Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR MANUFACTURING ALMGSI ALUMINIUM STRIP
The invention relates to a method for manufacturing a strip from
an AlMgSi alloy, in which a rolling ingot is cast from an AlMgSi
alloy, the rolling ingot undergoes homogenization, the rolling
ingot which has been brought to rolling temperature is hot
rolled then optionally cold rolled to final thickness and the
finished strip is solution annealed and quenched. Furthermore,
the invention relates to advantageous uses of an AlMgSi
aluminium strip which has been manufactured accordingly.
In particular in automotive vehicle construction, but also in
other application fields, for example, aircraft construction or
rail vehicle construction, metal sheets of aluminium alloys are
required which are not only distinguished by particularly high
strength values, but at the same time have a very good
formability and enable high degrees of deforming. In automotive
vehicle construction, typical application fields are the
bodywork and chassis components. In the case of visible painted
components, for example, metal bodywork sheets which can be seen
from the outside, additionally the forming of the materials has
to be carried out in such a manner that the surface is not
impaired by defects such as flow figures or roping after the
painting. This is particularly important, for example, for the
use of aluminium alloy metal sheets for producing bonnets and
other bodywork components of an automotive vehicle. However, it
choice of material is restricted with respect to the aluminium
alloy. In particular AlMgSi alloys, the main alloy constituents
of which are magnesium and silicon, have relatively high
strengths in the state T6 with, at the same time, good forming
behaviour in the state T4 and excellent corrosion resistance.
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AlMgSi alloys are the alloy types AA6XXX, for example, the alloy
type AA6016, AA6014, AA6181, AA6060 and AA6111. Conventionally,
aluminium strips are produced from an AlMgSi alloy by means of
casting a rolling ingot, homogenising the rolling ingot, hot
rolling the rolling ingot and cold rolling the hot strip. The
homogenisation of the rolling ingot is carried out at a
temperature from 380 to 580 C for more than one hour. Owing to
a final solution-annealing operation at typical temperatures
from 500 C to 570 C with subsequent quenching and natural
ageing approximately at ambient temperature for at least three
days, the strips can be delivered in the state T4. The state T6
is set after the quenching by means of artificial ageing at
temperatures between 100 C and 220 C.
It is problematic that, in hot-rolled aluminium strips of AlMgSi
alloys, coarse Mg2Si precipitations are present which are broken
and comminuted in the subsequent cold rolling by high degrees of
forming. Hot strips of an AlMgSi alloy are usually produced in
thicknesses from 3 mm to 12 mm and supplied to a cold-rolling
operation with high degrees of forming. Since the temperature
range in which the AlMgSi phases are formed is passed through
very slowly during conventional hot rolling, these phases are
formed in a very coarse manner. The temperature range for
forming the above-mentioned phases is alloy dependent. However,
it is between 230 C and 550 C, that is to say, in the range of
the hot-rolling temperatures. It could be proven experimentally
that these coarse phases in the hot strip have a negative
influence on the elongation of the end product. This means that
the formability of aluminium strips from AlMgSi alloys could
previously not be fully exploited.
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From the published European Patent Application EP 2 270 249 Al,
belonging to the same Applicant, it was proposed that the AlMgSi
alloy strip, directly after leaving the last hot rolling pass,
has a temperature of a maximum of 130 2C and is coiled at this
or a lower temperature. Owing to the quenching of the hot strip
with this method, aluminium strips in the state T4 were able to
be produced, which in the state T4 has an elongation at break of
A80 of more than 30 % or a uniform elongation Ag of more than
25 %. Furthermore, very high values for the elongation at break
in the state T6 were also produced. However, it has been found
that this temperature range at the outlet of the last hot
rolling pass leads to problems with respect to the surface
evenness of the hot strip so that the subsequent production
steps were impaired. Furthermore, the predetermined cooling rate
could be achieved only at reduced production speeds. Based on
this prior art, an object of the present invention is to provide
an improved method for manufacturing an aluminium strip from an
AlMgSi alloy, by means of which method AlMgSi aluminium strips
with very good formability in the state T4 may be produced in an
operationally reliable manner.
