 4a) a schematic cross section through a thermally treated flat
product
according to a fourth exemplary embodiment,
Figure 4b) an illustration of the hardness progression across the
thickness of
the thermally treated flat product in Figure 4a),
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Figure 5a) a schematic cross section through a thermally treated flat
product
according to a fifth exemplary embodiment,
Figure 5b) an illustration of the hardness progression across the
thickness of
the thermally treated flat product in Figure 5a),
Figure 6a) a schematic cross section through a thermally treated flat
product
according to a sixth exemplary embodiment,
Figure 6b) an illustration of the hardness progression across the
thickness of
the thermally treated flat product in Figure 6a).
A first exemplary embodiment for thermally treating a flat product (1) is
shown in
Figure la) in a schematic view. The flat product (1) consists of a
ferromagnetic steel
with a primarily homogeneous structure with a first hardness (1.1), for
example, of a
thermally treatable steel material with a ferritic/perlitic structure with a
thickness
between 3 and 80 mm, preferably between 6 and 20 mm, which is preferably made
of a
heavy plate. The flat product (1) has a length (L), a width, which is not
shown here
because of the sectional view and, for example, is many times smaller than the
length (L)
with regard to the dimension, and has a thickness and a total thickness (D).
The flat
product (1) is preferably thermally treated within the scope of a continuous
process at
least in sections, preferably across the entire width of the flat product (1)
and at least in
sections, preferably across the entire length (L) of the flat product (1). As
is shown in
Figure la), the flat product (1) is, for example, located on a roller conveyor
(R) and is
moved in the direction of a thermal treatment unit (W), symbolized by the
arrow shown.
The thermal treatment unit (W) comprises at least one heat source for the one-
sided
heating of the flat product (1), wherein at least one inductor (I) is
preferably used as a
heat source, and at least one cooling unit to cool down the heated flat
product (1), which
preferably comprises at least one water shower or water spray (B). The flat
product (1)
is heated to at least a temperature of at least Ac3 + 20 K via the inductor
(I) during the
heating process and held at this temperature for at least 1 s up to a maximum
of 60 s,
preferably a maximum of 10 s, wherein the area to be hardened (2.1) is fully
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austenitized within the flat product (1). Due to the heat conduction inside
the flat
product (1), it must be ensured that the area (2.1) to be hardened does not
exceed the
desired final thickness. Depending on the thermal treatment depth, the
austenitized area
to be hardened (2.1) is quenched via a water spray (B), wherein the cooling
speed > 30
K/s is selected in order to configure a hardening structure, for example, a
martensitic or
a martensitic/bainitic structure (2.2) in the edge section (2). Thereby, the
heating (I)
and the cooling (B) of the flat product (1) are coordinated with each other in
such a way
that a structure with a second hardness (2.2) is formed in sections across the
thickness
(D) of the flat product (1`), namely in the edge section (2), and the
structure with the
first hardness (1.2) remains the same at least in one section (1.1) across the
thickness
(D) of the flat product (1'), meaning that the section (1.1) is not or is not
significantly
influenced by the thermal treatment in a negative manner. As an alternative
and not
shown here, the thermal treatment unit and/or its units can be individually
arranged in
a movable manner across the flat product. The hardness profile across the
thickness of
the flat product (1') is shown in Figure lb) and shows that the edge section
(2)
comprises a structure with a second hardness (2.2), which is higher than the
section
(1.1) with the structure with the first hardness (1.2), wherein the hardness
difference is
preferably at least 100 HV10.
A second exemplary embodiment for thermally treating a flat product (1) is
shown in
Figure 2a) in a schematic view. In order to avoid repetitions, only the
differences in
comparison with the first exemplary embodiment will be explained. The flat
product (1)
shown on a roller conveyor (R) is moved in the direction of a thermal
treatment unit
(W), as is shown in Figure la). On the side facing away from the thermal
treatment unit
(W), there is a second thermal treatment unit (W'), which comprises at least
one heat
source, preferably at least one inductor (I') to heat the flat product (1) and
at least one
cooling unit, preferably at least one water shower or spray (B') to cool down
the heated
flat product (1). The heating takes place on both sides via at least one
inductor (I, I')
respectively, which preferably completely extends across the entire width of
the flat
product in order to cover the entire width of the flat product (1) and to
completely
austenitize the areas to be hardened (2.1, 2.1) within the flat product (1).
Depending on
thermal treatment depth, the austenitized areas to be hardened (2.1, 2'1) are
quenched
via water sprays (B, B'), wherein, for example, a martensitic or a
martensitic/bainitic
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structure (2.2, 2`.2) is configured in each case in the edge sections (2, 2').
Thereby, the
heating (I, I') and the cooling (B, B') of the flat product (1) are
coordinated with each
other in such a way that a structure with a second hardness (2.2, 2`.2) is
formed in
sections across the thickness (D) of the flat product (1`), namely in the edge
sections (2),
and the structure with the first hardness (1.2) remains the same at least in
one section
(1.2) across the thickness (D) of the flat product (1'), meaning that the
section (1.1) is
not or is not significantly influenced by the thermal treatment in a negative
manner and
this forms the core layer (1.1) of the flat product (1'). Thereby, thermal
treatment takes
place simultaneously on both sides. Alternatively and not shown here, the
thermal
treatment units can also be arranged offset to one another, whereby both edge
sections
can be created in a temporally offset manner. The hardness profile across the
thickness
of the flat product (1') is shown in Figure 2b) and shows that the edge
sections (2, 2')
have a structure with a second hardness (2.2, 2'.2), which are higher than the
section
(1.2) or the core layer with the structure with the first hardness (1.2).