According to a first teaching of the present invention, the
object set out for a method is achieved in that, immediately
after being discharged from the last hot rolling pass, the hot
strip is at a temperature of more than 130 20 to 250 C,
preferably to 230 2C and the hot strip is coiled at this
temperature.
In contrast to the known method with particularly low coiling
temperatures, it has surprisingly been found that the mechanical
properties with respect to the uniform elongation Ag which
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determines the formability did not change or changed only
insignificantly in spite of the changed coiling temperatures.
The AlMgSi alloy strips produced according to the invention in
the state T4 further showed a uniform elongation of more than
25 % in the tensile test according to DIN EN. Furthermore, they
had the very good hardenability in the state T6 as known from
the prior application of the Applicant. However, the production
method could be substantially stabilised and a higher production
speed could be achieved.
According to an advantageous embodiment of the method according
to the invention, this cooling process is carried out within the
last two hot-rolling passes, that is to say, the cooling to more
than 130 C, preferably from 135 C to 250 QC, preferably from
135 QC to 230 C, is carried out within seconds, at the most
within 5 minutes. It has been found that, in this method, the
increased uniform elongation values with usual strength and
yield point values in the state T4 and the improved
hardenability in the state T6 are achieved in a particularly
operationally reliable manner.
According to another embodiment of the method according to the
invention, operationally reliable cooling of the hot strip is
achieved in that the hot strip is quenched to the outlet
temperature using at least one plate cooler and the hot rolling
pass itself, loaded with emulsion. A plate cooler comprises an
arrangement of cooling or lubrication nozzles which spray a
rolling emulsion onto the aluminium strip. The plate cooler may
be present in a hot rolling installation in order to cool rolled
hot strips to rolling temperature before hot rolling and in
order to be able to achieve higher production speeds.
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If the temperature of the hot strip before the start of the
cooling process, which preferably takes place within the last
two rolling passes, is at least 400 2C, preferably from 470 QC
5 to 490 2C, according to a next embodiment of the method, it is
possible for particularly small Mg2Si precipitations to be
present in the quenched hot strip, since the largest proportion
of the alloy constituents magnesium and silicon are present in
the dissolved state in the aluminium matrix at these
temperatures. This advantageous state of the hot strip is
practically "frozen" by the quenching operation.
According to another embodiment of the method, the temperature
of the hot strip after the penultimate rolling pass is from
290 C to 310 C. It has been found both that these temperatures
enable sufficient freezing of the precipitations and, on the
other hand, at the same time, the last rolling pass can be
carried out without any problems.
If the rolled hot strip has, immediately after being discharged
from the final hot rolling pass, a temperature of from 200 2C to
230 2C, an optimum process speed can be achieved during hot
rolling, without the properties of the aluminium strip produced
being impaired.
The thickness of the prepared hot strip is from 3 mm to 12 mm,
preferably from 5 mm to 8 mm, so that conventional cold rolling
mills can be used for the cold rolling operation.
The aluminium alloy used is preferably of the alloy type AA6xxx,
preferably AA6014, AA6016, AA6060, AA6111 or AA6181. All the
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alloy types AA6xxx have the common feature that they have
particularly good formability, characterised by high elongation
values in the state T4 and high strengths or yield points in the
state for use T6, for example, after artificial ageing at
205 2C/30 minutes.
According to another embodiment of the method according to the
invention, the finished, rolled aluminium strip undergoes a heat
treatment, in which the aluminium strip is heated to more than
100 C after solution annealing and quenching and then coiled
and aged at a temperature of more than 55 C, preferably more
than 85 C. This embodiment of the method enables, after the
natural ageing, by means of a shorter heating phase at lower
temperatures the state T6 to be set in the strip or the metal
sheet, in which the metal sheets or strips formed to components
are used in the application. To this end, these rapidly
hardening aluminium strips are heated only to temperatures of
approximately 185 C for only 20 minutes in order to achieve the
higher yield point values in the state T6.