Preferably, the
hardness difference is at least 100 HV10.
In Figures 3a), 4a), 5a) and 6a), cross sections through flat products (1')
manufactured
according to the invention with the related hardness profiles across the
respective
thickness (D) in Figures 3b), 4b), 5b) and 6b) are shown.
In a schematic cross section through a flat product (1') thermally treated
according to a
third exemplary embodiment, a section (1.1), in particular a core layer with a
structure
with a first hardness (1.2), two edge sections (2, 2') with a structure with a
second
hardness (2.2, 2'.2), and, respectively, an annealing section (3, 3') with a
structure with a
fourth hardness (3.2, 3`.2) between the edge sections (2, 21 and the section
(1.1) are
shown. The annealing sections (3, 3') have in each case a lower hardness by at
least 10
HV10 in comparison to the section (1.1). The section (1.1) and the edge
sections (2, 2')
each correspond to 30% of the total thickness (D) of the flat product (1') and
the
annealing sections (3, 3') take up another 5% respectively; Figure 3a). The
symmetrical
hardness profile across the thickness (D) is shown in Figure 3b).
In a schematic cross section through a flat product (1') thermally treated
according to a
fourth exemplary embodiment, in comparison to the third exemplary embodiment,
the
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difference exists in the fact that the upper edge section (2) with a structure
with a
second hardness (2.2) is designed to be thicker, which corresponds to 50% of
the total
thickness (D) for example, and the lower edge section (2') with a structure
with a second
hardness (2'2) is designed to be thinner, which, for example, corresponds to
10% of the
total thickness (D); Figure 4a). The asymmetrical hardness profile across the
thickness
(D) is shown in Figure 4b).
In a schematic cross section through a flat product (1') thermally treated
according to a
fifth exemplary embodiment, a section (1.1), in particular, a core layer with
a structure
with a first hardness (1.2), and two edge sections (2, 2') with a structure
with a second
hardness (2.2, 2'.2) are shown. In both edge sections (2, 2`), the flat
product (1')
comprises a decarburized edge layer respectively or comprises an edge layer
(4, 4') with
a structure with a fifth hardness (4.2, 4'.2) respectively, which has a lower
level of
hardness in comparison to the edge section (2, 2'). The section (1.1) is 30
c/o and the
edge sections (2, 2') each correspond to 35 % of the total thickness (D) of
the flat
product (1'), wherein the decarburized edge section or the edge layer (4, 4')
can be
present up to a maximum thickness of 5% with reference to the total thickness
(D) of the
flat product (1'); Figure Sa). The symmetrical hardness profile across the
thickness (D)
is shown in Figure 5b).
In a schematic cross section through a flat product (1') thermally treated
according to a
sixth exemplary embodiment, a section (1.1) with a structure with a first
hardness (1.2),
an edge section (2) with a structure with a second hardness (2.2), and an
annealing
section (3) with a structure with a fourth hardness (3.2) between the edge
section (2)
and the section (1.1) are shown. In the edge section (2), the flat product
(1') comprises a
decarburized edge layer or comprises an edge layer (4) with a structure with a
fifth
hardness (4.2). The section (1.1) has a thickness of 35%, the annealing
section (3) has a
thickness of 5%, the edge section (2) has a thickness of 60%, from which a
thickness of a
maximum of 5% can be omitted for the edge layer (4), with reference to the
total
thickness (D) of the flat product (1'); Figure 6a). The asymmetrical hardness
profile
across the thickness (D) is shown in Figure 6b).
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14
The design of the sections with various levels of hardness is not limited to
the exemplary
embodiments shown. Rather, for example, one of the edge layers can comprise a
structure with a third hardness, wherein the structure with the third hardness
can have
a lower level of hardness than the structure with the second hardness,
however, a higher
level of hardness than the structure with the first hardness. In particular,
due to the
manufacturing process, the flat steel products can be used in the as-rolled
condition as
well as alternatively or cumulatively already with a homogeneous initial
hardness of at
least 300 HV10 for example. The flat products according to the invention,
which can be
optionally formed and/or cut into an end product, are either used as a part or
component of an armoring or as a part or a component with special
characteristics, in
particular, against the effect of wear influences. Other application fields
are also
conceivable, in which flat products or end products with at least one section
with a
structure with a first hardness across the thickness and at least one section
with a
structure with a second hardness across the thickness of the flat or end
product can be
used, wherein the second hardness is greater than the first hardness.
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Reference list
1 flat product
1' thermally treated flat product
1.1 section, core layer
1.2 structure with a first hardness
2, 2' edge section
2.1, 2%1 austenitized, heated area to be annealed
2.2, 2'.2 structure with a second hardness
3, 3' annealing section
3.2, 3'.2 structure with a fourth hardness
4, 4' decarburized edge layer, edge layer
4.2, 4`.2 structure with a fifth hardness
B, B' cool-down spray
thickness, total thickness
I, I' inductor
W, W' thermal treatment unit
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