The elongation at break values Ago of the aluminium strips
produced by means of this embodiment of the method according to
the invention in the state T4 are slightly below 29%. However,
the aluminium strip produced according to the invention is
further distinguished after the ageing in the state T4 by a very
good uniform elongation Ag of more than 25%. The term uniform
elongation Ag is intended to be understood to refer to the
maximum elongation of the sample, at which no necking of the
sample can be identified during the tensile test. The sample is
thus expanded in a uniform manner in the region of the uniform
elongation. The value for the uniform elongation for similar
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materials was previously at a maximum of 22% to 23%. The uniform
elongation has a significant influence on the formability since
it determines the maximum degree of forming of the material used
in practice. In this regard, using the method according to the
invention, an aluminium strip having very good forming
properties can be provided, and can also be converted into the
state T6 by means of an accelerated artificial ageing operation
(185 2C/20 min).
An aluminium alloy of the type AA6016 has the following alloy
constituents in percent by weight:
0.25 % Mg 0.6 %
1.0 % Si 1.5 %
Fe O.5%
Cu 0.2 %
Mn 0.2 %
Cr 0.1 %
Zn 0.1 %
Ti 0.1 %
and the remainder being Al and inevitable impurities, up to a
maximum in total of 0.15 %, individually a maximum of 0.05 %.
With magnesium contents of less than 0.25% by weight, the
strength of the aluminium strip which is provided for structural
applications is too low but, on the other hand, the formability
with magnesium contents of above 0.6% by weight becomes worse.
Silicon, together with magnesium, is substantially responsible
for the hardenability of the aluminium alloy and consequently
also for the high strengths which can be achieved in the
application, for example, after paint baking. With Si contents
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of less than 1.0% by weight, the hardenability of the aluminium
strip is reduced so that in the application only reduced
strengths can be provided. Si contents of more than 1.5% by
weight do not lead to an improvement of the hardening behaviour
of the alloy. The Fe proportion should be limited to a maximum
of 0.5% by weight in order to prevent coarse precipitations. A
limitation of the copper content to a maximum of 0.2% by weight
in particular leads to improved corrosion resistance of the
aluminium alloy in the specific application. The manganese
content of less than 0.2% by weight reduces the tendency for the
formation of relatively coarse manganese precipitations.
Although chromium ensures a fine microstructure, it is intended
to be limited to 0.1% by weight in order to also prevent coarse
precipitations. The presence of manganese in contrast improved
the weldability by reducing the tendency for cracking or
susceptibility to quenching of the aluminium strip according to
the invention. A reduction of the zinc content to a maximum of
0.1% by weight in particular improves the corrosion resistance
of the aluminium alloy or the finished metal sheet in the
respective application. In contrast, titanium ensures grain
refinement during the casting operation, but is intended to be
limited to a maximum of 0.1% by weight in order to ensure good
castability of the aluminium alloy.
An aluminium alloy of the type AA6060 has the following alloy
constituents in percentage by weight:
0.35 % Mg 0.6 %
0.3 % Si 0.6 %
O.1% Fe 0.3 %
Cu 0.1 %
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Mn 0.1 %
Cr 0.05 %
Zn 0.10 %
Ti 0.1 % and
the remainder being Al and inevitable impurities, up to a
maximum in total of 0.15%, individually a maximum of 0.05%.
The combination of a precisely predetermined magnesium content
with an Si content which is reduced in comparison with the first
embodiment and narrowly specified Fe content produces an
aluminium alloy in which the formation of Mg2Si precipitations
after hot rolling with the method according to the invention can
be prevented particularly well so that a metal sheet having
improved elongation and high yield points in comparison with
conventionally produced metal sheets can be provided. The lower
upper limits of the alloy constituents Cu, Mn and Cr
additionally increase the effect of the method according to the
invention. With respect to the effects of the upper limit of Zn
and Ti, reference may be made to the statements relating to the
first embodiment of the aluminium alloy.
An aluminium alloy of the type AA6014 has the following alloy
constituents in percentage by weight:
0.4 % Mg 0.8 %
0.3 % Si 0.6 %
Fe 0.35 %
Cu 0.25 %
0.05 % Mn 0.20 %
Cr 0.20%
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Zn 0.10 %
0.05 % v 0.20 %
Ti 0.1 % and
5 the remainder being Al and inevitable impurities, up to a
maximum in total of 0.15%, individually a maximum of 0.05%.
An aluminium alloy of the type AA6181 has the following alloy
constituents in percentage by weight:
0.6 % Mg 1.0 %
0.8 % Si 1.2 %
Fe 0.45 %
Cu 0.10 %
Mn 0.15%
Cr 0.10%
Zn 0.20 %
Ti 0.1 % and
the remainder being Al and inevitable impurities, up to a
maximum in total of 0.15%, individually a maximum of 0.05%.
An aluminium alloy of the type AA6111 has the following alloy
constituents in percentage by weight:
0.5 % Mg 1.0 %
0.7 % Si 1.1 %
Fe 0.40 %
0.50 % Cu 0.90 %
O.15% Mn 0.45%
Cr 0.10 %
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Zn 0.15 %
Ti 0.1 % and
the remainder being Al and inevitable impurities, up to a
maximum in total of 0.15%, individually a maximum of 0.05%. The
alloy AA6111 has in principle owing to the higher copper content
higher strength values in the application state T6, but must be
classified as being more susceptible to corrosion.
All of the aluminium alloys set out are adapted specifically in
terms of their alloy constituents to different applications. As
already set out, strips of these aluminium alloys, which have
been produced using the method according to the invention, have
particularly good uniform elongation values in the state T4
paired with a particularly evident increase of the yield point,
for example, after artificial ageing at 205 2C/30 min.
This also applies to the aluminium strips in the state T4 which
have been subjected to heat treatment after the solution
annealing.
Owing to the excellent combination between good formability in
the state T4, high corrosion resistance and high values for the
yield point Rp0.2 in the state for use (state T6), the object
set out above is achieved according to a second teaching of the
present invention by the use of an AlMgSi alloy strip produced
by the method according to the invention for a component,
chassis or structural part or panel in automotive, aircraft or
railway vehicle engineering, in particular as component, chassis
part, external or internal panel in automotive engineering,
preferably as a bodywork component. In particular, visible
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bodywork components, for example, bonnets, mudguards, etc., and
outer skin components of a railway vehicle or aircraft benefit
from the high yield points Rp0.2 with good surface properties
even after a forming with high degrees of deformation.
A rapidly hardening AlMgSi alloy strip having excellent
formability can therefore be provided by an aluminium alloy
strip which has been produced according to the invention and
which has been subjected to a solution annealing with subsequent
heat treatment after the production thereof. In the state T4, as
already set out, it has a uniform elongation Ag of more than 25%,
for example, with an yield point Rp0.2 of from 80 to 140 MPa.
With this variant, a rapidly hardenable and at the same time
very readily formable AlMgSi alloy strip can be provided. The
artificial ageing in order to achieve the state T6 can be
carried out at 185 C for 20 minutes in order to achieve the
required increase of the yield point.
According to a next embodiment, an aluminium alloy strip which
has been produced according to the invention has a uniform
elongation Ag of more than 25% in the rolling direction,
transversely relative to the rolling direction and diagonally
relative to the rolling direction so that a particularly
isotropic formability is enabled.
Preferably, the aluminium strips produced according to the
invention have a thickness from 0.5 mm to 12 mm. Aluminium
strips having thicknesses from 0.5 mm to 2 mm are preferably
used for bodywork components, for example, in automotive vehicle
construction, whilst aluminium strips having larger thicknesses
from 2 to 4.5 mm are used, for example, in chassis components
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for automotive vehicle construction. Individual components can
also be produced in a cold strip with a thickness of up to 6 mm.
In addition, in specific applications, aluminium strips with
thicknesses of up to 12 mm can also be used. These aluminium
strips with a very large thickness are conventionally provided
only by means of hot rolling.
The invention will now be explained in greater detail with
reference to embodiments together with the drawing.
The drawing shows in the single Fig. 1 a schematic flow chart of
an embodiment of the method according to the invention for
manufacturing a strip from an MgSi aluminium alloy having the
steps of a) producing and homogenising the rolling ingot, b) hot
rolling, c) cold rolling and d) solution annealing with
quenching.
A rolling ingot 1 is first cast from an aluminium alloy with the
following alloy constituents in percentage by weight:
0.25 % Mg 0.6 %
1.0 % Si 1.5 %
Fe 0.50 %
Cu 0.20 %
Mn 0.20 %
Cr 0.10%
Zn 0.20 %
Ti 0.15 % and
the remainder being Al and inevitable impurities, up to a
maximum in total of 0.15%, individually a maximum of 0.05%.
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The rolling ingot produced in this manner is homogenised at a
homogenisation temperature of approximately 550 C for 8 hours
in a furnace 2 so that the alloy constituents which have been
added by means of alloying are present in the rolling ingot in a
state distributed in a particularly homogeneous manner, Fig. la).
Fig. lb illustrates how the rolling ingot 1 in the present
embodiment of the method according to the invention is hot
rolled in a reversing manner by means of a hot rolling mill 3,
the rolling ingot 1 having a temperature of from 400 to 5502C
during the hot rolling operation. In this embodiment, after
leaving the hot rolling mill 3 and before the penultimate hot
rolling pass, the hot strip 4 preferably has a temperature of at
least 400 C, preferably from 470 C to 490 C. Preferably at
this hot strip temperature, the quenching of the hot strip 4 is
carried out using a plate cooler 5 and the operating rollers of
the hot rolling mill 3. Preferably, the hot strip is in this
instance cooled to a temperature of from 290 C to 310 C before
the last hot rolling pass. To this end, the plate cooler 5,
illustrated only schematically, sprays the hot strip 4 with
cooling rolling emulsion and ensures accelerated cooling of the
hot strip 4 to the last-mentioned temperatures. The operating
rollers of the hot rolling mill 3 are also loaded with emulsion
and further cool the hot strip 4 during the last hot rolling
pass. After the last rolling pass, the hot strip 4 has at the
output of the plate cooler 5 in the present embodiment a
temperature from 200 C to 230 C and is subsequently coiled at
this temperature by means of the recoiler 6.
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Owing to the fact that the hot strip 4 directly at the outlet of
the last hot-rolling pass has a temperature of more than 135 C
to 250 C, preferably from 200 C to 2302C, or optionally in the
last two hot rolling passes, using the plate cooler 5 and the
5 operating rollers of the hot rolling mill 3, is brought to the
temperatures mentioned, in spite of the increased winding
temperature, the hot strip 4 has a frozen crystalline
microstructural state which leads to very good uniform
elongation properties Ag of more than 25% in the state T4.
10 Nonetheless, owing to the higher coiling temperature, it can be
processed more rapidly and advantageously. The hot strip having
a thickness from 3 to 12 mm, preferably from 5 to 8 mm, is
coiled via the recoiler 6. As already set out, the coiling
temperature in the present embodiment is preferably from 135 C
15 to 250 C.
In the method according to the invention, no or only a few
coarse Mg2Si precipitations are now able to form in the coiled
hot strip 4. The hot strip 4 has a very favourable crystalline
state for further processing and can be decoiled from the
decoiler 7, supplied, for example, to a cold rolling mill 9 and
recoiled on a recoiler 8, Fig. 1c).
The resulting cold rolled strip 11 is coiled. Subsequently, it
is supplied to a solution annealing operation at temperatures
from 520 C to 5702C and a quenching operation 10, Fig. 1d). To
this end, it is decoiled again from the coil 12, solution
annealed in a furnace 10 and quenched, and again coiled to a
coil 13. After natural ageing at ambient temperature in the
state T4, the aluminium strip can then be supplied with maximum
formability. Alternatively (not illustrated), the aluminium
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strip 11 can be separated into individual sheets, which are
present in the state T4 after natural ageing.
With larger aluminium strip thicknesses, for example, with
chassis applications or components, such as, for example, brake
anchor plates, piece annealing operations can also alternatively
be carried out and the metal sheets can be subsequently quenched.
The aluminium strip or the aluminium sheet is brought into the
state T6 by means of artificial ageing at from 100 C to 220 2C
in order to achieve maximum values for the yield point. For
example, artificial ageing can also be carried out at 205 2C/30
min.
The aluminium strips produced in accordance with the embodiment
illustrated have after the cold rolling, for example, a
thickness of from 0.5 to 4.5 mm. Strip thicknesses from 0.5 to 2
mm are conventionally used for bodywork applications or strip
thicknesses of from 2.0 mm to 4.5 mm for chassis components in
automotive vehicle construction. In both application fields, the
improved uniform elongation values are a decisive advantage in
the production of components since in most cases significant
deformations of the metal sheets are carried out and nonetheless
great strengths are required in the state for use (T6) of the
end product.
Table 1 sets out the alloy constituents of aluminium alloys from
which aluminium strips are produced in a conventional manner or
according to the invention. In addition to the shown contents of
alloy constituents, the aluminium strips contain as remainder
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aluminium and impurities, individually at a maximum of 0.05% by
weight and in total a maximum of 0.15% by weight.
Strips Si % Fe % Cu % Mn % Mg % Cr % Zn % Ti %
by by by by by by by by
weight weight weight weight weight weight weight weight
251 1.3 0.19 0.06 0.3 0.01 0.02
252 1.3 0.19 0.06 0.3 0.01 0.02
491-1 1.39 0.18 0.002 0.062 0.30 0.0006 0.01 0.0158
491-11 1.40 0.18 0.002 0.063 0.31 0.0006 0.0104 0.0147
Table 1
The strips (samples) 251 and 252 were produced using a method
according to the invention in which the hot strip was cooled and
coiled within the last two hot rolling passes from approximately
470 C to 490 QC to from 135 C to 250 C using a plate cooler
and the hot rollers themselves. In Table 2, the measurement
values of these strips are designated "Inv.". Subsequently, a
cold rolling operation was carried out to a final thickness of
0.865 mm.
The strips (samples) 491-1 and 491-11 were produced using a
conventional hot rolling and cold rolling operation and
designated "Conv.".
The results of the mechanical properties illustrated in Table 2
clearly show the difference in the achievable uniform elongation
values Ag.
=
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Strips T4 T6 205 9C/30 Min.
Thickness Rp0.2 Rm Ag An Rp0.2 Rm Ag
(mm) (MPa) (MPa) (%) (95) (MPa) (MPa)
(%) Rp0.2
(MPa)
251 L Inv. 0.865 93 207 26.3 30.4
251 Q Inv. 0.865 86 203 26.4 29.0 193 249 12.4
107
251 D Inv. 0.865 87 203 27.0 30.0
252 L Inv. 0.865 93 206 26.1 31.5
252 Q Inv. 0.865 88 205 26.6 29.0 185 244 12.2
97
252 D Inv. 0.865 87 202 27.3 31.1
491-1 Conv. 1.04 92 202 23.1 27.8 180 235 10.7 88
491-11 Cony. 1.04 88 196 23.0 27.4 179 232 11.2 91
Table 2
In order to achieve the T4 state, the strips were subjected to a
solution-annealing operation with subsequent quenching and
subsequent natural ageing for eight days at ambient temperature.
The T6 state was achieved by means of artificial ageing which
followed the natural ageing at 205 2C for 30 minutes.
The samples designated L were cut out in the rolling direction,
the samples designated Q transversely relative to the rolling
direction and the samples designated D diagonally with respect
to the rolling direction. The samples 491-1 and 491-11 were each
measured transversely relative to the rolling direction.
It has been found that the advantageous microstructure, which
was adjusted in the strips 251 and 252 by means of the method
according to the invention, with an identical yield point Rp0.2
and strength Rm, enables a substantial increase of the uniform
elongation Ag. The uniform elongation Ag increased from 23.0% to
a maximum of 26.6% transversely relative to the rolling
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direction in the strips produced according to the invention in
comparison with the conventionally produced strips.
The microstructure configured with the method according to the
invention leads to the particularly advantageous combination of
high uniform elongation Ag of more than 25 % with very high
values for the yield point Rp0.2 from 80 to 140 MPa. In the
state T6, the yield point Rp0.2 increases up to at least 185 MPa,
the uniform elongation Ag further remaining at more than 12%. The
hardenability with a LRp0.2 of 97 or 107 MPa is furthermore very
good in the strips produced according to the invention.
In the state T6, the increase of the uniform elongation Ag in
comparison with conventionally produced strips was almost able
to be preserved.
The elongation at break values Ag and Ago, the yield point values
Rp0.2 and the tensile strength values Rm in the Tables were
measured in accordance with DIN EN.
